Evaporative Air-Conditioning: Applications for Environmentally Friendly Cooling

WORLD

(I

BANK

TECHNICAL

PAPER

NO.

421

EneegySeries

WTP421 March 1999

Work in progress for public discussion

Evaporative Air-Conditioning Applicationsfor Environmentally FriendlyCooling

GelitJan Bom Robert Foster Ebel Dijkstra Tummer-s AMIaija

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WORLD BANK TECHNICAL

PAPER NO. 421

EnergySeries

Evaporative Air-Conditioning ApplicationsforEnvironmentally FriendlyCooling

GertJanBom RobertFoster EbelD#jkstra Marja Tummers The WorldBank Washington,D.C.

Copyright © 1999 The International Bank for Reconstruction and Development/THE WORLD BANK 1818H Street, N.W. Washington, D.C. 20433,U.S.A. All rights reserved Manufactured in the United States of America First printing March 1999 TechnicalPapers are published to communicate the results of the Bank's work to the development community with the least possible delay. The typescript of this paper therefore has not been prepared in accordance with the procedures appropriate to formal printed texts, and the World Bank accepts no responsibility for errors. Some sources cited in this paper may be informal documents that are not readily available. The findings, interpretations, and conclusions expressed in this paper are entirely those of the author(s) and should not be attributed in any manner to the World Bank, to its affiliated organizations, or to members of its Board of Executive Directors or the countries they represent. The World Bank does not guarantee the accuracy of the data included in this publication and accepts no responsibility for any consequence of their use. The boundaries, colors, denominations, and other information shown on any map in this volume do not imply on the part of the World Bank Group any judgment on the legal status of any territory or the endorsement or acceptance of such boundaries. The material in this publication is copyrighted. The World Bank encourages dissemination of its work and will normally grant permission promptly. Permission to photocopy items for internal or personal use, for the internal or personal use of specific clients, or for educational classroom use is granted by the World Bank, provided that the appropriate fee is paid directly to Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923,U.S.A.,telephone 978-750-8400,fax 978-7504470.Please contact the Copyright Clearance Center before photocopying items. For permission to reprint individual articles or chapters, please fax your request with complete information to the Republication Department, Copyright Clearance Center, fax 978-750-4470. All other queries on rights and licenses should be addressed to the World Bank at the address above or faxed to 202-522-2422. ISSN: 0253-7494 Gert Jan Bom, Ebel Dijkstra, and Marja Tummers are development consultants at Ecozone, Haarlem, the Netherlands. Robert Foster is a project engineer at New Mexico State University, Las Cruces. Libraryof Congress Cataloging-in-Publication Data Evaporative air-conditioning: applications for environmentally friendly cooling / Gert Jan Bom . . .[et al.].

p. cm. - (World Bank technical paper; 421. Energy series) Includes bibliographical references (p. ). ISBN 0-8213-4334-3 1. Air conditioning. 2. Evaporative cooling. I. Bom, Gert Jan. II. Series. TH7687.E94 1998 697.9'3-dc2l 98-31273 CIP

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Contents

Foreword.............................................................

ix

Abstract.............................................................

xi

Acknowledgments .............................................................

xiii

Abbreviations, Symbols, and Glossary .............................................................

xv

1. Introduction.............................................................

l1 Benefits of Evaporative Cooling .............................................................. 2 Opportunities and Limitations .............................................................. 2 Environmental Benefits.............................................................. 2 Direct Evaporative Air-Conditioning .............................................................. 3 Residential Coolers .............................................................. 3 Indirect Evaporative Air-Conditioning .............................................................. 4 Desiccant-Assisted Evaporative Air-Conditioning .............................................................. 4 Commercial Evaporative Air-Conditioners .............................................................. 5 Comparing Vapor-Compression and Evaporative Air-Conditioning ................................................. 5 Outlook .............................................................. 5

2. Opportunities and Constraints.............................................................. 9 Climatological Factors ............................................................. 9 Comfort Issues ............................................................. 10 Expected Performance of Evaporative Air-Conditioning ............................................................. 12 Power Supply ............................................................. 13 Water Supply ............................................................. 13 Advantages of Evaporative Versus Vapor-Compression Air-Conditioning .................................... 13 3. Economics............................................................. Economics of Residential Coolers ............................................................. Investment Costs ............................................................. Market Situation .............................................................

