Wind-Diesel Systems: A Guide to the Technology and its Implementation

This book provides for the first time in a single volume the collective knowledge of many leading researchers on state-of-the-art wind-diesel technology. It contains the results and advice of numerous experts from many different countries, and has been carefully edited to provide a coherent reference volume. Wind has long been recognised as one of the most promising 'renewable' sources of power, and much development and commercialisation of the technology has taken place in recent years. The first half of the book discusses selection of an appropriate system from the different wind-diesel options available, taking into account the needs of a particular community and the available wind resource. It then goes on to describe in detail how a practical system should be designed. The second half of the book is concerned with getting the best out of a system once it is installed. It starts by presenting case studies to illustrate systems that work excellently, and some that have been disappointing. For complex systems, modelling can be very useful for getting the optimum configuration and these are discussed in a separate chapter. The following chapter discusses the installation, monitoring, and maintenance of winddiesel systems. The final chapter of the book is devoted to the economics of running a wind-diesel system. The book will be useful to all professional engineers and researchers who are interested in wind energy conversion. It is hoped that the collected knowledge of these leading experts will serve to hasten the development and application of winddiesel systems.

Wind-Diesel Systems

Wind-Diesel Systems A Guide to the Technology and its Implementation Prepared under the auspices of the International Energy Agency

Edited by Ray Hunter and George Elliot

CAMBRIDGE UNIVERSITY PRESS

Published by the Press Syndicate of the University of Cambridge The Pitt Building, Trumpington Street, Cambridge CB2 1RP 40 West 20th Street, New York, NY 10011^211, USA 10 Stamford Road, Oakleigh, Melbourne 3166, Australia © Cambridge University Press 1994 First published 1994 A catalogue record for this book is available from the British Library Library of Congress cataloguing in publication data available ISBN 0 521 43440 8 hardback Transferred to digital printing 2004

Contents

Editors' note

ix

Foreword

1

How to use this book

3

Chapter 1

5

Wind-diesel system options Remote power, Simple wind-diesel combinations, Energy storage - the advantages and disadvantages, Load management, Electrical generator options, System architecture options, Nature and size of market

Chapter 2

Matching the wind-diesel system to the community

27

Assessing the consumer load demand, Siting considerations, Climatic conditions, Legal and statutory considerations, Annoyance, Other environmental factors, Community impact Chapter 3

Assessing the wind resource

54

Introduction, Survey of available meteorological information, Inspection and selection of candidate sites, Simple 'Guidelines' terrain model, More sophisticated numerical terrain models, Other evaluation techniques, Wind measurement programme, Decision tree, Glossary Chapter 4

Designing a system System operation, Quality of power, Choice of generators, Choosing a diesel generator set, Wind turbine selection, Dump or auxiliary heat loads, Storage selection, System control, Electrical safety, Load management, Appendix Sample calculations for rating the diesel's generator

95

Contents

vm Chapter 5

Wind-diesel case studies

140

The Froeya wind-diesel demonstration, The Foula electricity scheme, The RAL/ICST wind-diesel research facility Chapter 6

Modelling techniques and model validation

165

Application of models, General description of time series modelling, Statistical modelling techniques, Modelling package summary, Trial validation of models considered by the authors, Guidelines for future validation exercises, Error propagation, Appendix - Simple example of use of statistical modelling, Appendix - Detailed treatment of start/stop cycling in statistical models, Appendix - Uncertainty analysis Chapter 7

Installation and monitoring of wind-diesel systems

209

The importance of verification tests, The pre-installation phase, The assembly and erection phase, Commissioning, Monitoring, Operation and maintenance Chapter 8

Assessing the economics

221

The economic incentives, Cost parameters, Indirect financial considerations of total environmental and social cost, Direct financial considerations, Economic appraisal methodology, Economic assessment methods, Economic assessment example Index

245

Foreword

The International Energy Agency (IEA) was formed as a result of the 1973 oil crisis to provide a means whereby member countries could co-ordinate their energy policies to ensure long term mutual safeguards to continuity of supply. Wind has been recognised for some time as one of the most promising 'renewable' sources of energy, and considerable development and commercialisation of the technology has taken place in recent years. There is in addition growing concern about carbon dioxide emissions and the detrimental effects which result from burning fossil fuels on our planet. The IEA has been active in encouraging interaction within wind technology, and has sponsored two major programmes, one (IEA LS WECS) has supported development of large machines, whilst the other (IEA R&D WECS) has encouraged more generic research. As part of this latter programme, a project was set in motion in 1985 to support research into the problems of integrating wind into small decentralised systems. In June 1985, a group of experts, gathering under the auspices of the IEA, met at the National Engineering Laboratory, UK, to discuss their common interest in the development of wind-diesel technology. A programme of work was set in motion, the formal technical aims of which were: a T o define cost effective models and techniques suitable for obtaining wind and electrical load data necessary for planning and specifying decentralised wind energy conversion system installations', and b T o apply and further develop models suitable for analysing the performance of wind-diesel systems, and to obtain a sound analytical basis for planning and designing wind-diesel systems'. At subsequent meetings, the desire was widely expressed to produce a work of reference which would convey to the wider engineering community the potential difficulties, and stage of development that wind-diesel technology had reached. This book is the result.

