Solar Domestic Water Heating: The Earthscan Expert Handbook for Planning, Design and Installation

Solar Domestic Water Heating The Earthscan Expert Handbook for Planning, Design and Installation Chris Laughton s e ries editor : f r a n k jac k son

p u b l i s h i n g fo r a s u s t a i n a b l e f u t u re

London • Washington, DC

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First published in 2010 by Earthscan Copyright © Chris Laughton, 2010 The moral right of the author has been asserted. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as expressly permitted by law, without the prior, written permission of the publisher. While the author and the publishers believe that the information and guidance given in this work are correct, all parties must rely upon their own skill and judgement when making use of them – it is not meant to be a replacement for manufacturer’s instructions and legal technical codes. Neither the author nor the publisher assumes any liability for any loss or damage caused by any error or omission in the work. Any and all such liability is disclaimed. This book was written using principally metric units. However, for ease of reference of readers more familiar with imperial units, the author has inserted these in the text in brackets after their metric equivalents. Please note that some conversions may have been rounded up or down for the purposes of clarity. Earthscan Ltd, Dunstan House, 14a St Cross Street, London EC1N 8XA, UK Earthscan LLC, 1616 P Street, NW, Washington, DC 20036, USA Earthscan publishes in association with the International Institute for Environment and Development For more information on Earthscan publications, see www.earthscan.co.uk or write to [email protected] ISBN: 978-1-84407-736-6 Typeset by Domex e-Data Pvt. Ltd. Cover design by Yvonne Booth A catalogue record for this book is available from the British Library Library of Congress Cataloging-in-Publication Data has been applied for.

At Earthscan we strive to minimize our environmental impacts and carbon footprint through reducing waste, recycling and offsetting our CO2 emissions, including those created through publication of this book. For more details of our environmental policy, see www.earthscan.co.uk. Printed and bound in the UK by Scotprint, an ISO 14001 accredited company. The paper used is FSC certified and the inks are vegetable based.

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contents Acknowledgements List of Acronyms and Abbreviations

vii ix

1

What is Solar Heating? 1.1 What solar water heating is about 1.2 A short history of solar water heating 1.3 What do solar water heating systems look like? 1.4 What the technology can achieve 1.5 Other solar technologies 1.6 Energy supply issues, peak oil, resources depletion 1.7 Climate change and CO2 emissions 1.8 Purchasing a solar heating system

1 1 1 2 3 7 9 10 11

2

Solar Radiation and Resources 2.1 Solar radiation and the solar resource 2.2 Quantifying solar energy 2.3 Solar geometry, angles and orientation

13 13 15 19

3

How Solar DHW Works 3.1 How solar domestic hot water systems work 3.2 Main system components and their functions 3.3 Energy and mass flows

23 23 28 29

4

Solar Collectors 4.1 Solar collector absorbers 4.2 Flat plate collectors 4.3 Evacuated tube collectors 4.4 Collector components 4.5 Self-build collectors 4.6 Unglazed collectors 4.7 Collector performance 4.8 Using collector performance test reports 4.9 Typical collector performances values

31 31 37 40 48 51 52 52 60 63

5

solar heating systems 5.1 System performance 5.2 Typical system performance values 5.3 Reduction of fuel bills and pollution 5.4 Measuring solar contribution 5.5 Required DHW temperature 5.6 Using system performance test reports 5.7 Collector and system selection

65 65 67 68 71 73 76 78

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iv   solar domestic water heating 6

additional components 6.1 Main system components 6.2 Heat transfer fluids (HTFs) 6.3 Pipes and pipe fittings 6.4 Solar storage tank 6.5 Heat circulation and pumps 6.6 Controls for safety, performance and information 6.7 Heat exchangers 6.8 Back-up sources of heat 6.9 The water supply

83 83 83 86 89 96 100 103 112 116

7

system layouts 7.1 System layouts 7.2 Integral collector storage (ICS) 7.3 Passive (thermosyphon) systems 7.3 Active direct fully filled systems 7.4 Active drainback indirect systems 7.5 Active fully filled indirect systems 7.6 Choosing the most suitable layout

117 117 118 119 122 123 124 126

8

designing a system 8.1 Overview of design principles 8.2 Technical survey 8.3 Site visit 8.4 Cold and DHW water pressure 8.5 Occupant’s DHW use routine 8.6 Collector location 8.7 Solar storage tank and other equipment 8.8 Roof coverings 8.9 Collector orientation, angles and shading 8.10 Distances between components 8.11 Retention of the building’s insulated structure 8.12 Circulation pumps and circulation rates 8.13 Expansion or explosion of components 8.14 Steam or scalding water 8.15 Bacteria 8.16 Freeze damage 8.17 Mineral deposits, silt and other debris from water supplies 8.18 Loss of cold drinking water quality 8.19 Loss of hot water quality 8.20 Future plumbing and electrical system maintenance issues 8.21 Indication of correct operation 8.22 Animal and insects 8.23 Ultraviolet, heat and vibration degradation 8.24 Other issues

129 129 129 130 131 131 133 134 134 137 139 140 140 140 144 145 146 147 148 149 149 150 151 151 152

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contents   v

9

Sizing System Components 9.1 System sizing 9.2 Manual sizing method for collectorand storage tank using data table 9.3 Estimating collector area based on DHW use and annual irradiation 9.4 Example calculation 9.5 Shading 9.6 Sizing other system components 9.7 Software calculation methods

153 153 153 157 158 160 160 163

10

installation 10.1 Health and safety 10.2 Overview 10.3 Solar storage tank 10.4 Collectors 10.5 Pipes, joints and insulation 10.6 Pumps, valves and vessels 10.7 Electrical work and controls 10.8 Final filling, commissioningand handover 10.9 Sample commissioning sheet 10.10 User information: How best to use system 10.11 Building codes and planning permits

165 165 166 169 170 173 174 179 179 184 187 187

11 the economics of solar water heating 11.1 How long does it take to install a system? 11.2 The market and marketing

189 192 192

12

197 197 197 199 201

other types of solar heating 12.1 Larger systems 12.2 Solar air collectors assisting space heating 12.3 Swimming pools 12.4 Solar cooling

13 Case Studies

203

14 Glossary

221

Further Information Conversion Tables Index

227 233 237

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acknowledgements The author would like to especially thank the following:

• • • • • • • • • • • • •

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Frank Jackson Kirsty Sedgmen Claire Maynard Centre of Alternative Technology Tom Lane Volker Quaschning Jean Marc Suter Jan Erik Nielsen Valentin Energie Software GmbH Norfolk Solar ESTIF The volunteers of the solar standards committees All image providers, who I hope are correctly credited next to their images.

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list of acronyms and abbreviations AM ANSI ASHRAE ASTM CEN CSP DHW DTC EF EPDM ESTIF ETC HTF IAM ICS ISO PV SEF SF SRCC SWH

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air mass American National Standards Institute American Society of Heating, Refrigerating and Air-Conditioning Engineers American Society for Testing and Materials Committee for Standardization (Europe) concentrating solar energy domestic hot water differential thermostat control energy factor ethylene propylene diene monomer European Solar Trade Industry Federation evacuated tube collector heat transfer fluid incidence angle modifier integral collector storage International Organization for Standardization photovoltaic solar energy factor solar fraction Solar Rating and Certification Corporation solar water heating

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1

What is Solar Heating? This chapter explains what solar energy is about, focusing particularly on solar water heating. It describes what solar energy systems look like, the general principles involved, why it is a good idea and how it fits into the overall matrix of solar energy technologies.

1.1 What solar water heating is about The sun’s energy is the source of life on the planet. It reaches the Earth’s surface in the form of radiation. Plants harness it via photosynthesis. It provides heat, which can be harvested, and it can be converted into electricity. When sunlight falls on a surface, some of the sunlight’s energy is absorbed and the surface warms up. Paint the surface a dark colour and more of the energy is absorbed. Put the surface plate in a box, glaze over the front and insulate the box and less heat will be lost to the surrounding air. This heat can be used to raise the temperature of the water that we use for cleaning, cooking and other processes. Solar-heated water can also be used for swimming pools, space heating and even to help cool buildings. The sun’s energy varies seasonally and, due to daily changes in the weather, is not always reliable. However, solar energy can be stored and used when the sun is not shining. Solar collectors can range from simple flat plate collectors with no glazing, to boxes or tubes covered with glazing, through to complex arrangements of mirrors. Solar collectors are usually found on the roofs of buildings but can also be fixed to vertical walls and balustrades or mounted on the ground. They are usually fixed to face in one direction but can also be fitted onto rotating tracking devices to follow the sun’s movement across the sky. Inside the solar collector, the dark-coloured absorber plate gets hot and transfers heat to fluids such as air, water or another medium. Through a sequence of pipes and ducts the heat is transferred to a storage vessel of hot water, usually located internally within a building – but it can also be integral to the collector

1.2 A short history of solar water heating Since the beginning of recorded history, humans have used hot water. The advent of glass production, from the Roman period onward, gave people the idea that it was possible to ‘catch’ sunshine inside buildings and boxes. Once economic production of flat plate glass and float glazing occurred at the end of the 19th century, solar water heaters began to be produced commercially, often

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2   solar domestic water heating Figure 1.1 Dark colours and transparent covers increase the temperature of materials heated by the sun

˚C

˚C

˚C

˚C

Transparent cover

Light

Dark

B

A

F C D

E Location Pitched roofs

Key A

B Flat roofs

Insulated

in areas of the world where other methods for heating water were either expensive or inconvenient. Early pioneers were California, Florida, Japan, Israel and Australia. The rapid rise in oil prices in the 1970s led to a particularly significant period of technological development in the field of solar water heating. The 21st century has seen further developments, fuelled by global interest in preserving depleted fossil fuel resources and minimizing carbon dioxide emissions.

1.3 What do solar water heating systems look like? Solar water heating systems can usually be identified from the solar collectors mounted on the roof of a building. Collectors can also be seen erected on frames alongside storage tanks. The large rectangular glazed flat plates or series of

C D

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Free Standing installation

E

Walls/balcony rails/balustrades

F

Figure 1.2 Solar collectors can be fitted in many ­positions Source: www.viessmann.com

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what is solar heating?   3

Figure 1.3 Evidence of commercially made solar collectors found from the 1900s

Figure 1.4 A solar collector on a roof is sometimes fitted with accompanying valves

Source: © 2010 Butti/Perlin Archive, All Rights Reserved

Source: www.ECS-Solar.com

glass tubes can appear similar from a distance. They can easily be confused with photovoltaic modules or large skylights. Collectors on commercial or historic buildings may well be hidden from view, placed high up on a flat roof. Sometimes collectors can be difficult to identify as they have been fixed flush with the roof covering and show no protrusions. A water storage tank – to store the solar-heated water – is an integral part of a solar water heating system, which, depending on the type of climate and local regulations, is either mounted outside or inside the building. Systems can have more than one storage tank. Other components, such as pumps and electric controls, are usually located inside the building, in lofts, cupboards and service ducts.

Figure 1.5 A collector with close-coupled storage on free-standing metal frame Source: www.ECS-Solar.com

1.4 What the technology can achieve Solar domestic water heating can significantly assist with the provision of domestic hot water for homes and places of work. Even in the cloudier and cooler parts of the world, it is reasonable to expect over half the annual demand for domestic hot water (DHW) to be provided. In some places, with suitable equipment, it is even possible to meet all hot water requirements. Most system owners can enjoy somewhere between 40 per cent and 90 per cent of their total annual DHW energy consumption from off-the-shelf solar equipment, depending on location. In summer, they can expect to receive 90–100 per cent of their hot water requirements.

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Figure 1.6 In some warmer climates, the solar storage can be located externally on the ground Source: www.apricus.com

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4   solar domestic water heating

Figure 1.7 In colder climates, the solar storage is mounted internally Source: German Solar Industry Association (BSW-Solar)

Figure 1.9 In very hot climates a simple unglazed system is sufficient

Figure 1.8 In some warmer climates, the solar storage can be located externally on the roof Source: German Solar Industry Association (BSW-Solar)

Figure 1.10 The ground can be used as a collector location Source: Thermomax/ESTIF

Source: South African Department of Energy

Solar heating systems provide individuals, communities and countries with a measure of fuel security and independence from conventional heating fuel price fluctuations. For example, Australia’s total current energy consumption per annum could be met by an area of 4000km2 (1544 sq. miles) of solar collectors.

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what is solar heating?   5

Figure 1.11 A vertical wall can be used as a collector location

Figure 1.12 The collector can be mounted flush with the roof line with no tiles beneath:

Figure 1.13 A typical flat plate collector fitted on top of the tiles Source: www.ECS-Solar.com

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6   solar domestic water heating

Figure 1.14 A typical tube collector fitted on top of slates

Uranium

Gas

Annual Solar Radiation

Oil

Coal Figure 1.15 The world’s primary energy consumption is easily exceeded by the solar energy received in one year by a factor of 10,000

World Energy Consumption

Source: DGS

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what is solar heating?   7

Figures 1.16 Solar collector arrays can be few or many, depending on if they are heating just domestic hot water or space heating as well Source: www.Wagner-Solar.com

If this were constructed as a real-life power station (with approximately 20 per cent rate of land coverage) it would measure just 138 × 138km (85 × 85 miles), which is about the area covered by all Australia’s dwellings. Twenty times this area would be required for the US and twice this area for the United Kingdom.

1.5 Other solar technologies Solar water heating is not the only way of harvesting solar energy. There are many others. Solar space heating The sun’s energy can be used heat buildings. Systems are similar to solar water heating systems; however, solar space heating systems are larger because space heating energy requirements are generally much greater. With solar space heating the demand for heat is quite seasonal, whereas domestic hot water is required all year round. Some types of solar space heating systems heat air in their collectors. Passive solar This technology is about the design and construction of buildings in order to make the best use of the sun’s energy, keeping them warm in winter and cool in summer. In a typical passive solar building, in winter, the sun’s radiation passes through the glazing in doors and windows and heats the interior fabric of the building. Similarly, in summer, sunshades can be used to keep a building cool. It is called ‘passive’ because there is no other source of energy used to move the heat, Figure 1.17 Passive solar heating uses larger windows and thermal mass such as pumps or valves or electricity. to store heat

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8   solar domestic water heating

Figure 1.18 Photovoltaic (PV) modules create electricity and are not intended for heating

Photovoltaic (PV) technologies generate electricity directly from the sun’s radiation. The collectors, called ‘photovoltaic modules’ are made up of waferthin photovoltaic cells. They are also called ‘panels’: a term that can sometimes cause confusion with the flat panels used for solar water heating. In fact, PV and solar heating (thermal) technologies are two fundamentally different technologies, but from a distance PV modules can look similar to solar water heating collectors. Active solar cooling Solar thermal technologies can also be used to cool buildings – and cooling is mostly needed when the sun shines. The technology used is absorption refrigeration – which uses solar heat to power absorption chillers to produce cooling energy (common in larger air-conditioning systems), rather than using a conventional fossil fuel. Solar cooling is a developing technology and is currently very expensive. Concentrating solar energy (CSP) Using mirrors and lenses it is possible to concentrate the sun’s radiant energy and achieve very high temperatures. This can be used simply to boil water in a pot through to raising steam to drive a turbine or sterling engine for electricity generation.

Figure 1.19 Mirrors or reflectors help raise ­temperatures much higher for industrial ­processes Source: ESTIF

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what is solar heating?   9

Disinfection, desalination and solar drying In strong sunlight conditions in warm climates, it is possible to disinfect or desalinate water with simple passive techniques. The sun can also be used for drying crops either directly or by warm air.

1.6 Energy supply issues, peak oil, resources depletion Globally, the energy used for heating water comes mainly from solid fuel, oil and natural gas. If fossil fuel use continues unchecked, all reserves of petroleum and natural gas will be depleted by the Figure 1.20 Solar heat can be used to raise air end of the 21st century with only coal reserves temperatures a few degrees: enough to help crop drying available for a longer period of time. Only the size of already-explored fossil fuel deposits are known. Additional reserves, yet to be discovered, can only be estimated. Even if new major fossil fuel reserves are to be discovered, this would not change the fact that the supply of fossil fuels is limited. ‘Peak oil’ refers to the point in time when the maximum rate of global petroleum extraction is reached, after which the rate of production enters into terminal decline. Some analysts believe that ‘peak oil’ is due to occur within a decade and cause large-scale disruptions to the global economy. Others believe that, as the cost of oil increases, new previously uneconomic reserves will be accessed. Uranium reserves for operating nuclear power stations are also limited. And, due to stringent safety and planning laws, new nuclear power stations can take decades to build. The future of short-term energy price trends is uncertain. There are many variables. Historically, energy prices have tended to follow the price of oil, which

$140 Strong demand and weak supply tighten the market

$120

$/Barrel

$100

OPEC shocks the market

$80 Prices are low and stable

$60 $40 $20 $0

1974

1981

1988

1995

Benchmark oil prices

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2002

2009

Figure 1.21 Cost of a barrel of crude oil over the last 30 years Source: data from IEA

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10   solar domestic water heating has in turn been strongly affected by general economic growth and international conflicts. Domestic energy prices in some industrialized nations surged 20 per cent during 2008 – closely related to a peak in oil prices of $140 per barrel (there are 159 litres in a ‘barrel’ of oil) – only to be followed by a rapid drop in oil prices to below $40 per barrel and a global economic recession. Government policies to enforce reductions in carbon-based fuels is likely to lead to carbon taxes and subsidies for non-carbon based fuels. The long-term prospect of fossil-based fuels is forecast to be one of increasing prices. In comparison, purchasing a solar water heating system can be thought of as purchasing a percentage of one’s heating fuel in advance for the next 20 years at a known fixed price. Where low interest rates exist, investing in solar water heating equipment can become a more attractive option than depositing money in a bank account.

1.7 Climate change and CO2 emissions The increased production of ‘greenhouse gases’, such as carbon dioxide, methane, chlorofluorocarbons, nitrous dioxide, ozone and water vapour, are causing a rise in average global temperatures, which in turn causes icecap melting, extreme weather events and sea-level rises. A drastic reduction in human-generated (anthropogenic) carbon dioxide emissions is needed. Replacing the fossil fuels used for hot water heating with solar energy represents one of the easiest and most reliable ways to achieve this.

16%

Heating

32% 50% 68%

84%

Energy consumption

Lighting, cooking, electrical appliances

50%

Hot water

CO2 emissions

Relative costs

16%

Figure 1.22 Domestic hot water energy represents a small proportion of overall energy use (UK figures)

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Energy Consumption showing DHW

11%

73%

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what is solar heating?   11

Domestic (sanitary) hot water production in industrialized regions represents around 5 per cent of total fuel use. In terms of delivered energy to homes it represents more like 10 per cent. The specific energy figure varies widely for each individual household, typically within a range between 500 and 5000kWh (1.7 million – 17 million BTU) per annum set. For many dwellings in colder climates, the most cost-effective way to reduce carbon dioxide emissions is to first reduce space heating requirements before adding solar hot water. Here the upgrading of fabric insulation and more efficient boilers are the first priorities. This will also help considerably with annual fuel bills. Once these improvements are made, solar water heating becomes the next biggest energy-saving measure.

1.8 Purchasing a solar heating system At first the choice of solar water heating products, systems and brand names can seem overwhelming. The terminology may appear unfamiliar and confusing, with every competing advertisement promising the ‘best’ performance. If you do not intend to become involved in the details of specifying, manufacturing or installing solar water heating then the best way to purchase a working system is to find an established, personable installation company located in your area. Because collectors are located externally and therefore visible, it is relatively easy to find and approach existing owners in order to share their experiences with installation companies. Established companies will generally be happy to offer a list of satisfied customers. However, the opportunity to support new installers should be considered. These may not necessarily have a track record in solar but may already be experienced in heating, plumbing, electrical or roofing and are willing to attend training courses on solar. They may be already familiar with the existing water heating of a particular building – which could qualify them for at least some involvement under a lead contractor. However, it is important to remember that unless a good quality installation can be assured then the end result will be disappointment. Building regulation (codes) need to be complied with. Government rebates and other fiscal initiatives may only apply if particular products and installers are used. Some distributers or manufacturers of solar products will keep preferred lists of approved installers. Company websites may list them. A considerate installing company will always try to personally view a property before giving a quotation, as assessments of the type and condition of a roof, hot water system and access are all critical for technical possibilities and worker safety. Doing-It-Yourself (self-build) may restrict which products are used, as some are available only to trade wholesale bulk buyers. There are opportunities to self-build key components, such as collectors and thermostatic pump controls; however, it is unlikely that such equipment will out-perform or cost less than the mass produced factory-made equipment manufactured to international standards of quality and safety.

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12   solar domestic water heating The route to obtaining a suitable solar heating system can be many and varied, but there is a significant role for a thoughtful building manager or owner who is prepared to survey, specify key equipment as well as manage skilled tradespersons or labourers. The following chapters will assist in this task, as well as providing guidelines for those wishing to become installers themselves.

Box 1.1 Temperature, energy and power units Energy refers to the ability to do work and can take different forms, such as heat (thermal), light, sound or movement (kinetic energy). It can also change from one form to another. For example, although a lightbulb’s function is to produce light, some of its energy is also transformed into heat. But the total energy going into and out of the lightbulb remains constant. The international unit for energy is the joule (J). One kilojoule (kJ) is equal to 1000 (103) joules, and 1 megajoule (MJ) is equal to 1,000,000 joules (106). In this book the units most used are: Watt-hours (Wh) = 3.6kJ Kilowatt-hours (kWh) = 3.6MJ Megawatt hours (MWh) = 3600MJ Other common energy units are: Calorie (cal) = 4.19J Kilo Calorie (kcal) = 4.19kJ British Thermal Unit (BTU) = 1.05kJ One joule of energy will raise the temperature of 1 gram of dry air by 1°Celsius. One calorie of energy will raise the temperature of 1 gram of water by 1°Celsius. One BTU of energy will raise the temperature of 1lb water by 1°Fahrenheit. Power is the rate at which energy is supplied or consumed (or energy per unit of time). The international unit for power is watts (W). One Watt (W) = 1J/second. One kilowatt (kW) is equal to 1000 (103) watts, and one megawatt (MW) is equal to 1,000,000 watts (106). Other units used for power include: Calorie per second (cal/s) = 4.19W Kilo Calorie per second (kcal/s) = 4.19kW British Thermal Unit (BTU) = 0.29W Horse power (hp) = 746W Temperature is a measure of how hot or cold an object is. The international scientific unit of temperature is the Kelvin (K). In this book the temperature units used are Celsius (°C) and Fahrenheit (°F). One degree difference Celsius is equal to 1°Kelvin. The Kelvin scale begins at the coldest theoretical temperature (absolute zero), which is equal to −273.15° on the Celsius scale and −459.67° on the Fahrenheit scale. Water freezes at zero °C (32°F). Water boils at sea level at 100°C (212°F).

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2

Solar radiation and resources 2.1 Solar radiation and the solar resource This chapter looks at the way solar energy reaches the earth, solar energy quantification, why the orientation of solar panels matters and what are the optimum angles. Most of the energy the sun emits does not reach the earth. When measured at the top of the earth’s atmosphere, the average solar irradiance is 1368W/m2. (When radiation strikes a surface it is described as irradiance or irradiation, as opposed to when it travels through air and space.) This means that each square metre is receiving 1368 watts of power. This is known as the ‘solar constant’. How much radiation reaches the earth’s surface depends on the solar elevation angle, which varies through the day as the sun rises and sets. Additionally, as the solar radiation passes through the atmosphere, it is partly scattered and absorbed by air molecules, impurities and water vapour. When the sun is directly above a point on the earth, the solar radiation travels the shortest distance through the atmosphere. This occurs only at certain times of the year between the Tropic of Cancer, the equator and the Tropic of Capricorn at around the middle of the day, or more precisely at solar noon. The thickness of the atmosphere the radiation has to pass through is described as the air mass (AM); AM1 is the shortest possible, that is when the sun is perpendicularly overhead; outside the atmosphere is AM0; AM2 is twice AM1, and so on. Solar radiation eventually reaches the ground in two forms: direct or diffuse irradiation. Direct irradiation comes directly from the sun and casts hard shadows. Diffuse radiation comes from no defined direction; it is light reflected from clouds around atmospheric particles. Measured on a cloudless day, at sea level and on a surface perpendicular to the sun around midday, radiation reaching the surface anywhere on the earth reaches an approximate maximum of 1000W/m2. The total solar radiation, the sum total of the direct and diffuse irradiance, measured on a horizontal surface on the ground is called global radiation (horizontal hemispherical). Some radiation is reflected from the earth’s surface back up to particles and clouds in the atmosphere and then sometimes back to earth again. Different coverings have different reflectance or ‘albedo’. Fresh snow reflects over 80 per cent, whereas water or forests less than 20 per cent. The average albedo for the earth is about 30 per cent. On a steeply inclined surface, some lighter coloured surfaces reflect the solar radiation back up to strike this surface. When allowing for reflections, it is possible to measure peak values of over 1200W/m2.

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14   solar domestic water heating

Figure 2.1 The earth receives only a small proportion of what the sun emits in total Source: www.aeronomie.be/ en/topics/atmospheres/ atmospheres.htm

AM0 1367 W/m2 Solar Constant

Edge Figure 2.2 The amount of irradiance reaching the surface of the earth is affected by how much atmosphere it passes through

AM1 1000 W/m2

of atm

osphe

re

AM1.5 950 W/m2

Planet Earth

Note: AM = air mass.

Diffuse Figure 2.3 An inclined surface receives three types of solar irradiation

Direct

Reflected

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solar radiation and resources   15 Solar Radiation Spectrum 2.5 UV

Visible

Spectral Irradiance (W/m2/nm)

2

Infrared Sunlight at Top of the Atmosphere

1.5

5250˚C Blackbody Spectrum

1 Radiation at Sea Level

H2O 0.5

H2O

O2 0

O3 250

H 2O H2O

500

750

1000

1250

1500

1750

Absorption Bands CO2 H2O 2000

Wavelength (nm)

2250

2500

Figure 2.4 The sun’s radiation is made up of a range of wavelengths, which are affected differently as they pass through the atmosphere Source: Wikipedia commons

The spectrum of solar radiation reaching the earth’s atmosphere is mostly in the visible (40 per cent) and infrared range (50 per cent) with some ultraviolet (10 per cent). By the time they reach the earth’s surface, certain wavelengths (especially ultraviolet and visible ranges) have been strongly reduced in intensity due to absorption by water, nitrogen, carbon dioxide and ozone.

Box 2.1 Irradiation and irradiance The sun’s energy travels through interplanetary space as radiation, which is a form of electro-magnetic waves or particles. Irradiance is the rate of energy reaching a unit of surface area. For solar irradiance, this is literally the power of the sun. The angle of the surface is usually stated: i.e. horizontal or perpendicular. The international unit of scientific measurement of irradiance is W/m2. Other units in common use for irradiance include: J/second per m2 Watts per sq. ft Calories per minute per sq. cm = 1 Langley Irradiation is the total energy over a set period of time reaching a unit of surface area. The international unit of scientific measurement of irradiance is W/m2 per hour, day or year. The sun’s direct irradiation reaching a horizontal surface on the earth is termed insolation. (Note: insulation is something different!)

2.2 Quantifying solar energy The main scientific instrument used to measure solar irradiance is a pyranometer, an instrument that senses the warming effect of solar irradiation on a black

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16   solar domestic water heating

Figure 2.5 A pyranometer measures accurately across many wavelengths of the solar spectrum but is slow to respond Source: Kipp and Zonen

Figure 2.6 A photovoltaic reference cell responds quickly but to only a small part of the solar spectrum

surface and creates a voltage proportional to the intensity of the irradiation via a ‘thermopile’. Photovoltaic sensors are also used. These generate current/ voltage in response to the solar radiation; however, they do not respond to the infrared part of the spectrum. Comparisons between data collected by both devices must be made with caution. Special shading devices make it possible to block out direct (beam) radiation in order to measure indirect radiation only. When predicting future solar irradiation, best results are obtained by employing data measured over the previous decade. Meteorological weather stations often provide the most reliable data. Data is usually provided for radiation incident to a horizontal surface – which needs to be adjusted because solar collectors are installed at angles, rarely horizontally.