15 15 16 18

4. Technology ............................................................. Direct Evaporative Air-Conditioning ............................................................. Indirect-Direct Evaporative Air-Conditioning ............................................................. Desiccant Cooling .............................................................

21 21 26 29

5. Choosing and Maintaining Equipment ............................................................. Available Equipment .............................................................

31 31

v

vi

Evaporative Air-Conditioning, Applications for Environmentally Friendly Cooling

Direct Evaporative Air-Conditioning Recommended Air Change Rate for Design Wet-Bulb (WB) Conditions .......................................................... Maintenance ..........................................................

33 33

6. Solar EvaporativeAir-Conditioning.......................................................... 37 The Market.......................................................... 37 Optimizing Evaporative Air-Conditioning Design for Solar Operation .......................................... 38 7. Introductionand Local Manufacturein Developing Countries......................................................... 41 Maintenance ........................................................... 41 Installation and Sizing.......................................................... 41 Manufacturing Requirements ........................................................... 41 Know-How .......................................................... 43 8. CommercialEvaporativeAir-Conditioning .......................................................... Commercial versus Residential Cooling .......................................................... Commercial Kitchen Evaporative Air-Conditioning .......................................................... Laundry and Dry Cleaning .......................................................... Extreme Heat Conditions .......................................................... Industrial Applications .......................................................... Factory Air-Conditioning Design Considerations ........................................................... Agricultural Applications-Poultry .......................................................... Greenhouses ..........................................................

45 45 46 46 46 47 47 48 49

Bibliography..........................................................

69

.

Annexes

1. 2. 3.

Introduction to Evaporative Cooling .......................................................... 53 Suitability of Evaporative Air-Conditioning in Different Climate Zones ........................................ 57 List of Manufacturers and Suppliers .......................................................... 63

Boxes 2.1 Relative Humidity and Wet-BulbTemperature .......................................................... 5.1 A Simple Sizing Example .......................................................... Figures

1.1 1.2 1.3 2.1 2.2 2.3 3.1 3.2 3.3 3.4 4.1 4.2 4.3 4.4 4.5

Typical Direct Evaporative Air-Conditioner .3 Roof-Mounted Downdraft Evaporative Air-Conditioning Unit, El Paso, Texas .4 Direct Evaporative Air-Conditioner for Transport Use .7 Modified Evaporative Air-Conditioning Comfort Zone Taking into Account Increased Airflow Compared with ASHRAE Comfort Zone Based on Vapor Compression Air-Conditioning .11 Annual Energy Use Summary: Vapor Compression Air-Conditioning (SEER= 9.5 for Phoenix, Arizona, USA).14 Annual Energy Use Summary: Indirect/Direct Evaporative Air-Conditioning (2,000scfm, for Phoenix, Arizona, USA).14 Typical Investment Costs for Evaporative Air-Conditioning in the United States .16 Typical Investment Costs for Evaporative Air-Conditioning in India .17 Typical Life-Cycle Costs: Evaporative Air-Conditioning versus Air-Conditioning for the United States .17 Typical Life-Cycle Costs: Evaporative Air-Conditioning versus Air-Conditioning for India .18 Simplified Evaporative Air-Conditioning Process .22 Psychrometric Process for Direct Evaporative Cooling, Mexico.22 Comnonly Available Rigid Cellulose Pads Provide Superior Saturation and Cooling Compared with Ordinary Aspen Pads .24 Close-up of Rigid Cellulose Pad Made of Corrugated Paper .24 Common Cabinets for Residential Coolers in India .26