2

Foreword

The authors comprise the foremost experts from the ten participating countries, who by discussion and information exchange have agreed upon the contents. The main concentration of wind research in many countries has been to provide supplementary power to main electricity networks. However, many millions of people in the world have no access to grid electricity, and it is on them that wind-diesel systems could have the greatest impact. This book aims to facilitate the development and application of wind-diesel systems by providing in a single volume the collective knowledge and advice of the leading researchers in the field, a group of people with whom it has been our pleasure to work over the past five years.

Ray Hunter and George Elliot (UK) Editors and Operating Agents for the IEA R&D WECS Annex VIII Project, National Wind Turbine Centre, NEL, East Kilbride, Glasgow, G75 OQU, Scotland. May 1992

How to use this book

Wind-diesel technology is at an exciting stage of development. Much research work on sophisticated wind-diesel systems is under way and encouraging performance characteristics have been demonstrated in the laboratory. A number of wind-diesel systems have been installed to serve real loads, and although the earlier ones did not save appreciable quantities of fuel, more recent installations are proving increasingly attractive, particularly those which include load control. The wind-diesel option is now an economic alternative to straightforward diesel at good locations The purpose of this book is twofold. For the interested researcher it sets out in simple terms the state-of-the-art, but secondly, and more importantly, it forms a first attempt at dissemination of knowledge to the wider non-expert community who may wish to consider wind-diesel for remote power applications. In the real world, wind-diesel must justify itself economically, and this book is designed to provide an indication of the tools necessary for an assessment to be made of wind-diesel potential at any given site. In Chapter 1, the reader is introduced to wind-diesel technology and shown by way of example the various system options that the installer may wish to consider. The various system building blocks are specified and possible system architectures are outlined. As for any power supply network, before installing generating plant, it is important to know the nature of the load in terms of its peak and mean values and also any daily or seasonal trends. The quality of power required must also be appraised. Chapter 2 indicates what to take into account. Also in Chapter 2 other community and local considerations are outlined. For instance, how easy would it be to install a turbine? Does the road and craneage infrastructure exist? Is there any likelihood of special grants being available? Are there any areas where the turbine could not be sited due to presence of transmitter stations? What impact will the system have on the community? For a wind power based system, the economic viability is dependent more than anything else upon the host site's wind climate and in particular its mean wind speed. Accurate wind appraisal is seldom simple and Chapter 3 gives a comprehensive overview of the techniques which can be used. Measurement methods are described, as

4

How to use this book

are a number of modelling techniques of varying complexity. A methodical approach to wind appraisal is proposed. Chapter 4 deals with the large number of technical design considerations which are relevant to wind-diesel. The object here is to help the reader decide what his or her particular system should include and what the approximate ratings should be for each element. To help show how these considerations fit together in a real system, a number of practical case studies are presented in Chapter 5. However, it is a somewhat complex matter to optimise the configuration and control of a wind-diesel network. Some degree of modelling is nearly always required, and Chapter 6 gives details of several types of simple model. Chapter 7 deals with the practical issues of installation, operation and reliability, and gives advice on testing, commissioning and monitoring system performance and behaviour. These chapters will therefore be of interest to the researcher, the system designer, the installation engineer, and the end user or operator. Chapter 8 indicates how the economics of a particular system can be assessed using standard economic appraisal techniques. Taken together with Chapters 4 and 6, an iterative optimisation loop can be adopted. Having decided at the outset on a 'sensible' system (Chapter 4), the performance can be assessed (Chapter 6), and the resultant power production estimates used as input to the economic calculations (Chapter 8). Thereafter, the designer can vary the input parameters one by one to see the effect on overall system economics.

1 Wind-diesel system options

REMOTE POWER Historically, until the advent of electricity, all power systems were decentralised, ie power was produced at the location where it was required. In the case of wind power, milling, pumping, and irrigation were popular applications. However, electricity brought about the development of grid networks with centralised generating capacity, and the demise of many decentralised power systems. Today, we tend to forget that there are still many locations in the world which do not have an electrical connection to a central utility network. Furthermore, in many places, due to remoteness and cost, it is unlikely that a main grid connection will ever be established. However the need for power still exists. Power systems which can generate and supply electricity to such remote locations are variously termed 'remote, decentralised, autonomous, or stand-alone'. The purpose of this book is to show that in many locations wind power can usefully be incorporated into, or form the basis of, such systems. Broadly speaking, there are three types of application for remote electrical power, these being: *

Power for specialised applications in remote areas, eg, communications, irrigation.

*

Power to remote communities in industrialised countries, and on islands.

*

Community power generation in developing countries.

