Wind energy conversion an introduction

P.J. Musgrove, B.Sc.(Eng.), Ph.D., C.Eng., M.R.Ae.S.Indexing terms: Wind power, Energy conversion and storage

Abstract: The global wind-energy resource is very large and widely distributed; and, within Europe, windenergy has the potential to provide an energy output equal to about three times the present electricity consump-tion. Although the wind is not very reliable as a source of power from day to day, it is a reliable source ofenergy year by year, and the main role for future wind-energy systems will be operating in parallel withelectricity grid systems or, in remote locations, in parallel with diesel engines, so saving fuel. Systems integrationstudies indicate that existing utility grid systems could accept a contribution of about 20% from wind turbines,although, with changes to the future plant mix, the potential contribution is substantially greater: and similarpercentage fuel savings are possible in remote locations with wind/diesel systems. Recent progress in the devel-opment of wind turbines is reviewed and the cost data now becoming available indicates that medium-sizedmachines, i.e. ~ 20-40 m diameter and with power ratings in the range 50-500 kW, offer the most attractiveeconomics for land-based applications in the near future, giving energy costs in the range 2.8-5.6 p/kWh, for atypical site where the annual average wind speed is 5.5 m/s (measured at the normal 10 m height); in windierlocations energy costs will be lower. Corresponding capital costs for installed wind turbines are in the range750-1500/kW (with average outputs equal to about 30% of the rated). The UK, in common with some othercountries, has a large offshore wind-energy potential, but, to be economically competitive, offshore systems willneed to use multimegawatt wind turbines with diameters of 100 m and larger. Prototype machines in this sizerange already exist, but considerable further development is needed before the construction of large offshorewind turbine arrays can commence, although this is a realistic prospect for the 1990s. The economics of wind-energy conversion systems are already encouraging, and commercial applications already in evidence, mostnotably in the USA and Denmark where more than 2000 wind turbines with a total installed capacity in excessof 150 MW have been installed in the past two years. However, further operational experience is required todemonstrate that reliable operation can be sustained over periods of many years. As this experience is accumu-lated, and as the cost benefits associated with quantity production are achieved, the market for wind turbinescan be expected to expand rapidly.

1 Introduction

Windmills have been used to harness the energy in thewind for more than a thousand years, probably muchmore, and the earliest clear evidence of their use is to befound in the Middle East and Afghanistan. By the MiddleAges, windmills were in widespread use around the Medi-terranean, and the type of design which we now regard asthe traditional Dutch windmill gradually evolved. In theirheyday, at the end of the 18th century, about 10000 ofthese windmills were in use in the Netherlands, with asimilar number in use in Britain. Many significant contri-butions to the development of the traditional windmillwere made in Britain, and the 18th century researches ofJames Smeaton [1] are particularly notable: Golding [2]includes a summary of the major steps in the developmentof the traditional windmill in his now classic book.

The traditional windmill most commonly had fourblades which, in their simplest form, as shown in Fig. 1,provided a lattice framework over which the miller couldspread the canvas sails. In light winds, the whole bladearea would be covered, but, in strong winds, the poweroutput could be limited by only covering part of theblades. With a diameter, typically, of about 25 m, the tradi-tional windmill could deliver a shaft power output ofabout 30 kW in a wind speed of about 7 m/s (force 4): in awell exposed location, it would give an average poweroutput of about 10 kW, corresponding to an energy outputof about 100 kWh per working day.

For many centuries, the only practicable alternative(unless one lived alongside a substantial river, whichcould provide water power) was the muscle power outputPaper 2912A. first received 21st October and in revised form 31st October 1983The author is with the Department of Engineering, Reading University, White-knights, Reading RG6 2AY. England

of people or animals. But a man needs to work hard allday to deliver 1 kWh of useful work. The traditional wind-mill could, therefore, do the work of a hundred men, ormore than a dozen oxen or horses; hence its importancefor so many centuries. However, steam engines becameprogressively more efficient and more economic as the19th century advanced, and, because these could alsoprovide power on demand, the use of windmills went intodecline. This decline was hastened by the later develop-ment of internal combustion engines and the growth, inthe early 20th century, of electricity supply systems.Although these other sources of power were dependent onthe availability of fossil fuels; first, coal and wood, then, oiland gas; these became progressively more readily availableand (in real money terms) less costly.