10 32

Contents

4.6 4.7 4.8 4.9 4.10 6.1 6.2 7.1 8.1 8.2 8.3 A1.1 Al.2 A1.3 A1.4 A1.5 A1.6 Al.7 A1.8 A2.1 A2.2 A2.3 A2.4 A2.5 A2.6

v2i

Cutaway of a Direct Evaporative Air-Conditioning ................................................................... 27 Plate-Type Indirect-Direct Evaporative Air-Conditioning ................................................................. 27 Indirect-Direct Evaporative Air-Conditioners on a Public School Rooftop, Colorado Springs, USA.................................................................. 28 Indirect-Direct Evaporative Air-Conditioning Process ..................................................................29 Ventilation Cycle Desiccant Cooling System .................................................................. 30 A Solar-Powered Evaporative Air-Conditioner .................................................................. 37 Evaporative Cooler Coupled with Solar Power (System installed by a homeowner in Chaparral, New Mexico, USA).................................................................. 39 Evaporative Air-Conditioners in Kamla Market, New Delhi, India ................................................. 42 Typical Evaporative Air-Conditioning Application for Poultry Houses .......................................... 49 Evaporative Cooling Pad Section of Rigid Cellulose Pads ................................................................. 50 External Evaporative Air-Conditioners on a Research Greenhouse, New Mexico State University, Las Cruces, New Mexico.................................................................. 50 Psychrometric Chart and Saturation Line .53 Complete Psychrometric Chart .53 Wet-BulbDepression of Ambient Air .54 Saturation Effectiveness for an 80 Percent Effective Evaporative Cooling Pad .54 Saturation Effectiveness of 80 Percent for Evaporative Cooling Pads at Different Ambient Conditions .54 Effectof Indirect Evaporative Cooling on Ambient Airstream .54 Effect of Combined Indirect Evaporative Cooling Coupled with Direct Section.55 Energy-Saving Effect of Using a Smaller Coil Coupled with Indirect and Direct Evaporative Cooling Sections.55 Suitability of Evaporative Air-Conditioning: Africa .57 Suitability of Evaporative Air-Conditioning: Asia .58 Suitability of Evaporative Air-Conditioning: Australia .59 Suitability of Evaporative Air-Conditioning: Europe .60 Suitability of Evaporative Air-Conditioning: North America.61 Suitability of Evaporative Air-Conditioning: South Arnerica .62

Tables 1.1 Vapor-Compression versus Evaporative Air-Conditioning ................................................................. 6 2.1 Effectiveness of Evaporative Cooling by Climate Type .................................................................... 9 2.2 Relation between Wet-Bulb Temperatures and Effectiveness of Evaporative Air-Conditioning .................................................................. 10 2.3 Evaporative Air-Conditioning Performance in Selected Locations at 1 Percent Cooling Design Conditions .................................................................. 12 2.4 Benefits of Evaporative Air-Conditioning Versus Vapor Compression Air-Conditioning ............13 5.1 Available Residential Evaporative Air-Conditioning Equipment ..................................................... 31 5.2 Useful Cooling Chart: Percentage of Useful Cooling for Direct Evaporative Air-Conditioning Output ................................................................... 34 6.1 Available Packaged Solar Evaporative Air-Conditioning Equipment .............................................. 38 6.2 Design Measures to Optimize Evaporative Air-Conditioning for Solar Power .............................. 38 7.1 Work Involved in Manufacturing Evaporative Air-Conditioning .................................................... 42

Foreword

Although evaporative coolers cannot be used in all countries and at all times, they are generally very much underutilized in places where they can be used successfully. This is unfortunate, both for the potential user, the country, and the global environment. Benefits include lower cooling equipment costs and a much reduced electricity bill for the user, reduced electrical energy and power demand at peaktimes for the country, and lower greenhouse gas and CFC/HFC emissions for us all. This handbook is designed for those who do not know evaporative coolers, but might be convinced to try using or promoting them. It provides the advantages and disadvantages of using evaporative coolers while comparing them to the commonly used, energy guzzling, and expensive vapor compression air conditioners. Existing markets where evaporative coolers are currently used, local manufacturing possibilities, operational aspects are discussed along economic and global aspects. A world-wide list of manufacturers and suppliers is included in the Annex.