Each has its own particular requirements and design constraints, for example system reliability can be more important than cost of power in unmanned locations, whilst communities in industrialised countries quickly develop high expectations of power quality and availability as well as competitiveness, whilst for communities in developing countries simplicity of maintenance is a prime consideration. Diesel networks At present, the most common way to supply electricity to remote loads, whether communities or special applications, is with a diesel engine driving a generator set.

6

Wind-diesel system options

For small loads a single diesel set might be appropriate, whereas for larger communities multiple diesels are commonly employed. In the latter case, one or more diesels, typically the most efficient, supplies the base load. Frequently, enough reserve capacity is provided so that at least one machine can be taken out for overhaul at any given time. Multiple diesel grids can run at high efficiency since it is possible to ensure that all running plant is highly loaded. Diesel manufacturers strongly advise operating above a minimum load, typically 40 per cent, in order to maintain high efficiency since fuel consumption can be significant and therefore wasteful, and to minimise engine wear. However, due to continually changing load patterns and the desire to always meet the demands of the load, many locations have diesels which are incorrectly sized or inefficiently controlled. This quite often results in the diesels not complying with the operational constraints. Nevertheless diesel plants offer reliable long term power supply. The main disadvantage with diesel electric grids is that the cost of power tends to be high, often many times greater than for larger capacity networks. Poor economics are traceable to the cost of diesel fuel, including the cost of transportation which is often the dominant factor and the cost of operation and maintenance in what is usually a remote location. Although diesel networks are simple, they need to be well maintained and like all plant, they need to be periodically replaced either due to unavailability of spare parts, to maintain reliability, or to take advantage of improved efficiency resulting from technological developments. However such factors are often neglected, and there are many ageing, unreliable systems in existence at present which are nearing the end of their useful lives. The major advantage of diesel systems is that they are extremely well proven, and if maintained correctly, highly dependable. The key point is that although diesel electric power is often reliable, it is also expensive. This situation is not likely to change in the future, indeed it is likely that costs will rise. Wind power Although wind power continues to be used for water pumping and irrigation, developments brought about by the oil crises of the 1970's have led to the creation of a new market for wind power, for electricity production. Modern wind turbine generators have been developed using highly complex sophisticated technology and differ from the traditional wind mill or wind pump in that they have few blades, typically two or three. This new generation of wind turbine also has much improved efficiency. Most recent work has concentrated either on developing very large prototype machines with rotor diameters of up to 100 metres and rated up to 3 MW for large utility grids, or on evolving competitive medium sized machines, typically rated at 200-500 kW with rotor diameters of between 25 and 50 m, for use in arrays or wind parks. Such

Wind-diesel system options

7

machines are designed to be integrated into large networks where the wind capacity represents a small proportion of the overall capacity. In grid connected mode wind power has proven itself to be extremely cost effective at good windy sites. Wind parks feeding large grids are now capable of supplying energy at a cost which can compete with more conventional forms of electrical supply such as coal or nuclear. Many places which require remote power are in regions of high wind energy potential, and it might seem strange that initial wind turbine generator developments have not taken place at such locations. The reason for this is primarily due to the great variation in available wind power which occurs from season to season, hour to hour, minute to minute, and indeed second to second. The power in the wind is proportional to the cube of the speed and hence the presence of short term wind speed fluctuations (turbulence) and the frequent passage of weather systems can lead to an extremely variable power availability. This would not be a problem if the load was well correlated to the energy availability, but unfortunately this is not often the case, and to supply all the load from wind would involve either vast excess capacity or alternatively expensive energy storage systems. Electrical requirements at decentralised sites The nature of the electrical load at a decentralised site is of primary importance and depends strongly on the type of electrical apparatus in the system. Two considerations are: *

The variability of the electrical load.

*

The required power quality or 'firmness' of the grid.

In the first case, a community load tends to vary in a more or less regular way over the day, often reaching a maximum sometime during normal waking hours and falling to a low in the very early hours of the morning. As well as the gradual changes, there are also fluctuations of much shorter duration caused by switching in or out large electrical equipment. There may also be substantial changes in the load with the time of week, or with the season of the year. From an economic point of view it is quite important to know if the annual variation in wind energy potential in any way corresponds to that of the load or if there is a substantial mismatch. In the first instance if a good match exists it would be possible to take full advantage of most of the wind energy potential, even from a relatively large machine. However, this might well be at the expense of having to run a diesel inefficiently at a low load. A poor match can give better advantage for certain systems since the diesel can be better loaded when demand is high although with the penalty of using more diesel fuel. The second consideration, the 'firmness' of the supply, is a little less obvious, but quite important nonetheless. In order to properly supply most electrical loads, the voltage