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solar radiation and resources   17

Box 2.2 Irradiance on tilted surfaces When a surface is tilted, its total annual solar irradiance will be different to when it is horizontal, usually greater. To calculate the total irradiance of a tilted surface from a figure known from a horizontal surface, three irradiation components must be calculated and then added together: the direct irradiance, diffuse irradiance, and ground reflection (not present for horizontal surfaces). Each component is affected by the angle of the tilted surface to the horizontal as well as the sun’s height (solar altitude or elevation) (see Figure 2.3). Maximum instantaneous direct (beam) irradiance occurs when the light from the sun is perpendicular to a surface. If the sun is directly overhead, the direct (beam) irradiance will be highest on a horizontal surface. Otherwise the maximum occurs at a specific Surface tilt angle. The chosen tilt geometrical line perpendicular to surface angle becomes more important nearer the poles z Surface tangent and away from the equator. Sun to earth Because the sun alters its solar altitude through the days and seasons, no single fixed position is perfect all the z time. However, there is usually Limits of earth s atmosphere one fixed tilt angle which gives the optimum performance Hypothetical surface over a year. This is best perpendicular to the sun's rays worked out with a computer Earth simulation program or using pre-worked tables. Automatic tracking systems can also be used but they are not usually Figure 2.7 The intensity of beam irradiation is reduced when a surface faces used with solar water heating away from perpendicular to the sun systems. ‘

When designing solar hot water systems it is the average irradiation data measured over months or a year that is needed. It is the average accumulation of energy that is important for most calculations. The measurement of sunshine hours (duration) is generally not very useful; it is the average intensity of the sun over time that really matters. It should be considered that if an average over time includes the night, then ignoring clouds, the average ‘insolation’ for the whole earth is approximately 250 watts per m2 assuming the lower radiation intensity in early morning and evening and its near-absence at night. In poor irradiation regions, this value drops to below 100 watts per m2. Many meteorological stations measure only the total solar irradiance. However, many calculations for solar thermal energy systems require a separation of direct and diffuse irradiance because collectors react differently to each type. In general, the diffuse irradiation component is larger than the direct component in milder, cloudier climates; whereas in sunnier climates, nearer the equator, the direct irradiation component predominates. Nearer the poles there is a large difference between winter and summer direct irradiation. However,

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18   solar domestic water heating latitude can only give a rough indication of the annual irradiation because there are local Ratio high: lowest Range kWh/m effects that can have a major impact. Care month of irradiation should be taken when comparing different 10:1 Sub 950 data sources for irradiation, as definitions 5:1 950–1300 can vary. 3:1 1300–1800 Global primary energy demand could in 1.3:1 Over 1800 fact be provided by collecting the annual solar energy received by 18,700 square miles (48,500km2) of the Sahara, an area slightly larger than Switzerland, or one-ninth that of California. Theoretically, total global energy demand could be provided solely by solar energy.

Table 2.1 Annual irradiation measured horizontally Low solar radiation Temperate maritime Temperate continental Tropical/Sunbelt

2

Global Irradiation measured horizontally per annum for different cities 50 45

Sydney Upington Bombay Cairo LA Rome London Berlin Bergen

30 25 20 15 10 5 e ag

ec D

Av er

ct

ov N

O

p

g

Se

ly

Au

ne

Ju

ay

r

Source: Data from Palz and Greif, 1996; Nasa, 2003

Ju

M

Ap

b

ar M

n

0 Fe

Figure 2.8a Irradiation measured horizontally per annum for different cities

35

Ja

KWh/m2 day

40

Months

300

Global Irradiation for Summer vs. Winter

KWh/m2 month

250 200 Winter Summer

150 100 50

Figure 2.8b Irradiation in three different regions showing split between summer and winter

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0

Australia

Equatorial regions Region

Europe

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solar radiation and resources   19 Jun 21 Lat 54°N

th

st

e gh

un

pa

s

Solar Noon

Hi

Mar Sep 21 11

Feb Oct 21

10 9

1

2

Jan Nov 21

8

7 6

un

we Lo

s st

Dec 21

th

pa

3 4 5 6

S

Figure 2.9 The paths of the sun can be seen through a solar site selector – northern hemisphere

2.3 Solar geometry, angles and orientation When designing solar energy systems it is essential to take into account the changing position of the sun at different times of the year at your location – the angle of the sun’s height (solar altitude or elevation, measured in degrees) and the azimuth (direction facing away from true north, also measured in degrees). In the tropics and equatorial zones, the sun is overhead for much of the day with little difference between the seasons. This means that a solar collector does not need to be tilted to any great extent. Away from these zones, it becomes more important to tilt the collector towards the equator – that is, facing south in the northern hemisphere, and facing north in southern. The path the sun takes across the sky each day at each location on the earth varies according to the latitude and time of year. A sun-path diagram can be used to plot this. Sun-path diagrams show the path the sun takes across the sky during the longest and ϒs shortest days of the year – the solstices, αE in December and June. Diagrams can also be shown for the two equinoxes, in September and May. Solar site survey devices can Figure 2.10 The sun’s relative position in the sky to an observer can be instantly show how much shading defined from two values will occur at the proposed site of Note: gS = solar elevation; aE = azimuth. collectors and help choose the location

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20   solar domestic water heating

Figure 2.11a A hand-held device shows the sun path through a viewer

Figure 2.11b Looking down onto a perspex dome of a tripodmounted site. Selector shows a reflection of the sky and a compass

that has the least. These devices work even on cloudy days. Most can be used in conjunction with a camera to record the data at the site, and these data can later be analysed using a computer. For best accuracy, measurements are taken from each corner of the intended collector-array surface. Shade on collectors is more likely (and critical) when the collector is fixed at steeper tilt angles.

Box 2.3 Solstice and equinoxes The earth’s rotational plane is not perpendicular to its orbital plane. For half the year (March to September) the northern hemisphere tilts towards the sun while the southern hemisphere tilts away. This is the reason why the winter and summer seasons occur at higher latitudes. (a) 

Figures 2.12a/b The earth is tilted in relation to the rotational plane around the sun

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solar radiation and resources   21

(b)

Low density of incident rays (northern winter)

Earth

Night

tor Day

ua

Eq

High density of incident rays (southern summer)

Sun

Figures 2.12a/b The earth is tilted in relation to the rotational plane around the sun (Cont'd)

The shortest and longest days of the year are called solstices. These occur twice each year. At the extremes of the Arctic and Antarctic circles, during the solstice periods of June and December, the sun will either disappear for weeks on end or always remain in the sky. The sun appears directly above the earth (the zenith) in the equatorial zones at solar noon (midday) only on the March and September equinoxes. The length of day and night is equal across the globe during the equinoxes.

If a solar collector tracks the sun so that its angle of incidence to the direct radiation is always perpendicular, the energy yield will be the maximum possible. Double-axis trackers in the mid-latitudes can increase solar energy collection by approximately 30 per cent, with single-axis trackers (azimuth only) it is closer to 20 per cent. However, tracking systems are expensive and have higher operating/maintenance costs than fixed mounted collectors. The

Figure 2.13 An automatic tracker turns the collectors to follow the sun across the sky Source: Lazer2 Solar Tracker from Solar UK

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22   solar domestic water heating

40°

600 1205

700

30° 20°

800

10°

900 >900

0° East

South

West

Angle of azimuth aE kWh/m2 –180° –150° –120° –90° –60° –30° 0° 30° 60° 90° 600 800 1000 1200 1400 1600 1800 >2000

1400 1600 1700 1800 1900 2000

North

120° 150° 180° 90° 80° 70° 60° 50°

>2200

40° 30°

2300

Solar elevation gS

North

S

East

Solar elevation g

South West North Angle of azimuth aE kWh/m2 –180° –150° –120° –90° –60° –30° 0° 30° 60° 90° 120° 150° 180° 90° 900 80° 950 1000 400 70° 1050 1100 60° >1150 500 50° North

20° 10° 0°

Figure 2.14 The annual useful energy produced by a fixed collector is affected by its angle in relation to the latitude and its compass direction (azimuth) Source: Volker Quaschning

advantages rarely compensate for the disadvantages and so these are not often used for solar thermal. In general, the tilt angle of a solar collector is not that critical to annual performance. Local practicalities such as adjoining roof pitches, dust build-up, wind loading and ‘footprint’ often mitigate the theoretical ideal. In most cases a tilt angle of between 10 and 50 per cent up from horizontal is an acceptable compromise.

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3

how solar DHW works 3.1 How solar domestic hot water systems work This chapter describes how typical solar domestic water heating systems work, the main components and the functions in the system. In a solar DHW system there are two circuits:

• a ‘primary’ circuit that collects the solar energy and transfers it to a water tank in which it is stored:

• a ‘secondary’ circuit that transfers the heat stored in the tank to the domestic hot water supply to be consumed at taps etc.

The primary and secondary circuits sometimes use the same water – simply moving it from the solar collector via pipes and tanks to the taps. This arrangement is called a ‘direct’ system. However, in most systems the primary and secondary circuits use different liquids (water or water solutions – this is discussed in detail later) and transfer the heat from one liquid to the other via a heat exchanger. Heat exchangers are normally constructed of a series of metal pipes or plates. This arrangement is called an ‘indirect’ system. There can be more than one heat exchanger. The water is circulated over and over again in a loop, but the heat moves in one direction only. Heat exchangers can be inside the storage tank, inside the collector or separate. The direction of higher temperature liquid leaving a solar collector is called the ‘flow’ whereas the direction of lower temperature liquid returning to a heat generator is termed the ‘return’. These terms are often confused but the trick is to consider where the source of heat is as this is where the heat starts to ‘flow’. In the same way, the hottest pipe leaving a gas or oil boiler is called the ‘flow’. A solar water heating ‘primary’ circuit consists of:

• • • • •

a collector to capture the solar radiation; a heat transfer fluid to move heat to the secondary system; a separate storage tank or a collector-integrated storage tank; a heat exchanger (in some systems); pipework to circulate the fluid.

A solar water heating ‘secondary’ circuit consists of:

• • • •

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a cold fresh water (drinking quality) source; pipe or heat exchanger to connect with primary circuit; a back-up heat source (usually); discharge points (taps, showers etc.).

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24   solar domestic water heating

UPSTREAM

BOILER PRIMARY

COLD FEED

SECONDARY CIRCUIT

Figure 3.1 The movement of heat in different parts of a system

DOMESTIC HOT WATER APPLIANCE

SECONDARY CIRCUIT

TO TAPS

SECONDARY CIRCUIT

PRIMARY

SECONDARY

(b)

Imaginary line between primary and secondary

THESE TWO TYPES OF STORAGE CAN BE COMBINED TOGETHER INTO ONE TANK OR LEFT AS TWO SEPARATE TANKS

(a)

Figure 3.2 The movement of liquid in different parts of systems; a) example indirect tank; b) example direct tank

HEAT FROM BOILER OR IMMERSION HTR

DEDICATED SOLAR PREHEAT STORAGE

PRIMARY CIRCUIT

COLD STORE OR DIRECT FEED

DOWNSTREAM

SOLAR PRIMARY

HEAT FROM SOLAR

SECONDARY

PRIMARY

Source: Solarpraxis

The cold fresh water (drinking quality) source can be from a rising street main (city water), a cistern (water container/tank), a pumped well or a natural spring. Back-up heat sources can be gas-fired boilers, oil-fired boilers, solid fuel or electricity. These can include separate storage tanks, but may also be an instantaneous heater (tankless) – gas combi-boiler or electric shower for example. The primary or secondary circuit should include a tank dedicated to storing solar-heated water. The position or absence of a heat exchanger will determine if this solar storage tank is part of the primary or secondary circuit. Solar heat warms the temperature of incoming fresh cold water – usually inside the solar storage tank. This is called ‘solar preheating’; any subsequent back-up heating is known as ‘after-heating’. The solar storage tank will mainly contain the water to be ultimately consumed as domestic hot water (DHW).

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how solar works   25

(a)

(b) SECONDARY

SECONDARY

PRIMARY

PRIMARY

PRIMARY

(c)

(d) PRIMARY

SECONDARY

SECONDARY

PUMP

PUMP

Heat exchanger types (a) Internal coil (c) Coil in tube

Stand-alone heat exchangers, also called external heat exchangers, are shaped like a small box or long tube with insulation and four connecting pipes. Sometimes, where two external heat exchangers are used, one fixed each side of the solar storage tank, an intermediate circuit (indirect) is created and so the solar storage tank will no longer contain water that is to be ultimately consumed. Solar radiation varies according to the weather, seasons and time of day. It does not always match DHW requirements. This is the reason why back-up heat sources are needed. At lot of the time what is

CH-03.indd 25

(b) Tank in tank (d) Compressed plates

(a)

(b)

(c)

(d)

Figure 3.3a–d Indirect systems have a heat exchanger separating the liquid in-between the primary and secondary systems Source: Solarpraxis

Figure 3.4 Cold water can be provided from various sources, (a) well, (b) natural spring, (c) city water or (d) high-level cistern

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26   solar domestic water heating

Figure 3.5 This heat exchanger is made up of a series of plates pressed close together

PRIMARY

INTERMEDIATE

SECONDARY

Figure 3.6 More than one heat exchanger can be used to reduce limescale risk and quantities of antifreeze in a large scale system Source: Valentin software

happening is that the solar is preheating the water in the solar storage tank – if it does not reach the required temperature, the back-up heating system kicks in to add the extra heat required. This reduces fuel use. The type and location of back-up heat sources makes systems more complex. Back-up heat sources (or after-heaters) should be capable of providing all the required DHW if solar heat is not available. Relying on just solar heat all year round is rarely practicable, though sometimes it is possible in summer. Back-up heat sources include: gas or oil boilers heating a tank, gas instantaneous (tankless) boilers, electric resistance heaters inside a tank, solid fuel stoves and cooking ranges. Most back-up heat sources are thermostat-controlled. Thus the

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how solar works   27

maximum temperature limit can be adjusted. The same thermostat is able to detect if there is enough solar-heated water and, if there is, then it switches off the back-up. It is very important not to waste back-up heating. The method of integration with the solar storage tank must be carefully considered. If it is not done

(a)

(b)

(d) (c)

E

E

(e)

= Electric resistance element = Cold water feed = Pump

Type of back-up heating (a) On top of solar storage tank and indirectly by boiler (b) Separate tank indirectly by boiler (c) Instantaneous boiler (d) and (e) Electric resistance element on top of solar storage tank

Number of Systems: 1

Figures 3.7a–e Common methods for adding back-up heat after the solar heating Note: Circuits for space heating and heat exchangers not shown for clarity. Source: Valentin software

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28   solar domestic water heating correctly, the back-up heat can interfere with the solar performance by accidentally entering the solar storage. This will force the solar collector to work harder and become hotter, hence losing useful heat. An even worse scenario is useful heat being wasted accidentally by back-up heat circulating through the collector.

3.2 Main system components and their functions The main component groups in solar water heating systems are:

• • • • • •

a solar collector; a water storage tank of solar heat; pipes containing water or other fluids; controls for safety, efficiency and information; a water supply; back-up heating and electricity.

Collectors can take various shapes and forms, ranging from simple darkcoloured surfaces to glazed panels, tubes or a combination involving mirrors and lenses. They are best fitted where no shading exists, but can work acceptably if shading is minor, intermittent or seasonal. A larger collector array (field) may be made up of smaller individual collectors. Because the collector gradually heats a store of water the solar heat can be used at times when solar irradiation is low or non-present. Once the solar irradiation has been converted into heat, the heated liquid moves into the storage tank and then on to other locations by either water or water containing chemical additives such as antifreeze. All pipes and ducts usually require insulating to help retain heat. The heat generated by the sun’s radiation can cause solid and liquid materials to reach high temperatures well over 100°C (212°F). Such temperatures

Solar radiation

Cold water supply

A solar collector fixed externally Pipes containing water or other fluids

Electricity for pumps and controls

Figure 3.8 A complete solar water heating ­system is made up of a sequence of functional groups

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A water storage tank of solar heat

Controls for safety, efficiency and information

Back-up heating Domestic hot water (DHW) distribution inside building

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how solar works   29

cause expansion and so pressure can build up. A liquid can become a gas (e.g. water to steam), and its volume can increase a thousand-fold. Correctly designed equipment will be designed to withstand this, but safety controls are also needed to ensure the physical limits of materials are not exceeded and that the risk of scalding and explosion is reduced. Controls can be a combination of mechanical (passive) or electrical. They can also improve the solar collector efficiency by altering pumped circulation fluid rates and changing the order of heating extra storage tanks. Controls can also provide an indication of correct functioning and provide extra performance information. However, water is vulnerable to freezing and bacterial growth so is not necessarily used in all parts of the system. In an indirect system plain water is not allowed to flow in the collector or any building-external pipes. Other fluids, which contain antifreeze or corrosion inhibitors, are used, with heat exchangers separating the different parts of the system from each other. The back-up source of heat not only provides any shortfall of heat but also ensures sterilization of the bacteria that are always present in untreated cold water.

3.3 Energy and mass flows Solar water heating systems obey the laws of conservation of mass and energy: the total mass of water entering and leaving the system will be equal, aside from any small losses of evaporation in open vessels. However, because the density of water varies as it is heated, the volumetric flow rate can appear slightly different. In a closed circuit, the same mass of fluid remains in the loop (assuming no leaks). The energy flow is more complex. Thermal gains or losses from electrical equipment such as pumps, as well as the ambient surroundings – particularly around the solar storage – play a part. The cold water entering the property will vary in temperature through the day and seasons, always containing some energy if its temperature is above absolute zero. Small amounts of energy are also lost up as heat and noise from the loss of pressure flowing through the pipes. Cold water left standing in pipes and vessels also picks up ambient heat. As the solar heat passes to the heat transfer fluid thermal losses occur to the ambient surroundings if the fluid is at a higher temperature. Back-up heating added will involve further losses.

Cold water supply

Evaporation

CH-03.indd 29

Domestic Hot Water

Leaks

Figure 3.9 The mass of cold ­water entering a solar system equals the domestic hot water drawn off less any lost ­evaporation. In good quality systems these will be kept very low

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30   solar domestic water heating Heat loss Boiler E Boiler E Elec – Heat Element

Figure 3.10 The collected solar energy integrates with energy from other heat sources and the surrounding ambient conditions. Once allowing for some losses, useful DHW heat arrives at the outlets Source: Valentin software

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E Coll Loop G Active Sol, Shade

E DHW

Energy Loss – Pump Heat Loss – Pipes

Heat Loss – Tank

Heat Loss Coll – Optical Heat Loss Coll – Thermal

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4

solar collectors This chapter discusses the main types of solar thermal collectors, their components and collector efficiency. Solar collectors are designed to absorb solar radiation and convert it into thermal energy. They are available in many forms, shapes and sizes. Although the collector is a crucially important component, the overall performance of the system depends on the quality of all the components as a good collector cannot make up for other components that function poorly.

4.1 Solar collector absorbers The part of the collector that receives the solar radiation is called the absorber. This is dark-coloured, often black, since lighter colours tend to reflect rather than absorb. Special coatings on the absorber surface assist the collector’s efficiency by reducing ‘re-emittance’ – the amount of radiation that is lost from the absorber as it gets hot – just like when you hold your hand over a car on a sunny day, you can feel the heat ‘bouncing off’ the surface and being lost to the air. These special coatings are called ‘selective’ coatings. Black paint is a cheap option for creating a darkened absorber surface; however, this can also result in high re-radiated losses and can lose adherence over time and peel away from the absorber if it becomes hot. Selective coatings are preferable. These are factory-applied and form a chemical bond at the surface of the metal, sometimes with a blue/black hue with no visible thickness. Although darker colours are better than light colours at absorbing radiation, dark colours also make good radiation emitters when hot. By combining a dark, selective surface and maintaining a lower absorber temperature, collector efficiency can be optimized. One interesting phenomenon is that on clear, dark nights, a black-coloured absorber can become colder than the ambient air or other lighter materials surrounding it. This happens when the night sky is ‘blacker’ than the absorber, producing a net loss of radiation. This can waste heat should a pump be accidentally left switched on. Glazed collectors are predominately used for solar domestic hot water (DHW) heating because they can easily operate at a much higher temperature than the surrounding ambient air. This is because the front of the absorber is covered with a transparent sheet that reduces air movements and consequent heat loss. This cover, often made of glass or plastic, allows most of the solar

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32   solar domestic water heating

Box 4.1 Selective coatings

He

at

Copper sheet

Black paint

Black chrome

TINOX

Lig ht

88%

ABSORBER Conventional black lacquer coating

% Absorption

Reflection

18% more effective

Radiation spectrum of the sun

12% ht

Heat radiation

at He

Lig

100

0 0

2.5

5 Wavelength λ in µm

Figure 4.1 The wavelength spectrum of solar radiation is different to that of radiation re-emitted from hot absorber surfaces Source: DGS (2010) Planning and Installing Solar Thermal Systems, second edition, Earthscan, London

ABSORBER Selective coating

25% more effective

at He

Figure 4.2 Selective coated absorber surfaces improve the emission behaviour Source: www.wagner-solar.com

Lig

ht

5%

ABSORBER Highly selective vacuum coating (dark blue) e.g. TINOX

When solar radiation reaches the absorber, it is mainly received at wavelengths shorter than 2.5 micrometres. The solar radiation is then partly re-radiated or reflected back out at longer wavelengths, mostly at greater than 2.5 micrometres. A selective coating presents a structured, layered surface that ‘traps’ the escaping re-radiation. Common selective coatings include black chrome, black nickel and TINOX. There is little difference between the ‘absorbing’ properties of selective and non-selective coatings; however, there is nearly a tenfold difference in the re-emitting properties above wave lengths of 2.5 micrometres.

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solar collectors   33

Box 4.2 Radiation and heat Heat transfer occurs between two objects when one is at a different temperature to the other – from the higher temperature object to the lower-temperature object. Heat can be measured in joules, watt-hours and BTUs. Heat is transferred in three different ways: by conduction, convection and radiation. Conduction describes heat transfer that is predominant in solid materials. It also occurs in fluids but here convection predominates (see below). Conduction describes how molecules pass energy to each other without moving past each other. At a microscopic level, they vibrate and can pass their tiny movements to their neighbours, hence passing along the energy each time. A material’s ability to conduct heat is known as its thermal conductivity. It can range widely when comparing, say, metals and plastics. Conductivity is compared in different materials using the same temperature difference and material thickness per unit of cross-sectional area. The international unit of scientific measurement of thermal conductivity is watts/(metre. Kelvin) = W/(m.K). Other common units for thermal conductivity are: British Thermal Unit per (hour.foot. Degrees Fahrenheit) (BTU/(hr.ft. oF)) 1 BTU/(hr.ft. oF) = 1.73 W/(m.K) Convection describes heat transfer that occurs in fluids but not in solids. It takes place through conduction combined with the flow of molecules past each other. Natural convection is also called ‘thermosyphoning’. Radiation is an electro-magnetic phenomenon that describes heat transfer that occurs through gases or in a vacuum. It is occurs across a range of wavelengths, including visible light, ultraviolet and infrared. If a material is transparent, radiation can be transmitted through it. The ratio of incident radiation as it strikes a material can be divided into three components: transmittance, reflectance and absorbance. These components are affected by the wavelength of the radiation, the colour, surface roughness and incident angle. The international unit of measurement of wavelength is metres (m). One millimetre (mm) is equal to a thousandth (10-3) metre, one micrometre (μ) is equal to a millionth -6 (10 ) metre, one nanometre (nm) is equal to a billionth (10-9) metre. The frequency is inversely proportional to the wavelength. The international unit of measurement of frequency is hertz (Hz).

0%

CH-04.indd 33

10

radiation to pass straight through but also reduces the amount of re-emittance heat loss from the absorber. In colder climates, the temperature of DHW that is to be heated is significantly higher than the ambient air so only glazed collectors can be used. In some very hot climates, glazing may not be needed. Glass covers have the advantage of durability and ease of recycling, but tend to be heavy and can sometimes crack when impacted. Plastic covers have the advantage of lower weight, but can suffer from thermal and ultraviolet degradation in the long term; the cover becomes discoloured and causes reduced solar radiation transmission.

Reflection off the glazing 8% Absorbtion by the glazing 1%

Convection 15%

Reflection off the absorber 5% Heat radiated by the absorber 8% 60%

Useful heat leaving the collector

Heat lost through insulation 3%

Figure 4.3 Only part of the incident solar irradiation usefully reaches the absorber. The higher this is, the more efficient is the collector Source: www.wagner-solar.com

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34   solar domestic water heating

Figure 4.4 Covers of collectors can be glass or plastic

The solar radiation arriving at the solar collector contains different wavelengths (or frequencies) of light. The transparent cover allows most of the shorter ultraviolet wavelengths to pass through and be received by the absorber causing it to heat up, but tends to absorb or reflect back radiation at longer infrared wavelengths. When an absorber gets hot then some of this heat starts to be re-radiated back out. The glazing traps the longer wavelengths inside the collector, allowing the absorber to get hotter – the same principle as the ‘greenhouse effect’. The cover also protects selective surfaces that may be sensitive to moisture, contaminants, wind and physical damage. Some types of glass, such as those with low iron content, are better at transmitting solar radiation and over 90 per cent efficiency can be achieved; it is often used in more expensive solar collectors. Double glazing is rarely used in solar collectors intended for DHW heating but it is sometimes used in higher temperature applications, such as with space heating or active cooling. Experiments on selfshading glazing prototypes, using chemical coatings, indicate that there could be control of overheating once a selected temperature has been reached. Some solar collectors are without transparent covers, so the absorber is fixed externally on its own with no protection. These are used for low-temperature applications, such as solar heating of swimming pools. Collectors are also subject to extremes of weather, including high winds and freezing temperatures. A well-designed collector should be able to survive extreme weather conditions safely and reliably and without specialized maintenance for over 20 years. There are two main types of glazed solar collector for DHW heating: flat plate collectors and tube collectors.

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solar collectors   35

Absorbers contain manufactured pipes or passageways. These contain fluid that is designed to heat up inside the absorber. The construction of the absorber affects the rate at which heat is transferred to the fluid. There are many types of absorber construction, some relatively cheap and simple. Others types are more complex and expensive although these tend to be more efficient at absorbing radiation and transferring heat to the fluid. During periods of high irradiation, if this fluid circulation does not take place then no heat will be extracted from the absorber. Under these circumstances the absorber surface can reach a temperature of between 150°C and 300°C (300°F and 570°F), depending on its construction. This is known as the maximum ‘stagnation’ temperature. Stagnation means that the liquid in the absorber is no longer circulating. The insulation materials chosen around the absorber in a glazed collector must be able to withstand the stagnation temperature. The part of the absorber that faces towards the sun normally consists of thin metal sheet or metal strips. Metals are mostly used because they have a high conductivity. The fluid passageways, also metal, are bonded immediately behind these sheets. The rate of heat transfer relies significantly on the type of bond. Simple absorbers may only rely on a friction bond between pipes and absorber sheet, whereas complex absorbers tend to be soldered, brazed or welded. The width of the absorber sheet (the fin) in relation to the number of pipes is carefully designed to minimize heat loss. Typically, the pipes are spaced no more than 150mm (6") apart. Some absorbers are manufactured with the fluid passages integral to the absorber sheets, such as when two sheets of metal spot are welded together to form a ‘pillow’ construction. Absorbers can also have complex parabolic shapes, which use Figure 4.5 Flat plate absorbers can have different ­constructions reflective surfaces to reflect and focus solar Source: www.wagner-solar.com radiation onto pipes. Absorbers using non-metallic materials, such as synthetic polymers (plastics), generally have lower heat-transfer rates, and are often used unglazed in low temperature applications, such as for swimming pools.

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36   solar domestic water heating

Figure 4.6 An infrared camera shows the heat distribution across different absorbers. Evacuated tubes clearly show their advantage losing less heat. Source: upper: D. Dovic, S. Švaic and A. Galovic, 'Estimating heat losses in solar collectors by IR thermography and numerical simulations'; lower: Thermomax

Zig Zag absorber

Figure 4.7 Some absorbers have complex shapes that reflect and ­concentrate radiation into a central receiver Source: Solarfocus GmbH

When fluid moves through pipes, the natural resistance from the sides of the pipes cause friction. How much resistance is determined by the pipe diameter, the type of fluid, the direction of movement and the number of bends. The higher the resistance, the slower the fluid will move, potentially requiring a bigger pump. However, high resistance often means there is also a high level of turbulence in the fluid. Turbulence is desirable as it creates a better environment for exchanging and transferring heat. A smooth, laminar flow is undesirable as it can permit some fluid to pass unheated through the absorber. Smaller diameter pipes will create more turbulence, and the circulation is likely to be pumped to obtain an adequate flow rate. The high flow rate further encourages turbulent flow. A higher flow rate also permits a greater removal rate of heat and keeps the absorber cooler, which in turn reduces heat loss and encourages better collector efficiency. The shape and layout of the pipes in the absorber affect how easily trapped air can be removed when filling the system. Pipe sections can be connected in series, in effect creating one long pipe that forms a serpentine shape; or the absorber pipes can be perpendicular to a larger diameter pipe that forms a manifold at either the top/bottom or sides of the collector (called a header/ riser assembly or ‘harp’ shape). The layout and direction of pipes

Figure 4.8 Absorbers with fins and pipes can have different internal layouts Header/riser absorber

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Source: www.wagner-solar.com

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solar collectors   37

restricts which external system layout can be used to connect other components. For example, collectors that rely only on natural convection (thermosyphoning) without a pump are best designed so that the absorber fluid passages run upwards – and with larger diameters to reduce the resistance to moving fluid. But in collectors that regularly drain-down and drain-back, the absorber pipes must avoid trapping air or pockets of fluid. Collector manufacturers normally specify which type of external system can be connected.

Box 4.3 Heat transfer fluid (HTF) Heat transfer fluid (HTF) is the liquid that transfers heat from a collector to a heat exchanger or storage tank. The specific heat capacity is the measure of the heat energy required to increase the temperature of a unit quantity of a substance by a certain temperature interval. Its value can vary with temperature. The international unit for specific heat capacity is in joules/(gram. Kelvin) = J/(g.K) Other common units for specific heat capacity are: Watt-hours per (gram.Kelvin) = Wh/(g.K) British Thermal Unit per (pound. Degrees Fahrenheit) = (BTU/(lb. °F)) Calorie per (gram. Degrees Kelvin) = cal/(g.K) The specific heat capacity of water @ 20°C is 4.186J/(g.K) = 1.16Wh/(kg.K) One calorie of energy will raise 1 gram water by 1°Celsius @ 20°C. One BTU of energy will raise 1lb water by 1°Fahrenheit @ 68°F. Plain water has one of the highest specific heat capacities of any common liquid. This means it can carry more energy with a smaller rise in temperature and hence has fewer losses during transmission. This also means it can have a relatively low flow rate compared to aqueous solutions or oil.