James Bond Director Energy, Mining and Telecommunications Department

ix

Abstract

As the harmful environmental effects of chloro-fluorocarbons (CFCs) and greenhouse gases have become better known, interest has grown in environmentally friendly cooling technologies. Evaporative air-conditioning (EAC) is such a technology. Whereas conventional vapor compression air-conditioning (VAC)uses CFCs as cooling liquids, EAC uses water. EAC technology is simple, functional, and has both residential and commercial applications in industrialized and developing countries. EAC can provide superior cooling and ventilation while consuming less energy (and hence contributing less to greenhouse gas emissions) than VAC. EAC works best in hot, dry climates, but it can be used in more humid climates as well. This paper elucidates some of the technical characteristics and fields of application for EAC and outlines the climatic conditions under which EAC can be most effectivelyemployed. The document begins with a general outline of the applications and limitations of EAC and explains the differences between "direct" and "indirect" EAC. Chapter 2 discusses the applicability of EAC in different climates and explains the use of wet-bulb temperature as a useful tool for predicting the applicability of EAC. Chapters 3 and 4 discuss the economics of EAC versus VAC in terms of energy consumption, required investments, and life-cyclecosts. Production costs, the paper points out, are low enough so that EACs can be manufactured relatively easily in the developing world, as is now being done in South Asia and the Middle East. Chapters 5 and 6 review the market for EACs and try to show how EAC can increase individuals' "feeling of comfort." Chapter 7 explains the basic technology of EAC.The difference between direct and indirect coolers is elaborated on through the use of a psychrometric chart. The hardware components of the EAC are explained: pads, motor, pump, and fan. Chapter 8 lists the equipment available on the market. It also points out that the capacity of the cooler and the size of the room to be cooled are key elements in selection of an EAC.A simple example is given to aid in sizing. Like any sort of mechanical equipment, EACs need to be maintained regularly to perform well and last longer. Maintenance requirements for each component are discussed in chapter 9. EACs require little energy, and because the presence of strong sunshine coincides with the need for cooling, a link with solar energy appears to be attractive. In chapter 10 the usefulness of solar EAC and the present market situation are outlined. EAC is an attractive cooling solution, for industrial as well as for less developed countries too. The requirements for the introduction of a relatively new technology like EAC are discussed in Chapter 11.In Chapter 12 the usefulness of EAC for commercial applications is outlined. Commercial kitchens, laundry and dry cleaning and industrial applications are three areas where EAC could be useful.

xi

Abbreviations, Symbols,and Glossary

Design temperatures: outdoor temperatures at a fixed percentage more temperate than worst-case figures, which are a standard air-conditioning system design parameter. Enthalpy: total heat content of air-water vapor atmospheric gas. Not altered by adiabatic cooling. Evaporative air-conditioning: lowering of dry-bulb temperature as air passes over water. Two methods using evaporating water to cool air: (1) direct, which is adiabatic and humidifies the air; and (2) indirect, which is nonadiabatic and cools the air being treated. Indirect evaporative air-conditioner. a heat and mass transfer device used to sensibly cool a primary airstream, without addition of moisture, by means of an evaporatively cooled secondary airstream. Since the secondary air provides wet-bulb depression, it represents a heat sink to the primary air. Latent heat load: heat carried by water vapor in air; varies with humidity. Wet-bulb temperature is an index to latent heat. Saturation (cooling) effectiveness: the primary air dry-bulb temperature reduction divided by the primary air entering dry-bulb temperature less the entering secondary wet-bulb temperature. Temperature, dry-bulb: the air temperature measured by a dry temperature sensor. Temperature, wet-bulb: the temperature measured by a temperature sensor covered by a water-moistened wick and exposed to air in motion. When properly measured, it is a close approximation of the temperature of adiabatic saturation.

xv

Introduction Evaporative air-conditioning (EAC) technologies are being used increasingly in residential and commercial applications worldwide. EAC technologies-which rely on water as a coolant rather than on chemical refrigerants-are economical to produce and use and have important environmental benefits. This paper introduces the technical aspects of EAC, reviews EAC's scope of application, and surveys the specific climatic conditions under which EAC can be used most effectively in industrialized and developing countries. Under the right conditions and applicafions, EAC can provide excellent cooling and ventilation with minimal energy consumption using water as the working fluid and avoiding the use of ozone-destroying chlorofluorocarbons (CFCs). Policymakers in particular should become better informed about EAC because of the opportunities it affords to reduce the use and emission of CFCs and hydrofluorocarbons (HFCs), the reduction in CO2 emissions that come from the energy efficiency of the technology, and the potential for mitigating problems of peak electricity demand during the hot season in many countries. The viability of using EAC will depend on the particular application and on the local climatic conditions. For example, for comfort cooling, EAC is most suited to dry regions, although technical improvements such as indirect/direct and desiccant-assisted systems widen the zone of applicability. On the other hand, some commercial applications of EAC are suitable even in humid climates. In general, several sectors have significant reasons for considering employing EAC technologies: * Utilities.Dissemination of EAC appliances can serve as a significant demand-side management (DSM) tool for utilities. Power savings of EAC technology versus VAC are on the order of 70 percent for direct EAC and 50 percent for indirect EAC. This differential presents substantial peak-saving opportunities for utilities that can promote the use of EAC within their service areas. * Governments.For goverrnent agencies and planners, cost savings from reduced electrical consumption can be realized directly by incorporating EAC technology into buildings and other installations. In addition, government planners should encourage use of EAC technologies as a relevant technology alternative to VAC that will save consumers money, reduce overall electrical demand, reduce pollution emissions, and help meet international treaty obligations related to reducing pollutant emissions. 1