8

Wind-diesel system options

and frequency of the power should stay within appropriate limits. Accomplishing this is usually fairly straightforward when only diesel generators are used. When other sources of power, such as wind machines are added, the problem is more difficult. This is due primarily to the short term fluctuations (in the order of seconds to minutes) in wind speed which cause corresponding variations in wind power generation. Being able to accommodate these fluctuations is a necessary requirement of any wind-diesel system. The potential of wind diesel The obvious solution to the problem of using wind power capacity at a remote location would be to compensate for the variability of the wind by using a diesel electric generator to make up any shortfall. Such a system would have the benefit of using the free wind resource, of saving on existing levels of fuel consumption, and of providing power on demand to meet the consumer load. Ideally the diesel could be used to provide firm power during periods of insufficient wind, and the wind turbine could be used to save diesel fuel when wind energy is plentiful. Such basic systems exist and supply firm power while fuel is being saved. However, as will be shown later some combination of dump load, short term storage, and load control will often improve power quality and fuel savings, but at present not necessarily economics. For example, the use of basic load management techniques can allow excess wind energy to be well used for low quality, high capacity loads such as heating, so helping to enhance the standard and quality of living of the community. Different definitions of 'wind-diesel system' are possible, many of them depending upon total rating of the system. However, for the purposes of this book it will be assumed that a wind-diesel system is one in which the wind energy penetration is sufficiently high to require special control strategies to be adopted to maintain the continuity and quality of the supply. It should always be borne in mind however that wind-diesel systems can only be economic and workable if an adequate wind resource exists. In many applications solar-diesel or hydro-diesel or charge-cycle battery systems might be more viable options.

SIMPLE WIND-DIESEL COMBINATIONS The simplest approach to incorporating wind derived power into a diesel system is to connect a wind turbine to the network in the same way as would be done in a conventional large electric grid. Ideally, operation should be straightforward. When the wind is blowing, the effective load on the diesel(s) is reduced and, if the wind machine is large enough, the diesel(s) may be shut off altogether. The result will be significant but not necessarily very large fuel savings relative to what there would be with no wind turbine in the system. While this approach might seem like a good idea, in reality the situation is not that easy. When large fuel savings are desired, the system must generally contain

Wind-diesel system options

9

a variety of additional components and control systems. Conversely, it must be borne in mind that one of the main concerns is nearly always the reduction of total costs for generating the electrical power. Thus the value of the fuel savings must not be overshadowed by the capital cost of the system. The pattern of wind power availability The pattern of wind power availability has a dramatic impact on the overall economics as well as the design details of any wind-diesel system. The most significant aspect of this pattern is the variability of wind, which occurs on various time scales, ranging from 4 long term' changes (hourly to seasonally) to 'short term' turbulent fluctuations (seconds to minutes). In considering long term variability, it can frequently be desirable from an economic point of view if the long term wind speed patterns are fairly similar to those of the load. For example, a site which has high winds as well as a high load in the winter may be a better candidate for a wind-diesel system (all other things being equal) than one in which the winds are highest in seasons of lowest load, such as on some resort islands. While the correlation between long term wind power and load may significantly affect the economics of a system, it is the short term fluctuations which have the greatest impact on the system design. Most of the problems with the simple wind-diesel combination are traceable to this level of variability in the wind. The rapidly varying wind speed results in correspondingly drastic changes in wind power. This means that the amount of power which can be depended upon over a moderate time interval (an hour, for example) can be much less than the average over that interval. Because of the random nature of wind fluctuations, absolute minimum dependable power levels cannot be guaranteed, but probabilities of exceedence and confidence levels are useful concepts in this regard. Power variability is highly site dependent. It is affected primarily by the local turbulence, which in turn is influenced by the terrain (eg, hills and mountains), the surface conditions (eg, presence of trees), and the climatic conditions. Power variability may also be affected by the type of wind turbine used. Regardless of the cause or the magnitude, the basic point remains the same: the wind power available from a single machine may vary drastically from one minute to the next, even while the average remains relatively constant. The severity of the problem will decrease if a number of wind turbines are used, but it will not disappear. The pattern of diesel loading Because the wind power varies so much, the diesel power must also vary in a complementary way in order to meet a relatively steady load. As long as the maximum instantaneous wind power is less than the load, operational problems should be minimal. The wind turbine will act as a negative load. Thus what the diesel 'sees' is the load less the available wind power. In addition to the variability of the wind, the load itself may