Since 1973 this trend of declining fuel prices has beensharply reversed, and it is now accepted that the era ofvery cheap fuel has ended. Many countries have conse-quently allocated substantial funds, over the last decade, tothe study of alternative and/or renewable energy sources,including wind energy, wave energy, direct solar radiation,tidal energy and geothermal energy. And, although windenergy was not initially regarded as having any particularpromise (except perhaps by a very small group ofenthusiasts), it has now emerged as one of the most prom-ising, if not the most promising, of the renewable energytechnologies; see, for example, the recent comparativeassessment [3] of renewable energy sources published bythe UK Department of Energy.

However, it is important to recognise that, despite thevery substantial fuel price increases experienced in the lastdecade, energy costs are still relatively low when seen in anhistoric perspective. One can still purchase the energyequivalent of a man working hard all day, in the very con-venient form of a kilowatt-hour of electricity supplied to

506 IEE PROCEEDINGS, Vol. 130, Pt. A, No. 9, DECEMBER 1983

one's home, office or factory, for just a few pence. Theoutput of a traditional windmill, valued on this basis,would be insufficient even to pay for the attendant's wages,

Fig. 1 Traditional windmill

and would give no surplus to provide a return on thewindmill's capital cost. If the use of wind energy is toreturn on a significant scale in the future, as many who areactive in the field now believe, one needs designs that aremuch more cost effective than the traditional windmill.Moreover, societies today require power on demand, andso one must consider carefully how to integrate the poweravailable from an intermittent source such as the wind.One must also determine whether wind energy can make awidespread and substantial contribution to present dayenergy needs, or whether its role is a limited, localised one.It is also important to ensure that the means for harness-ing wind energy are themselves environmentally accept-able.

2 The wind-energy resource

Wind energy is an indirect form of solar energy, as thewinds result from the fact that the Earth's equatorialregions receive more solar radiation than the polarregions, and this causes large-scale convection currents inthe atmosphere. The detailed processes are, inevitably,complex, but the recent WMO report [4] on meteorologi-cal aspects of wind energy provides a useful introductorydiscussion. The total amount of solar energy receivedannually by the Earth is extremely large. For example, theworld's reserves of fossil fuel, i.e. the coal reserves, whichgiven present trends of consumption can be expected to

last for about 300 years, plus the oil and gas reserves(which, given present trends of consumption, can beexpected to last for about 40 years) have a total energycontent equivalent to that received by the Earth as solarradiation in only 10 days. A small proportion of the inci-dent solar power, estimated to be about one per cent, isconverted by atmospheric convection processes into windpower, and one per cent of this (i.e. about one hundredthof one per cent of the incident solar power) is equivalent tothe power presently provided world wide by burning fossilfuels. The practical problems of harnessing the power inthe wind on anything approaching this scale are, of course,enormous, but, as the technology of wind-energy conver-sion systems develops, it is encouraging to know that theresource is so very large. And although global estimates ofthe wind-energy resource have relevance, it is important tomake more detailed and localised assessments. Such astudy has recently been undertaken within the EuropeanCommunity [5], and concluded, even after making allow-ance for the many siting constraints that exist in practice,that there were sites in Europe for about 400 000 megawattscale wind turbines; enough, in principle, to provide up toabout three times Europe's present electricity consump-tion, and equivalent to an oil output (nondepletable) ofabout 16 million barrels per day.