2

Evaporative Air-Conditioning: Applications for Environmentally Friendly Cooling

* Consumers.Consumers who use EAC at home can save money on cooling costs. The typical capital, installation, and operation costs are significantly lower for EAC technologies than for VAC technologies. Moreover, EAC technology is simple enough so that most homeowners can maintain their own units. * Privateenterprise.The manufacture and sale of EAC appliances presents significant opportunities for both small and large enterprises. It is particularly suited to manufacture even in relatively poor developing countries because-unlike the comparatively complex technical requirements for production of chemical air-conditioners-EAC production requires only the basic infrastructure and skills mix related to sheet metal, motor, pump, and fan fabrication. Hence, marketers of EACs can underbid VAC prices while maintaining comparatively high profit margins. In the right climates, EACs can gain far more than a "niche" market: in some of the larger cities in the southwestern United States and northern Mexico, for example, 95 percent of the residential airconditioning market is taken by EAC units, most of them manufactured locally.

Benefits of Evaporative Cooling The following benefits of EAC can be cited: * * * * * * * * * * * *

Significant local fabrication and employment Substantial energy and cost savings No chlorofluorocarbon (CFC)usage Reduced peak demand Reduced CO2 and power plant emissions Improved indoor air quality Life-cyclecost effectiveness Easily integrated into built-up systems Wide variety of packages available Provide humidification when needed Easy to use with direct digital control (DDC) Greater regional energy independence

Opportunities and Limitations EAC works best for comfort cooling where it is hot and dry. EACs are widely used in the Middle East, Australia, the Indian subcontinent, Eastern African, northern Mexico, and the southwestern United States. Residential EACs are known in India as desertcoolers,and in such desert or dry-steppe climates EACs do give "significant relief" during the hot months. "Significant relief" is considered to be provided when the final supply-air temperature leaving the EAC is about 20' to 250C (680to 77°F). Even in a tropical savanna climates such as in the northeast of Brazil, the Sahel region of Africa, the southwest Dominican Republic, EAC can be useful in some comfort cooling applications and also for many commercial applications such as greenhouses and poultry houses. A limiting factor for the application of EAC is the definition of comfort. A residential cooler bringing down the temperature from 450to 30°C(1130to 860T) may still be appreciated even if it does not provide "significant" relief. Environmental Benefits EAC technologies represent significant enviromnental benefits related to reducing CFC/HCFC use and for obviating C02 and other emissions, as well as for reducing peak electrical demand. For example, the

Introduction

3

4 million EAC units in operation in the United States provide an estimated annual energy savings equivalent to 12 million barrels of oil and an annual reduction of 5.4 billion pounds of CO2 emissions. They also avoid the need for 24 million pounds of refrigerant traditionally used in residential VAC systems. Similar energy savings and environmental benefits are also made by commercial applications of evaporative cooling technologies in the United States and elsewhere. Through increasing use of EAC technologies, countries can save energy, reduce power plant emissions, obviate CFC usage, and improve indoor air quality. Basic air conditioning with water is a relatively simple process. Direct Evaporative

Air-Conditioning

Direct EAC is the simplest, the oldest, and the most widespread form of air-conditioning. This system typically uses a fan to draw hot outside air into a dwelling through a porous wetting medium. Heat is absorbed by the water as it evaporates from the porous wetting medium, and the air thus leaves the EAC at a lower temperature. The amount of cooling provided is determined by efficiency of the wetting medium, the fan, and the overall design and construction of the unit. A critical component in EAC is the use of water. This may vary from a few liters per day in small residential coolers to perhaps a hundred liters or more in pad-and-fan EAC systems in greenhouses and complicated duct-systems in laundries and hotel kitchens. Residential

Coolers

A residential EAC typically consists of a cubical box of sheet metal or plastic containing large vertical filter "pads," an electric-motor-driven fan, a water pump, a water distribution system, and a water sump at the bottom. As Figure 1.1 and Figure 1.2 show, the fan draws in warm outside air through the wetted media, cooling the air. The water pump lifts the water from the sump through the distribution system to the top of the pads from where it trickles down by gravity back to the water sump. The cooled air is then delivered either directly through a grille into a single room or into a duct distribution system. This is a "direct" EAC in which the cooled and saturated outside air flows into the room, displacing the hot air. It is simple and cheap but is not sufficient for indoor comfort cooling once ambient wet-bulb temperatures reach 21°C (69.8°F).