10

Wind-diesel system options

also change dramatically over a short time interval. With progressively smaller systems, the switching on or off of any single item will have an ever more noticeable effect. When the available wind power can exceed the load, the control situation becomes significantly more difficult, as is discussed below. Fuel consumption The impact of the wind turbine on diesel fuel consumption is also not as obvious as it might at first seem. The simplest way to estimate fuel savings would be to determine first how much fuel is required to produce a kWh of electrical energy under rated conditions of the diesel. One might then assume that for every kWh produced by the wind turbine there should be a pro rata drop in diesel fuel consumption. This is not the case, however, because the efficiency of the diesel generator decreases at decreased loadings, causing more fuel to be consumed for a given amount of energy produced. To a first approximation, fuel consumption at no load is 15-30% of the full load value, and the relationship for intervening loads is linear. Note that the no load fuel consumption to rated fuel consumption ratio is engine dependent, with the larger values corresponding to smaller engines. A decrease in power generation will indeed result in a reduction in fuel use, but it is typically closer to 2/3rd of the amount that would be calculated in the simple way described above. The actual fuel saving at decreased loadings depends on such factors as the size, type, and age of the diesel engine. An additional complication is that some engine manufacturers recommend that diesels should not normally be allowed to operate for long periods below some minimum acceptable power level (typically 40 per cent of rated power). This may mean that at times not as much of the available wind power could be used as might be expected. Improvements can be expected in systems with multiple diesels where the ability to switch diesels on and off allows for greater flexibility and possibly greater fuel savings. However, the operational control strategy must be carefully designed to reap the benefits of this option. Furthermore, such strategies must take into account the warm-up time that may be required for the engines. Diesel engine starts and stops If the instantaneous wind power is greater than the load it might be thought desirable to turn off the diesel, thereby saving more fuel, although this would require some modifications to the system architecture. However, unless the load is always less than that supplied by the wind turbine, the diesel would not be able to stay off for long, because the wind power would on occasion drop to less than the load. The effect of trying to stop the diesel whenever the wind power is above a certain level is to create an operating strategy in which the diesel cycles on and off very frequently, possibly hundreds of times in an hour. Such a strategy has little to recommend it. It would put excessive strain on the engine and the starter motor, possibly resulting in much shorter component lifetimes. The rate of cycling could be

Wind-diesel system options

11

reduced by an appropriate start-stop strategy, but this would be at the cost of increased fuel use. In practice, the desirability of maintaining a minimum run time also complicates the issue. It should also be noted that rapid load changes, especially in small grids, may have a similar, and at times more serious, effect than that of the fluctuating wind power. Use of dump loads When the maximum instantaneous wind power is greater than the load (less the minimum acceptable diesel power if the diesel is on), some operational problems may appear. In this case it is at least necessary to provide a method of dissipating the excess power. A number of methods have been advanced to deal with this excess power (in a system with no storage). They are basically of two types: power control and machine control. The first type includes so-called 'dump loads'. These usually comprise resistors which may be controlled by frequency or power sensitive relays or some other form of power converter. When possible, the dumped power is put to some secondary use, such as space or domestic hot water heating. The second method involves power regulation of the wind machine itself. Two of the techniques that have been considered are blade pitch control and generator and hence speed control. The latter can for instance be achieved on machines with synchronous generators by adjusting the field excitation. Use of mechanical systems such as pitch control which is only possible with some wind turbines may also have to be accompanied by a control system capable of a response rapid enough to accommodate drastic power changes, ie a dump load.

ENERGY STORAGE - THE ADVANTAGES AND DISADVANTAGES To minimise the frequent start-stop cycles which would be associated with turning the diesel on and off it may be advantageous to use some kind of short-term energy storage. In addition such storage may be used to ensure continuity of supply or to improve frequency stability. There are four types of short-term storage which show promise for this purpose. They are: batteries, hydraulic accumulators, flywheels, and hydrostatic (pumped) storage. These are described in more detail below. Batteries This type of storage system includes storage batteries, usually lead-acid traction units, together with a rectifier and inverter for power conditioning. So far, this has been the most common type of storage system for wind-diesel applications. (Other types of batteries are discussed in Chapter 4.) Battery technology has the advantage of being well proven, although not in this context where they are being tried out. Batteries are readily available in most parts of the world. The disadvantage is that they are best suited to longer term energy storage, of the order of hours or more. What is really needed to minimise start-stops is a store which contains relatively little energy (a few minutes duration of the rated load), but which has a high power transfer rate. Batteries are not ideally suited to this task. They can be used for short term storage, but in order to keep

12

Wind-diesel system options

the power transfer rates to and from each battery down to acceptable levels, and to maximise battery bank lifetime, a relatively large number of batteries will generally be needed. This can result in high cost. The associated cost of the power converters (for ac systems) will also significantly increase the price of the complete system. Factors which must be considered when batteries are to be used include lifetime, methods for monitoring the state of charge, in-out efficiency, equalisation charging, and maximum charging rates. Lifetime is to a large extent a function of the design and construction of the battery, but in any case it is also strongly dependent on how deeply the battery is regularly discharged. Ensuring a reasonably long life requires a method of assessing the state of charge. This cannot be measured directly. Rather it must be inferred, usually on the basis of measurements of voltage and currents, and the battery's charge/discharge history. The battery must also be charged periodically to higher than a normal level ('equalising charge') to enhance its longevity. One important restriction on batteries is the maximum charging current they can accept. This amount decreases as the state-of charge increases. A 'rule of thumb' for lead-acid batteries is that the charging current in amps should never exceed the ampere hours that are left to be filled. This is important for wind-diesel systems because it means that as the battery level becomes full it can take progressively less of the excess wind power that may be available. Thus some other mechanism must be provided to reduce or dissipate that energy, such as a dump load. Hydraulic/pneumatic accumulators Hydraulic/pneumatic accumulators are devices which allow energy to be stored as compressed gas in a pressure tank and recovered later when needed. The process is facilitated by use of a hydraulic pump/motor which is used to transfer the energy to and from the gas. These are described in more detail in Chapter 4. A prototype system has been installed in Machynlleth, Wales and is described by Slack and Musgrove (1987, 1988). Flywheels Flywheels are storage devices which are particularly well suited to high power applications, but not for holding large amounts of energy. Conceptually, then, they should be a good match to the requirements of wind-diesel systems for smoothing out the turbulence induced power fluctuations. Two types of flywheel storage have been applied to wind-diesel systems and are discussed by Infield, et al. (1988) and Davies, et al. (1987). In the first, and simplest, case the flywheel is attached to the shaft of the diesel's synchronous generator. When a clutch is used, the flywheel must be on the generator side. A power dissipator, such as a dump load, is also generally included for power fluctuations which are too great for the storage to absorb. The system works in the following way: when the wind power exceeds the load by some specified amount, the diesel's generator is disconnected from the engine, which