4.2 Flat plate collectors A flat plate glazed collector is usually constructed using flat sheets of absorber material and flat glazing. The absorber contains pipes or passageways that in turn contain fluid, which allows the heat to be transported away from the absorber. The absorber and pipes are normally made of metal, such as copper, aluminium or stainless steel. It is usually contained within an insulated enclosure box, usually of aluminium, but can be plastic or even wood. The glazing may be glass, rigid plastic or thin films of plastic. Most flat plate collectors are rectangular, but can be made to custom shapes. The gap between the absorber and glazing is usually approximately 2cm (1"). If this gap is too narrow, radiation will be re-emitted from the absorber back through the glazing and the heat losses will increase; if is too wide there will be an increase in

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38   solar domestic water heating (a)

(b)

(c) Absorber types (a) Fin with tube (b) Welded plates (c) Moulded polymers

Figures 4.9a–c Some different constructions of flat plate collectors Source: (b) Filsol Ltd

unwanted warm air circulation, causing heat loss. Most commercial flat plate collector absorbers will have selective coatings. Under normal circumstances a flat plate collector operates at around 40 to 80°C (100–180°F). The lower the absorber temperature, the less the heat loss and so the more efficient the operation of the collector becomes. Some flat plate collectors are fitted in enclosures from which the air is partially evacuated, which further reduces heat loss. For a flat plate collector, maximum stagnation temperatures can vary from 130°C to over 200°C (260–400°F) – at which point any liquid in the collector will have evaporated into a gas. Commercial flat plate units are normally supplied fully assembled, but some allow installers to remove the glazing and absorber. This has the advantage of reducing weight during lifting and facilitating future maintenance. The absorber pipe connections – that connect to the rest of the system’s plumbing – protrude out of the collector towards either the sides or the back of the collector. This is important for selecting suitable pipes and system layouts. For example, if a collector is fitted flush with the roof covering then it would be desirable to have the pipe connections facing the rear. On the other hand, if sequences of collectors are to be connected together side by side, then having pipe connections at the sides will be useful. There are also a variety of

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absorber layouts that can be fitted inside the collector, some of which are more suited for specific types of plumbing systems (see later sections). Flat plate collectors should be to tilted at least 15 degrees from the horizontal to facilitate air removal from the absorber and drainage of rain water from the collector glazing. As the sun becomes lower in the sky, the flat glazing will tend to become more reflective and eventually the sides of the enclosure box will shade the edge of the absorber. Some models have ventilation holes or slots in the box enclosure to reduce the risk of condensation building up behind the glass. High temperature flexible seals are used around pipe entries to protect against dust and insect ingress. In pumped systems, purpose-made sensor pockets can be used to measure accurately the temperature of the absorber. Flat plate collectors may be designed with integral storage tanks mounted behind or immediately above the absorber; the

Figure 4.10 Flat plate collectors can be mounted above or integrated into the roof. Tube collectors are mounted only above Source: Viessman Werke GmbH & Co.

Figure 4.11 Sensor pockets are often threaded into the edge or back of the collector

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40   solar domestic water heating (a)

collector is normally fixed in position via the box enclosure, which may be designed with purpose-made mounts. Due to freezing risk and high heat loss, this method is only suitable for frost-free climates.

4.3 Evacuated tube collectors

(b)

Figure 4.12a/b Absorbers inside evacuated tubes can be shaped as (a) tubular or (b) a flat fin

An evacuated tube collector (ETC) is a glazed collector made up of a series of glass tubes mounted in rows and plugged into a manifold box. Inside each tube there is an absorber – usually a strip of metal or glass with a thin metallic coating, normally with a selective coating. The absorber contains pipes or passageways that in turn contain fluid, which allows the heat to be transported away from the absorber. The glass tube surrounding the absorber normally contains a vacuum that acts as insulation and reduces heat loss. A distinguishing feature of an ETC is the way in which the vacuum seal is contained. One method is to use two concentric walls of glass to create a vacuum flask with a ‘hollow’ centre that contains the absorber at atmospheric pressure. The other method is to create a weld joint at one end of the glass tube between a metal pipe and glass so that the absorber is held within the vacuum. Some manufacturers shape the absorber as a long, thin metal fin attached to a narrow-bore metal pipe that exits the glass tube. In some models, the installer can rotate the glass tube so that the flat-fin absorber is at the optimum angle to face the sun. Alternatively, the absorber may itself be also shaped as a cylinder or pipe inside the glass. This can then hold the heat transfer fluid. A cylindrical or tubular absorber automatically presents the same absorber area to the sun

Figure 4.13 Evacuated tubes being fitted into a manifold box (see Chapter 10.1 for safety considerations) Source: Michael R. Broglie, MRB Solutions, LLC

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Figure 4.14 Examples of how the vacuum in an evacuated tube is held. A glass double-walled flask (left) or a single-wall tube with a metal end-cap welded to the absorber (right)

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solar collectors   41 A

B

C

D A B C

b a D A

B

C

D

E

F

C

A B C D E F

Mounting rail Vacuum tube Mounting bracket Connection housing Fixing plates Substructure

E Towards equator

Figure 4.15 Flat fin absorbers inside tubes rotated at different angles during installation to suit each location Source: Viessman Werke GmbH & Co.

throughout the day, whereas a flat-fin absorber presents a different aspect to the sun throughout the day. Each glass tube in the collector array will eventually shade its neighbouring tube as the sun becomes lower in the sky, so some tube types have reflective mirrors (facing the sun) at the rear of the tubes to mitigate this. These mirrors can be curved to concentrate the radiation back onto a cylindrical absorber and may be located either inside or outside the glass tubes. Each glass tube is connected to a manifold box at one or both ends of the tube. The manifold, normally made of metal and insulated, contains the heat transfer fluid (HTF). The tubes can be connected vertically or horizontally. In this latter case, the manifolds will be to the side of tubes, not above them. With some types, the installer has a choice of either option. Two variants of ETCs are available, with ‘dry’ and ‘wet’ connections to the manifold. The ‘dry’ variant has an absorber pipe that is sealed at both ends and the liquid inside each pipe is separate from the liquid that flows through the manifold box. A single pipe runs up the central axis of the glass tube and contains a small amount of liquid under low pressure. These are called ‘heat pipes’. This liquid vaporizes inside the heat pipe as temperatures rises above about 25°C (77°F) and becomes a gas that rises up to the top of the heat pipe. A ‘condenser bulb’ in the manifold (to which it is connected) condenses the gas back into a liquid and absorbs the heat. The condensed liquid then falls back to the bottom of the heat pipe to begin the cycle again. The HTF can be water (at low pressure) or a low-boiling-point liquid like an alcohol. These tubes must be mounted at an incline of at least 20 degrees so that the manifold box and condenser are at the top. The ‘dry’ joint can be convenient during maintenance

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42   solar domestic water heating (a)

(b)

(d)

(c)

Figure 4.16a–d Mirrors can be used inside or outside a tube collector Source: (c) Ritter Solar/ESTIF; (d) Schott-Rohoglass/ESTIF

(a)

or replacing a damaged tube, although this does come at the expense of some efficiency. The condensers of these tubes will become hot when the tube is put out in the sun, even if it is not attached to the manifold box. Heat pipes are nearly always found in ETCs but in theory they could also be laid out together as if into a flat plate. Most heat pipes are made of metal but some all-glass forms also exist. In the ‘wet’ variant the tubes and manifold share the same HTF. Two concentric metal pipes, one within the other or a U-shaped metal pipe, run the whole axis of the glass tube so that the HTF can be pumped down one part of the pipe and up the other. These techniques allow the glass tube to be installed in any orientation, including horizontally. These are sometimes called ‘direct-flow’ tubes. They will not heat up in the sun if they are empty of fluid. Some forms of ‘wet’ tube have no internal metal pipes but are made simply as a glass flask. Maximum stagnation temperature can range from 170°C to over 300°C (340–570°F). In order to limit maximum temperature in the manifold box some heat-pipe ETC tubes have a special thermostatic spring inside the condenser to restrict the hot heat-pipe gases reaching into it. Tubes are available with a choice of maximum temperatures – usually in the 95°C (200°F) to 130°C (270°F) range.

(b)

(c)

Figure 4.17a–c Tube collectors can have their manifold box in various positions depending on the type of construction, (a) at bottom, (b) at top, (c) at side Source: (a) Green shop (Consolar); (b) Ritter Solar/ESTIF; (c) Thermomax

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Figure 4.18 (left) A metal heat-pipe ETC showing the sealed end welded to a sealed single-wall glass tube; (right) a metal concentric pipe ETC with ‘wet’ joint and single-wall glass tube

Because the absorber in an ETC is situated in a vacuum, external ambient temperatures have a minimal effect on performance. This means on a cold sunny day, an ETC will generally outperform a flat plate collector of the same absorber area. And at a high temperatures, such as when heating above 60°C (140°F), the ETC would again outperform. (a)

(b)

Figures 4.19a/b ETC that do not use a heat-pipe can often be fitted horizontally or flat, (a) balcony, (b) flat roof Source: Viessman Werke GmbH & Co.

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44   solar domestic water heating (a)

(b)

(c)

(d)

(e)

Figure 4.20a–e Different constructions of ETC. (a) Metal heat-pipe in singleand double-wall glass tubes; (b) ETC with metal U-tube in open double-wall glass flask; (c) ETC with metal concentric tube in open double-walled glass flask; (d) open flask with double glass walls; (e) glass heat pipe combiming single and double wall glass tubes Source: www.alliedsolar.ie.; bottom left photo by Pat Healey, http://dogstarsolar.net

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Figure 4.21 The metal heat pipe in this ETC is sealed and connected into the absorber with a flexible end. The glass is welded to the heat pipe

(a)

(b)

Figure 4.22 A metal heat pipe in an ETC can be a ‘wet’ joint or a ‘dry’ joint at the manifold box; (a) dry joint (heat pipe); (b) wet joint (concentric pipe) Source: Viessmann Werke GmbH & Co.

(a)

(b)

Figure 4.23a/b The pipe connections to an ETC can be at (a) the top of the manifold or (b) the ends Source: Viessmann Werke GmbH & Co.

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46   solar domestic water heating

Figure 4.24 Sensor pockets for ETCs are sometimes located inside the manifold Source: Viessmann Werke GmbH & Co.

Figure 4.25 A solar storage tank is sometimes connected straight onto an ETC like a large manifold Source: FuelFreeForever

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Box 4.4 Solar collector test standards The standards organization with the largest number of participating countries is the International Organization for Standardization (ISO). In Europe there is the Committee for Standardization (CEN); in the US the American National Standards Institute (ANSI), and also the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) or the American Society for Testing and Materials (ASTM). Certification organizations exist to ensure that standard product test methods are fairly applied, using regular and random samples of solar collectors from different manufacturers. These include the Solar Keymark in Europe and the Solar Rating and Certification Corporation (SRCC) in the US. In general, the product test standards for solar water heating are divided into two categories: one for solar collectors and one for complete systems or other system components. For solar collectors, the most well-known test standards are the series based on EN 12975, ISO 9806, ASHRAE 93-77, 93-1986 or 96-1980. The collector test standards are often divided between performance tests (efficiency) and durability (reliability) tests. Durability tests include: • • • • • • • • • • • • • •

internal pressure (absorber leakage and distortion); high temperature (high irradiance without fluid); exposure (30 days without fluid/absorber soundness); external shock (sun then heavy rain); internal shock (sun then cold fluid intake); rain penetration (water spray/retention); freeze resistance (freeze/thaw cycles); positive pressure on cover (wind and snow); negative pressure cover (wind uplift); negative pressure on mountings (wind uplift); cover impact and corrosion resistance (hailstones, fireworks); deterioration of internal insulation material; condensation risk; documentation.

Some units are supplied completely assembled, with the tubes already fitted to the manifold; some with the tubes separate to the manifold box. ETCs are designed to be located above the roof covering or on a frame. It is therefore rarely possible to integrate ETCs flush with the roof. External manifold-box pipe connections for the system are either at the sides or top. This can assist pipe layout. There are limitations as to which system layouts can be used on: some types cannot be used on thermosyphon (natural circulation). Where the absorbers in the tubes permit the HTF to travel down inside them, such as ‘direct flow’ types, this will not naturally drain itself even if the rest of the system has been emptied. ETC units are available for frost-free climates in which the collector and solar water storage tank are fixed immediately above the tubes – via a metal frame that supports the tubes and manifold box.

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Figure 4.26 Poor quality plastic glazing on solar collectors can prematurely degrade

Figure 4.27 When the absorber sheet and the fluid passages start to separate, the heat transmission will be reduced

4.4 Collector components The length of time before a collector is expected to reach a major failure is termed its ‘design life’. This is affected by climatic conditions such as proximity of sea salt or strong winds at high altitudes, and by how often the collector stagnates at high temperatures. But the main thing that affects a collector’s design life is the materials used in its manufacture. Typical reasons for collectors having reduced performance or failing include:

• transparent glazing cover cracking or yellowing;

• loss of bonding between absorber and fluid

Figure 4.28 The absorber is starting to corrode, which will reduce its efficiency due to increased reflection

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• • • •

passages; corrosion of absorber; loss of adhesion or flexibility of seals; dust or insect ingress; cracking or warping of absorber or box enclosure;

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Figure 4.29 The flexible seals around the pipe entries are damaged, allowing moisture or insects in

Figure 4.30 Insects and plants have entered into the collector, affecting the absorber and insulation

• mould growth on transparent glazing cover or reflectors; • animal attack on thin plastic components. Traditionally, solar collector components have principally been metal and glass – due to their established track record, availability, reasonable cost and recyclability. These materials also allow low-cost prototypes to be built. More recently, there has been an increased use of polymers (plastics) for components such as transparent glazing covers, box enclosures, absorbers and pipes. Polymers with rubber-like qualities are often referred to as elastomers. Polymers can provide cost advantages in mass production and lower overall collector weight. However, some polymers have a considerably shorter design life

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Figure 4.31 The collector box has reached the end of its life and is starting to become loose at its corners

Figure 4.33 Birds have damaged a plastic collector cover

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Figure 4.32 Mould growth is ­building up on this transparent collector due to high levels of condensation

Figure 4.34 Modern polymers are increasingly being used in collectors for covers, absorbers and cases such as with this integrated storage collector Source: © 2008 Harpiris Energy

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solar collectors   51

compared to traditional materials, due to their susceptibility to thermal degradation, ultraviolet light and work-hardening from vibration, causing problems including cracking and reduced flexibility. Also, some polymers are currently difficult to recycle. Collectors can also be damaged by rodents, or by birds seeking nesting materials. Hard materials such as metal and glass tend to survive such attacks far better than polymers. As regards vandalism, some polymer transparent covers resist breakage better than glass. The term ‘embodied energy’ describes the energy originally used to manufacture, transport and install the collector. If embodied energy is too high, a solar collector may never ‘pay itself off’ in energy terms. Where traditional materials are used, such as metal and glass, the embodied energy of a solar collector can be expected to be recouped in less than five years in sunny climates. The CO2 produced during its production can be offset against the amount of CO2 not emitted / saved during the operation of the collector.

4.5 Self-build collectors Some people build their own collectors, constructing them from relatively simple and readily available components. It is rare that the design life and performance of do-it-yourself (self-build) collectors exceeds those of factorymade models. Government subsidies are not always available for their installation because their performance cannot be guaranteed. Sometimes a kit of factory-made parts can be purchased for self-build assembly. In some countries there are local ‘Solar Clubs’ that provide members with training and reduced material, equipment and installation costs. Self-build absorbers are often made from strips of copper approximately 15cm (6") wide, which can found second hand at scrap metal dealers. These are then soldered along their entire length to copper pipes of 10–15mm (3/8–1/2") diameter and painted black on the side exposed to the sun. A very simple combined collector storage unit can be created from a mild-steel drum painted black. However, this is likely to corrode and must not be used without treating the internal metal or using a heat exchanger for the DHW. Aluminium, plastics and stainless steel are difficult materials for self-build absorbers as they require specialist jointing methods. Where the climate is particular sunny and hot it is possible simply to use long coils of black plastic pipe or black plastic bags. These provide small batches of hot water suitable for a short shower or washing up dishes, but care should be taken to avoid potential contamination of clean water from unwanted chemicals. Flat plastic transparent glazing of 5mm (1/4") is often used on self-build collectors. This is low weight and easy to cut, but sheets of over two square metres (20ft2) tend to have insufficient rigidity and require mid-span support. This can be achieved with proprietary aluminium T-shaped glazing bars purchased with plastic glazing. These fit under or between sheets and add strength. Multi-wall polycarbonate sheets provide superior rigidity. Ordinary window glass is easily obtained; however, its weight and sharp edges make it more difficult to handle. The boxed enclosure can be made from timber screwed together with aluminium or steel angles, which can also be used to hold the glazing in place. Glass fibre can also be used, but drain and ventilation holes

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Figure 4.35 Unglazed collectors are typically used for swimming pool heating or for DHW in sunbelt regions Source: Solar in Spain

need to be provided. Mineral wool is a readily available heat-resistant insulation material and can be placed behind and to the sides of the absorber. If rigid expanded foam insulation is used, it should be foil-faced. This has the added advantage of serving as a lightweight back panel to the collector.

4.6 Unglazed collectors An unglazed solar collector is one of the simplest and cheapest constructions. This is essentially a bare absorber, but can also be integrated into a large unit combined or close-coupled with a storage tank. For low-temperature applications up to 30°C (86°F) they provide solar heat at comparatively low cost. They are seldom used for DHW preparation – which requires around 60°C (140°F). They easily lose heat, especially in windy conditions. They are more likely to be found in sunbelt regions, deserts and the tropics.

4.7 Collector performance The term ‘collector efficiency’ refers to the ability of the collector to convert solar radiation to useful heat. It is often used solely as a means to compare solar collectors; however, it is important also to consider other factors that affect overall collector performance. These include:

• The stagnation temperature at maximum solar irradiance. • The effective thermal capacitance. Some collectors are made up of lightweight components that heat up quickly. These will have a low ‘thermal capacitance’, or low ‘inertia’, meaning they respond swiftly to

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Box 4.5 Collector efficiency The efficiency of a solar collector refers to the collector’s ability to convert solar energy into heat energy. It is expressed as the ratio between the useful output (solar-heated water) and the input (incident solar energy), expressed as a percentage or a fraction between 0 and 1. The symbol η is often used. It is theoretically impossible for a collector to have an efficiency exceeding 100 per cent since there will always be losses from the collector. Typical maximum values for factory-made collectors range from 50 to 90 per cent. Efficiency is best measured when the collector is in a steady state condition – when the collector irradiation, direction, wind, ambient temperature and cold inlet temperature all remain stable over many minutes. Each factor can be altered to a new value in a series of steps, and graphs can be plotted. This is often called a plot of instantaneous efficiency and is in effect the measurement of the power of the irradiation against the power of the heat gained at a given instant of time. When a collector is working in a real system, its efficiency is always changing due to changing temperatures and position of the sun. Solar collector efficiency can be measured using artificial solar radiation generated by electrical lamps that provide a spectrum (range of wavelengths) that mimics the ‘real’ sun. Any statement of efficiency should indicate the level of irradiation at which the test was performed, the temperature differences between the absorber and ambient, and the basis of measured collector area. Different test standards measure the temperature of the absorber in one of two ways: either at the centre of the absorber or the inlet. It is always preferable to compare test results using the same standard.

•

changes in weather. The opposite is high inertia, which occurs in collectors that do not respond quickly to external changes. Resistance to pumping. This refers to the pressure drop that a pump has to overcome at a certain flow rate. It is sometimes called a ‘dynamic pressure drop’ or loss of ‘pump head’.

If it were possible to fix the efficiency of a solar collector at one value under any conditions, then most applications offer the choice of whether to use a small number of high-efficiency collectors or a larger area of lower efficiency collectors. Lower efficiency collectors are usually cheaper, but higher efficiency collectors generate more hot water for the same collector area and with fewer materials. If the roof area is limited then there is often no other option but to use a highefficiency collector as these take up the least space. In climates with relatively low solar radiation with low ambient temperatures or where large volumes of water at temperatures in excess of 60°C (140°F) are required, then a high-efficiency collector will also be best because it will be able consistently to deliver higher temperatures. For high-irradiation climates, milder ambient temperatures or for low-temperature applications, then a low-efficiency collector can easily suffice. The term ‘instantaneous efficiency’ refers to the efficiency measured at a particular instant. It is not advisable to choose a solar collector based solely on this because the instantaneous efficiency of a collector in real conditions is in fact

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54   solar domestic water heating 100% ηο

90%

Optical losses

Collector efficiency

80%

Figure 4.36 The efficiency of a solar collector depends on its construction, temperature difference at the irradiance level shown 200–1000W/m2

Thermal losses

70% 60% 50% 40%

200 W/m2

30%

400 W/m2

600

1000 W/m2 800

20% 10% 0%

0

10

20

30

40

50

60

70

80

90

100

Temperature difference between average absorber and ambient (K)

always changing. It really depends on a few factors. Most importantly, it is affected by the difference between the ambient air temperature and the temperature of the absorber. This temperature difference is called the ‘Delta T’. When the Delta T is zero, the ambient and collector temperature are the same and no conducted or convected heat is lost from the collector. When the efficiency of a collector with a Delta T of zero is measured, this is called the ‘optical’ or zero-heat-loss efficiency of the collector. It is expressed with the symbol ηo and is the highest theoretical efficiency that the collector can achieve. As the Delta T increases, the thermal losses increase in addition to the original optical loss. This means the collector starts to work less efficiently. The amount of time a collector spends at a particular range of Delta T in a real working system is strongly related to the system layout. For example, if heating a swimming pool that is cooler than 25°C (77°F), there will be Delta T of only a few degrees between this and ambient. This means the collector works near its best efficiency of ηo. However, when the same collector is heating DHW at 60°C (140°F), then there will be a Delta T of over 30°C (60°F) and so the collector works less efficiently. Because solar collectors are sold in different sizes, care is required when making a cost comparison. One very simple way is to compare the cost of the same ‘effective’ area – which is the aperture area multiplied by the zero-heatloss efficiency ηo. Here the aperture area is multiplied by the zero-heat-loss

Box 4.6 Solar collector areas Due to the different ways used to describe their surface areas, care should be taken when comparing the relative efficiency of collectors. There are at least three ways to define collector surface areas: • • •

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Gross area refers to the overall collector dimensions. Aperture area is the unshaded opening that lets light in (or reflects light in). Absorber (net) area is the surface optically presented to the solar radiation.

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Flat plate Absorber area Aperture area Gross area Figure 4.37 The area of a collector can be measured in three ­different ways

Calculating the areas of tubular absorbers with rear-mounted reflecting mirrors requires particular care. In this case, the area of the absorber is the entire cylindrical area inside the glass because both front and rear of the absorber are irradiated due to the reflector; and the aperture is the projected area or ‘shadow’ of the reflectors as it would appear onto a horizontal surface behind the collector. Collector performance, cost-effectiveness and test comparisons should always be made using the same reference collector area. In the US it is common to use gross area, whereas in Europe the ‘active’ area (the absorber or aperture area) is used. Accurate comparisons can only be made using the same test standard. It is often easier to compare the value for money of collectors by comparing equivalent areas using the results of international standard tests done by independently certified laboratories.

Figure 4.38 Method of measuring the areas of tubular absorbers with reflectors Source: Solarpraxis

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Box 4.7 Comparing collectors Examples • Collector A costs $600, has an aperture area of 3.0m2 (32ft2) and a zeroheat-loss efficiency ηo of 78 per cent. Using metric values to illustrate this results in an effective area of 3.0 × 0.78 = 2.3m2. If this costs $600, it therefore effectively costs $260 per m2 . • Collector B costs $700, has an aperture area of 3.5m2 and a zero-heat-loss efficiency ηo of 82 per cent. This has an effective area of 3.5 × 0.82 = 2.8m2 and therefore effectively costs $250 per m2. Collector B offers a better performance value for money by $10 per m2, assuming that all other factors were the same, i.e. the quality of the manufacturer’s service back-up etc. However, this method is not detailed enough for complete performance comparisons, particularly in colder climates. A slightly improved method to normalize collector performance is to divide the optical efficiency (ηo) by its linear heat loss coefficient (a1). This ratio (a1/ηo) gives a score typically between one and ten: the lower the better. This score is called the collector performance ratio, with units of W/m2.K. The collector performance ratio combines two of the most important collector performance factors. This can then be divided into the aperture area to give a comparative score. Using the same examples as above (metric values only): •

•

Collector A has a linear heat loss coefficient (a1) of 5.0W/K.m2. So its collector performance ratio is 5.0/0.78 = 6.4W/m2.K (the lower the better). The revised area comparison score is 3.0/6.4 = 0.47 (the higher the better). The revised cost score per unit area is $600/0.47 = 1276 (the lower the better). Collector B has a linear heat loss coefficient (a1) of 3.5W/K.m2. So its collector performance ratio is 3.5/0.82 = 4.3W/m2.K (the lower the better) The revised area comparison score is 3.5/4.3 = 0.81 (the higher the better) The revised cost score per unit area is $700/0.81 = 864 (the lower the better).

Collector B continues to offer better performance value for money according to this improved scoring method. However, this still does not account for any difference in the rest of the system. Comparing whole systems is considered in the next chapter.

efficiency ηo to give the ‘effective’ area. The unit cost of the collectors is then divided by the ‘effective’ area of each collector to give a simple comparison. A high-performance collector stays reasonably efficient even at large temperature differences between the collector and the ambient. This is achieved through good insulation. An evacuated tube has a very good insulation value because without air around its absorber is in a vacuum. A low-performance collector loses heat rapidly at high temperature differences. It should be noted that unglazed absorbers show surprisingly higher optical efficiencies ηo when

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solar collectors   57

Typical collector efficiencies

Instantaneous Efficiency [%]

100 80 60 40 20 0

0

10

20

30

40

50

60

70

80

90

Temperature of absorber-Temperature of ambient [K] Evacuated tube

flat plate

Un-glazed

100

Figure 4.39 The ­efficiency of different solar collector types measured against the temperature ­difference of the absorber to ambient Source: Jan Erik Nielsen/ESTIF

compared to more sophisticated and expensive collectors. This is because they have no glazing, which otherwise reflects and absorbs radiation, and so initially the absorber receives more solar radiation. However, as soon as the temperature increases these show increased heat loss. Many collector efficiency tests are performed with radiation falling perpendicular to the collector. However, in reality the sun moves across the sky throughout the day and seasons, and so is rarely perpendicular to the collector. This movement affects the efficiency of the collector; firstly because the irradiance is only at its maximum in the middle of the day, and secondly because there is more incident reflectance from some forms of flat collector glazing at lower sun angles (altitudes). Cylindrical absorbers and glazing respond quite differently to flat plates, as do some forms of specially coated glazing and mirrored reflectors. Some collector tests compare the difference of efficiency at the same irradiation between perpendicular and 50° from vertical to produce a dimensionless factor called the incidence angle modifier (IAM). Tests are normally performed in two directions for tube collectors, since different effects occur depending on the direction of the tubes. One direction is up and down the length of the tube, the other across all the tubes. The IAM figure is sometimes stated separately for direct (beam) and diffuse irradiation because some collectors react to each type differently. The higher the IAM figure, the less the angle of the sun affects performance and so the better the collector performs when the sun is lower in the sky. The rate of removal of useful heat from a collector via its heat transfer fluid affects the instantaneous efficiency, because this in turn affects the build up of heat and therefore temperatures in the absorber. Under the same irradiation and ambient temperature conditions, a higher rate of heat removal will increase the efficiency and lower the absorber temperature. This phenomenon is most noticeable in collectors with poor insulation, such as unglazed absorbers. It is also noticeable in system layouts that do not use a pump (thermosyphoning). Here a pump will be expected to increase collector efficiency as this increases the rate of heat removal. However, when a pump is used, then the higher efficiency and hence thermal energy gains have to be accounted for against the electrical energy used

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58   solar domestic water heating

Box 4.8 Solar collector heat loss The rate of heat loss of an absorber strongly affects the efficiency of the whole collector. This loss increases at the same rate as the temperature difference increases between the absorber and the ambient temperature. Different insulating materials help resist this heat loss. Heat loss can be measured and expressed in terms of the power loss per degree of temperature per unit area. This is known as the heat loss coefficient. The lower the value of the heat loss coefficient, the less heat is lost and the better the collector’s performance will be. Where heat loss is measured for typical building materials, it is called the ‘U-value’. This value is the inverse of the R-value, which is a measure of thermal resistance. The higher the value of thermal resistance, the less heat is lost and the better the collector’s performance will be. The international unit for heat loss coefficient is watts per degree Kelvin per square metre (W/K.m2).

The effect of the quadratic heat loss coefficient

Instantaneous Efficiency [%]

100 80 60

Without quadratic heat loss

40 With quadratic heat loss

20 0

0

10

20

30

50

60

70

80

90

100

Temperature of absorber-Temperature of ambient [K] With coefficient

Without coefficient

Figure 4.40 The heat loss coefficient increases at high temperature differences causing an efficiency curve to bend downwards Source: Jan Erik Nielsen/ESTIF

Another unit for heat loss coefficient is: British Thermal Unit per degree Fahrenheit per square foot (BTU/°F.ft2). The standard international tests always calculate the heat loss coefficient when the absorber fluid temperature is equal to ambient temperature. This is called the first order coefficient (or linear heat loss) with units of W/K, often represented by the letters ‘a1’ or ‘k1’. Sometimes this is represented as the slope of efficiency against temperature difference, which is plotted as a straight line on a graph. When there is a large temperature difference between the absorber fluid temperature and ambient temperature, the 1st order coefficient gets slightly larger. This effect can be measured in some tests and is called the 2nd order heat loss coefficient (or quadratic heat loss), with units of W/K², often represented by the letters ‘a2’ or ‘k2’. This causes a slope of efficiency plotted against temperature difference to become a curve on a graph.