Figure1.1. TypicalDirect EvaporativeAir-Conditioner Distribution Manifold

Inlet Air

d. Wetted Media

Conditioned Air Recirculation Pump

Source:Authors.

4

Evaporative Air-Conditioning: Applicationsfor Environmentally Friendly Cooling

Figure1.2. Roof-MountedDowndraftEvaporativeAir-ConditioningUnit, El Paso,Texas

7."~

~

~

~

~~~~.

Source: R. Foster.

Indirect EvaporativeAir-Conditioning Indirect-direct EAC is a method established only over the past 15 years. It is not as widely used as direct EAC, but it is gaining in popularity because it cools air more than direct EAC, and cools the air down from higher wet-bulb temperatures. hindirect EAC accomplishes these effects by building an additional step into the cooling process. That is , the incoming air is cooled first with a normal air-to--airheat exchanger. This is the "indirect" stage because it does not add moisture to the supply air. Instead, only one side of the heat exchanger is cooled with evaporating water as the supply air passes through the other side, dropping in temperature

as it does. Only then, as it passes through the direct EAC stage, is the

supply air moisturized. The final air leaving an indirect-direct EAC unit is generally 3.5C (6.30 F) cooler than what could be achieved with a direct EAC unit alone. Because it cools the air first without moisturizing it, the indirect-direct process also allows the EAC unit to provide more comfort in slightly more humid areas. Commonly these units achieve 65 percent indirect stage efficiency (performnancefactor), which allows an ambient wet-bulb temperature of up to 250 C to provide acceptable room temperatures for real comfort. Two-stage air-conditioners combinLingindirect and direct EAC are becoming popular in the United States and Australia, particularly in locations where slightly higher wet-bulb temperatures (i.e.,conditions of higher ambient humidity) do not permit sufficientlycomfortablesupply-air temperatures via direct EAC. On the downside, however, the two-stage units have higher construction and maintenance costs.

Desiccant-Assisted EvaporativeAir-Conditioning The use of dehumidifying chemicals (e.g., desiccants such as silica gel) further widens the scope for EAC. Desiccant technologies can widen the scope for comfort cooling to even the most humid regions. In such systems, the desiccant is used first to dehumidify the ventilation air to a desired state; then, EAC (either direct or indirect or a combination thereof) is used to cool the air to the desired supply-air temperature.

Introduction

5

CommercialEvaporativeAir-Conditioners Commercial EAC applications are of several types. Commercial comfort cooling applications are used for offices,retail establishments, and so on, as determined by local climates and comfort preferences. In other commercial applications, EAC may be used to moderate the effects of an additional internal heat source that does not depend (only) on the climate or the time of the year. For example, temperatures may rise inside warehouses or buildings because of the operation of ovens, machines, or the presence of livestock. These heat sources sometimes exacerbate already high ambient temperatures. Although the cooling requirements differ as a matter of degree, so to say, cooling of buildings affected by both internal and external sources of heat does require a somewhat different approach from residential cooling to moderate high outside ambient temperatures. For one thing, such commercial EAC systems may well need to be designed for operation the year round rather than just in a "hot season." A commercialkitchen or bakery, for example, might need cooling year-round. Moreover, the internal cooling requirements may be quite localized within the building (e.g., spot-cooling in a manufacturing plant). Another difference between commercial and comfort cooling with EAC is that EAC in some commercial applications is the only practical alternative; that is, where VAC technologies cannot function or compete effectivelybecause of high operating costs. The most salient example here is the cooling towers in a power plant, but on a smaller scale, EAC is the only real alternative in agricultural applications such as greenhouses, where VAC is both inappropriate and far too costly Common commercial applications for EAC include the following: * • • * * -

Commercial kitchens Hotels and restaurants Hospitals Other institutions Laundry and dry cleaning Industrial applications Agricultural applications Poultry sheds - Greenhouses * Schools and offices * Transit buses (Figure 1.3) * Industrial applications - Warehouses - Spot cooling - Factories