Wind-diesel system options

13

is assumed to then stop. The synchronous generator and flywheel continue to spin. In order to use the rotating inertia for short term storage, the rotational speed of the dieseFs generator (and hence the grid frequency) is allowed to change within a certain range. The synchronous generator and flywheel accelerate as energy is absorbed and decelerate when energy is delivered back to the system. Generally in this arrangement the flywheel can store the energy equivalent of up to a minute's operation of the wind turbine at rated power. Note that in this scheme one function of the synchronous generator is to supply reactive power to the network (when a wind turbine fitted with an induction generator is used), whether or not the diesel is operating. Thus it must continue spinning at all times if the wind turbine is to continue operating. An advantage of this configuration is that the flywheel can be readily incorporated as a clutch needs to be installed in any case to uncouple the diesel engine from the generator. The disadvantage is that the frequency must be allowed to vary and this may not always be acceptable to the electrical equipment making up the load. Furthermore, space limitations may prevent a flywheel being installed between the diesel engine and its generator. An additional consideration is that special provisions may also be needed to start up a diesel which is coupled to a relatively high inertia flywheel. A second type of flywheel storage which is still at the experimental stage uses a separate flywheel driven by an a.c. motor which operates asynchronously. By allowing large speed variations the amount of energy which can be removed from a given size of flywheel increases dramatically. This is the main advantage. This configuration consists of an ordinary induction generator, the flywheel, and a variable speed regenerative a.c. motor drive. In this system the grid frequency does not have to vary for the stored energy to be used. Disadvantages include the introduction of added complexity and increased cost associated with the power electronics of the motor drive. Pumped storage With this type of storage excess power from the wind turbine(s) is used to pump water into an elevated storage reservoir. When power is required by the load, the stored water passes back from the reservoir through a water turbine, which in turn drives an electric generator. This system has the advantage of including a very controllable storage medium from which energy can be extracted at a high rate. The capital costs are quite high and there are not many locations where such systems could be built. A disadvantage is the overall inefficiency since energy is lost when the water is pumped and again when it passes back through the turbine. Additional losses occur in the piping through which the water must pass. Such a system is in operation on the Island of Foula, in the Shetland Islands of Scotland (Somerville and Stevenson (1987)).

14

Wind-diesel system options

LOAD MANAGEMENT Load management involves adjusting the load rather than the supply, to take best advantage of the available wind power. The possibilities for load management are very dependent on characteristics of the demand at the specific site and these must be examined thoroughly. To be applicable load management requires that some loads be deferrable in time. Heating loads, which incorporate thermal storage are ideal for this. Other applications which are not time critical include water pumping, purification, ice making etc. Load management is discussed in more detail in Chapter 4. The main advantage of load management is that it provides an attractive alternative to regenerative energy storage. The disadvantages are that it cannot always be used and additional capital equipment may be involved. The system will invariably require load control electronics which introduce additional complexity.

ELECTRICAL GENERATOR OPTIONS The types of generators that may be considered for wind-diesel systems include: a) synchronous generators, b) induction generators and c) direct current (d.c.) generators. In some cases they may be combined with power electronics to allow variable speed operation. These different generators and their applications are described in more detail in Chapter 4. As a very broad generalisation most, but by no means all, simple, standalone diesels are fitted with synchronous generators whereas most wind turbines, which are designed primarily to integrate with existing grids, have induction generators.

SYSTEM ARCHITECTURE OPTIONS As shown in Figure 1.1a generalised wind-diesel system consists of the following major components: *

One or more wind turbines.

*

One or more diesel generator sets.

*

A consumer load.

*

An additional controllable or dump load.

*

A storage system.

*

A control unit (including load management).

Actual systems depend on circumstances. One way to describe a scheme is shown in Table 1.1, and is based on a four part classification adapted from Manwell and McGowan (1988). A number of wind-diesel systems from throughout the world which are listed in Table 1.2 have been categorised according to this classification in Table 1.3.