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solar collectors   59

(a)

% 100

Flat absorber under flat glass

(b)

75

58

25

29

0

0

80° 70° 60° 50° 40° 30° 20° 10° 0° 0° 10° 20° 30° 40° 50° 60° 70° 80° Both directions

%

Flat absorbers in glass tubes

87

50

(c)

% 116

Curved absorbers in the glass tubes

80° 70° 60° 50° 40° 30° 20° 10° 0° 10° 20° 30° 40° 50° 60° 70° 80° Crosswise

(d)

Curved absorber in glass tube with rear reflector

% 112

152 84 114 56

76

28

38

0 80° 70° 60° 50° 40° 30° 20° 10° 0° 10° 20° 30° 40° 50° 60° 70° 80° Crosswise

0 80°70°60°50°40°30°20°10° 0° 10°20°30°40°50°60°70°80° Crosswise

a) Efficiency remains flat for most of day. b) Efficiency drops slightly in middle of day as some radiation is lost between gaps in tubes. c) Strong peaks exasperated by curved absorbers d) Reflectors catch radiation between tubes.

Figures 4.41a–d The incidence angle modifier (IAM) measures how the efficiency changes as the sun changes its angle in relation to different collector types Source: Valentin Software

Points of view from the sun – dawn/dusk to midday At midday, both collectors are perpendicular to the sun’s rays

Flat plates

Tubes Midday 0 degree angle of incidence. Sun is shining directly on the collectors.

Flat Plate Solar Panel

Tubes are perpendicular to the sun’s rays

As the sun moves, only tubes are perpendicular to the rays

Afternoon/ Morning 40 degree angle of incidence. Sun is shining slightly across collectors.

Flat PlateSolar Panel

Flat Plate Solar Panel

only the tubes are perpendicular to the sun’s rays

only the tubes are perpendicular to the sun’s rays

Sunrise/Sunset 90 degree angle of incidence. Sun is only striking the side of the collector.

Figure 4.42 The angle of the sun affects collectors differently depending on their construction Source: www.apricus.com

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60   solar domestic water heating by the pump. These electrical ‘losses’, called ‘parasitic losses’, typically represent less than 5 per cent of the total solar thermal energy contribution. For standardized international tests such as EN 12975 and ISO 9806, a collector manufacturer can choose the circulation rate. This must then be stated in the report. If this same circulation rate is very different to that obtained under normal conditions, then the reported efficiency and overall performance can be also expected to be different. Some standard tests measure the pressure difference across the absorber as it is pumped at a selection of flow rates. This is called the ‘dynamic pressure drop’ and indicates how much electrical power will be required for the pump to run at various settings. This information is particularly useful when multiple collectors are joined together in series since the cumulative pressure drop becomes more significant. Another useful test result is a collector’s thermal capacitance or thermal inertia. The lower this value, the quicker the collector responds to changes in solar radiation, which can be beneficial in cloudy conditions. It is measured as the temperature rise per unit area in per unit of heat. It can also be measured in terms of a ‘time constant’, the number of minutes it takes the collector to reach 63.2 per cent of its highest temperature from cold on a sunny day. The quicker the collector responds, the lower its thermal capacitance. When comparing the efficiency test results for different collectors, it is important to ensure that the average wind speed and irradiance levels are similar. High air speed increases heat loss and reduces efficiency. If a test is performed with a high irradiance then the collector will appear to have a higher efficiency, assuming the circulation rate is altered to maintain a constant temperature difference. Measuring and comparing solar collector performance is a complex subject. There are many other variables to consider that will affect the collector’s overall performance. Computer simulation programs provide a much more accurate representation of what can be achieved. And the complete assembled system, not just the collector, should be considered.

4.8 Using collector performance test reports Performance test reports from different organizations are generally not compatible with each other, unless they have been performed according to the same international test standard. They also provide only theoretical performance figures with many variables fixed in a way that rarely represents a real situation. Hence, generic performance reports are used to compare collectors and systems but never to predict individual performance.

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solar collectors   61

Box 4.9 The Solar Keymark The Solar Keymark in Europe is a voluntary mark developed by the European Committee for Standardization (CEN). It has an online database of randomly inspected solar collectors tested to EN 12975 and is available at www.solarkeymark.org. In the example data sheet, the following points are highlighted: •

The manufacturer and product name should be carefully confirmed, as there can be many similar sounding models. • The indicated dimensions and calculated area of the collector should be confirmed as either aperture, absorber or gross. •  The zero loss efficiency is the optical efficiency based on the calculated area normally assumed as the aperture and the mean temperature of the absorber. The higher the better. •  The units of heat loss should be checked to avoid confusion between the linear and quadratic coefficients. ‘K’ indicates units of Kelvin equivalent to Centigrade. The lower the better. •  Two values of IAM are given, the higher the better; the beam (direct) value is shown at a selected angle of incidence normally 50°. The diffuse value applies from all directions. Tube or concentrating collectors will show values for transverse and longitudinal directions. •  The thermal capacitance units should be checked to ensure the value is specific to the area. •  The nominal flow rate may be different to that used when installed in a system. •  The technical data is used in other calculations or simulation programs to calculate the collector Figure 4.43 An example Solar Keymark data sheet for a collector to EN 12975 energy yields. Source: www.solarkeymark.org

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62   solar domestic water heating

Box 4.10 The Solar Rating and Certification Corporation The SRCC is an independent third-party organization used by many US state subsidy programmes. It has an online database of randomly inspected solar collectors tested to OG100 and is available at www. solarkeymark.org. In the example data sheet, the following points are highlighted: •

The ratings provide the expected daily solar energy yield. They assume a south-facing collector tilted up at 40° measured from sunrise to sunset under laboratory conditions. It is assumed the latitude is 40°N and the sunpath is at the time of the equinoxes. Three irradiation-level options are given and explained in the following table:

Figure 4.44 An example SRCC data sheet for a collector to 0G100 Source: www.solar-rating.org

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solar collectors   63

Daily irradiation

Daily irradiation

Daily irradiation

Clear sky

23 MJ/(m .d)

2000 BTU/(ft .d)

6.4 (kWh/m2.d)

Mildly cloudy

17 MJ/(m2.d)

1500 BTU/(ft2.d)

4.7 (kWh/m2.d

Cloudy sky

11 MJ/(m .d)

1000 BTU/(ft .d)

3.0 (kWh/m2.d

•

2

2

2

2

The following table shows a choice of categories and their applications. Categories C and D represent the most common DHW applications: CATEGORY

APPLICATION

A

–5°C

(–9°F)

Certain types of solar assisted heat pumps. Swimming pool heating.

B

5°C

(9°F)

Liquid collectors with certain types of solar assisted heat pumps. Swimming pool heating. Space heating – air systems.

C

20°C

(36°F)

Service hot water systems. Space heating – air systems.

D

50°C

(90°F)

Service hot water systems. Space heating – liquid systems. Air conditioning.

E

80°C

(144°F)

Space heating – liquid systems. Air conditioning. Industrial process heat.

• •

The zero loss efficiency is based on the gross area and the inlet temperature of the absorber. The ratings can be divided by the cost of the collectors to give the energy per unit of currency spent for comparison.

4.9 Typical collector performances values Table 4.1 Typical solar collector performance values in metric units Optical efficiency

Optical efficiency

Linear heat loss

Quadratic heat loss

Incidence angle modifier

Thermal capacitance

Collector performance factor

ηo Based on absorber area

ηo Based on gross area

a1

a2

Φ @ 50°

C

a1/ ηo Based on absorber area

%

%

W/m2.K

W/m2.K2

%

Wh/K.m2

W/m2.K

Evacuated tube

60–85

35–60

1.2–3.0

0.004–0.009

84–95

0.8–13.8

2–5

Flat plate glazed

75–83

65–72

3.0–6.0

0.006–0.020

88–95

0.8–13

4–8

Unglazed

90 +

80 +

20 +

0.040 +

97–100

0.8–13

Above 10

Units Collector type

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64   solar domestic water heating Table 4.2 Typical solar collector performance values in US units Optical efficiency

Optical efficiency

Linear heat loss

Quadratic heat loss

Incidence angle modifier

Thermal capacitance

Collector performance factor

ηo Based on absorber area

ηo Based on gross area

a1

a2

Φ @ 50°

C

a1/ ηo Based on absorber area

%

%

BTU/ft2.°F.hr

BTU/ft2.°F2.hr

%

BTU/°F.ft2

BTU/ft2.°F.hr

Evacuated tube

60–85

35–60

0.21–0.53

0.041–0.092

84–95

0.15–2.45

0.35–0.88

Flat plate glazed

75–83

65–72

0.53–1.06

0.061–0.2

88–95

0.15–2.45

0.7–1.4

Unglazed

90 +

80 +

3.52 +

0.408 +

97–100

0.15–2.45

1.76 +

Units Collector type

The Y-axis indicates the instantaneous efficiency of the collector. The units may be stated as a fraction 0.0–1.0 or as a percentage 0–100%

The intersection with the Y-axis represents the theoretical maximum efficiency η0 at zero temperature difference.

The three lines indicate the efficiency plots according to the different ways to measure a collector shown in different colours below the graph.

Relative efficiency η 1.0 The slope of the plot is made up of the linear heat loss a1 (straight line) and quadratic heat loss a2 (a slight curve downwards).

Solar irradiance G = 800 W/m2

0.8

The intersection with the Xaxis is not always shown but it represents the theoretical maximum temperature difference at 0% efficiency, i.e. at stagnation

0.6 0.4 0.2

The X-axis indicates the mean difference between the average of the absorber and ambient. This is normalized per square area and per unit of irradiance.

0.0 0 0.02 Reference

0.04 0.06 0.08 Gross Aperture Absorber

ηo a1 [WK–1m–2] a2 [WK–1m–2]

0.598 0.756 1.12 1.41 0.0023 0.0029

0.524 0.98 0.0020

∆T

The units of the x-axis can vary but are often shown in the range 0.0–0.10 in K.m2.W–1.

Figure 4.45 A set of typical European instantaneous efficiency characteristics for a glazed collector Source: www.solarenergy.ch

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5

solar heating systems This chapter discusses overall system efficiency and performance.

5.1 System performance The term ‘system’ refers to the complete arrangement of components that together collect, store and deliver useful solar heat, and includes the solar collector, as well as pipes, tanks, pumps and controls. System efficiency refers to the efficiency of the system as a whole with regards to its ability to convert solar radiation to useful heat. Efficiency calculations are often used alone to compare one system to another. However, it is important to consider the other factors that affect overall performance, which include:

• • • •

proportion of solar energy saved against other energy sources; consistency of domestic hot water (DHW) temperatures; time lag of solar heat reaching hot water outlets; loss of energy from other energy sources.

A key performance feature is the solar ‘fraction’ (SF), or solar ‘coverage’. This refers to the percentage of energy provided by solar as measured against the total energy it takes to heat the water (DHW), measured over the year. The SF is normally expressed as a percentage, or as a fraction between 0 and 1. For example, if half the energy used to heat water was supplied by solar, the SF expressed as a fraction would be ½, or 0.5; if a quarter, the SF would be ¼, or 0.25. The higher the fraction, the greater the quantity of useful solar energy. If all the water were to be heated by solar the solar fraction would be 100 per cent. However, the solar fraction is not always the most reliable way to measure performance, as its calculation may not fully account for heat losses in storage and DHW circulation. Care is also required when comparing solar fractions from different sources. The amount of conventionally heated DHW energy replaced by solar is called the ‘displaced’ fuel. This can be measured in two ways. The first is to measure conventional energy as it actually goes to heat water. This is called ‘net’ energy, which is the ‘raw’ energy replaced by the solar system after any back-up appliance, such as a boiler, has heated the water. The second method is to measure the conventional energy purchased before use in any appliances. This is then called the ‘gross’ energy supply. When comparing net and gross energies for different fuels, for example in the case of gas, the gross energy use will be a higher value because some heat is lost up the flue in the form of hot gases. The difference between the lower net and the higher gross value represents the losses of the back-up appliance.

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66   solar domestic water heating

[KWh] per week

Solar Energy Consumption as Percentage of Total Consumption

Figure 5.1 The solar fraction can be visualized over the total DHW energy requirements Source: Valentin Software

70 65 60 55 50 45 40 35 30 25 20 15 10 5 Feb

Mar

Apr

May

Jun

Solar Contribution 1458kWh

Jul

Aug

Sep

Oct

Nov

Dec

Total Energy Consumption 3094kWh

The term ‘system efficiency’ or ‘utilization’ refers to the amount of solar irradiation that is actually utilized to heat up water. It is the ratio of ‘useful’ solar energy output to the amount of solar irradiation that falls on the collector, and is a similar concept to collector efficiency, except that the ‘useful’ output is measured as it leaves the solar storage unit instead of the collector. Any heat loss occurring in the pipes connecting the collector to the storage, as well as losses in the solar storage tank itself, are included in the system efficiency calculation. System efficiency is normally expressed as a percentage or as a fraction between 0 and 1. Because solar heating is weather dependent, if a system is not properly controlled erratic temperature fluctuations in the DHW can occur. When solar irradiation is insufficient, a back-up source switches in to achieve the desired temperature. When too much, automatic controls prevent scalding temperatures from reaching the DHW outlets. In some systems there is a time lag before the solar-heated water reaches the taps. This is especially common with separate tanks of heated water or with systems that work without pumps. Where only one tank is used, combined with a back-up heat source, there is a risk of accidental loss of energy from the back-up energy source. This can occur when hot water is accidentally pumped up to the collector in cold weather or at night.

Box 5.1 Performance terminology The ‘collector loop’ is the part of the system that circulates heat transfer fluid. It includes the collector and pipes up to the point at which they meet the solar storage tank. The solar energy output reaching the hot water tank from the collector loop is called the ‘yield’. The term ‘specific yield’ refers to the energy per unit of collector area. This is measured in kWh/m2 (BTU/ft2). Typical values range from 300 to 700kWh/m2 (0.095–0.22MBTU/ft2) dependent on the performance of the collector and the relative rate of DHW use. ‘Parasitic’ energy refers to energy losses from pumps and controls, though over 50 per cent of the electric heat created in a pump is passed into the liquid it is pumping. Parasitic losses also include conventional heat losses from accidental circulation back up to the collector.

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solar heating systems   67

5.2 Typical system performance values If the performance of a same-sized system with the same hot water requirement was compared in different locations throughout the world, one would find that the solar fraction would vary between 30 and 100 per cent, with three times the difference in energy yield. However, the system efficiency would remain remarkably similar, at between 30 and 40 per cent. This is because, if located in a high-irradiance region, a system designed for low-irradiance regions will be grossly oversized. When a system is over-sized it cannot utilize all the existing solar irradiation. If the same system were in one location, this time altering only the volume of DHW that is drawn off daily, one would also note a significant effect on the performance. In much of Europe and the USA the solar fraction would increase to between 30 and 55 per cent simply by halving the hot water demand. However, this would decrease system efficiency from 50 to 30 per cent because greater heat losses would be incurred. The higher the average daily amount of DHW use per unit of area of collector, the greater the system efficiency and the lower the temperatures in the system, but the lower the solar fraction.

Box 5.2 The effect of DHW draw-off rate, collector area and solar tank storage on performance The ‘draw-off rate’ is the rate at which DHW is taken out of the storage tank. The draw-off rate of DHW relative to collector area can be expressed as a ratio – the volume of DHW used per day to the area of the collector. This ratio allows different sizes of systems to be more easily compared. The term ‘collector area’ can refer either to the gross area, the aperture area or the absorber area. These are three very different figures and the choice of which must be stated with the calculation of the ratio (see Chapter 4 for further information). When comparing ratios of different systems the solar collectors should all have the same ‘orientation’, and the DHW draw-off temperature should be the same in each. The draw-off rate is described in litres per day per square metre of collector (litre/day.m2) or gallons per day per square foot of collector (gall/day.ft2). When designing a system, the volume of daily DHW required is needed. The more DHW used, the greater the area of collectors it will be worth fitting. If the ratio between these two variables is kept constant, the solar fraction and system efficiency will be similar. For example, a small household with a small collector area and a big household with a big collector area in the same location can both end up with similar solar fractions and system efficiencies, even though the big household uses far more energy. Typical values for draw-off rate in a temperate climate with DHW at 45oC, relative to solar collector aperture area, range from 25 litres/day.m2 up to 70 litres/day.m2 (7–18 gallons/day.ft2). In sunbelt or tropical climates these figures tend to be much higher, at over 100 litres/day.m2 (26 gallons/day.ft2), because the irradiation is twice as much. Average use is 35–65 litres (9–17 gallons) DHW per person per day. When making performance comparisons, another important factor to consider is the solar tank storage volume. This must also remain in proportion to the collector area and volume of daily DHW use in order to achieve similar performance of solar fraction and system efficiency. Therefore, the greater the daily DHW use, the greater the solar collector area, and the bigger the solar tank storage must be. If these are not all kept in proportion it becomes much more difficult to compare different systems. Computer simulation programs can be useful in this regard.

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Figure 5.2 The solar fraction of a DHW system is inversely related to the system efficiency when increasing the number of collectors Source: Valentin software

%

68   solar domestic water heating 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0

Efficiency Solar Fraction

2

4

6

8

10 12 14 16 Number of Collectors

18

20

22

24

Allowing for the wide range of system sizes, typical annual average system efficiency of most solar DHW systems lies between 30 and 60 per cent. In general, system efficiency and the solar fraction are inversely proportional to each other – the higher the system efficiency the lower the solar fraction, and vice versa. What you gain in performance (in the case of cars, speed and acceleration) you lose in efficiency (fuel economy). To relate this to solar systems, if ten collectors were placed on the roof of a house they would produce a lot of heat and so a very high solar fraction. However, the efficiency of the system would be much lower, because as the collector, pipes and storage become increasingly hotter the amount of heat lost from them increases. Hence, as a system gets closer to providing 100 per cent of the DHW demand, the more inefficient it becomes. The system efficiency is also affected by the size of the solar storage water tank. In the summer months a small tank will heat up very quickly and reach saturation point, and be unable to receive any more solar energy, no matter how sunny the day. The collectors will then start to stagnate, likely to reach their highest temperature. Systems that have relatively modest collector areas with large solar storage in respect to their daily DHW usage tend to have better system efficiencies. This means that the collectors are giving their users better value for money. Deciding between increased efficiency, value for money or a high solar fraction is a fundamental design choice. The decision is best made early in the design process, and will be affected by the type and size of the collector, the capacity of the solar storage tank and the degree of insulation of the pipes and storage. Systems with high solar fractions tend to have relatively larger collectors and hence are more expensive than high-efficiency systems. Many households prefer a high solar fraction because it means they are more likely to be able to switch off their conventional heating. Many businesses prefer high system efficiencies because it gives the best financial return.

5.3 Reduction of fuel bills and pollution An important feature of any solar water heating system is the proportion of conventional fuel it replaces, or the amount of ‘purchased energy’ it saves. This is known as the ‘fractional energy saving’. It is normally expressed as a percentage or as a fraction between 0 and 1. For example, if a household without a solar system

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solar heating systems   69 Before solar heating

With solar heating Energy in DHW

Energy in DHW Original Gross Energy for DHW heating

Net conventional energy before fitting solar

Net back-up energy after fitting solar

Gross energy after fitting solar Back-up heat source

Original DHW heating Storage Tanks

Storage Tanks Solar Collector

Figure 5.3 The fractional energy saving is the gross energy reduction of the DHW fuel bills comparing before and after the implementation of a solar heating system

used four times as much gas for DHW as it now uses with a solar system, the fractional DHW energy saving would be ¾, or 0.75. This is a similar concept to the solar fraction saving except that it takes into account the heat lost from back-up appliances or fixtures (e.g. the boiler). Fractional energy saving calculations account for the efficiency of other appliances that would otherwise heat hot water. The average annual fuel bill can be multiplied by the fractional energy saving to calculate money or energy savings. For example, if an existing heating system for three people required the owner to purchase 3000kWh (10 million BTU (MBTU)) of energy each year just for the DHW, and a prospective new solar DHW system is calculated to have a fractional energy saving of 50 per cent, it is easy to calculate the amount of fuel saved. The purchased energy for DHW would then be halved to 1500kWh (5MBTU) after the solar heating is fitted. However, whilst the cost of the fuel bill for DHW would be halved, any fuel just used for central heating would not be affected. So if the central heating used 17,000kWh (58MBTU) per year on top of the DHW, the total conventional Table 5.1a Example energy balance using solar DHW using metric measurements Household of 3 people

Hot climate

Temperate climate

Cold climate – poor insulation or big house

Cold climate – good insulation or small house

Annual purchased energy kWh DHW

4000

3500

3000

3000

Space heating

1000

10,000

17,000

7000

Total Typical solar DHW fraction Fractional energy saving on total fuel bill

CH-05.indd 69

5000

13,500

20,000

10,000

70% = 2800

55% = 1925

50% = 1500

50% = 1500

56%

14%

7.5%

15%

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70   solar domestic water heating Table 5.1b Example energy balance using solar DHW using US units Household of 3 people

Hot climate

Temperate climate

Cold climate – poor insulation or big house

Cold climate – good insulation or small house

Annual purchased energy MBTUs DHW

13.64

11.94

10.24

10.24

Space heating

3.41

34.12

58.00

23.88

Total Typical solar DHW fraction Fractional energy saving on total fuel bill

17.06

46.06

68.24

34.12

70% = 9.55

55% = 6.56

50% = 5.11

50% = 5.11

56%

14%

7.5%

15%

energy bill would be 20,000kWh (68MBTU). The saving on the total fuel bill would therefore be 1500/20,000 (5/68), which equals 7.5 per cent. The smaller the energy used for space heating, the greater the influence of solar water heating on the original energy bill. People living in warmer climates notice more significant savings, because they have a smaller total fuel bill and also get better solar energy production due to higher irradiation. Table 5.1b shows some typical fractional energy savings in different climates.

Box 5.3 Solar system test standards In Europe, the internationally recognized system test standards include EN 12976 and prEN 12977. These in turn refer to ISO 9459. In the US there is ASHRAE 95-1981. Besides performance tests (efficiency) there are also durability (reliability) tests. The durability tests include: • • • • • • • • • • • • • • •

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freezing and freeze damage resistance; over-temperature of hot water; water contamination; safety expansion and pressure relief; electrical safety; lightning protection; reverse circulation prevention; resistance to weather and animal attack; corrosion resistance of metal frames and pipes; steam escape; pipe and tank insulation; wind and snow loads; sound and vibration; auto-ignition and flammability; documentation.

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solar heating systems   71

For new buildings there will be no previous fuel bills. In this case, it is possible to use a theoretical ‘reference’ hot water system and then estimate savings of a solar water heating system against this. Some computer simulation programs can do this.

5.4 Measuring solar contribution To find the solar energy contribution to the overall system energy consumption, one needs to measure it. This can be done at the following points:

• leaving the collector; • arriving at the solar storage (solar yield); • arriving at the point of back-up heating. The solar energy contribution can be expressed as a fraction of any of the following:

• • • • •

all energy in the property generated for DHW and space heating; conventional energy generated at the power station / gas or oil field; conventional energy used (consumed in the property / purchased energy); conventional energy reaching the DHW; secondary circulation losses or DHW storage losses.

When comparing the performance of different systems and system components it is important to use the same method. Most figures are given for over a period of a year, but figures for shorter periods can be used – to show, for example, the summer months. Energy from electric immersion and pump Energy from back-up boiler

Energy in DHW Energy from primary circuit

Solar Radiation Heat Loss – External piping Heat Loss – Internal piping

Heat Loss – Optical Collector Heat Loss – Thermal Collector

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Heat Loss – Tank

Figure 5.4 The flow of energy through a solar heating system can be visualized by different colours and sizes of arrows from left to right Source: Valentin Software

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72   solar domestic water heating Energy Conversion Chain and Losses for Water Heating with a Gas Cooker

Effective energy Final energy gas 90% (230 Wh)

Primary energy source Natural gas 100% (311 Wh)

Natural Gas

Heating water 31% (97 Wh)

Heating water 19% (97 Wh)

Losses 10% (31 Wh)

Energy Conversion Chain and Losses for Water Heating with an Electric Cooker

Primary energy source e.g. coal 100% (515 Wh)

Grid Electricity

Figure 5.5 Typical energy conversion losses from central energy generation in order to boil a pan of water

Final energy electricity 34% (175 Wh)

Heating water 19% (97 Wh)

Effective energy

Waste heat of power plant 66% (340 Wh)

Waste heat of cooker 15% (78 Wh)

Source: Volker Quaschning

%

%

Tropics

100

100

80

80

60

60

40

40

20

20 0

0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

North of Tropics

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Time Period 1/1/-31/12/

Time Period 1/1/-31/12/

DHW Solar Fraction 60%

DHW Solar Fraction 92% % 100

South of Tropics

80 60 40

Figure 5.6 The typical solar fractions of each month can be compared in different regions Source: Valentin Software

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20 0

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Time Period 1/1/-31/12/ DHW Solar Fraction 78%

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solar heating systems   73

5.5 Required DHW temperature The proportion of fuel that the solar will replace varies according to the intended DHW temperature, sometimes called the ‘target’ DHW temperature, and is measured at DHW outlets. The temperature produced may vary between these outlets, so an average is assumed. Most people use DHW between 40°C (104°F) and 60°C (140°F). The DHW in the distribution pipes, tanks and boilers will be at a higher temperature than the target DHW, which is why cold water is often added to ‘mix down’ the temperature. Sometimes the target DHW temperature is fixed by the supplier or manufacturer of the taps and the shower mixer valve to reduce the risk of scalding. More often, the temperature is adjusted by the user. The higher the target DHW temperature the more total energy will be required, and so the smaller the solar fraction and fractional energy saving for a given size of solar collector. In general, both annual fractions will reduce by about 10 per cent when water is at 60°C (140°F) as compared to the lower temperature of 45°C (113°F). Ideally, the target DHW temperature should always be stated alongside any solar performance figures. (Check manufacturers assumptions regarding required delivered temperatures.) When comparing theoretical solar performance between systems then the target DHW temperature should be assumed to be the same, even though in reality this differs depending on personal preference. Another factor affecting the proportion of solar-replaced fuel is the temperature in the distribution pipes, tanks and boilers. If it is higher this will incur greater losses, which also tends to reduce both fractions by increasing the overall energy requirement. The temperature of the cold ‘feed’ water can vary between winter and summer, as well as between different locations. This has two effects. If the feed temperature is low, for example in winter, the total energy demand for DHW will increase and therefore both fractions will be reduced. However, a colder feed temperature improves solar efficiency as there will be less heat loss from the collector. This results in a complex balance that may actually cancel itself out. Only a computer simulation can accurately predict the outcome.

Box 5.4 Other solar analytical terms and methods The ‘energy factor’ (EF) of a water heater is the ratio of useful hot water leaving the heater measured against the heater energy input (purchased energy), such as gas, oil or electric, plus any electrical energy input from pumps and controls. This measure is commonly used in the US for conventional hot water appliances or fixtures. Typically, an electric auxiliary tank has an EF of 0.9 and a gas heated tank an EF of 0.6. The ‘solar energy factor’ (SEF) is the ratio of solar energy leaving the solar storage as measured against the energy input (purchased energy) of the back-up heating system. This is commonly used by SRCC rating organizations in the USA. Typical SEF values range from 1 to 10: the higher the better.

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74   solar domestic water heating

Figure 5.7 An example Solar Keymark data sheet for a complete system to EN 12976 Source: www.solarkeymark.org

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solar heating systems   75

Figure 5.7 An example Solar Keymark data sheet for a complete system to EN 12976 (Cont'd)

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76   solar domestic water heating

5.6 Using system performance test reports Performance test reports from different organizations are generally not compatible with each other, unless they have been performed according to the same international test standard. Also, they can give only theoretical performance figures with many of the variables fixed. This rarely represents a real situation. Hence, generic performance reports are used to compare collectors and systems but never to predict the performance of individual systems. The Solar Keymark in Europe has an online database of randomly inspected solar systems tested to EN 12976 available at www.solarkeymark.org. In the example data sheet, the following points are highlighted:

• The solar collector details are taken from EN 12975 test results. • The system is constructed in a laboratory and inspected for safety, durability and reliability.

• This test does not consider the efficiency of the conventional back-up •

• •

heating, but does consider if it is large enough to back-up if the solar is not providing enough heat to meet the draw-off profile. For solar systems where the solar tank has no other heat source, the performance is given as the heat (kWh) delivered by the solar heating system QL, the solar fraction (%) fsol and the parasitic energy Qpar (kWh). The solar fraction does not include conventional storage losses. For solar systems where the solar tank does have another other heat source, the performance is given as the net auxiliary energy demand Qaux net (kWh) and the parasitic energy Qpar (kWh). It uses a computer simulation for performance prediction, assuming ideal conditions as follows: – Collector orientation south, collector tilt angle 45°. – Temperature of integrated auxiliary heating 52.5°C (126.5°F). – Climate and cold water referenced to a stated location. – Daily load pattern 100 per cent at 6 hours after solar noon. – Draw-off flow rate 10 litres/min @ 45°C (113°F) stated at various daily volumes.

The Solar Rating and Certification Corporation (SRCC) has an online database of randomly inspected solar collectors tested to OG 300, available at www.solar-rating.org. In the example data sheet above, the following points are highlighted:

• The solar collector details are taken from OG 100 test results. • This test does account for the efficiency of the conventional back-up heating. • The performance is given as the solar energy factor (SEF): the higher the better.