Although the wind is not a reliable source of powerfrom one day to the next (a fact which strongly influencesthe way that the output from wind turbines is used, as isdiscussed later), it is a much more reliable source of energy,year by year. The annual energy in the wind at a givenlocation depends on the wind-velocity-duration distribu-tion, which, in general, can be expressed mathematically asa Weibull function, which involves two parameters, i.e. ashape parameter and a characteristic speed, see Reference4. The Rayleigh distribution, which is a special case of theWeibull distribution when the shape parameter is equal to2, is a simple single-parameter function which is nowwidely used to describe the wind (especially when detailedsite-specific long-term wind data is unavailable). The Ray-leigh distribution provides a reasonable approximation tothe wind-velocity-duration distribution over flat terrain inWestern Europe and elsewhere, and states that the prob-ability p, that the wind speed exceeds a certain value V, isgiven by

(1)

where Vm is the annual average wind speed. This equationindicates that wind speeds in excess of Vm can be expectedfor 46% of the year, whereas wind speeds above 2.4 Vmoccur for less than 1 % of the year.

At any given wind speed V the power in the wind P perunit area (measured perpendicular to the wind direction) issimply the product of the kinetic energy per unit massy^2,and the mass flow rate pV, where p is the air density; i.e.

P e r u r n t a r e a (2)Given a Rayleigh distribution of wind speeds, it can thenbe shown that the annual average power in the wind isequal to 0.95pV^. In the absence of more detailed infor-mation, for a particular site, it is usual to assume p =1.225 kg/m3. Given a typical European near-coastal loca-tion, one can expect Vm ~ 5.5 m/s at the standard meteoro-logical height of 10 m. This then gives an annual averagepower density of 190 W/m2, corresponding to 1700 kWh/m2 annually.

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The retarding effect of the Earth's surface results in aboundary layer which is typically several hundred metresdeep, through which the wind speed progressivelyincreases with height; see, for example, Reference 4. Overlimited ranges of height, this variation of wind speed canbe represented using a simple power law, e.g.

K1 0

(3)where Vh is the velocity at height h and Vl0 is the velocityat h = 10 m; the magnitude of the exponent n depends onthe terrain roughness, but for flat open country n isapproximately one-seventh. Large modern wind turbineswill have hub heights of about 60 m and the wind speed atthis height is, consequently, about 30% higher than ath = 10 m; as the power is proportional to the cube of thewind speed, this means that the wind-power density at thishub height is more than double the wind-power density at10 m height.

For a reasonably good site, where the annual averagewind speed Vl0 ~ 6 m/s and where the hub height annualaverage wind speed Vm ~ 8 m/s, the annual average windpower density is approximately 600 W/m2, correspondingto 5200 kWh/m2, annually. The economics of renewableenergy systems are naturally very dependent on howconcentratedor how diffusethe energy source is, and itis therefore worth noting that power densities of this mag-nitude compare favourably with the annual average solarradiation power densities, which, in Britain and NorthernEurope, average approximately 100 W/m2. Moreover, inEurope most solar radiation is received in the summermonths, when the need for energy is least. By comparison,the seasonal availability of wind energy in Europe correl-ates closely with the seasonal demand for energy, withhigher wind speeds in the winter; and this positive correla-tion significantly enhances the value of wind-energysystems.

It must also be noted that average wind speeds mayvary significantly over relatively short distances; e.g. loca-tions only a few kilometres apart may experience wind-power densities that differ by an order of magnitude, if onehas good wind exposuresuch as on a hill topand theother is sheltered by trees or buildings or in a valley.Wind-turbine site selection should, therefore, be under-taken with care, and provisional site selection confirmedby wind-speed measurements made over an extendedperiod, preferably a full year. This sensitivity to the localterrain does, however, mean that one can frequently sitewind turbines in locations where the wind-power density issignificantly higher than the regional average.