ComparingVapor-Compressionand EvaporativeAir-Conditioning Table 1.1 compares the basic characteristics of VAC with those of EAC.

Outlook Worldwide, the potential for EAC is much greater than is currently realized. Investment, operation, and replacement costs can be lowered significantly by foregoing or replacing VAC technologies and using EAC.The potential applications are manifold: from buildings and homes to buses and kitchens. In some developing regions of the world where air-conditioning has scarcely arrived, EAC could bring comfort, as VAC may not be affordable by many because of its high investment and operating costs. Even where the conventional electric grid service is available, EAC may be a viable and economically attractive option, particularly in conjunction with the use of solar photovoltaic (PV) modules.

6

EvaporativeAir-Conditioning:Applicationsfor EnvironmentallyFriendlyCooling

Table 1.1.

Vapor-Compression

versus Evaporative Air-Conditioning

Basic characteristics

Coolant Production residential coolers Sensitivity to humidity for comfort cooling applications Ventilation (indoor air quality) Energy use in a typical residential air conditioner for a 100m3 room. Investment for a residential cooler

Maintenance Annual accumulated costs including power, maintenance, depreciation

Vapor compression AC

Evaporative AC

CFCs/HFCs Small and large scale Applicable in all climate types

Water Small and large scale Applicable in dry hot climates for comfort cooling 100%outside air 350 kWh/yr

20% outside air 1,000kWh/yr Developed country US$1,000-1,600 Less developed country US$600-1,400 Change filters every 2 years In USA: US$500 In India: US$500

Developed country US$200-700 Less developed country US$60-300 Annual pad change for aspen sump coat every 2 years In USA: US$170 In India: US$37

Source:Authors.

Some options expanding and realizing the benefits of EAC are noted below: Low energy use/solar. Small EAC units using solar photovoltaics (PV) are available in several commercial and prototype models. Manufacture and dissemination could be done through commercial channels providing cost-efficient cooling in grid-and non-grid settings. Transfer I of technology. EAC technologies are a fertile field for South-South transfer of technology, in particular with regard to small residential coolers and some agricultural applications. .Support possibilities. EAC has substantial applicability as a demand-side management tool, in government offices and schools. Technical assistance to developing countries, pilot programs, and demonstrations all may provide further opportunities for EAC.

Introduction

Figure 1.3. DirectEvaporativeAir-ConditionerforTransportUse

Note

he EAC unit(onthe

forklift at right) wasbut

by

imatran andisbeinginstalledontoatytransitbusinDenverColorado.

More than 400 buses in the United States and more than 1,200in Australia use evaporative air-conditioning. Source:R. Foster.

7

Opportunities and Constraints

Climatological Factors Unlike vapor-compression air-conditioning, which can work under virtually any climatic conditions, evaporative air-conditioning varies in applicability and efficiencywith the relative humidity of the outside air: that is, the drier the air, the more suitable EAC is and the better it cools. The general climatic parameters for applying EAC for comfort cooling can be superimposed on the world map in terms of three types of climatic zones that are, respectively, highly, moderately, and marginally suitable for EAC (Annex 2 contains maps showing these zones of applicability of EAC for each continent). The climate types are listed in Table 2.1, and for each type the effectiveness of EAC is indicated. This effectiveness is rather constant for desert climates, but for both the steppe and savanna climates, a generalization about applicability masks what may be significant month-to-month variations in the actual comfort derived from EAC. It should be emphasized, moreover, that this sort of zoning provides only a rough indication of suitability; each zone may contain areas that are better or worse suited for EAC than their assignment to the zone would suggest. Moreover, some specialized EAC applications (e.g., in greenhouses or poultry houses) are effective and commonly used in even the most humid of climates outside of these zones. EAC is already popular in the desert climate zones such as the arid southwestern United States, Mexico, Australia, Iran, Iraq, Jordan, Libya, Spain, Sudan, Egypt, India, Pakistan, and South Africa.These Table 2.1. Effectivenessof EvaporativeCoolingby ClimateType Climatetype Desert Steppe Savanna