• The system is constructed by the supplier on a site and randomly inspected for safety, durability and reliability.

• It uses a computer simulation for performance prediction, assuming ideal

conditions as follows: – A 4733Wh/m2  – day (1500BTU/ft2  – day) solar radiation profile between 08:00 and 17:00.

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solar heating systems   77 SOLAR WATER HEATING CERTIFICATION AND RATING

CERTIFIED SOLAR WATER HEATING SYSTEM SUPPLIER:

Viessmann Manufacturing Company (US) inc. 45 Access Road Warwick, RI 02886 USA (401) 732-0667 (401) 732-0590 Fax (800) 288-0667 SYSTEM NAME: Vitosol 200F Combi-Package SRCC OG-300

SYSTEM TYPE: Indirect Forced Circulation LOCATION:

IL-CHICAGO

Description: Glazed Flat-Plate, Differential Controller, Not found Not found, No Load Side Heat Exchanger. Freeze Tolerence: -31F, Fluid Class II, Electric Auxiliary Tank Cost System Cost 300* Date. Model_name

Collector Fund Manufacture

Collector Panel Name

5

54.3

303

Aux SEF Annual Annual Solar Aux Savings Solar Panel Panel Panel (k Whr)Function Vol(g) Vol(I) Vol(g) 80

2.8

2610

.56

AV

2-200F-80RE 2007032A 13- Vienuna Manufacturing Company (US) Inc. SV2, SH2 JAN09

Total Total Solar Panel Panel Panel area area Vol(I) (Sq-m) (Sq-m)

Ex P

C

AV

Solar Storage Tank Cold supply OG-300 System Reference: 2007032A

SVG Diagram Display

February, 2010 Certification must be renewed annually, For current status contact SOLAR RATING & CERTIFICATION CORPORATION c/o FSEC 1679 Clearlake Road Cocoa, FL 32922 (321) 638-1537 Fax (321) 638-1010

Figure 5.8 An example SRCC data sheet Source: www.solar-rating.org

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78   solar domestic water heating – Auxiliary set temperature 57.2°C, water mains temperature 14.4°C. – Draw-off flow rate of 11.34 litres/min (3.0 gpm) to total of 243l (64.3 gal) to meet total energy draw (QDEL) 43,302kJ (41,045BTU) – Six draws at the beginning of each hour starting at 9:30 a.m.

5.7 Collector and system selection When selecting a solar collector, the first thing to consider is the primary purpose the solar energy will be used for. For example, will it be used only for domestic hot water (DHW)? Or will it also be used for central heating or swimming pools? Then the other system components need to be selected. A successful integration of the system into the building’s existing heating equipment is as important as having the ‘ideal’ collector. The main objectives can be established using a checklist, such as the following:

• • • • • • • • • • •

What proportion of annual heating from the sun will be realistic? Will value for money be more important than having the ‘best’? How important are consistent water temperatures? Will the glazing or collector structure present a safety risk? Does the system have to work with or without a utility supply of water and electricity? How long does the equipment need to last without attention? Do the overall life cycle and environmental benefits matter? Is simplicity of operation important or is owner intervention acceptable? Is performance required in the summer or all year round? Can the roof / attic support the weight and size of components? Are the aesthetics important?

Where a professional designer is involved, it is best they are given a set of requirements by their client as per as the list above. When put in writing, this gives future recourse to the owner should the system prove unsatisfactory. Many problems occur when solar hot water systems are ordered without asking clearly for a minimum level of performance. It’s not enough to simply ask for ‘solar heating’ without further detailing expectations. As a general rule, the higher the required solar fraction, the bigger and more expensive the system will be. The best way to anticipate accurate performance is to use computerized simulations that consider each particular aspect of system options. These programs take the individual test performance figures of a collector and calculate the solar fraction, efficiency and the saved back-up energy, which can then be compared against the cost of the equipment. Standard performance ratings for collectors, which give an estimated performance in test laboratory idealized conditions, are sometimes published by government-funded bodies. These sometimes provide annual energy performance ratings based on different applications or climates, and so the closest match to these conditions can be found. When choosing between collectors it makes sense to keep in balance the two most important factors: optical efficiency (go) and linear heat loss (a1). Optical efficiencies are better when high and heat losses are better when low. From

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solar heating systems   79

Figure 5.9 Computer simulations are increasingly used to anticipate results more accurately Source: Valentin Software

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80   solar domestic water heating

Figure 5.10 Some tube collectors can be fitted in a horizontal row for low appearance on flat roofs Source: www.solarworks.co.uk

these two measurements the ‘collector performance ratio’ (a1/go) can be calculated, by dividing optical efficiency by linear heat loss. A good (low) score occurs when both factors are optimized. A score of below 5 indicates a superior class of collector, which can be expected to be relatively more expensive. The less sunny the climate, the harder it is to achieve solar fractions of over 50 per cent without a superior class of collector. An example of a particularly poor climate for solar hot water heating will have a combination of annual irradiation measured horizontally below 1000kWh/year, an average ambient temperature below 10°C (50°F) and a large difference between summer and winter hours of daylight. Adequate solar storage tank volume is always necessary to achieve a high solar fraction. A superior collector should always be matched with a superior solar tank to get the best out of it. The extra expense of a superior collector will be lost unless matched by a superior system. Further guidance on sizing main components is given in later chapters. In temperate climates, and where a modest solar fraction of approximately 30 per cent is acceptable, an evacuated tube absorber will give equivalent performance to a glazed flat plate absorber so long as the flat plate absorber area is at least 20 per cent larger. However, if a high solar fraction is required, say over 60 per cent, then at least a 35 per cent increase in the glazed flat plate absorber area will be needed. The best value for money and overall system efficiency generally lies with modest solar fractions, below 50 per cent in northern Europe and USA. Nearer the equator it will be easier, at lower cost, to achieve high solar fractions. This is due to the higher solar irradiance and smaller difference in daylight hours between summer and winter. Where solar water heating is required only in summer, a lower class of collector would achieve acceptable results.

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solar heating systems   81

In areas of the world lacking reliable utility services of electricity and water, simple system designs are preferable – systems that operate without controls or a pump. However, this will often result in lower efficiencies and reduced hot water temperature control. When fitted onto dwellings, the aesthetic appearance of solar collectors becomes relevant. There is sometimes the option of different colours for the frame edges of flat plates and manifolds of tube collectors. Some flat plate collectors can be fitted flush into the roof without evidence of pipes or valves, which gives a cleaner look. Tube collectors can sometimes be fitted horizontally, which may suit the appearance of some dwellings better; these can also reduce reflected sunlight ‘flashes’ due to their rounded format. Where significant snow is expected in winter, this can lie for long periods on evacuated tube collectors due to the high level of insulation and compaction between the tubes. On flat plates, the snow and ice melts more quickly because more heat is reflected and the snow slides easily off. With flat plates, condensation can form on the glazing after a cold night.

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6

additional components This chapter deals with the principal solar water heating system types and discusses the main issues associated with integrating solar heating into existing heating systems.

6.1 Main system components A solar collector must be accompanied by a system composed of other components. For solar domestic hot water (DHW) these components can be categorized according to their main functions as follows:

• • • • • • • •

heat transfer fluid; pipes and connections to connect to a storage tank; a solar storage tank containing water; a means of circulation such as a pump; controls for safety, performance and information; a heat exchanger (in most systems); back-up sources of heat; sources of water and electricity.

6.2 Heat transfer fluids (HTFs) Heat transfer fluids (HTFs) carry heat from the collector to the rest of the system. Plain water is common but has limitations. It can contain impurities that corrode some metal absorbers or restrict the movement of heat. Fresh drinking-quality water in particular contains dissolved oxygen that is released when heated, forming bubbles. The oxygen can promote corrosion in some materials and the bubbles can rise to the highest point and then obstruct water movement. With fresh water, this oxygen is continually replenished. In systems in which water is used indirectly, the oxygen bubbles only form once and can be removed. Impurities such as minerals can form limescale that eventually blocks pipes and the absorber. However, water remains the first choice in many systems because it is cheap, readily available and able to absorb and transfer heat very well. In cold climates, plain water can easily freeze solid in the collector or pipes and rupture rigid metals and plastics. The ice also prevents safety valves from working and can trap melted water inside a collector, which on a cold sunny day can cause unwanted boiling. This is why some systems use indirect circuits,

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84   solar domestic water heating Solar collection

Controls and information Source of electricity

Safety Back-up system

Sensors

Circulation

Solar heat storage DHW

Figure 6.1 A solar water heating system can be grouped into main ­functional groups

Expansion and fluid level indicator

Fluid fill and drain Source of water

filled with an HTF that is a mixture of chemicals, including antifreeze. Antifreeze or chemical corrosion inhibitors must never be used without a heat exchanger. The most common active chemical that prevents freezing belongs to a family of chemicals called glycols. There are two common types. One of these, ethylene glycol, is not suitable for use in solar circuits; this is the antifreeze often used in automobiles and has toxic properties and poor longevity. The correct antifreeze for solar systems is based on propylene glycol, which is sold with corrosion inhibitors especially for solar heating circuits. These are considered non-toxic when new. It is mixed with water to a concentration that offers protection against freeze damage typically down to –30°C (–86°F). Sometimes the chemicals are sold with water already added. In this case, the water will have been specially prepared to reduce mineral content and be free of dissolved oxygen. Chemicals are also sold as a concentrate and water is added at the point of use. However, the added water can introduce unwanted minerals as well as dissolved oxygen, which must be removed later through air vents. Some new antifreeze products are non-glycol fluids based on stabilized vegetable extracts. Antifreeze must always be used with corrosion inhibitors. In some indirect systems antifreeze may not be required, but a corrosion inhibitor may be. Antifreeze tends to degrade over time. This can happen within weeks if the antifreeze is continually allowed to get very hot, which can happen in some indirect systems

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additional components   85

(a)

(b)

Figures 6.2a/b Antifreeze solutions are often brightly coloured in new containers but will discolour inside systems due to chemical degradation at high temperatures Source: Tyforop Chemie GmbH

when the pump is left switched off on a hot summer season. Regular testing of acidity and glycol levels is required to check that corrosion protection and freeze protection are maintained. This can be done with a small kit which includes pH strips and a refractometer. Although propylene glycol has low toxicity and is readily biodegradable, old antifreeze should not simply be disposed of in storm drains or public sewers without checking with the local authorities. When filling a system, the more antifreeze added the harder a pump will have to work in order to circulate the same heat because antifreeze has less heat capacity and a higher viscosity than water. All HTFs should be able to change readily between their liquid and evaporation phases without leaving behind insoluble deposits. This is why minerals in plain water can cause problems. Some local building regulations (codes) require chemicals in an indirect circuit to be at a lower pressure than the fresh water supply in order to reduce the risk of contamination caused by a heat-exchanger leak. Sometimes this is achieved by including two layers of metal in the heat exchanger. Some HTFs are nonaqueous (without water). These include a variety of mineral oils. But they are rare.

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Figure 6.3 A few drops of antifreeze from a system permit it to be checked for its quality using a refractometer and pH strips

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86   solar domestic water heating

Box 6.1 Special requirements for HTFs In the US, a single-wall heat exchanger is permitted if the heat transfer medium is taken from a potable water source or is distilled water suitable for domestic use. At the time of writing, any additives must be listed in the Code of Federal Regulations as a substance generally recognized as safe. Labels shall mark all drain and fill valves in the system. Each label shall identify the fluid. The location of fluid handling instructions shall be referenced. The label shall list the heat exchanger type and heat transfer fluid class as defined by the American Water Works Association. The label shall include a warning that fluid may be discharged at high temperature and/or pressure.

6.3 Pipes and pipe fittings Most of the pipework associated with the solar system will be regular plumbing; however, some of the pipework will be subject to more extreme conditions, such as:

• • • • • • •

higher temperatures; higher pressures; antifreeze chemicals; animal and insect attack; sunlight when external; impact and abrasion when external; freezing in some climates.

Copper piping is particularly common, a range of sizes and fittings are available. It is available in a soft form as coiled loop, as well as in solid straight lengths. Stainless steel pipe is available as straight lengths as well as flexible pipe with integral bellows. Some antifreeze compounds will react with metals like aluminium and galvanized steel, so pipes made from these materials are not usually recommended. Non-metallic pipes made from polymers, plastics and rubber are also occasionally used. However, some of these have failed prematurely when used in strenuous solar heating applications. Unless clearly recommended by the manufacturer, these materials should not be used near collectors. Pipes and fittings should always bear an indelible mark as a means to identify their make and model. Apart from with very simple unglazed collectors, it is reasonable to expect that temperatures exceeding 150ºC (302ºF) will occur in the pipework connected to the collector at some point during the system lifespan. This may be because of a fault, or it might occur in some systems during normal operation. Such high temperatures are capable of causing injury by scalding. Most HTFs will vaporize to a gas (steam) at these temperatures, and excess pressure can simultaneously occur.

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additional components   87

(a)

(b)

Figure 6.4a/b Flexible and rigid pipes made from metal are very common in solar systems

Any pipework passing externally or inside building voids is at risk of attack by animals, birds and insects, often seeking nesting materials or warmth from the pipes. Ultraviolet light in sunlight has a weakening affect on many non-metallic materials. There is the danger of impact against external pipework during storm winds or during other roof work. Metal pipes withstand all these effects best and metal pipework has a proven history. Joints in pipes connect pipes to other pipes as well as to the tanks, valves and various controls. These joints are subject to the same stresses as the pipes. In some sealed systems, the maximum pressure can reach 6 bar (87psi), which will exceed the strength of ordinary plumbing joints. However, many other system layouts are intended to operate at much lower pressures. Soldering or brazing is a traditional way of joining copper piping – melting another metal (a tin alloy) that then bonds the copper together. However, some solar collectors can now reach over 300ºC (572ºF), which exceeds the melting point of some solders. Another problem is that during the melting process, some corrosive chemicals are released so the pipes must be carefully cleaned before use. Compression joints use a metal clamping ring or olive, which is compressed onto the pipe to form a joint. These joints rely on the strength of the tube to resist the compression process without distortion, but in some cases the tubes and clamping ring are not strong enough for sealed systems. A higher grade of tube or joint is then required. Some pipe fittings require a threaded joint that can be later dismantled. Some of these use a high-temperature fibre washer, which forms a seal between the two flat faces of the joint. Other modern thread sealants include thin Teflon

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88   solar domestic water heating

Box 6.2 The measurement of pressure Pressure is the force per unit area measured perpendicular to the surface area. The international unit is the pascal (Pa), which is equal to one newton of force per square metre (N/m2). One kilopascal, kPa, is equal to a thousand (103) pascals. Other common units for pressure are: Pound (force) per square inch = psi mBar = equal to one thousandth of a bar = 0.1kPa = 1 hectopascal (hPA) Pressure is also sometimes described as being above atmospheric pressure (many pressure gauges show zero when they actually mean atmospheric pressure). At standard sea level, atmospheric pressure is 1013.25 mbar although it is common to use 1 bar instead. 1 bar = 0.987 atmospheres = 14.5psi. Another way to express pressure is to compare it with the height of an imaginary column of water. For example, 0.981 bar is equal to 10 metres (32.81ft.) of water column. For small gas pressures, inches of water are often used instead. Where in doubt, the following terms can be used: • • •

‘Absolute’ pressure is zero in a vacuum, so it is equal to gauge pressure plus atmospheric pressure. ‘Gauge’ pressure is zero at ambient air pressure at sea level. ‘Differential’ pressure is the difference in pressure measured between two points.

Figure 6.5 Typical joints for copper pipes in solar systems

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additional components   89

tape or liquid glue that sets hard, both of which need care to ensure they are sufficiently temperature-rated and suitable for glycol antifreeze. A less traditional joint for pipes is a crimp fitting, which uses a power tool to force the joint together. These can incorporate a manufacturer-approved EPDM (ethylene propylene diene monomer) rubber O-ring seal made especially for solar heating. If using non-metallic pipes ensure that the pipes and the fittings are suitable for solar heating system. Pipe supports should be metallic where in contact with metal pipes. Metal pipe fixings that clamp over pipe insulation reduce heat loss, but make sure they do not unduly compress the insulation. Components made of different metals should be selected so they will not corrode when in contact with each other. Pipe insulation should also be selected for its high-temperature resistance. Two main types of pipe insulation are used: foil-faced mineral wool and flexible EPDM high-temperature rubber. Both are susceptible to rodent attack and, externally, to damage by birds. Hence, extra mechanical protection such as aluminium, PVC cladding or petroleum wrap should be used. Mineral wool can only be used internally since it will otherwise absorb moisture.

6.4 Solar storage tank Water is the most common medium used to store solar heat because it is cheap, abundant and has an excellent capacity to contain heat. Hot water tanks (also called ‘cylinders’) are usually made of copper, coated steel or stainless steel. However, they can also be made from glass-fibre, polymers or even concrete. In most countries the construction of water tanks in buildings is regulated by codes. Normally tanks should be insulated to reduce heat loss, using materials such as mineral wool and expanded foam. Some solar tanks only work with indirect circuits and so contain water that is continually recirculated. These can be made cheaply with mild steel. The warmer the climate the more likely the solar storage tank will be located outside, and in some cases it will be integral with the collector. The solar tank may not be the only hot water tank connected to the water system, since back-up heating equipment can also use tanks. Some tanks will be heated by solar alone, but some will also have other internal heat sources (electrical resistance immersion elements, heat exchanger coils). These dual heated, twin-coil or bivalent tanks have to be designed so that interference from one heat source by the other is avoided. In these tanks the return pipe to the collector is best connected as low as possible in the tank and as

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(a)

(b)

(c)

(d)

Figure 6.6a–d Solar storage tanks are found in different locations depending on regional climatic conditions and local building codes. (a) External on ground (Aus); (b) internal on ground (US); (c) internal 1st floor (UK); (d) external roof (Asia) Source: www.chromagen.biz; www.apicus.com

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90   solar domestic water heating (a)

(b)

(c)

Figures 6.7a–c (a) A specialized tank designed to be heated by solar from the bottom and a boiler from the top indirect coil; (b) and (c) demonstration tank with cut-away in side Source: Viessmann Werke GmbH & Co.

(a)

(b) The return pipe connected at the top of the tank is very bad for efficiency

(c) The return pipe connected in the middle of the tank restricts efficiency

(d) Figure 6.8 Examples of choices to connect a solar collector to work at its most efficient by designing the return leg to be taken from the coldest part of the system Note: Circuits for space heating and heat exchangers, and safety equipment not shown for clarity.

The return pipe connected low down in the tank keeps the efficiency higher

= Instant gas boiler The cold feed entering low down in the tank keeps the efficiency higher

= Instant electric boiler = Cold feed = Pump

Source: Valentin Software

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additional components   91

close as possible to the fresh cold water feed (if present) in order to reduce the risk of losing heat from other heat sources back out to the solar collector. The most important consideration for solar tanks is the volume of cool water available to be heated by the solar collector at the start of the day. This should be large enough to hold the heat produced on a hot sunny day without the temperature rising above safety limits. As DHW is used up it is replaced by fresh cold water, allowing more heat to be added. This means that the more people who use hot water during the daytime, the smaller the tank volume can be. If people are normally absent during the day, a larger tank is required. A

Box 6.3 Buoyancy and stratification in tanks When water is heated inside a tank it becomes less dense. The warm water will rise upwards to the top of the cooler water. This is known as its buoyancy. Heated water tends to form layers of different temperatures with the hottest at the top and the coldest at the bottom. This phenomenon is called stratification. In solar-heated tanks, stratification is highly desirable because it allows the bottom of the tank to remain cold for a long time. If this water keeps the pipe returning to the collector cold, the efficiency of the collector will be improved – because the colder something is, the easier it is to transfer heat to it. More solar energy can then be converted into heat. A tall, thin solar tank will stratify better than a short squat one, even if both hold the same volume of liquid. Stratification is also improved by minimizing water movement inside the tank, such as by using heat exchangers or reducing the speed of any pumps. Where cold water enters the tank, it can also create unwanted water currents, which can be reduced by pipes and plates called diffusers and lances.

Flap valve

Temperature zones 50 – 60°C 40 – 50°C 30 – 40°C

Funnel

60° from solar collector

10 – 30°C

20° to solar collector

Figure 6.9 Specialized solar storage tanks may contain inner partitions to assist layering of the heat, minimize mixing and hence improve collector efficiency

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92   solar domestic water heating tank that is too small gets hot too quickly and wastes the available solar energy. This makes the solar collector less efficient. Solar tanks are available in volumes of between 50 and 500 litres (13 and 130 gallons). Diameters range from 300 to 600mm (12–24 inches), heights from 900 to 2000mm (35–79 inches), in either vertical or horizontal formats. Several tanks can be linked together to create the necessary total volume.

Box 6.4 Dedicated solar tank volume The volume of water that can be heated solely by the solar collector is called the ‘dedicated solar volume’. This does not include water that is actively heated from other heat sources. The dedicated solar volume is always located in the coldest part of the system. This is where the fresh, cold water enters the system. In some cases, the solar heat is added to a tank with no other heat source. These are called preheat tanks. The complete volume of a preheat tank is then the dedicated solar volume. In a tank that includes other back-up heat sources, the dedicated solar volume is that which is unaffected by the other active heat sources. This means that it lies below those heat sources, because warm water rises. Extra allowances can be made for any solar heat that can be stored above the dedicated solar volume. This represents an increase in total capacity to store solar heat in excess of the dedicated solar volume. The total of the two is called

Figure 6.10 Where a tank contains heat from both solar and other heat sources, the volume that is lowest is called the dedicated volume

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additional components   93

the ‘effective volume’. This is only available if the solar-heated water is hotter than the water above it, or if the other heat sources are switched off. The fraction of extra volume in the upper part of the store available to store solar heat depends on the type of timing and thermostatic control of the back-up heater. It can be estimated as follows: Schedule of back-up heater in upper part of dual heat-source tank

Extra volume available for solar in upper part of tank above dedicated solar volume

Always on

Less than 10%

Heated overnight only

Less than 30%

Heated twice daily

Less than 30%

Heated once daily late afternoon

Less than 70%

If the effective volume is too small or large, more back-up energy will be required and there may be a risk of overheating. The required total effective volume should represent between once or twice the average daily hot water use, dependent on the climate, collector performance and collector area (see also Chapter 9). The ideal total volume of a dual heat-source tank should be the sum of the dedicated solar volume (at the bottom of the tank) and the volume to be heated by any other heat source (upper part). Examples of typical tanks are shown in the following table. Total litres

Persons

1–2

2–3

4–5

Household DHW use per day

30–100

60–150

120–250

Tank only heated by solar

50–130

75–200

140–300

Another heat source heats upper part of tank

90–180

110–300

300–500

Total US gallons Persons

1–2

2–3

4–5

8–26

16–40

32–66

Tank only heated by solar

13–34

20–53

37–79

Another heat source heats upper part of tank

24–48

29–79

79–132

Type of tank

Household DHW use per day Type of tank

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94   solar domestic water heating (a)

(b)

When water is heated it expands, and excess pressure is released through a safety temperature pressure relief valve

When water is heated, it expands into a high-level open-top cistern

Expansion

(c)

When water is heated, it expands and excess pressure is released through a safety temperature pressure relief valve

Float valve

City, mains or boost pump water Over-pressure safety valve

Expansion

Isolating valve

Over-pressure safety valve

Expansion City, mains or boost pump water

Cold feed

Isolating valve, filter, pressure reduction, check valve

Figure 6.11a–c There are three common methods to accommodate expansion of water when heated

City, mains or boost pump water Isolating valve, filter, pressure reduction, check valve

Note: Circuits for space heating and heat exchangers, and safety equipment not shown for clarity.

Box 6.5 Solar tank specifications When a solar tank is purchased or when sizing one in a computer simulation program, it is necessary to specify particular technical details. When a solar tank is sold together with a solar collector, it is better to compare the whole system together with another similar system before deciding on which. Where the solar tank is purchased separately, the following details need to be checked before deciding on which one to install: • • • • • • • • • • • • • •

manufacturer; model number, year of construction and serial number; suitability for fresh drinking water; material and corrosion protection (only in case of drinking water); weight (empty); height and diameter; total volume of stored water; volume heated by any back-up heating; dimension, heat transfer rate and construction of integral heat exchanger (if present); heat resistance and thickness of tank insulation; maximum operation pressure; maximum operation temperature; diameter and type of connections; vertical positions of the temperature sensor locations.

European standard EN 12977-3 specifies the following characteristics for solar tanks, above that of ordinary tanks: • •

thermal stratification during stand-by; thermal stratification during discharge.

In the US the Solar Rating and Certification Corporation (SRCC) uses TM1 SDHW system and component test protocols.

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additional components   95

Attempting to use a tank that can be heated completely from other heat sources runs a high risk of poor solar contribution. This is because if the water is already hot, it becomes very hard to add heat more without causing overheating or high heat losses. Whilst it is theoretically possible for the owner to adjust switches and timers to avoid too much hot water in the tanks at the start of the day, this can prove unreliable unless the owner is very committed. If a tank that is connected to a solar heating system has to be heated from other non-solar sources, then this is best done late in the afternoon to minimise the negative effect on solar contribution. When hot water is heated in a tank its expansion can cause components to rupture or explode. Local building codes often require extra safety components to either absorb the hot water expansion or allow it to discharge to waste. The extra volume of expansion of heated water is not normally allowed to go back into the cold water supply pipe, as this could cause contamination of drinking water. If this pipe is fitted to the tank, contamination is prevented by spring-loaded one-way valves or a separate tank of cold water that is filled by a float valve. A key difference between hot water tanks is whether they are designed to operate using a pipe open to the atmosphere or kept sealed and under a higher pressure. A solar tank will have purpose-made locations for attaching temperature sensors. It is useful to have at least two sensors, one near the top and one near the bottom of the tank. These sensor locations have small ‘pockets’ or tubes built into the side of the tank. They are left open at one end to insert the sensor. Alternatively, the tank will have open threaded holes that can be fitted with special sensor pockets. If sensor locations are not used, they can be blocked off. Tanks can corrode and eventually the metal will fail and leak depending on the water quality. Special metals, chemicals or coatings are sometimes added to inhibit corrosion. If local water is a problem, local plumbers and plumbing shops can give advice. Alternatively, a water sample can be tested at specialist (a)

(b)

Side entry

3 2

3

Optional corrosion protection rod

2

Tall and thin stores have for best temperature stratification

1

Insulation thickness exceeds 50 mm (2")

12 Optional back-up heat sources with thermostats 11 Electric immersion heater heads behind covers (if permitted)

13

14 11 15

14 Vertical separation of back-up heat source from dedicated solar volume 13 Immersed sensor pockets for accurate temperature measurement

8

Return to solar collector is located in coolest part of the tank

A Volume dedicated to solar heat input not normally heated by any other source 8

Cleaning hatch

17 Baffle plate over cold feed entry

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1

3 13 12 11

14

16

16 Volume of water around solar heat exchanger in coolest part of tank

16 17 1

2

15 Large surface area of heat exchangers unless direct or external 6

5

Top entry

8

Figure 6.12a/b A purpose-made solar tank will have special features Source: Viessmann Werke GmbH & Co.; RHEEM

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96   solar domestic water heating

Box 6.6 Multiple tanks and other heat loads A solar tank is an integral part of a solar heating system. Other heat loads can include swimming pools, underfloor (radiant) heating or other solar tanks. Combinations of these require increasingly complex layouts. Pumps and controls are used to manage the sequence, prioritize each heat load and control maximum temperatures. Some heat loads can be used to disperse excess heat when other heat loads have been satisfied. These are called heat ‘dumps’, ‘heat shedding’ or ‘shunt’ loads, and are sometimes used to reduce the effects of stagnation during the peaks of summer when a E building’s occupants may also be on holiday. Figure 6.13 A schematic layout of solar collectors heating multiple heat loads Source: Valentin Software

laboratories and the results discussed with tank manufacturers. Some manufacturers will not guarantee their tanks unless the water quality is within specified limits. If limescale is a problem, it is very likely to form when heated in a solar tank. The most reliable way to reduce its effect is to be able to inspect and clean inside the tank. An inspection hatch fitted in the lower part of the tank can enable this. These are bolted or screwed onto the tank’s side and opened after draining the water out. These hatches can also allow removable internal heat exchangers to be fitted and cleaned. The colder the climate, the more sensible it is to keep the storage tank inside the building. Not only does this reduce the risk of freeze damage but any heat loss will usefully add heat to the building. Internally located tanks are more efficient. Solar tanks often require annual inspection and maintenance, for example to check for leaks or build-up of limescale inside, so the tank needs to be accessible.

6.5 Heat circulation and pumps Solar heat can be moved from collector to tank by an electrical pump, a natural thermosyphon pressure, incoming cold water pressure or self-pumping phasechange. Electrically driven pumps are often used to circulate heat transfer fluids (HTFs) in a closed loop (circuit). Pumps can also be used in open-ended systems, for example to boost the flow rate and pressure of a shower. Because a circuit or pipe-loop is closed, the HTF always comes back to its starting point

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additional components   97

Figure 6.14 Typical AC circulating pumps

Figure 6.15 Typical DC circulating pumps Source: www.ECS-Solar.com

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98   solar domestic water heating so the pump doesn’t need continually to work as hard as in open-ended systems. Pumps are not simply left running all the time – this would waste electricity and disperse useful heat already in the storage tank. Pumps have a range of specifications that need to be considered. Most pumps in solar circuits are ‘centrifugal’ pumps. They use a rotodynamic centrifugal mechanism such as an ‘impeller’ shaped like a wheel with angled blades. The term ‘centrifugal force’ refers to an outward force away from the centre of rotation. The centrifugal force in a pump drives the HTF against the sides of the spinning wheel. The HTF is then caught by the angled blades of the impeller, which compresses the HTF and shoots it forward in a jet of liquid inside the outlet port. Centrifugal pumps are especially common in solar circuits due to their ability to spin freely, even if the circuit becomes closed due to a valve or other blockage. Positive displacement pumps, which must use a safety bypass valve to relieve pressure in the event of a circuit blockage, are not common.