3 Wind turbine characteristics

The technological advances that have taken place since theearly 19th century zenith of the traditional windmill aresuch that modern 'windmills' differ in many major featuresfrom their predecessors, and it is convenient to make thisdistinction clear by referring to present day designs aswind turbines, or aerogenerators. However, the modernwind turbine functions in exactly the same way as the tra-ditional windmill, albeit more efficiently, and their bladeshave much in common with the wings on an aeroplane.Aeroplane wings are designed to interact with the air flowpast them, in such a way as to produce a large transverseforce, called the lift, which normally just supports theweight of the aeroplane. On a wind turbine or windmill theblades are designed to produce a similar transverse force,which is used not to support the blade weight (the tower

meets this requirement) but to force the blades to rotatearound a central shaft, and in the process to deliver auseful shaft power output.

This is illustrated more clearly in Fig. 2. If, for simpli-city, one assumes that the blade is untwisted and has nopitch offset, then Fig. 2a shows the view looking radially

bladespeed

wind speed at rotor

lift L

downwindforce fd

crosswindforce fc

Fig. 2 Aerodynamic forces on wind-turbine bladeinwards along the blade of a conventional horizontal-axiswind turbine. The circumferential blade speed cor is theproduct of the wind turbine's rotational speed co and theradial distance r from the hub to the section of blade beingconsidered; Vr is the wind speed at the rotor. For modernwind turbines cor is several times larger than Vr, even morethan is indicated by the lengths of the arrows in Fig. 2a;the resultant air flow relative to the blade therefore meetsthe blade at a fairly small angle a, as shown in Fig. 2b. Aswith aeroplane wings, which have a similar aerofoil cross-section, a large transverse lift force L is produced, perpen-dicular to the relative air flow, together with a small dragforce D which is parallel to the air flow. These forces canthen be resolved into crosswind and downwind com-ponents, as shown in Fig. 2c.

The crosswind force fc = L sin a D cos a provides atorque fc r about the hub and delivers a shaft power outputfc rco as the blade rotates. Integration along the entireblade length, from hub to tip, and multiplication by thenumber of blades then gives the overall shaft poweroutput. The downwind force fd = L cos a -I- D sin aimposes loads on the blades and tower of the wind turbine,and the designer must ensure that the structure is strongenough to withstand these loads, even under severe stormconditions. Clearly, good aerodynamic design is required,so as to give a high lift and relatively little drag. But goodstructural design is even more vital, so that the loads onthe wind turbine may be resisted without failure over a

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prolonged period (more than 20 years) by a structurewhich permits low-cost construction.

Associated with the downwind force fd on the blades,there is an equal and opposite force experienced by theapproaching air flow, which reduces its momentum (and,hence, velocity) as it approaches the wind-turbine rotor.The air velocity Vr at the rotor is, therefore, somewhat lessthan the wind speed V, well upstream. It is important tonote that, as / d is increased, e.g. by increasing the speed ofrotation, or the number of blades, the ratio VJV decreases,and beyond a certain point this will lead to a reduction inthe overall power output. More detailed analyses of wind-turbine performance, e.g. References 6 and 7, indicate that,for maximum output power, fd should be just sufficient toreduce the approaching air speed to Vr ~ \ V.

The two parameters most widely used to describe theperformance of wind turbines are the tip speed ratio k andthe power coefficient Cp. The former is defined as

coR^~ V (4)

where R is the rotor radius, measured to the tip of theblade, co is the rotational speed and V is the wind speedwell upstream. The power coefficient is defined as

(5)where P is the output power, A is the rotor swept area(= nR2 for most wind turbines) and p is the air density; Cpgives a measure of the wind turbine's power output relativeto the power in the wind passing, without obstruction,through an area equal to the rotor area.