Generaleffectivenessof EAC Realcomfortduring the wholecoolingseason(e.g.,offices,homes,libraries,restaurants) Realcomfortduring the dry period of the hot seasonand moderatereliefcoolingduring more humid periods Onlycan provide reliefcoolingduringthe hot season(e.g.,warehouses,greenhouses, poultryhouses).

Source:Authors.

9

10

EvaporativeAir-Conditioning:Applicationsfor EnvironmentallyFriendlyCooling

Box 2.1.

Relative Humidity and Wet-Bulb Temperature

Apart from using the rough measure of climate zones or the level of humidity, one can predict the effectivenessof EAC for a particular location fairly accurately using the locally prevailing wet-bulb temperatures (WB).Table 2.2 shows how these are measured. In brief, by adding about 5-C (9°F)to the WB,one knows the effectiveroom temperature that can be reached with EAC. Becausethe WBvaries over seasons and during the course of the day,it does not suffice to use average WB. Rather, one should consider the WB at the time when cooling is most important-for example,around noon.

areas have in common high summer temperatures coinciding with low humidity-that is, high ambient temperatures combined with low wet-bulb temperatures. This combination means that EACs can be very efficient and can provide real indoor comfort (see Table 2.2 and Box 2.1 for a range of benefits). A total of about 20 million EAC units are presently in use worldwide. EAC is largely unknown, however, in many areas with steppe or savanna climates, even though it could constitute a real alternative to VAC. Comfort Issues "Human comfort" depends on a range of factors ranging from temperature, humidity, and air movement to clothing and culture. What is comfortable for one person in one society may be entirely uncomfortable for another. Someone who has long lived without VAC may find an artificially air-conditioned environment uncomfortable, whereas people who take VAC for granted in their homes and workplaces may avoid being outside during hot weather all together. Standards

Comfort zones are often shown on standard psychrometric charts and have been developed to indicate regions where a person is "comfortable." In the United States, the American Society of Heating, Refrigerating and Air-conditioning Engineers (ASHRAE)has developed comfort zones based on psychrometric charts. However, these standard types of comfort charts have more limited relevance related to evaporative air-conditioning. First, standard comfort zones are based on air velocities typical of vapor-compression air-conditioning systems, not the higher air velocities used with evaporative air-conditioners. Second, the traditional comfort zones used today (unlike those of the past) have horizontal, constant humidity-ratio (constant dew point) lines supposedly aimed to minimize respiratory diseases, mold growth, and similar problems. Relative humidity boundary lines are just as effective (and were previously used) and would distort comfort analysis less. Tests have shown that human comfort is a continuum, not confined between dewpoint lines. Consequently, the standard comfort zones commonly used face shortcomings relative to EAC.

Table 2.2. Wet-bulb temperature

15-210 C 21-230 C 23-270 C

Source:ECI.

Relation

between

Wet-Bulb

Temperatures

and Effectiveness Typical

Type of EAC Unit

Direct Direct Indirect / direct Direct Indirect / direct

of Evaporative

supply air temps (Dry-bulb)

17-230 C 23-250 C 22-230 C 25-300 C 23-260 C

Air-Conditioning

Cooling

effectiveness

Real comfort Moderate relief Real comfort Some relief Moderate relief

Opportunitiesand Constraints

11

TheModified Comfort Standard for Evaporative Air-Conditioning The effect of a given air stream on a person can be determined by an effective temperature chart, as is commonly used when calculating wind chill. By increasing the velocity of movement, air feels cooler.For evaporative air-conditioning, it is more reliable to consider a comfort zone bounded by relative humidity and extended to take into account the cooling effect of increased airflow, as shown in Figure 2.1.

Figure2.1. ModifiedEvaporativeAir-ConditioningComfortZone Takinginto Account IncreasedAirflow Comparedwith ASHRAE ComfortZone Basedon VaporCompressionAir-Conditioning 23.9

Wet-Bulb Temperature

(°C) 18.3

90_L_g ModifiedComlbrtZone

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JUNE 1997

Annex 2: Suitabilityof EvaporativeAir-Conditioningin DifferentClimateZones

61

FigureA2.5. Suitabilityof EvaporativeAir-Conditioning:NorthAmerica(Shadedareasindicatesuitability)

R8

1

EvaporativeAir-Conditioning:ApplicationsforEnvironmentallyFriendlyCooling

62

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