Box 6.7 Pump terminology ‘Peak power’ refers to the maximum power that can be provided by the pump, and is measured in watts (W). The pump converts the electrical power into hydraulic energy, which can be measured in two forms: flow rate (how quickly the liquid travels along the tube) and pressure (head). The pump’s efficiency can be calculated by dividing the power on its way out of the pump (the ‘hydraulic’ power) by the power on its way in to the pump (the ‘electric’ power). The difference is accounted for by heat or noise produced. (A successful pump can be expected to work quietly, while a noisy pump often indicates a fault.) Higher efficiency pumps will use less electricity to carry the same amount of solar heat. In some countries, there are requirements to label pumps with their efficiency rating. A pump in a closed loop (circuit) creates a zone of relative positive pressure in front (at the outlet) and a zone of relative negative pressure behind it (at the inlet). The inlet of a pump circulator should normally be maintained above atmospheric pressure. This is because pump circulators provide very little suction. Pump manufacturers refer to this as the ‘minimum inlet pressure’. This minimum inlet pressure can be created either by pressuring the fluid above atmospheric pressure in a sealed system, or by connecting a sufficiently high open-topped tank (cistern). The flow rate and dynamic pressure at the outlet of a pump are inversely related to one another. This means they can either produce a high positive pressure or a high rate of flow. The combination of these two figures is called the ‘pump characteristic’. It is important that a pump with the correct characteristic is chosen to match the requirements of the solar system.

Central heating pumps are often used but they tend to be over-sized for solar systems; however, lower-powered pumps are now becoming available. The pump needs to be suitable for the HTF used in the system. An alternative to using a pump is to use natural circulation or ‘thermosyphon pressure’, which uses the tendency of warm water to move naturally upwards in the pipes and round in a continual circuit. The lower part of the circuit must be hotter than the upper, so the solar storage tank must be positioned higher than the collector. Thermosyphoning will work whether the circuit is open or closed, but it does work relatively slowly compared to pumps. (Thermosyphoning can

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additional components   99 System Pressure Characteristic 8 7 Possible working points for typical circulating pumps.

Head (metres of water)

6 5 4 3

Typical pump curves

2 1 0 0

.5

.25

.75

1.0

1.25

1.5

1.75

2.0

2.25

2.5

2.75

Flow Rate v (m3 /h)

Figure 6.16 A circulating pump characteristic shows that as the flow rate increases then a system will need a higher pumping head whereas most pumps only produce a high head at low flow rates

Vent

Header Cistern Roof

Temperature Safety Solar collector

valve

Hot water store DHW with blending valve

Return

Drain Valve

Figure 6.17 If the collector is lower than the solar tank, then natural thermosyphoning can usefully occur. There is a high risk of overheating DHW unless extra precautions are taken Note: Safety equipment removed for clarity.

also occur inside just one pipe: the heat can go up the centre of the pipe and the colder HTF drop down around the outside, leading to heat loss at night.) Without a pump it is difficult to control the system, leading to a loss of collector efficiency and even the risk of over heating during high irradiation periods. Thermosyphoning also does not work well in highly diffuse light conditions, because a sufficient temperature difference may not occur. Pumps

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100   solar domestic water heating provide more flexibility in the solar layout, higher rates of heat circulation and improved controls such as overheating prevention and frost protection.

6.6 Controls for safety, performance and information Controls enhance a system’s operation. They sense deviations in a particular characteristic of the system (for example, an unwanted rise in temperature) and then either set off an alarm or automatically initiate action. Control devices are used to improve safety and performance or to provide information. They may be electrical or mechanical, automatic or require manual operation. They can also anticipate operational problems and provide long-term monitoring of performance. Where a control is a safety feature, it should be readily accessible and verifiable in operation during commissioning and maintenance. All solar water heating systems require temperature-based safety controls to reduce the risk of very hot fluids reaching vulnerable components and water outlets (like showers). Electrically powered differential thermostat controls (DTCs), which use temperature sensors fitted into the collector and solar tank to control the pump, are very common. They monitor and compare the information provided by these two sensors, and then use this information to decide when to turn the pump on or off. The sensors are wired to the DTC with thin cable and work at a low voltage. The DTC uses a small lamp diode to indicate whether the pump is on or off, and can also show an electronic readout of temperatures. If the solar storage becomes too hot, the DTC can also switch the pump off. Figure 6.18 The layout of a differential pump control sensing temperatures

High sensor

Readout Grid mains high voltage E N L

Low voltage sensors

High

Measurement Low

L N E

Measurement sensor

Pump

Solar Cylinder

Low sensor Live (Phase) Neutral Earth (Ground) Fused double pole isolation

KS put cylinder here and re-join wires

L N E

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additional components   101

DTCs can also record total pump-run hours and temperatures over previous weeks in a memory chip, both useful options for anticipating problems before they become too serious. Advanced DTCs can vary pump speed to optimize collector efficiency, control any back-up DHW heating and even connect to computers and the internet. For a DTC to function correctly, the temperature sensors must be firmly in contact with the fluid-carrying components, but are themselves not normally allowed to get wet. They are mounted on the outside of tanks, pipes or absorbers or inside sealed metal pockets. A manual switch is normally provided to override the automatic settings to engage the pump for testing after installation. In systems without pumps, mechanical valves can be used to control tank temperatures. They either dump hot water to a drain or automatically mix in cold water. For climates with occasionally frost, mechanical valves can prevent freezing by automatically dripping next to the Figure 6.19 A typical differential temperature controller collector. Analogue temperature (on-electrical) gauges that do not require electricity can sometimes be used, but these are not very practical for showing the temperatures of roof-mounted collectors. It is possible to use a small photovoltaic module to control or power a DC pump. It works on the assumption that if there is enough irradiation to generate power then there will be enough heat in the collector. Pump speed will be determined by the level of irradiance. Sensor and pump must be carefully matched to ensure a reasonable collector efficiency. Potential problems with this arrangement are pumping-out of useful stored heat at the end of a summer’s day or overheating the store, or on a cold sunny day further heat can be lost from the solar store up to the collector as the pump operates regardless of the temperature difference. It is essential to test for correct circulation in all solar primary systems during system commissioning to ensure there are no obstacles or air ‘locks’ and set the correct pump flow rate for maximum efficiency. A mechanical flow indicator, combined with an adjustment regulator, can enable this to be done. For greater accuracy on large systems, electronic counters, showing the total fluid quantity, are used. When combined electronically in a DTC with

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102   solar domestic water heating

Box 6.8 DTC functions Solar collector temperature varies throughout the day. The temperature in a solar storage tank varies according to the DHW demand. Because of these two variables, simple thermostats such as those used in ordinary central heating will lead to losses of useful solar energy. But a DTC is able to compare two or more remote locations simultaneously and then use relay switches to operate pumps and valves. The pump is turned on when the difference between the temperatures of collector absorber and the lower part of the solar tank is great enough to overcome any heat losses in the pipes. This ensures that the exchange of heat is greater than the power used by the pump. If the pump switched on before the necessary temperature difference was reached then the pump would use more power than it saved thermally. A typical switch on of the pump would happen at around 8°C (15°F), and would then switch off at around 3°C (6°F) These two values are adjusted during test and commissioning, but they must never be too close together, otherwise the pump will switch off too early and leave useful heat in the pipes. In the morning the collector absorber may only need to be about 25°C (77°F) to bring the pump on. Later in the afternoon, after a long summer’s day when the solar tank is already quite hot, the collector absorber is likely to be over 60°C (140°F) before the pump would be worth turning on. If the solar storage or collector is too hot, the pump will also not switch on, reducing scalding risks. More complex DTCs allow control of several pumps and valves, and regulate other heat loads such as extra storage tanks. DTCs with pump-speed control options can rapidly vary the voltage given to the pump using a semiconductor relay, and thus HTF flow rate through the collector to maintain a steady temperature difference between the collector and the solar tank, usually at around 10°C (18°F). In warmer climates with only very occasional freezing, a DTC can be programmed to run the pump when the temperature approaches 0°C (32°F). Many DTCs can indicate a fault using a flashing light or buzzer.

(a)

(b)

Figure 6.20a/b A tempering valve automatically limits DHW temperature by blending hot and cold water. (a) Normally located close to the draw-off point; (b) has three pipe connections and an adjustment wheel Source: CIPHE and RWE

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additional components   103

temperature sensors, heat quantity can also be calculated. To avoid unwanted reverse circulation during the night, some systems include a valve that shuts off liquid movement in the pipes leading up to the collector. The mechanical forms of these are called one-way or ‘check’ valves and are shaped internally to close via a spring once the pump switches off. This can also be achieved using an electric solenoid or motorized valve. In closed systems, the pressure is displayed by a mechanical gauge or an electronic sensor connected to a DTC. For safety, a mechanical spring-loaded safety valve can release any excess fluid if the pressure reaches a maximum limit. The outlet of these valves can emit very hot liquid or vapour if there is a fault on a sunny day – so they need to be installed with extra care.

6.7 Heat exchangers Heat exchangers keep the fluids of a system separate whilst allowing heat to pass from one fluid to another. They permit the use of corrosion inhibitors and antifreeze in collectors without contaminating the fresh water. They also reduce the problem of minerals and debris in the water Figure 6.21 A flow indicator allows the rate of liquid movement to be set accurately

(a)

(b)

(c)

Figure 6.22a–c A one-way valve stops heat naturally circulating upwards at night. (a) Spring loaded (any direction); (b) flap (must be vertical); (c) sinking ball (must be vertical) Source: Pegler Yorkshire

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104   solar domestic water heating (a)

Solar single-Coil Tanks Heat Trap specially designed built-in hot water outlet pipe preventing unnecessary heat loss.

Hydrastone Lining A seamless 1/2" thick lining which has proven to be the most effective method of preventing tank failure due to corrosion.

Temperature and pressure relief valve (included) Plastic Jacket Rust-free, all plastic outer jacket. Easy to clean and no maintenance.

Dense Urethane Foam Insulation completely surrounds the tank to significantly reduce heat loss and increase energy savings Cold water inlet Diffuser introduces cold water at the bottom of the tank and prevents turbulent mixing with heated water this ensures desired layering of hot water through the tank.

Removable Heat Exchanger Single or dual coil for easy maintenance. Bronze Fittings at all water openings eliminates corrosion due to dissimilar metals. No dielectric unions or anode rod required.

Approved with many collectors to obtain 0G300 certification

7 Years Limited Warranty

(b)

Removable Heat Exchanger Conveniently located and simple 8-bolt flange design for ease of installation and service. This superior design allows for periodic cleaning of coil to maintain maximum performance. Rubber gasket Metal screw Steel flange cover plate

Steel tank Insulation Heat exchanger coil

Figure 6.23a/b A tank with an integral coil heat exchanger with method of allowing this to be removed for cleaning linescale Source: www.vaughncorp. com/SEPCO.pdf

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Slotted steel Flange Acess panel

1/4" hex nut, 1 1/4' x 1/4" bolt, one of eight one of eight Nylon bushing one of eight

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additional components   105

Figure 6.24 A tank with an external coiled heat exchanger Source: Rheen Solaraide HE and Round Solar Servant HE

Figure 6.25 A tank within a tank Source: www.wagner-solar.com

(b)

(a)

Figure 6.26 A tank with a double layer of walls (mantle)

Figure 6.27a/b An external heat exchanger with double tubes and a pump

Source: www.infinenergy.can3

Source: http://HeatSwapper.com

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106   solar domestic water heating reaching the collector and the chances of bacteria growing, prevent steam passing directly from the collector to the hot water and reduce the risk of leaks affecting the whole system. The downside is that they add expense and reduce the efficiency of the system, although they are an essential feature of most system types. Where a heat exchanger separates the DHW from the solar primary system, this is called an indirect system. If a heat exchanger is not fitted between the primary and secondary parts of the system, there may be additional regulations / codes to comply with. Heat exchangers are made of metal and can be external or internal to the tank. Typical materials include stainless steel, coated steel and copper. Some internal Figure 6.28 Schematic to show heat exchanger types are ‘fixed’ – that is, they are manufactured into the with second pump tank. Others are removable for inspection and cleaning. As a general rule, the greater the heat exchanger Note: Safety equipment removed for clarity. surface area, the easier it is to transfer heat into the tank Source: Valentin Software and the lower the temperature of the heat transfer fluid when returning to the collector, improving system efficiency. This particularly favours the tank-within-tank and double-layer tank wall exchangers sometimes called ‘mantle’ exchangers. One advantage of immersed-coil

Figure 6.29 An external heat exchanger with double tubes using natural convection Source: www.willisrenewables.com

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additional components   107

(b)

(a)

Figure 6.30a/b An external heat exchanger with compressed plates and a pump. (a) Schematic layout of pre-plumbed kit, (b) insulated cover over plates

(b) (a)

Figure 6.31a/b An external heat exchanger works best when the different liquids move in opposite directions across the exchanger to each other. (a) Tubes in shell exchanger; (b) compressed plate exchanger Source: Solarpraxis

(b)

(a)

Flow

Roof

The hottest pipe leaving the collector is called the 'Flow' pipe

Temperature/Pressure safety valve Flow Cold Feed

DHW with blending valve

Return

Drain Valve E

Return The cooler pipe returning to the collector is connected lowest at the tank for best efficiency

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Cold Feed

Figure 6.32a/b The orientation of the flow and return connections strongly affects the performance. Flow is the highest and hottest pipe and the return is the lower cooler pipe in a circuit. (a) Thermosyphon; (b) pumped Note: Safety equipment removed for clarity

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108   solar domestic water heating exchangers is that they can be designed to be easily removed and cleaned via a hatch; however, some types are fixed and cannot be easily removed. Immersedcoil types can use smooth pipe, although finned types provide a greater surface area. Tank hatches also enable inspecting and cleaning of double-layer tank walls and wrapped-coils exchangers. In general, cleaning is only needed on the side of the exchanger facing the fresh water. If cleaning is not performed then the heat exchanger performance reduces over time, as debris such as limescale builds up on its surfaces. External heat exchangers are normally mounted close to the solar storage inside the building. Both sides of them can be pumped separately, improving the transfer efficiency. With internal heat exchangers this is not possible so the heat transfer is slower. External exchangers of compressed plates bolted together can also be cleaned easily and do not have to be replaced if the tank fails. They work best if the two liquids are moving in opposite directions on either side of the metal exchanging the heat. This encourages the coldest return temperature to the collectors, which improves system efficiency. Some external exchangers only use one pump on the solar primary side, with natural circulation transferring heat to the solar tank. External heat exchangers can be used in the solar tank and DHW distribution. These are called ‘load-side’ heat exchangers and are used to reduce the

E

CH-06.indd 108

Figure 6.33 Schematic to show back-up DHW heating by an immersed electric resistance element within a DHW tank combined in a single vessel with the solar storage at the bottom

Figure 6.34 Schematic to show back-up DHW heating by integral coil heat exchanger within a DHW tank heating indirectly from a gas boiler combined in a single vessel with the dedicated solar storage at the bottom

Source: Valentin Software

Source: Valentin Software

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additional components   109

E

Figure 6.36 Schematic to show back-up DHW heating by a separate DHW tank. Dedicated solar storage is in a separate tank Figure 6.35 Schematic to show back-up DHW heating by direct fired burner within a DHW tank combined in a single vessel with the dedicated solar storage at the bottom

Note: Safety equipment removed for clarity. Source: Valentin Software

Note: Safety equipment removed for clarity. Source: Valentin Software

Figure 6.37 Schematic to show backup DHW heating by DHW instant heater separate to the dedicate solar storage Note: Safety equipment removed for clarity. Source: Valentin Software

Figure 6.38 Schematic to show back-up DHW heating by a combined DHW/CH instant heater separate to the dedicated solar storage Note: Safety equipment removed for clarity. Source: Valentin Software

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110   solar domestic water heating

s

Figure 6.39 Schematic to show back-up DHW heating by an electric DHW only instant heater separate to the dedicated solar storage Note: Safety equipment removed for clarity. Source: Valentin Software

(a)

(b)

amount of stored water, which minimizes the problem of mineral or bacterial build-up. When a solar collector is connected to a solar tank, the height of the connections into the tank affects the system efficiency. The most important connection is the return to the collector, which should be almost the lowest connection in the tank, where the water is (c) coldest. This is true for both direct and indirect systems. The flow pipe from a collector or intermediate heat exchanger is always the hottest pipe. It is typically connected (d) halfway up the tank, which helps retain the tank stratification without disturbing the hot water above. If connected too high, this can disturb the hottest water that will have already risen up to the top of the tank. The flow pipe also should not be connected too Figure 6.40a–d An immersed electrical resistance heater and electrical instant shower heater can be difficult to pre-heat with low, otherwise it would disturb the colder solar. (a) Electrical instant shower heater; (b) near-sink electric water necessary to retain collector efficiency. water heater; (c) and (d) immersed electrical resistance heater. It can be very advantageous in terms of reducing plumbing joints if all connections Source: www.bristan.com to a tank are made above it. This is enabled when the tank manufacturer fits relevant pipes and connections at the factory. However this arrangement can lead to difficulty in draining a circuit completely unless drain valves are also provided.

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Figure 6.41 Natural piped gas can often be found in urban areas for DHW back-up using a wallmounted boiler Source: Worcester Bosch

Box 6.9 Heat exchanger assessment There are a few ways to assess and compare solar heat exchangers. The simplest is to compare exchanger areas. But this is only an approximate comparison since different materials and shapes transfer heat differently. The best way is to calculate the overall coefficient of heat transfer. This has the abbreviation of either ‘k’ or, more commonly, ‘U’ or ‘U-value’. When considering a heat exchanger of known area ‘A’, the power transmitted through the heat exchanger can be expressed per degree of temperature. This is known as the ‘kA’ or ‘UA’ value, or the ‘performance coefficient’ of heat transfer. The performance coefficient of heat transfer is measured in watts per degree Kelvin (W/K), not to be mistaken with the heat transfer coefficient which is measured in units of watts per degree Kelvin per square metre (W/K.m2). Other common units are: British Thermal Unit per hour per degree Fahrenheit (BTU/h.°F), or Million-British Thermal Units per hour per degree Fahrenheit (MBTU/h°F). Using these units heat transfer coefficient is measured in BTU per degree Kelvin per square foot (BTU/°F.ft2). 1W/K = 1.90BTU/h.°F 1W/K.m2 = 0.176BTU/h.°F.ft2 Note: 1MBTU/h = 1,000,000BTU/h It is best to have at least 100W/K performance coefficient of heat transfer for every square metre of solar collector area (or 4.9BTU/h.°F per square foot). The average temperature difference across the two heat exchanger fluids for small systems when operating under sunny conditions would ideally be 10K (18°F) For a plain coil immersed in the water, this equates to 0.2m2 (2.2ft2) for every square metre of solar collector area. With larger systems it is important to know how hard the pump has to work to overcome the friction of the heat transfer fluid as it moves through the heat exchanger. This effect can be measured and is sometimes indicated by manufacturers as the drop in pressure at a specified flow rate. The higher the drop, the bigger the pump needed to keep the same heat transfer rate.

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6.8 Back-up sources of heat Additional sources of heat energy are almost always necessary in solar water heating systems. These are called ‘auxiliary energy’ or DHW ‘after-heaters’. These commonly use fossil fuels, such as natural gas, propane, oil and coal. However, back-up heating can also be provided by burning wood or liquid biofuels. Electricity and district heating are other options. The back-up energy supply can also be the building central heating energy supply. Ideally, the back-up energy supply will lower its power output or switch automatically as soon as solar heat becomes available. This will give the best overall efficiency. But there can be complications: very high temperature water from solar storage tanks can trigger safety controls; and some back-up systems can lag behind the availability of solar heat to the extent that they use unnecessary fuel – this is particularly frequent in systems where the back-up energy source is fitted a long distance from the solar storage tank. Existing back-up heating systems often require specialist knowledge and inspection before their suitability for solar preheating can be determined. Frequently, the optimum layout for whole system performance is best achieved by a complete system replacement to ensure optimum matching, but this is usually not an option. Some back-up heat sources are specified as ‘solar-ready’ by their manufacturer, meaning they are especially suitable for solar preheating. These can be installed prior to and independently of a solar primary system. When a solar system is added later, there is often a maximum temperature stipulation that is achieved by choosing a suitable layout and controls for the solar systems. It is always preferable to reduce the distance between the solar storage and back-up heat-supply equipment. This avoids so-called ‘dead-leg’ losses: that is, where heat remains in a pipe each time after DHW is drawn off. Distance reduction also helps to reduce bacterial risks that may form in pipes. In any case, good pipe insulation on hot water pipes is always recommended. In small household systems the optimum arrangement is usually to combine back-up heating in the same vessel as the solar storage tank. This provides the shortest possible distance and lowest losses.

Electricity An immersion (resistance) element is fitted in the tank. An adjustable thermostat fitted alongside the resistance heating element can respond to the solar energy quickly if located above the solar energy in a tank. Where the electricity is converted to heat, such as in an immersion (resistance) heater, the efficiency within the tank can be considered to be near to 100 per cent, but this is deceptive: if the whole sequence of electricity generation is considered right back to the power station, then the overall efficiency is actually below 20 per cent.

Gas This can be provided either from the street mains (natural or city gas), bottled or even biogas (from composted biological material). There are a range of options:

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• • • •

boiler and hot water tank separated with a heat exchanger; boiler and hot water tank combined with no heat exchanger; instantaneous after-heater (tankless) for DHW only; instantaneous after-heater for DHW and room heating.

An adjustable thermostat, usually fitted in the back-up heater, responds to the temperature of solar energy by reducing the power or switching it off. Instantaneous gas water heaters do not immediately respond to the solar heat from the tank unless there is a thermostat connected to the tank. Without this thermostat, there will be a delay as the solar-heated water passes through the pipes. Instantaneous gas water heaters are sometimes called tankless heaters, geysers or post-gas.

Oil Oil boilers are similar to gas boilers, but there are very few instantaneous afterheaters that use oil. An adjustable thermostat, usually fitted in the boiler, responds to the temperature of solar energy by reducing the power or switching off. It is better to have an additional thermostat connected to the solar tank to avoid the boiler switching on when the solar-heated tank is already hot enough.

Coal The fuel is burnt in a fireplace with a back-boiler or utility/kitchen boiler along with a hot water tank separated by a heat exchanger. The boiler is often manually loaded and lit by the householder. A thermostat is not always fitted and they are slow to automatically respond to the solar energy. A flue (vent) is required. Installation and maintenance costs are high.

Figure 6.42 Bottled gas for DHW back-up can come as propane or butane in steel vessels Source: Calor

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114   solar domestic water heating (a)

(b)

Figures 6.43a/b Oil boilers for heating DHW also require and oil tank Source: www.harlequinplastics.co.uk

Wood logs The logs are burnt in a fireplace with a back-boiler or utility/kitchen boiler along with a hot water tank separated by a heat exchanger. The boiler is manually loaded and lit by the householder. A thermostat is not always fitted, in which case they are slow to automatically respond to the solar energy. Where the logs are burnt in a utility boiler, a large steel buffer store allows the wood heat to be stored. In this case an adjustable thermostat is usually fitted that responds to the solar energy quickly if located above the solar energy in a tank. Wood is considered a renewable energy source. However, there can be problems with smoke emissions.

Wood pellets and chips Figure 6.44 The traditional open fireplace is sometimes used with a back-boiler to heat DHW Source: www.ecostoves.co.uk

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Both are used in similar appliances to oil-heaters, but wood pellets can also be used in wood-pellet fireplace stoves. Wood chip systems are usually too large for single households, but wood-pellets systems of suitable sizes are available. The pellets,

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which are about the size of animal feed pellets, can flow like a liquid and are very clean to handle. They can be delivered by wood-pellet tanker or in bags. Control is via an adjustable thermostat.

Heat pumps These extract solar heat from the ground, water or the air. Electricity is used to drive the pumps and compressor. Heat is passed to a water filled circuit similar to an oil or gas boiler. They do not work efficiently when heating DHW up to high temperatures and it is not always easy to combine heat pumps with solar. An adjustable thermostat is usually fitted that responds to the solar energy from a solar collector quickly if located above the solar energy in a hot water tank. Pipes are required in the ground or nearby water source. Installation costs are very high but maintenance costs are low.

District heating systems Hot water is pumped from building to building, the Figure 6.45 Modern log wood boiler provides both heat coming usually from a combined heat and power DHW and central heating plant but also from wood-fired boilers located some distance from the building. The hot water tank will have a heat exchanger or a series of heat exchangers, and an adjustable thermostat (usually located above the solar energy in the storage tank) to control heat intake. (a)

(b)

Figures 6.46a/b (a) wood fuel as pellets; (b) can be delivered in bulk for automatic boilers Source: Sonnen-Pellets

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6.9 The water supply A clean, reliable cold water source that provides flow rate and pressure for all appliances or fixtures in the building without requiring boost pumps is ideal for a solar water heating system. Where a good quality cold water supply is not available, then extra sterilizing or cleaning equipment may be required and only system layouts with a heat exchanger can be recommended. (b)

(a)

Figures 6.47a/b Electricity can be used in a heat pump to recover solar energy from the ground, water, or air near a building. However, this is difficult to combine with solar DHW. (a) Horizontal trenches; (b) vertical borehole Source: Bundesverband Wärmepumpe (BWP) e.v.

(a)

(b)

(c)

Figure 6.48a–c District heating from a central source. (a) Via underground pipes; (b) transfer station inside building; (c) inside a transfer station Source: Aberdeen Council, Danfoss

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7

system layouts This chapter discusses the range of different system layouts available.

7.1 System layouts A system layout describes the relationship of the key components in a solar water heating system, including where the solar storage tank is located in relation to the solar collector, how the heat is circulated, whether a heat exchanger is present and the methods of temperature and pressure control. Although there are a great many varieties of individual design, it is possible to group some of the key features together under a few categories. The key distinguishing features of the mainstream categories of system layout are:

• • • •

passive or active; direct or indirect; fully filled or drainback; solar storage tank external or internal.

Passive vs. active Passive systems:

• contain very few moving parts; • use no pump or electronic controls; • store and circulate the heat by natural means such as thermosyphoning or from the incoming cold water pressure;

• are less complex and cheaper to construct but there are location restrictions and reduced efficiency.

Active systems:

• use electricity (grid or a small photovoltaic module) to power a pump and controls;

• allow more choices on component locations and enable better solar heat management.

• sometimes known as forced or split systems.

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118   solar domestic water heating

Direct vs. indirect Direct systems use the water that enters the building as a heat transfer fluid. Indirect systems use heat exchangers and a separate heat transfer fluid (see Chapter 6).

Fully filled vs. drainback In fully filled systems:

• heat transfer fluid is present in collectors all the time – even when there is no useful gain of solar energy to be made;

• all air is removed from the collector and pipes. In drainback systems:

• t he fluid is drained from the collector once the pump is switched off; • the collector is higher than the pipes and tanks; • some air is always retained.

Cold feed safety group

Over-pressure release valve

There is some overlap in the categories given above. For example, a drainback system implies there must also be a pump. Some system layouts are predominant in particular regions due to the characteristics of the climate, but also occasionally due to building regulations (codes). The system arrangements described below are some of the most common internationally.

Warm pipe

Temperature or pressure gauge

Cold feed Hot pipes

7.2 Integral collector storage (ICS) Categories: passive; direct; fully filled; solar storage external Typical climate: non-freezing

Expansion vessel

E

Some safety devices are not shown

E

Thermostatic tempering valve

Electrical resistance element

Figure 7.1 Schematic of a typical Integrated Collector Storage System with electrical back up

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ICS units combine the solar storage and collector into one externally mounted unit that can be roof- or ground-mounted, and are sometimes known as ‘batch’ heaters. There is no pump or integral temperature control. They feature an absorber of particularly high volume located behind glazing. The whole arrangement contains just plain water. Normally they are connected directly to the fresh incoming cold water, which pushes the hot water out when an outlet

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is opened in the house. Some are manufactured with an integral heat exchanger, which permits a separation of the fresh water to reduce problems of limescale. Because the tank is part of the unit any back-up heating system needs either to be an instantaneous appliance or have a separate tank. There is a risk that the water will be heated to a high temperature so an extra safety protection device may be required, such as an automatic blending (tempering) valve. ICS units are intended for use in non-freezing climates where pipe lengths between the storage, any back-up DHW (domestic hot water) heater and the draw-off points such as taps and showers are short. Long pipe runs can cause significant heat losses when drawing off small amounts of DHW. ICS units can survive light freezing conditions due to their high water content, although adjoining small diameter pipes are still at risk. ICS units work best when hot water demand is mainly in the late afternoon or early evening. Outside these times the solar heat is left in the ICS and is easily lost on cold nights, and may even reduce the temperature of incoming fresh cold water. They do not perform well if most of the hot water is required in the morning. In climates with occasional hard freezing conditions, the ICS unit and especially the pipes needs to be drained down or protected by automatic drip valves that open before freezing can take place, and maintain enough water movement in the pipes to prevent ice forming. However, some building codes do not require them. Care must be taken to ensure any useful heat from a back-up heater or internal ambient air is not lost upwards by unwanted thermosyphoning in the pipes. This can be prevented by spring-loaded check valves, a manually operated valve or automatic motor valve. That said, this technique of ‘losing’ heat can occasionally be used as a form of freeze protection by manually overriding these valves, allowing hot water from the building to flow into the unit. A rise in pressure, due to the expansion of water when heated, can occur in the whole hot water system when an ICS unit is connected to it. Special springloaded safety valves are normally required. When the hot water system of the building is drained down, this can create a temporary negative pressure in the ICS unit. If the pipes and unit are not made of a strong enough material, they are at risk of collapsing under atmospheric pressure. A device called a vacuum breaker, fitted at the top of the system by the ICS unit, will prevent this by allowing air into the ICS when it is drained down. ICS collectors are thicker and heavier than regular flat plate collectors. They are the least expensive type of system and one of the most popular on the world market for warm climates.