Flow similitude considerations (or dimensional analysis)indicate that, for a given wind turbine design, Cp is a func-tion of k, and the Cp/k characteristic for any wind turbinecan be calculated using the methods described in Refer-ences 6 and 7; Cp = 0 when / = 0 (as k = 0 corresponds tono rotation) and, typically, then increases to a maximum ofCp =- 0.40 to 0.45 at k ~ 6, thereafter reducing (as thedownwind force fd increases beyond its optimum) to Cp =0 at k ~ 10. The presence of the rotor, and the axial forcefd which it produces, makes the streamlines approachingthe rotor diverge, so that some of the oncoming wind flowsround the rotor, rather than through it (hence also Vr < V).It can therefore be shown that Cp = 0.45 corresponds tothe wind turbine delivering a useful power output equal to68% of the kinetic energy in the wind which actually flowsthrough the rotor. Modern wind turbines such as theBoeing MOD 2 shown in Fig. 3 are therefore fairly effi-

cient devices, even though they have a blade area (as seenby an observer looking along the axis of rotation) which isonly 5% or 10% of the rotor swept area A. The ratio ofblade area to swept area, known as the solidity, doesexceed 50% for the familiar multibladed water-pumpingwindmills, but this is only because these have to provide ahigh starting torque at a low rotational speed. High-solidity wind turbines are less efficient than low soliditymachines, and, as the latter can be made for a lower cost,they are preferred for most applications, especially elec-tricity generation.

Although traditional windmills and most modern windturbines have blades which turn about a horizontal axis,the last decade has seen considerable progress in the devel-opment of vertical-axis wind turbines. The basic Darrieuswind turbine is shown in Fig. 4, and has curved aerofoil-

Fig. 4USA

17 m diameter Darrieus wind turbine at Sandia Laboratories,

Fig. 3 Boeing Mod 2,91 m diameter, 2.5 MW rated

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cross-section blades which rotate about the central verticalsupport. Aerodynamically, vertical-axis wind turbinesfunction in a very similar way to horizontal-axis windturbines. As each blade crosses the wind, both upwind anddownwind of the tower, the velocity and force vector dia-grams are essentially the same as in Fig. 2, with each bladecontributing almost equally to the torque and, hence, tothe shaft power output. However, as each blade continuesto rotate about the central tower, the angle between the airflow and the blade decreases until, a quarter of a revol-ution later, each blade is running parallel to the wind, andthe instantaneous torque (and, hence, power) outputdecreases to zero. After a further rotation of a quarter-revolution, each blade is once again moving across the

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wind and delivering its full torque and power output. Thisfluctuating power output can be smoothed either by using3 blades, instead of 2, or by utilising the rotor inertia via acompliant torque transmission system. Theoretical calcu-lations, confirmed by field tests, indicate that vertical-axiswind turbines give power coefficients fully comparablewith horizontal-axis wind turbines. They are of courseomnidirectional, unlike horizontal-axis wind turbineswhich must be continually turned to face the wind, andtheir proponents argue that vertical-axis wind turbineshave the potential to deliver energy at lower cost thanhorizontal-axis wind turbines. However, the developmentof the relatively novel Darrieus wind turbine, and itsstraight bladed derivatives, is still at a fairly early stage,and the balance of advantage and disadvantage betweenhorizontal-axis and vertical-axis wind turbines will not beresolved before the 1990s.

High wind speeds give very high wind-power densities,but occur for only a small fraction of the year (see eqn. 1),and so contain only a small proportion of the annualenergy. It is, therefore, usual to limit the power output ofwind turbines in high wind speeds, and this may be accom-plished in horizontal-axis wind turbines by changing thepitch on all or part of the blades. The lowest wind speed atwhich a wind turbine will deliver its rated, maximumpower output is called the rated wind speed VR, and foroptimised modern wind turbines VR is usually about 1.5Vm. A higher value of VR would give a higher rated poweroutput, but this higher power level would only be pro-duced relatively infrequently, and the extra costs associ-ated with the consequential higher loads on the blades, thetower and the transmission are not usually justified by thesmall gain in annual energy output. Above a wind speed ofabout 25 m/s, which for most sites is exceeded only rarely,wind turbines are normally shut down; and, when shutdown, a well designed wind turbine will withstand windspeeds of up to 60 m/s or more.