Wind-diesel system options

Wind energy

15

Control Unit

Diesel fuel

Diesel Generator (S)

Wind Turbine Generator (S)

L

Storage Unit

r* -i I Additional i I Controllable I L ad

""__J L__ ° __JJ

Consumer Loads

Fig 1.1 Schematic of generalised wind-diesel systems

Table 1.1 System classification method Wind turbine generator type I II III

Induction Synchronous Induction or synchronous with a.c/d.c./a.c. power conversion

System power control1 A B C D E

None Dump load Storage Load management Turbine rotor

1 2 3 4 5 6

No storage Battery Flywheel Hydraulic/pneumatic Pumped storage End use

a b c d

One WTG/one diesel One WTG/multiple diesels Multiple WTGs/one diesel Multiple WTGs/multiple diesels

Storage

Configuration

Table! .2 List of various real wind-diesel systems Country Australia Brazil Canada ••

Location

Oper. Dates

Rottnest Island 985Fernand de Noronha 19861987AWTS 1986Calvert Island 1987Cambridge Bay Ft. Severn 1985Cape Verde Sal Island 1985•• 1987Sta. Catarina 1987Tarrafal Denmark 1984Riso France Domaine de Las Tours 1987Germany 1987Helogoland Schnittlingen 19831984Greece Kythnos Island Cape Clear Ireland 1985Inis Oirr 1981 Italy Calbria 1986Netherlands ECN 1982Norway Froeya 1989Spain Bujaraloz 1986Sweden Askeskar 1984" Chalmers University 1982Switzerland Martingy 1985UK Fair Isle 19821987Falkland Islands •• 1982Lundy Island " Machynileth 1986•* RAL 1983•i Shetland Islands 1988U.S. Block Islands 1979-1982 Clayton, N. M. 1977-1982

WTG Size (kW) 20, 50, 55 2-5 37.5 2-3 4-25 60 55 55 30 55 10-12 1200 11 5-22 2-30 1 -63 20 2-30 55 25 18.5 22 160 55 10 55 15 16 750kW 150 200

Diesel Size (kW) 100 50 2-50 2 4:380 - 760 85,125,195

Load Range (kW) 90 - 460 200 max 0-100 0.5-3.5 2375 max 50-150 30-90 11-45

Storage

Yes-batteries Yes-batteries Yes-batt./fly Yes-batteries No No Yes-batteries Yes-batteries 125 Yes-water 70 Yes-batt./fly 35 100 max 152 No Yes-water 1000-3000 2-1200 1 -15 No 25 31.4 Yes-batteries Yes-batteries 15-100 65 1-12,1-26,1-44 No Yes-batteries 2-20 Yes-batteries 50 50 15-50 Yes-batteries 50 Yes-water 16 Yes-batteries 8.1 Yes-batt./fly 20 60-80 No 130 No 1-20,1-50 Yes-batteries 10 3-6,1-27 No Yes-hydraulic 10 7 Yes-flywheel 30MW No 1800 max No 1-225,400,500 No 1-400,1700;2-1000; 3-1250 1000-3500

Load Control No No Yes Yes No No Yes Yes Yes Yes No No Yes Yes No No No Yes No No No No Yes Yes Yes Yes Yes No No No

Notes

w/PV

Biogas fuel input

Table 1.3 Architecture classification of various real wind diesel systems Country Australia Brazil Canada •• ••

Location

Rottnest Island Fernand de Noronha AWTS Calvert Island Cambridge Bay Ft. Severn Cape Verde Sal Island Sta. Catarina •• Tarrafal Denmark Riso France Domaine de Las Tours Germany Helogoland •• Schnittlingen Greece Kythnos Island Ireland Cape Clear Inis Oirr Italy Calbria Netherlands ECN Norway Froeya Spain Bujaraloz Sweden Askeskar Chalmers University Switzerland Martingy UK Fair Isle n Falkland Islands Lundy Island Machynileth RAL M Shetland Islands U.S. Block Islands Clayton, N. M. M

Wind Turbine Generator Type 1 1 III 1 II III III 1 1 III II 1 1 II III III 1 II III III 1 II III 1 II II II II II

System Power Control C C B C A A B C D,B B E E B.D B,E C,E D B,C B B,C B B,C B,C B B.D C B,D C B A A A

Storage

Configuration

2 2 2,3 2 1 1 6 2 6 2,3 1 6 1 2 2 1 2 2 2 6 2,6 2,3 1 6 2 6 4 3 1 1 1

d c b c d b

a a a a c b a c c b b c

a a a a a b a b

a a b b b

Wind-diesel system options

18

However, as summarised by McGowan, Manwell, and Connors (1988), another approach to categorising systems is by their basic architecture or configuration. This approach is being adopted by the American Wind Energy Association (1989). Examples of common wind-diesel system types according to this classification are as follows:

Battery storage/cycle charge This type of system is predominately used for smaller applications. As shown in Figure 1.2, it incorporates a diesel generator set and a wind turbine, both fitted with synchronous generators. Most of the power is fed through a rectifier to charge a bank of storage batteries. The load uses either direct current directly or is supplied with alternating current through an inverter. Normally the diesel gen set is only used to charge the batteries and not to supply the load directly. When it operates, it runs near rated power and for a substantial period of time. Its fuel efficiency and wear rate are therefore kept near to the optimum. Other advantages of this type of system include simplicity, reliability and the use of proven components. A major disadvantage is the added cost of having batteries and the reduction in efficiency caused by cycling the energy through the batteries and the power electronics. Such systems are most commonly applied where the chief concern is reliability of the power supply and not where the cost of energy is the primary consideration. A substantial number of systems of this type have been installed in remote telecommunications stations.

Voltage Regulator

Wind turbine

Dump Load

H

DC Load Dist.

Synchronous Generator

System Controller

Diesel Control Voltage Regulator

DC/AC Inverter Synchronous Generator Diesel

Battery Bank

AC Dist

i

Rectifier

Aux. AC loads

Fig 12 A cycle charge wind-diesel system

Auto Transfer Switch

AC Load Dist.

Wind-diesel system options

19

Basic system with no storage This system uses a conventional diesel engine with synchronous generator to supply the consumer demand. It is augmented by a wind turbine, which essentially acts as a negative load. If there is excess power from the wind turbine it may be shunted to a resistive secondary or dump load or otherwise dissipated. The diesel engine is not allowed to stop. When the wind turbine is small in comparison with the load this type of system has the advantage of being relatively simple. On the other hand, when the wind turbine is large, the fraction of useful energy, relative to the total generated energy, may be relatively small. However, total fuel savings might still be significant. Examples of this type of system have been installed at a hotel in Sal, Cape Verde (Hansen, Madsen, and Lundsager, 1986) and at a sewage plant at Martigny, Switzerland which uses biogas to operate the diesel generator. Basic system with flywheel This type is similar to the basic system with no storage, except that the diesel engine is separated from the synchronous generator by a clutch, and a flywheel is connected to the generator. When there is surplus wind power the diesel is stopped and disconnected from its generator. The latter continues to run as a synchronous condenser, providing reactive power to the wind turbine's induction generator. The flywheel/generator's rotating inertia compensates for the wind turbine's power fluctuations and can be used to start-up the diesel when it is needed. In this system the flywheel stores the energy equivalent of between a few seconds to a few minutes of operation at rated power. Examples of this approach are the experimental systems at the Netherlands Energy Research Foundation, ECN (De Bonte, et al.y 1985) and the RIS0 National Laboratory in Denmark (Madsen and Greisen, 1987, Lundsager and Norgaard, 1988). Basic system with batteries This system is the same as the basic system with no storage except that storage batteries and associated power converters are also included. A rectifier is used to charge the batteries when there is excess a.c. power from the wind turbine. A conventional line commutated inverter supplies alternating current back to the grid from the batteries when there is a drop in wind power. The addition of the batteries allows some power to be used which would otherwise be dumped. Basic system with flywheel and batteries A system installed on Cape Verde as part of a United Nations/DANIDA project has been configured to include both mechanical and electrical storage (Hansen, Madsen and Lundsager, 1986).

Wind-diesel system options

20

Dump load control

Consumer Load

Wind turbine

Dump Load

Induction Generator

Synchronous Generator

Diesel Diesel control

Frequency control Synchronous Generator

MicroProcessor

I

Accumulator Buffer Cylinder

Pump/ Motor

Pressure control Displacement control

Fig 13

. J

An hydraulic accumulator wind-diesel system

Basic system with hydraulic accumulator As mentioned earlier and described in more detail in Chapter 4 and by Slack and Musgrove (1987) this type of system includes an hydraulic accumulator to provide short term, high-power energy storage. A schematic diagram of this type of system is shown in Figure 1.3.

Wind Turbine

Voltage Regulator

Generator

•*• Dump Load

Inverter

Rectifier

1

I Battery Voltage Regulator

Diesel

Synchronous Generator

Fig 1.4 An integrated wind-diesel system

Consumer

Wind-diesel system options

21

Integrated wind-diesel system In such integrated systems, various components mentioned in the above systems are amalgamated, so that they can no longer be regarded as discrete units. An example of such a system is the integration of a battery bank into the power electronic conversion system of the wind turbine. As illustrated in Figure 1.4, a system can be designed in which the power from the wind turbine can feed the load directly or alternatively the current can be temporarily diverted and stored in batteries. Although the batteries are fed from an a.c. machine and in turn feed an a.c. load, they do not require their own dedicated rectifier/inverter power electronics as they can make use of those of the wind turbine conversion system. Development of systems of this type has been carried out by the Chalmers University in Sweden (Linders et al.9 1987) and by the Netherlands Energy Research Foundation, ECN(DeBonte,