7.3 Passive (thermosyphon) systems Categories: passive; direct and indirect system; fully filled; solar storage external and internal Typical climate: when direct, occasional freezing; when indirect, all Passive systems use natural circulation (thermosyphoning). The solar storage tank is always located above but near to the collector. On flat roofs or on the ground, sometimes a small frame is used to raise the solar tank the required height. Units with an integrated solar tank – so called ‘close-coupled’ systems – are

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120   solar domestic water heating

Anti-vacuum valve

Over-pressure safety valve

Anti-frost drip valve Hot pipes

E

Temperature gauge Thermostatic tempering valve Warm pipes

Drain valve Electrical resistance element

Cold feed

Temperature or pressure gauge

Expansion vessel

Cold feed safety group Some safety devices are not shown

Figure 7.2 Schematic of typical thermosyphon collector with external solar storage tank

also available. (What distinguishes ICS systems from a passive system is that ICS systems have the solar storage under the collector glazing.) The storage tank is normally connected directly to the building’s cold water supply. Some models have integral heat exchangers, making them indirect systems. When a directly heated solar tank is used, a rise in pressure can occur in the whole hot water system. Special spring-loaded safety valves will normally be required to discharge this; but some building codes do not permit wastage of water in this way. The alternative is an expansion vessel incorporating an integral flexible membrane (see Chapter 8). When the hot water system of the house is drained down, this can create a temporary negative pressure in the solar tank – and in the collector if there is no heat exchanger. A vacuum breaker (as in ICS units above) may be necessary. The solar heat reaches the back-up heating, usually located inside the building, from the pressure of the fresh cold water pushing through the tank after opening a DHW draw-off point. This can cause a delay as the solar heat passes though the pipes. If the back-up heating has its own tank, care should be taken that fuel is not wasted heating up this second tank when there is useable solar-heated water remaining in the solar tank. An electrical heating element also can be fitted into the solar storage tank. The risk that the water can be heated to a very high temperature in a thermosyphon system is high. Because there is no pump the heat continues to circulate into the solar tank even if it is already hot. The storage tank needs to be fitted with a mechanical valve set to open at a maximum temperature limit to discharge high temperature water to a drain and draw in fresh cold water. However, some building codes do not permit this.

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Solar storage tanks can be fitted outside the building’s insulated structure, but this is generally only done in non-freezing climates. As short a distance as possible between the external solar storage and the DHW draw-off points inside the building is recommended to avoid heat loss. Systems with solar tanks outside the building work best when the hot water demand is mainly in the late afternoon and early evening. Outside these times solar heat is easily lost, especially on cold nights. Sometimes external tanks may even decrease the temperature of the incoming fresh cold water – when the air temperature is less than that of ground, as can happen on clear cold nights. In occasional freezing conditions, the collector and especially the pipes need to be drained down, or at least protected by automatic drip valves, and heat can be lost by unwanted thermosyphoning (as in ICS units, see above). Heat exchangers in thermosyphon systems need to be specially designed not to inhibit circulation. When mounting a tank on a roof, the weight of the tank (considerably heavier than a collector on its own) needs to be considered. Some roofs will not be suitable. Some of the cheapest forms of passive solar heating systems in undeveloped countries are where the owners are willing to intervene once a day in the morning. Here they operate a valve to fill an external tank until it visually overflows onto the roof. Then after returning from work, they use up the hot water running under gravity pressure from the roof into the house. They always make sure all water is removed from the tank and external pipes at night in case of freezing. They repeat this sequence every day.

T

Anti-frost drip valve and anti-vacuum valve

Flow

Some safety devices are not shown

Return Differential temperature control

Check-valve or similar

Expansion vessel DTC

T

Temperature gauge Instant DHW boiler

T Pump

Drain valve Temperature sensor

Thermostatic tempering valve Cold feed safety group

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Figure 7.3 Schematic of typical pumped collector with storage on ground

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122   solar domestic water heating

7.3 Active direct fully filled systems Categories: active; direct; fully filled; solar storage external and internal Typical climate: occasional freezing or non-freezing With active systems (a pump is used for circulation), there is more flexibility as to where the collector and tanks can be located. They can then be some distance from each other and at different heights. Heat can be circulated downwards or along horizontal pipes, not normally possible in a thermosyphon system. The storage tank can be placed inside the building and the collector either on the roof, the walls or the ground. Pumped systems allow the collector to work more efficiently. Pump controls generally either use temperature sensors to compare the difference between the collector and the storage, or irradiation level sensors (see Chapter 6 for a fuller discussion). To protect against freezing, drip valves can be used (see above); or the collector and especially the pipes can be drained down; or freeze-resistant polymer pipes and absorbers can be used (externally); or the pump may be switched on. To prevent heat loss from a back-up heater or solar tank through the pipes to outside by unwanted thermosyphoning, spring-loaded check valves, a manually operated valve or automatic motor valve can be used. A fully filled direct system does not have a heat exchanger. It is completely filled with water – with bubbles of air and other gases removed – at the same pressure as the water supply, which is mostly above atmospheric pressure (see

Temperature sensor T

Flow Return Pressure or temperature gauge

Drainback vessel Over-pressure DTC safety valve

Expansion vessel

Boiler

T

T Pump

Figure 7.4 Schematic of typical drainback collector and internal solar storage tank

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Drain valve Some safety devices Cold feed are not shown safety group

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Chapter 6 for a discussion of water pressure). The whole system remains fully filled with the pump on or off. During a drain-down negative pressure may be a problem, and a vacuum breaker valve may be necessary (see above). Active (pumped) systems allow more solar tanks to be added easily, providing extra solar storage volume.

7.4 Active drainback indirect systems Categories: active; indirect, drainback, solar storage external and internal Typical climate: all The main advantage of drainback systems is that it is easy to control overheating and freezing. When the pump is turned off, the collector is left empty of liquid. The collector then becomes very hot, which it is designed to do. They are popular in climates with any risk of freezing. Simple drainback systems are active (pumped) systems, but the collector needs to be higher than the solar storage tank, and it must be mounted at an angle so it can properly drain back. There is some noise during the actual drainback, similar to a trickling waterfall sound. Some air is always left in the system. This naturally rises into the collector and all other external parts of the system when the pump is off. Any liquid then drops below this. When the pump is on, the heat transfer fluid (HTF) is pushed up into the collector, displacing the air downwards. The air gathers into a special drainback vessel located above the pump. If the vessel is installed internally, the HTF can be plain water. The vessel is fitted with a hole that allows liquid to be filled up to maximum level and to be subsequently checked. The hole is plugged during normal use. The liquid is filled approximately to atmospheric pressure. It is essential that the pipes between the collector always drop down towards the drainback vessel, otherwise air will become trapped. Antifreeze with corrosion inhibitors, while not always necessary in drainback systems, is sometimes used as double protection; but if the drainback vessel is installed externally antifreeze is essential. Residual air trapped in the indirect circuit acts as a cushion to absorb the expansion of liquid and air when heated. A special spring-loaded valve that prevents the pressure becoming too high in the circuit is still required. The pump, typically above 30 watts, must work harder – than in other systems – each time the system starts, because it must push the air out of the collector every time. Once the liquid reaches the top of the collector, the pumping requirement reduces. Sensors to detect the temperature difference between the collector and the storage are required – not irradiation sensors controls, as these will not manage the pump effectively on sunny cold days. Drainback systems do not have the problem of useful heat being lost upwards through the pipes up to the collector by unwanted thermosyphoning, because the pipes are empty of liquid overnight. Back-up heating requirements are similar to those for active direct systems.

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124   solar domestic water heating Temperature sensor

T

Flow

Pressure gauge

Return One-way check valve

Over-pressure safety valve Differential thermostat control

Expansion vessel

DTC

T

T

Figure 7.5 Schematic of fully filled pumped collector and internal solar storage tank with boiler back-up

Pump Drain valve

Cold feed safety group Some safety devices are not shown

7.5 Active fully filled indirect systems Categories: active; indirect; fully filled; solar storage external and internal

Typical climate: all Active fully filled indirect systems allow a high degree of flexibility with regard to locating components. They are very popular in climates where there is a risk of freezing and can easily be scaled up to very large systems. There are very few restrictions on the pipe layout. A fully filled system is completely – and at all times – filled with HTF, with air and other gases removed. In freezing conditions the pump has to stay switched on, or a non-freezing fluid used, or flexible freeze-resistant components used (externally). If there is heat exchanger the system is also indirect. The HTF is usually filled to above atmospheric pressure. A metal expansion vessel, incorporating a flexible membrane, acts as a cushion to absorb the expansion of liquid when heated; but a special spring-loaded safety valve to prevent the pressure becoming too high in the circuit is required. To control overheating of the solar tank the pump is switched off. During sunny conditions the collector becomes very hot, but it is designed for that. The HTF will eventually evaporate into a gas, pushing out all remaining liquid into the expansion vessel. The collector remains in this state until the irradiation levels drop and the HTF returns to liquid in the collector. This sequence can repeat many times over the years as long as the HTF does not degrade. It is possible to use both types of pump controls (see Chapter 6).

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(a)

Fluid in vessel when cold. Dotted line indicates position when initially filled

Fluid

Flexible membrane

Steel vessel Nitrogen gas Vessel as delivered

Vessel when system is cold

Vessel when system is hot

(b)

Figures 7.6a/b An expansion vessel is used in some systems to absorb expansion of the liquid when it gets hot. (a) Schematic cut-away, (b) metal bracket supporting vessel is hidden

If the back-up/after-heater uses a separate tank, care should be taken that it is not unnecessarily heated when there is useable solar-heated water remaining in the solar tank, which should be a short distance from any back-up/after-heater and the DHW draw-off points. Long pipe runs between them can cause significant heat losses when small quantities of water are drawn off. Otherwise,

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126   solar domestic water heating requirements of back-up heating are similar to those for active direct systems. With internally located solar tanks, care should be taken to ensure any useful heat is not lost upwards through the pipes by unwanted thermosyphoning at night. This can be prevented by one-way spring-loaded check valves, a manual operated valve or an automatic motor valve (see section 6.6 and Figure 6.12).

7.6 Choosing the most suitable layout Table 7.1 Layout points of comparison Passive or active

Direct or indirect

Fully filled or drainback

Solar storage

Typical climate

Integrated collector (ICS)

Passive

Direct or indirect

Fully filled

External

Non-freezing

Passive (thermosyphoning)

Passive

Direct or indirect

Fully filled

External & internal

Occasional / non-freezing

Active direct fully filled

Active

Direct

Fully filled

External & internal

Occasional / non-freezing

Active drainback indirect

Active

Indirect

Drainback

External & internal

All

Active fully filled indirect

Active

Indirect

Fully filled

External & internal

All

Figure 7.7 The probability of freezing in the next 20 years in the US Source: Figure courtesy of Jay Burch of NREL

One of the first things to do when deciding on which is the most suitable system is to assess the freezing risk. This information can be obtained from national meteorological centres, and is available in some software simulation programs. Maps can often only offer a level of probability of freezing. Table 7.2 indicates

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Figure 7.8 Potential frost days in Australia, based on past 30 years Source: www.bom.gov.au/jsp/ncc/climate_averages/frost/index.jsp

Table 7.2 Systems and climates

Table 7.3 Active/passive systems

All climates

Non-freezing or occasional freezing climates

Active (pumped, electricity required)

Passive (no pump)

Active drainback indirect

Integral collector storage (ICS)

Active drainback indirect

Passive (thermosyphon)

Active fully filled indirect

Active fully filled indirect

Passive (thermosyphon)

Integral collector storage (ICS)

Active direct fully filled

the system types most likely to be found in each climate. If there is no – or an erratic – electricity supply a passive system may have to be decided on. In frequently cloudy climates this can reduce in thermal performance, compared to an active pumped system. And full thermostatic control requires an active pumped arrangement. Table 7.3 indicates the options.

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Active direct fully filled

Table 7.4 Direct/indirect systems Direct systems

Either direct or indirect

Indirect systems

Active direct fully filled

Integral collector storage (ICS) Passive (thermosyphon)

Active drainback indirect Active direct fully filled

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128   solar domestic water heating A heat exchanger may be required to prevent fresh water entering the collector because of local building codes, or poor quality water with high limescale, silt or other debris. Where no accidental release of fluid or scalding vapour from a system is permitted, systems must be hydraulically secure (i.e. indirect). Where external aesthetics are paramount or local planning laws require it, the solar storage tank is likely to be located internally. There may also be restrictions on the collector’s location; for example, it may need to be mounted flush with the roof. This might mean that the system has to be active and have a flat plate collector. Where a system is likely to be expanded at a future date or heat supplied to multiple heat loads, an active fully filled system provides the greatest flexibility control. Some collectors are restricted to particular system types and so care should be taken to ensure they will be compatible to any restriction of a given site.

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8

designing a system This chapter gives an overview of system design and discusses the main issues involved. It needs to be read in conjunction with the other chapters, where system components are discussed in more detail. System sizing is dealt with in the next chapter.

8.1 Overview of design principles Designing a solar water heating system means bringing together the best size and type of components to suit a particular site into a cohesive layout. It also involves the anticipation of potential faults and errors that can occur during use. The design process can be subdivided into three themes: 1 Technical survey and site visit. 2 Energy conservation and solar efficiency. 3 Things that can go wrong. The first thing to do is carry out a technical survey and site visit. There is a distinction – buildings that are yet to be built cannot be visited. The person carrying out these tasks should be appropriately experienced and be sufficiently knowledgeable to asses the plumbing, electricity and building-structure issues they are likely to come across. Safety has priority over all other design issues. This may result in a system with reduced performance but few people will object to this. Reliability and durability are also high priorities. Planning and attention to detail is crucial. Unanticipated problems that only become apparent later can add significantly to the whole project cost.

8.2 Technical survey A technical survey always needs to be carried out and preferably completed on site; but if this is not possible drawings or photographs of the site can be used, the only option if the buildings have yet to be constructed. But with new-build it is possible to design the building and building heating system with solar water heating in mind – a great advantage. Key points to consider are the available roof area, any potential shading and the form of back-up heating planned/required.

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8.3 Site visit A site visit enables a full assessment of what is possible, and potential problems. Distances and spaces available for manoeuvring equipment should be indentified and measured; key points to anticipate include width of doorways and access hatches. It is also important to consider how to switch off water and electrical supplies safely in case of emergency. Survey forms, such as the one shown here, can be very useful. If the occupant of the building is available to answer questions, this can greatly improves the quality of the survey.

Box 8.1 List of items that need to go on site survey form • Project reference

• Date

• Project name

• Address

• Contact details

• Customer name

• What is the solar heating to be used for?

• Budget limit

• Other energy sources heating the DHW

• Times of the year the building will be used

• Existing fuel bills for DHW and space heating

• Building code or planning restrictions

• What fraction of the annual DHW use is to be provided by solar?

• How many occupants regularly in the building?

• Sketch of the building indicating dimensions of the roof or other location to mount the collectors. Indicate north and south and the pitch angle of the roof or flat roof or ground.

• Type of roof surface and any object in the roof including chimneys and skylights

• Draw a sketch of the horizon showing positions and heights of main objects that may shade the collectors.

• Restricted location immediately behind the roof such as attic rooms

• Height of roof from ground to roof edge (eaves)

• Suggested high-level access equipment

• Location for solar storage tank

• Distance from collector to solar storage tank

• If internal, size of room or cupboard for solar storage tank

• Other locations for solar equipment

• Other DHW storage tanks or instantaneous water heaters

• Origin of cold water supply

• Is the DHW fed direct from the water mains or is there a pump or cold water storage tank?

• List of appliances that use DHW and can be fed from solar storage tank

• List of appliances that use DHW and can not be fed from solar storage tank

• DHW use per person that can be fed from solar storage tank

• Requirement for secondary DHW circulation

• Timer or programmers for back-up hot water

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Box 8.2 Tools for site survey Useful tools that can be used during a site survey: • • • •

• • • •

A compass or map to anticipate the direction of the sun, although this may not always be immediately apparent if it is a cloudy, overcast day. A Solar Site Selector tool (sometimes called a Sun Path Indicator) helps anticipate shading. See Chapter 2 for more detail. A tape measure will allow accurate calculation of the available spaces for storage and collectors. A ladder may be useful if there is an existing roof or attic, as it may be necessary to gain temporary access to high places in order to inspect such locations in more detail. Torches, flashlights or inspection lamps come in handy where there is poor natural illumination. A digital camera will help record the site visit for future reference, as will written notes. A protractor can help calculate the pitch angle of a roof if held up in front of the eye from a distance. A bucket and stopwatch can help measure water flow rates, along with a pressure gauge that clamps onto a tap.

8.4 Cold and DHW water pressure The intended domestic hot water (DHW) pressure and the type of cold feed to the solar tank – an open tank (cistern) or direct connection to the incoming cold water to the building – greatly influence the choice of solar tank storage and back-up appliances. In the case of private cold water supplies, a small reservoir or open tank is often essential. This can reduce the necessary size of the cold water pump. However, storing cold water on site necessitates a considerably larger space requirement. This may also need to be kept cool for reducing bacterial growth and high up to maintain a reasonable cold-feed pressure. Where fresh water is to be taken to the highest point in a tall building, a test to check that the pressure is high enough may be required. Sometimes an electric booster pump is needed where the incoming pressure is too low. When deciding on DHW pressures and the associated pipe bore diameters, the peak DHW rates of showers normally present the highest typical flow-rate and so must be considered with the greatest care.

8.5 Occupant’s DHW use routine The way in which occupants of the building use hot water will have a noticeable influence on system performance. The total amount used, the place of use, the time of use and the required temperature need to be noted. If the building is not yet occupied or built, these have to be assumed. It should be possible to identify whether the use will peak in the morning or the evening, all through the day,

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Box 8.3 Measuring water flow rate Water moves in a pipe because it is under a higher pressure at the inlet of the pipe than it is at the outlet. When the water is not moving, the difference in pressure is known as the ‘static pressure’ or ‘static head’. When the water does move, this pressure is reduced due to the friction of the pipes. This is called the ‘dynamic pressure loss’ or ‘head loss’. This is why water often moves at a higher pressure when a tap is first opened, the pressure then quickly reducing. Once the water reaches the pipe outlet the easiest method is to measure the rate of flow per volume: for example, calculating how long it takes for the water output to fill up one litre. This can be done with a bucket, a stopwatch and a measuring jug. So if it took 20 seconds for the water to reach the one-litre mark on a measuring jug, the rate of flow would be a litre per second, or 0.05 litre/sec (m/s). The static and dynamic pressure can also be measured. Dynamic pressure measurement should be performed at another tap at the same time as measuring the rate of flow per volume. This provides a reading during normal use, and not just when the water is switched off. It is not accurate to use the same outlet for both measurements. Water speed is measured in either metres per second (m/s) or feet per second (ft/s). 1 foot per second (ft/s) = 0.3048m/s. Water mass flow rate is measured in either kilograms per second (kg/s) or pounds per second (lb/s). 1 pound per second (lb/s) = 0.453kg/s. Water volumetric flow rate is measured in either litres per second (litres/s) or gallons per second (gall/s). 1 gallon per second (gall/s) = 3.8 litres/s.

or even a continuous 24 hours per day. Significant vacation periods or other times of low occupancy should also be noted. Although fresh-water meters may be present in some buildings, the quantity of hot water used through these can be difficult to distinguish from cold water use. Certain hot water equipment (e.g. household appliances) may not cope with ‘preheated feed’ (filling with hot water), as these are ‘cold fill’ only. Examples include washing machines and dishwashers, which use internal electric resistance heating elements to heat the water up inside the appliance – though ‘hot fill’ models are available. The average daily hot use is the most relevant figure – occasional visitor peaks can be generally ignored. Where only a weekly or annual hot water figure is known, the relevant total numbers of days is used to divide the figure to obtain a daily average in litres (or gallons) per day. Where DHW use is not known it can be assumed at 45ºC (113ºF) with between 35 and 65 litres (9 to 17 gallons) used per day per person – depending on the number and types of household appliances. Where DHW peak demand occurs between lunchtime and evening, this optimizes solar energy efficiency, requires a smaller dedicated solar tank size and permits a wider choice of system types. Where DHW peak use is in the mornings, this requires larger and better insulated solar storage to retain the

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(a)

(b)

Figure 8.1a/b A washing machine that has only one water connection will be difficult to accept solar-heated DHW Source: www.lets-do-diy. com

190

600

152

500

135

400

108

300

81

200

54

100

27

0

0

1

2

3

4

5

6

7

8

No. of people

solar heat overnight from the previous days. In general a higher evening load is assumed for domestic dwellings, whereas business premises often have peak lunchtime/afternoon loads.

9

10

Low usage (35 litres per person) High usage (65 litres per person)

US Gallons

Litres used

DHW use per no. of householders 700

Figure 8.2 DHW use varies with the number of occupants

0

% 100

75

50

8.6 Collector location It may not always be possible to install collectors in the ideal place. Access to the collector location will be required not only during installation but also for future maintenance. Typical obstacles include conservatories, power/phone lines, porches and dormers. The presence of roof skylights of sufficient size can simplify future inspections. Critical system components such as pressure safety valves, temperature sensors, photovoltaic light

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25

0

0

2

4

6

8

10

12

14

16

18

20

22

Time of day

Figure 8.3 DHW use in a typical household varies throughout the day with strong peaks Source: Valentin Software

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134   solar domestic water heating switches, vacuum beakers and drip valves should also be in accessible locations, particularly if these must be inspected as part of the maintenance regime. Other workers who may work on the roof in the future should also be considered; they may need to pass over or near the solar external components. Some systems are only used seasonally and may require attention at each year’s first start-up and drain-down. Purpose built, non-corrosive embedded roof steps are commercially available and are designed to provide permanent, safe access to steep roof pitches for inspections. An alternative, especially for flat roofs, is a cable fastening-line for Figure 8.4 If future maintenance on the roof is expected, purposemade steps can be fitted into the roof personal harnesses. These are sometimes found on new commercial buildings. Source: Klober Safety Step, www.klober.co.uk Where maintenance is expected inside a roof loft a walkway could be constructed, particularly above any thick layers of ceiling insulation.

8.7 Solar storage tank and other equipment The structural stability and weight loading must be considered first. In very low freezing-risk climates the solar storage tank can be located externally. This would include locations where the external ambient temperature is regularly above 20°C and possibly where air conditioning is used internally. Where the solar tank is located internally, the floor’s structural loading should be considered along with the height and diameters of the space. Sometimes the insulation around a storage tank can be made removable to improve access possibilities. Space is required around the tank to allow for further pipe connections, insulated pipes, valves and other system controls. In particular, safe and code-compliant routes need to be planned for any overflow or pressure relief discharge pipes that may eject very hot water or steam under fault conditions. An accessible electric supply may be required. Deciding on the location of the solar equipment must include careful consideration of the possible options for DHW back-up heating (see Chapter 6), as this may affect the size, weight and length of pipe runs. Access for maintenance should also be considered.

8.8 Roof coverings The roof covering is supported by a sub-structure that holds it in place. The weight of the collector as well as the wind forces upon it must be passed through to the sub-structure whilst maintaining the weather tightness of the covering. The roof sub-structure needs to be able to withstand these loads.

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An ideal roof location for fixing a solar collector would have a readily removable roof covering (e.g. interlocking concrete tiles) giving simple structural and pipe/cable access – with a clearance gap of no less than two metres (six feet) to each side of the intended collector position for working space during installation and future maintenance. Some types of collector can be partially submerged or integrated within the roof covering instead of being mounted above it on brackets or a frame. Integrating the solar collector allows pipe and cables to exit the collector out of view, as well as reducing wind chill and improving thermal performance. Tiles on a pitched roof can be removed and the collector set almost level with the roof line, much like a skylight. It may even be possible to achieve a totally flush finish. Normally only flat plate collectors are suitable for in-roof applications. Care should be taken that the ‘flashing’ that weatherproofs the collector is compatible with the roof covering. Problems of access can occur if the underside of the roof is restricted. Where the roof structure has warped over time, integrating a rigid collector in-roof can be difficult. Interstitial condensation should also be considered in those locations where humid air is expected to rise towards the pipe building exits from the outside. Further considerations include: allowance

Figure 8.5 A solar storage tank mounted externally Source: www.apricus.com

Figure 8.6 A solar collector integrated flush with the roof line Source: Wagner & Co/ESTIF, www.wagner-solar.com

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136   solar domestic water heating for roof component thermal expansion, differential negative lifting on adjoining components and sufficient weathering overlap of roof components. Typical worst-case scenarios occur with the combination of snow, heavy rain and high winds, and snow melt or storm water drainage and must be considered with any roof-mounted features. When there is any doubt as regards the ability of any roof or any other part of a building to support collectors or tanks a structural engineer should be consulted. On a pitched roof a collector will require at least four mounting points. The structural loading is ideally taken through the roof covering without imparting loads onto the roof covering itself. If the roof covering is subjected to excessive weight loading, particularly during snow and wind storms, then not only would the roof covering potentially exceed its design limitations but adjoining roof components may lift unequally causing rupture of interlocking components. Traditional timber battens or lathes are normally insufficient to withstand the loading and are vulnerable to long-term corrosion. Flat roofs require a similar minimum number of fixings but greater care to avoid excess loading, as this can cause long-term deflection of the roof, leading to unwanted rainwater build-up. With wall- or ground-mounted collectors weatherproofing requirements are less onerous, but potential wind shear, windblown debris and snow build-up needs to be assessed. Flat-roof mounted systems can be held in place using ballast weights and lateral restraints, removing the need to puncture the roof covering. On new roofs purpose-made structural mounting points can be provided in advance. Routes for the pipes and any control cables joining the external and internal components should be planned beforehand. Care should always be taken with materials expected to be adjacent to fluid-carrying system components, as these can easily reach melting, charring or even auto-ignition temperatures. Prolonged temperatures in excess of 150ºC (302ºF) can be reached by metal components or by emitted steam in high irradiance conditions.

Figure 8.7 In some mountainous regions, the depth of snow on the ­collector will add significant weight Source: Radiantec, www.radiantsolar.com

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Figure 8.8 There is a wide variety of roof types

8.9 Collector orientation, angles and shading Many pitched roofs automatically provide an acceptable collector orientation. In any case, there is a generous leeway before orientation losses become excessive (see Chapter 2). A metallic frame may be used to elevate and optimize the collector pitch or change direction on a flat roof or the ground, although for many pitched roofs this is either visually unattractive or not cost-effective. With some evacuated tube collectors it is possible to simply rotate individual tubes to gain improved angles, although adjacent tubes should not be set to overshadow each other and typically a maximum rotational gain of 20 degrees is available. Some tube collectors already have cylindrical absorbers that do not need rotating. Less than optimum angles can sometimes be compensated for by collector over-sizing. If the available surfaces are too small or unfavourable, the use of split collector arrays can be considered. When collectors are divided on either side of a pitched roof that does not otherwise have an optimum direction, this is

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Box 8.4 Roof coverings Roof covering type

Notes

Ceramic tile

Very hard to cut, often available only in small sizes with frequent wooden battens that can interfere with pipe runs

Asbestos cement sheets

Long fragile sheets, structural fixing difficult

Built-up bituminous felt or asphalt

Larger areas without an overlap or seam mean penetrations have to be carefully sealed, often with hot tar

Profiled metal

Undulating profile of long metal sheets have to be fixed only through the crests to avoid leaks

Sheet metal (lead, copper, zinc)

Long lengths of flat sheets of metal that have raised seams must be strong enough to fix to avoid leaks

Conservatory or greenhouse patent glazing roofs

Fragile glass or plastic is difficult to work on with few places to make weatherproof connections

Wooden shingles or shakes

Can be relatively uneven, may warp further over time

Single ply flexible membrane and polymers

Requires specialist equipment to make repairs

Coated expanded foam

Requires specialist equipment to make repairs

Thatch

Surface is relatively weak, requires extra sub-structure to hold the collector off

Turf and other living materials

Surface requires sunlight to maintain growth, which a collector would shade

Figure 8.9 A collector mounted vertically on a wall Source: Batec/ESTIF

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known as an ‘east–west split’. However, these can be more expensive as they require independent circulation controls to avoid unwanted circulation through the nonilluminated aspect. The build-up of dirt/dust/grime on the collector surfaces should be avoided. A nominal 5 per cent loss of energy yield is expected in all conditions without cleaning: however, this will increase at pitches of less than 20 degrees. In areas where high build-up is expected, for example from sea salt, high density traffic or tree sap, the problem will be exacerbated with collectors set at a low pitch angle. Causes of shading include trees, chimneys, mountains and higher buildings (see Chapter 2). Partial or seasonal shading can also have adverse effects, with localized overheating of the fluid in one part of the collector or the shading of any sensors. Where shading is unavoidable, it should not:

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(a)

(b)

(c) Figure 8.10a–c Collectors mounted as (a) hanging from balcony; (b) a brize soleil; (c) on a balustrade Source: ZEN solar/ESTIF

• cover more than 10 per cent of the collector area; • take more than an hour to pass completely across the collector; • effect the collector between May and October in the northern hemisphere,

between October and May in the southern hemisphere, and throughout the year in the tropics.

Allowance for the future growth of nearby trees should be taken into consideration. Where shading is expected this will affect system design / performance predictions. The effect on some collectors will depend on whether it occurs predominately under conditions of direct or diffuse radiation. With pumped systems, one option is to use a controller to switch on the pump for a fixed, time-limited period once every 30 minutes, subject to temperature control. This can overcome unfavourable sensor positions during shading. Computer design software packages readily calculate the consequences of shading. Existing sites can be assessed with a compass and a transparent sheet indicating the tracks of the sun seasonally and daily. Similarly, solar site selectors allow a thorough examination even in cloudy or evening conditions.

8.10 Distances between components The length of pipe and volume of liquid that sits in pipes affects the amount of solar heat that is lost each time a tap or pump is switched off. This is because the heat is ‘trapped’ inside the pipes once the liquid stops moving. This also affects the length of delay before the heat reaches the next component. So it is always preferable to locate the main system components close to each other. Ideally, heat-carrying pipes should have a narrow diameter, thereby reducing the volume of liquid they carry. However, this also increases the pipe’s resistance and builds up friction, which either slows the flow rate down or requires an

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140   solar domestic water heating increase in pump energy. Some pipes leading to expansion vessels and safety valves must be kept large enough to maintain a high enough flow rate in fault conditions. Keeping distances between components short is not always possible. Where there is a choice, it is usually best to locate the solar storage and back-up heating nearer to the taps that are used most often – for example, the kitchen. Where distances between the collector and solar storage exceed 20 metres (65½ ft), a pump and its control can be adjusted to move the liquid faster through the pipes and at a higher temperature. Sometimes a second pump can be used to first heat up the collector and pipes without heating the solar tank until these are hot enough to switch the pump to heat the tank as normal.

8.11 Retention of the building’s insulated structure In cold climates the building will, hopefully, be well insulated against heat loss. This will include measures to seal the air movements in the joints of the structure. Where solar water heating pipes pass through the building to outside, it may be necessary to seal around the pipes. These are best sleeved with a high temperature grade of insulation.

8.12 Circulation pumps and circulation rates In general, the higher the circulation rate through the collector, the lower the absorber temperature and the greater the collector efficiency due to reduced heat loss (see also Chapter 4). However, the pump’s energy requirement will then be higher, so it should only operate at its highest level when necessary; with some pumps it is possible to vary the power with a manual selector switch. During periods of peak irradiation the lowest speed setting should be chosen. This will give sufficient circulation without risk of boiling. It is also possible to use a valve to adjust and slow the circulation down, although this does not always reduce the pump power. Once the rate of circulation has been manually adjusted, there are then two ways to further vary the pump power and speed. When using a temperature control it is also possible to use a pump control to vary the pump speed according to the temperature difference between the collector and the solar tank. The pump control will then try to achieve an ideal temperature difference. Where a photovoltaic module is directly coupled to a pump, this will provide the power and the control according to irradiation levels – the stronger the irradiation, the faster the speed of the pump, and vice versa. The power used by the pump should not use more than 2 per cent of the peak thermal power that is being gained by the system. This means no more than 15 watts of electric power should be used per square metre of collector area (see also Chapter 5).

8.13 Expansion or explosion of components When components are heated they will expand. Liquids can become a gas. During stagnation, when the collector reaches its highest temperature, the absorber can contain super-heated fluid or vapour at pressures several times

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Figure 8.11 The pressure and temperatures in a solar system can exceed the capability of ordinary plumbing components and cause premature failures

atmospheric pressure. If expansion cannot be accommodated or relieved, the pressure will rise and there will be a risk of significant leaks or even an explosion. Water-based liquids will expand around 5 per cent of their volume across the temperature range, from near freezing to near boiling point. The boiling point of water is 100°C (212°F) at atmospheric pressure at sea level, and rises to over 150°C (300°F) at five times atmospheric pressure. The volume of steam (in the atmosphere) is many hundreds of times the original volume of the water. A tank of water under high pressure can cause a steam explosion capable of blowing a house apart. If an indirect solar heating system can accommodate all expansion including steam formation under all conditions of sunshine, then it is termed ‘hydraulically secure’. Many solar system designs are not hydraulically secure, which means that under high temperature conditions very hot liquid or steam is released during very sunny conditions. This release is made to a safe location such as the roof or a drain. Common methods used in solar water heating to accommodate expansion are:

• • • • •

an open-topped tank (cistern) with an unobstructed route to the atmosphere; a vessel with an inert gas chamber with separating flexible membrane; a vessel with an integral air chamber; back flow; flexibility of components.

At least one of these methods should be used in each part of the system where there is a division between heat exchangers, or any risk of part of the heating system being accidentally isolated by a shut valve. An open-topped tank provides the traditional method of expansion. This is placed at a high level in the system and allows the level of liquid inside to vary without restriction. However, this can also enable excessive evaporation, which can lead to degradation of antifreeze or corrosion inhibitors. It can also allow

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Figure 8.12 The expansion of water can be accommodated in an open-topped container of water called a cistern. There may be a few required for other parts of the heating system. In some countries, building codes require covers for cisterns, freezing prevention and full support for weight and stability Source: Adapted from Polytank

Figure 8.13 An expansion vessel contains a flexible rubber membrane surrounded by a gas-filled chamber. This example contains a cut-away for display

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(a)

Working volume Level with pump off

(b) Expansion air pocket above liquid Drain / Fill indicator Figure 8.14a/b (a) A drainback vessel contains air that is in contact with the heat transfer fluid. (b) They are often found as a separate vessel near the solar storage tank

Total volume Level with pump on

Source: www.ECS-Solar.com

the liquid in high performance solar collectors to reach its boiling point too easily, since the pressure is not much above atmospheric pressure. An alternative is to use an expansion vessel with an indirect system sealed from the atmosphere. This is a metal vessel that contains a gas chamber filled with an inert gas such as nitrogen. The inert gas is separated from the water with a flexible membrane of a specially treated rubber. The gas pressure is adjusted to suit each system and can be set so that the system pressure is higher than atmospheric, thus raising the system’s boiling point. Where the vessel contains only trapped air and does not have a flexible membrane, the system pressure is not adjusted. Back flow is only technically possible in an open system where the building’s cold water enters the solar heating directly. When heated, the water passes backwards into the cold supply. This technique is not permitted by many building codes as it can lead to drinking water contamination. Back flow can also be stopped accidentally by switching off the incoming cold water valve. It is not generally recommended. Some system components such as thin or non-metallic pipes and tanks can flex and accommodate some expansion. However, this can lead to long-term stress building up in the materials, and subsequent failure. If a method of accommodating liquid expansion is not present or fails, excess pressure can be limited by allowing steam or hot water to exit from the system to the atmosphere by a spring-loaded safety relief valve (over-pressure limit) or a high-level safety vent pipe with no obstruction. Systems that are open to the atmosphere and use an opentopped tank tend to use a high-level safety vent pipe. Systems that

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(a)

(b)

Figure 8.15a/b A safety valve releases excess pressure in the system via a spring-loaded plunger. (a) Brass level; (b) plastic knob

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144   solar domestic water heating have no opening to the atmosphere (i.e. closed or sealed) will have a springloaded safety relief valve. For both these methods the following needs to apply:

• no possibility of freezing water blocking the pipe route from the collector to the safety valve or vent;

• no restrictions, valves or risk of pipe deformation in the pipe route from the collector to the safety valve or vent;

Figure 8.16 An open vent releases excess pressure in the system. If left over a cistern, the excess fluid can be retained Source: www.plumbersdiary. com

• a continuous rise of the pipe route from the collector to the safety vent; • final termination of the safety vent or valve to a safe location. Expansion and maximum pressure are affected by the initial cold filling pressure. If this is too high when cold, it will more likely to be too high when hot. In indirect systems the pressure is independent of the incoming cold water and can often be adjusted. A direct system is more dependent on the pressure of the building’s incoming water. Where an open-topped tank is used, this maintains a constant static pressure. Components made of solid materials also expand with temperature. For rigid metal pipes, an allowance of 3mm linear expansion per metre (1/8" per yard) should be allowed for in circuits above atmospheric pressure. Thus pipe supports should not be too rigid. Inside some collectors, the expansion is accommodated with pipe joints that slide over each other. This problem is also reduced in flexible metal pipes.

8.14 Steam or scalding water The most likely injury that can be caused from using a solar water heating system is scalding or steam burns. Collectors can reach temperatures of between 150ºC and 350ºC (302ºF and 662ºF). In a well-designed system, very hot liquids or gases like steam are safely contained. There are also safety controls for the solar storage tank and DHW distribution, which limit temperatures well before they reach any taps or shower heads. However, some collectors under fault conditions can generate steam that will eventually pass along pipes reaching into the building. The use of heat exchangers will prevent this. Overheating is most likely to occur during hot, sunny conditions when the building is not occupied or when there is a fault. Systems in which the collector is located above the storage, and which use a pump and a heat exchanger provide the best overheating control. The pump simply switches off at the correct maximum solar storage temperature – normally between 60°C and 80°C (140ºF and 176ºF). Switching the pump off thermostatically can only be designed into active systems that can either accommodate the steam generation into a steel expansion vessel or drains back the liquid out of the collector. Such systems are best designed to be hydraulically secure so there is no possibility of escape of steam. Some building codes specify a maximum DHW temperature. Blending or ‘tempering’ valves can be used to mix cold water automatically to the correct temperature. These valves should be fitted close to the DHW draw-off point and may require annual servicing to check correct function.

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During maintenance, it is advisable to cover the collector with a textile cover to prevent it getting hot. It is also possible to reduce temperatures by manually running off hot water, although extremely hot water can damage some sanitary equipment. Some systems incorporate extra heat storage or heat emitters that can be automatically switched on to divert heat away from the solar storage cylinder.

8.15 Bacteria Water quality can vary widely and sometimes biological material can enter the building’s water supply. In the vast majority of cases, this material passes through the building harmlessly. In general, if the water is maintained below 20°C (68ºF) or above 60°C (140ºF) the biological material remains dormant and has little effect. If bacteria are present in water and ingested, they are normally rendered harmless by the body’s digestive system. But there are risks and these need to be taken seriously. In very rare cases, if the bacteria are inhaled into the lungs serious illness can develop. To become harmful, the bacteria present in water have to colonize into greater densities than normal. They can do so on surfaces where a biological film can develop but this process can take many days and has little chance of occurring if the water is moving or outside the main breeding temperatures, or if sterilizing chemicals such as chlorine or natural biocides such as copper are present. Bacterial growth is most likely in water that remains stationary for many days between 20°C and 46°C (68ºF and 115ºF), and is situated near porous, non-metallic surfaces with water containing nutrients. These conditions are most likely to exist in poorly maintained DHW pipes and fittings that are seldom used. These could include pipes that lead to rarely occupied bathrooms, or junctions in pipes that have been capped off. In all cases, it is pipes that lead to shower fittings that deserve the most attention since the main risk is deeply inhaling a mist of bacteria. In theory, all tanks and pipes can present an increased risk of bacterial growth if the water is not used or heated for example during winter holiday periods. This same risk is also possible in the pipes and collectors of some direct passive solar heating systems. Although there are few cases in which solar heating has been shown to be connected to bacterial infections, it is sensible to ensure all solar-heated water passes through at least 60°C (140°F) for a some hours to reduce the level of any bacteria that are present. This can be achieved by using a back-up heater. This is connected with a thermostat so that it is used only if the solar-heating has not already achieved at least 60°C (140°F). A higher temperature of 70°C (158°F) will sterilize water in a few minutes but has a very high risk of scalding. Some building codes require the solar storage and DHW pipes to be sterilized by temperatures of over 60°C (140°F) at least once a day. This can be achieved by using extra DHW pipes that form a complete loop, including a pump that starts at the back-up heating and circulates to the furthest draw-off point and then back again to the solar storage. This displaces any cool water sitting in DHW pipes and keeps them constantly hot. However, this technique dramatically increases the energy loss of both backup and solar heat, especially if the DHW distribution pipes are not insulated. It will also dramatically decrease the dedicated solar storage and reduce solar performance, unless the pump is only used once a day after the main solar gains

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146   solar domestic water heating in the afternoon. To minimize the effect on solar gains, the pump should then be used only between 4 and 6pm immediately prior to the peak evening draw-off. Where there is a perceived high bacterial risk, the problem can be completely minimized by the use of extra heat exchangers. These can be placed so that the solar tank does not contain water used directly for DHW. In this way, no water that is to be consumed as DHW is stored. Instead this is heated instantaneously in the heat exchanger. Special attention should be given to cold water originating from unclean water tanks without lids, or where there are porous, suspicious or unknown fittings in contact with the water.

8.16 Freeze damage Freezing can be disastrous in rigid pipes and components containing plain water, since the water will expand as it becomes ice. It is imperative to assess the risk. If in doubt, it is generally better to assume the worst: that freezing could occur at least once in the expected lifetime of the system (see also Chapter 6). The ice can also block important safety devices and pipe routes that allow for expansion and pressure limitation. In general, the larger the diameter of pipe, the less likely they are to freeze. The greater the risk of freezing, the more automated the means of frost protection should be. If the solar system is used seasonally, meaning only in the summer, one solution would be to drain the system of water during the colder months. When warm water from inside the building is allowed back up to cold pipes and a collector to prevent freezing, then useful heat is in effect being lost. This is called ‘heat export’, and can also be used at night to intentionally cool the

Figure 8.17 Freeze damage affects rigid components like pipes and pumps Source: http://preparetoday newsletter.blogspot.com

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designing a system   147

water in solar tanks. When heat is used in this way, it becomes almost impossible to distinguish heat that was originally derived from the solar heating system from that which came from other fuel sources. This heat loss should be set against the gains from solar made at other times. If freezing occurs often this will waste a lot of useful heat. It is not permitted by some building codes. Another method is to fix electrical resistance heating cable to the water pipes, which is switched on when near freezing but this can consume quite a lot of electricity. Some materials maintain flexibility during freezing and accommodate the expansion; but accompanying rigid pipe fittings and components remain at risk and require additional freeze-protection methods. If a solar system is allowed to freeze, this may temporarily block pipe routes to any safety vents or relief valves. When the collector starts to defrost on sunny cold days, these pipes can remain blocked causing pressure to rise beyond the limits of the collector. Applying insulating materials to pipes can slow the onset of freezing water in pipes, but this does not eliminate the risk. Where used externally, insulating materials can compound the problem if there has been an ingress of moisture, as this impairs the insulation.

8.17 Mineral deposits, silt and other debris from water supplies Sometimes inorganic materials can enter the building’s water supply – in the form of dissolved minerals or suspended physical particles. Heating fresh cold water can in some areas produce limescale due to particular calcium and magnesium minerals (‘water hardness’) that can result in a significant build-up of hard chalk-like deposits inside system components and cause a gradual decrease in storage volume and heat exchange efficiency as well as reduced circulation rates and blockages. Installations in hard-water areas can eventually provide disappointing energy gains and give rise to premature replacement of components. The risk of limescale can be assessed from prior experience at a particular location, or in advance from water utility companies. Water samples can also be analysed for ‘hardness’ at laboratories or using self-test kits. Limescale also affects back-up heating, although solar heating can especially exacerbate the situation due to high temperatures both in collectors and solar storage. Water can be treated for limescale prior to entering a solar water heating system. This is called ‘water softening’, although some methods either require regular maintenance, purchase of chemicals or increased electricity use. It is possible physically to remove scale deposits during maintenance, as long as the relevant locations are accessible. This could necessitate cleaning hatches in storage tanks, particularly around heat exchangers. Limescale deposition tends to accelerate rapidly in water temperatures above 60ºC (140ºF). A careful balance has to be maintained between the risk of scalding and the reduction of bacteria and limescale. Some solar system layouts can reduce the problem by limiting the maximum temperature of the solar storage. The provision of sufficiently large volumes of dedicated solar storage can assist.

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148   solar domestic water heating (a)

(b)

Figure 8.18a/b Limescale can build up in solar storage vessels, pipes or inside solar collectors that have no heat exchanger Source: www.hydropath. com/casestudies-in-action4. html

One of the key benefits of an indirect system is the presence of a heat exchanger. These prevent problems caused when fresh water supplies are continually reintroduced into the solar collectors. Absorbers often have narrow internal and inaccessible locations that can not be easily cleaned. Direct solar systems are particularly prone to limescale forming within the absorber. The choice of an indirect circuit should always be considered as the first step to reducing the risk of limescale inside the collector absorber. Fresh water supplies also carry a degree of sludge and other fine debris that can accumulate in components. Filtering into the cold water supply helps, but filters require regular inspection, cleaning or replacement. Water acidity, particularly in some rural water supplies, can dissolve certain metals in storage tanks, including copper. This requires a careful specification of materials and system layout. The use of heat exchangers can also greatly reduce this problem.

8.18 Loss of cold drinking water quality Once cold water has been heated, it is no longer considered equivalent to the quality of drinking water. This means that the water in a solar water system should not be allowed to go back through any pipes where it can risk entering the cold drinking water supplies. Where a heat exchanger is present, the solar system may contain a heat transfer fluid that contains chemicals. Sometimes there are materials used in indirect systems that discolour or taint the water. Contamination could occur from backflow from the solar storage tank due to expansion or during draining down and backflow from an indirect system during filling. Backflow from solar storage tanks can be prevented using a spring-loaded one-way valve. Alternatively, the water can be supplied via an open-topped tank that uses a vertical gap of air between the water level and the filling point. Backflow from temporarily filling an indirect system can also be prevented with a spring-loaded one-way valve, or by having an intermediate vessel to contain the water and then using a filling pump. A simple hosepipe should never be connected directly from a drinking water tap to a solar system in case the solar system is at a higher pressure.

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8.19 Loss of hot water quality Some materials are not suitable for containing hot water because they can corrode or leach out chemicals when heated. Solar collectors can reach very high temperatures in fault conditions. Where a solar system does not have a heat exchanger, the same water in the collector will eventually pass through to the DHW draw-off points such as the taps, shower or bath. These chemicals could be accidentally ingested, possibly by children. The most common materials used in contact with hot water are those with a long trusted track record. These include copper, brass and stainless steel. Non-metallic materials that also have a good record include glass and glazed ceramics. Where more modern materials are used, such as polymers and rubbers, care is needed to ensure they have been tested to the maximum expected temperatures in the system in which they are to be used.

8.20 Future plumbing and electrical system maintenance issues

Figure 8.19 Where cold water is used to fill an indirect solar system, a temporary filling connection can be used to avoid backflow contamination of the drinking water

During the life of the solar water heating system it is inevitable that work will be done on the building to which it is fitted. This means that either the water supply will be stopped or drained down or the electricity will be switched off. This work may be performed by people who do not understand solar heating. Therefore, there should a simple and safe way to shut down the system. Instructions for shutting down should be left on site, perhaps in the form of a laminated sign/small poster. The key components that must be operated in order to switch off should also be clearly labelled. Wherever possible, a solar collector should be covered before switching off. This avoids unexpected high temperatures. Collector manufacturers sometimes provide purpose-made covers with straps that will stay on even in high winds, but textile covers, sheets of wood or cardboard sheets can be used subject to wind conditions. Electrical controls and pumps should be readily capable of being switched off to make them safe and avoid operation when the system contains no fluid. A wall-mounted disconnect (isolator or socket) located near to an internal solar storage tank is ideal. Accessing difficult locations and isolation methods that involve tools can both cause significant problems in emergency situations. A photovoltaic module used to provide power to a pump should also be covered. The parts of the solar system that contain liquid are drained using valves that require tools. This avoids accidental opening by children. Provision for

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150   solar domestic water heating

Figure 8.20 All parts of the system should be capable of being manually drained using a valve. This same valve may also allow refilling and so can be used with a filling pump

attaching a hosepipe or funnel may be useful to capture heat transfer fluid that could be used again. If the solar collector and the solar storage tank are in different locations, these should be drained down separately.

8.21 Indication of correct operation One simple method to discover if a solar water heating system is working is to switch off all other back-up heating. After a day or two, it will become apparent how much hot water is available and at what temperature. However, this can be inconvenient and may only reveal problems long after they have occurred. When the system is first commissioned it is important that some measurements are taken and recorded so as to verify correct function. This can then be compared to other installations or future performance. It is very useful for monitoring how well a system is working to have enough gauges and meters designed in at the outset. While this does add some cost and complexity, in the end it will allow the owner to get the best from their system. The easiest monitoring equipment to add is temperature gauges. These can be fitted into the solar storage tanks and the pipes to and from the collector. The simplest types are dial gauges that clamp onto pipes. These can be slow to react to changes but suit passive systems, which also have slow changes in the

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way they work. With pumped systems, better accuracy and response can be obtained with digital meters, often incorporated into pump controllers. These also allow easy monitoring of roof-mounted collector temperatures. Pump controllers also provide a simple indication of pump activity, usually with a small lamp or digital display. They can also flash a warning if there is a malfunction. Pressure gauges are essential on sealed systems. These show if the system has been correctly filled or if there has been a leak. For some systems, such as drainback, a piece of clear tube can act as a sight gauge to show the correct level. A similar device, using a small float and a spring, indicates the rate of circulation.

(a)

(b)

8.22 Animal and insects A system’s performance can be significantly reduced or even damaged by local wildlife. The most vulnerable components are soft and non-metallic, such as coatings for electric cables and pipe insulation. These are particularly attractive to biting mammals looking for nesting materials, especially as they are often located externally on roofs. Careful choice of system layout can reduce the vulnerability of these components by designing them away from external locations. Collectors can be integrated into roofs and all vulnerable components located internally. Thin plastic film collector glazing can be punctured by birds with their beaks, letting rainwater in. Insect colonies can grow inside solar collectors where they can access drainage holes. Insect-secreted acid can damage selective coatings on absorbers and degrade collectors. Ground-mounted collectors are best located on frames to make it more difficult for small non-flying insects to enter drainage holes.

Figure 8.21a/b A temperature gauge provides useful information about the system’s operation. These can be fixed in the system or portable Source: www.ECS-Solar.com

8.23 Ultraviolet, heat and vibration degradation Exposure to solar radiation degrades many plastics, rubbers and polymers. This can cause not only discolouration, but also brittleness and cracks. Materials need to be suitable for external use; and where possible have a successful time-served history. Heat can have a similar effect, and this can transfer

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Figure 8.22 A clear tube on a drainback system gives an indication of the correct liquid content and flow rate Source: www.ECS-Solar.com

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(a)

to other building materials internally. For example, where pipes from a collector pass tightly through airtight building membranes in the roof this can cause them to shrink or even melt. In this case, the pipes should be routed through sleeves with high temperature seals. Where there is vibration from pumps, this can cause some materials to build up stress and form cracks. This is called work-hardening. Flexible pipes near pumps can be prone to this.

8.24 Other issues (b)

Other issues that need to be considered when designing a system include:

• Collector performance ratio – see Chapter 4. High

quality collectors should be matched in quality with other components and the design. • Solar storage volume and solar tank insulation – see Chapter 6. Aside from sufficient volume, a high quality solar tank should normally have an insulation thickness exceeding 50mm (2 inches). Refer to local codes. • Pipe insulation – see Chapter 6. The wall thickness of the insulation should be approximately that of the pipe diameter. Refer to local codes. • Avoiding heat waste. Ways in which useful heat can be lost through the solar water heating system is discussed in Chapters 6 and 7, and include: pump left manually on; pump automatically switched on for frost; pump automatically switched on for cooling storage; pump on light control only; and accidental reverse thermosyphon. Figure 8.23a/b Many types of ordinary pipe • Warning devices to indicate energy loss. Pump controls insulation do not survive biting rodents in can provide a warning when unexpected temperature lofts or exposure to ultraviolet in sunlight gains occur at night-time, when stored heat has accidentally risen up from a store to a cold collector by unwanted thermosyphon. • Thermostatic control and locations for backup heaters – see Chapter 6. • Contamination of absorber and heat exchangers. For further detail on preventing contamination of absorbers and heat exchangers from inorganic and organic deposits, see Chapter 6.

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9

sizing system components 9.1 System sizing This chapter discusses the sizing of solar water heating systems and their constituent components, and gives a basic sizing method. Software simulation programs are also discussed. All the key components in the system require careful sizing, not only the collector and the storage tank. Complete system kits / packages are available that include correctly sized equipment under a single brand name. Often it is just a case of selecting between two or three preselected combinations according to the geographical region and the number of people using domestic hot water (DHW) in the building. The method outlined here is a reasonably straightforward manual method for sizing basic systems and can be used – but it is based on a few assumptions (outlined below) that may not always be the case. System suppliers and manufacturers should always be consulted and the method below is not a substitute for manufacturers’ sizing recommendations – manufacturers will know exactly how their systems are likely to perform in particular climates and under a range of applications. The use of computer simulation software will be dealt with later.

9.2 Manual sizing method for collector and storage tank using data table Certified ratings for collectors often provide values for the amount of energy per day they will produce under laboratory test conditions. These figures are excellent for comparing collectors but are difficult to use for reliable sizing or predicting overall annual system output. Where components are purchased separately, it is possible to use approximate or rules-of-thumb methods of sizing. This assumes that a building’s occupants, equipment and location are typically average. If this is not the case then a computer simulation may be the only route to a satisfactory calculation. The rule-of-thumb method described below assumes:

• • • •

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collector unshaded and near to optimal orientation and tilt; a household of 3–5 people; DHW usage per person is 35–60 litres (9–16 gallons) per day; DHW usage is mainly in the morning and evening;

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CH-09.indd 154 180°0'0" 160°0'0"W 140°0'0"W 120°0'0"W 100°0'0"W 80°0'0"W 60°0'0"W 40°0'0"W

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10 -2 2 0 0 23 230 0 24 240 0 25 250 0 26 260 0 27 270 0 28 280 0 -2 90

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Figure 9.1 Most of the populated regions of the world can be grouped into five irradiation zones 4/28/2010 12:43:20 PM

Source: © Mines ParisTech/Amines 2006

Realized by Michel Albuisson, Mireille Lefèvre, Lucien Wald. Edited and produced by Thierry Ranchin. Date of production: 23 November 2006. Centre for Energy and Processes, Ecole des Mines de Paris / Armines / CNRS. Copyright: Ecole des Mines de Paris / Armines 2006. All rights reserved.

sizing system components   155

• • • •

the cold water starts at 10oC (50oF); pipes and storage tank are insulated; commercial collector in middle of performance range (see Chapter 4); system solar tank is thermostatically controlled.

In the following tables, suggested sizes are given for the collector area, and solar storage tank volume required per person is given for four climate regions. Where a manufacturer provides a complete kit already sized for a particular region and household size then the following tables are not applicable. Low solar regions are above about 50 degrees north latitude. They are not necessarily cold areas but are likely to be often cloudy, with short winter days but long summer days. Examples are Alaska, United Kingdom, Scandinavia and northern Russia. It is particularly difficult in low solar regions to achieve an even spread of solar contribution through the year, and system performance tends to be more sensitive to collector performance than in other regions. Temperate areas include many maritime and continental locations that lie between the tropics and the polar circles. The climate in these regions is not usually extreme for long periods. Most of the world’s population lives in these areas. Tropical and sunbelt regions lie between the tropics and have the highest irradiation and daytime temperature values. Winter and summer daytime hours hardly vary, and so the smallest collector areas are required. It is normally necessary to consider only lower performance collectors in these regions. In general, the higher the annual irradiation, the less collector area and higher solar storage tank volume per person. And the more equal the lengths of day are between summer and winter, the higher the solar fraction can be. Collectors and solar storage tanks are sold in a few fixed sizes. If collectors are purchased slightly over-sized then a larger solar storage tank should also be chosen. Similarly, if a higher performance collector is used then it is best to choose either a larger storage tank or a smaller collector – or use a lower performance collector. Solar storage volumes are, of course, additional to any DHW storage if present. Tables 9.1 and 9.2 use different collector area definitions. The first table shows metric units, and the active area is either the absorber or aperture areas. The second uses US units where the gross area is normally used. In systems with poor thermostatic control of the storage tank temperature, the lower end of the range of recommended collector area will cause overheating less frequently in the summer than those at the higher end of the range. The tables give estimates for solar storage volume per person in the household. Example: if a glazed flat plate collector used in a low solar climate required an active area of 1–1.5m2 (or a gross area of 15–22ft2) per person, and a solar storage tank volume of 35–50 litres (or 9–14 gallons) per person, the same collector used in the same climate in a household of four people would require an active area of 4–6m2 (or gross area of 60–88ft2) and a tank volume of 140–200 litres (36–56 gallons). Adjustments should be made to take into account patterns of current and anticipated DWH use. Estimating the amount of DHW that can potentially be heated by solar is best done on a weekly basis, as some appliances are only used every few days (e.g. a washing machine). However, it is also useful to calculate the average daily figure per person, as this can be compared to typical averages. Hence, the

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156   solar domestic water heating Table 9.1 Collector and storage tank sizing table (metric units) Climate region

Annual horizontal irradiation kWh/m2

Collector type

Active collector area per person m2

Solar storage Solar tank volume per fraction person % litres

Low solar