With good design the average power output frommodern wind turbines is given by

Pm~0.25pVi (6)see Reference 8. For Vm = 5.5 m/s this gives Pm cz50 W/m2, equivalent to 450 kWh/m2 annually. ForVm = 8 m/s the average power output Pm ~ 160 W/m2,corresponding to 1400 kWh/m2 annually. These figures arerelative to the rotor swept area A; relative to the bladearea, the power densities are at least an order of magnitudelarger. These average power densities are such that calcu-lated energy recovery periods are relatively short, typicallyabout 12 months; i.e. the energy invested in the construc-tion of a modern wind turbine will be returned within thefirst 12 months of its operational life.

As the energy output from wind turbines may varymarkedly from one day to the next, in response to chang-ing wind speeds, careful consideration must be given to theway it is used. Modern wind turbines will find their majorapplications producing electricity, and operating in paral-lel with conventional power stations or, in smaller moreisolated communities, diesel engines. The coal or oil burntin the power stations or diesel engines provides the hourby hour certainty that there will be sufficient power tomeet the demand; the wind turbines contribute energy tothe system, as and when the wind blows, so that the overallfuel consumption can be significantly reduced. Recentstudies, e.g. References 3 and 9, indicate that the UK gridsystem in its present form could accept a contribution ofapproximately 20% from wind energy. Further assess-ments indicate that, with modification to the future plant

mix (e.g. by replacing with low-cost rapid-response gas-turbine generating sets some of the existing slow responsesteam-turbine power stations, as they become obsolescentover the next two decades), the potential wind-energy con-tribution could be substantially greater. Similar studies ofwind/diesel systems for isolated communities have shownthat fuel savings of up to about 50% appear feasible. Theprimary value of the energy output from wind turbines is,consequently, the value of the fuel that they save; diesel oilin small isolated communities and fuel oil and/or coal forlarger-scale applications. The cost of wind energy, whichincludes the amortisation of the wind turbines' capital cost,as well as their direct operating and maintenance costs,should therefore be competitive with the local costs ofthese fuels.

Although the generation of electricity, either for uselocally or for distribution via the grid system, willundoubtedly provide the main applications for futurewind-turbine systems, some traditional applications (and,in particular, water pumping) will continue to be impor-tant in many countries. The development of lower-costmultibladed water-pumping windmills could indeed havethe same beneficial effects in many developing countriesthat it had in the USA, before rural electrification, whenthe use of water-pumping windmills, literally by themillion, greatly assisted farming development. Water-pumping applications are facilitated by the fact that onecan compensate for day-by-day fluctuations in the wind byusing water storage tanks.

It is, of course, important that the environmental conse-quences of using wind turbines be carefully examined. Themain areas of potential concern are noise, electromagneticinterference and visual intrusion. Well designed machineswill be inaudible above the background noise level (whichitself increases as the wind speed increases) beyond dis-tances of 100-200 m. In most countries with active wind-energy programmes, it is presumed that medium or largewind turbines, i.e. with diameters upwards from 20 m, andratings above 100 kW, will not be located closer than200 m to any habitation. Infrasound, i.e. pressure waves atbelow audible frequencies, must also be considered as apotential nuisance, although only one wind turbine (theAmerican Mod 1, see the following text) has actuallycaused infrasound problems. Electromagnetic interferencecould be a problem if a large wind turbine or group ofwind turbines were placed between a residential area andthe TV transmitter serving the area. With care in siting,this type of problem can usually be avoided; if not it maybe necessary to instal a TV signal repeater. Some visualintrusion is unavoidable, because wind-turbine sites musthave good exposure to the prevailing winds, but visualimpact can be minimised by careful siting, and manymodern wind turbines are quite elegant in appearance.There is, moreover, the comforting knowledge that when,at the end of its working life, a wind turbine is dismantled,there are no continuing adverse environmental conse-quences.

4 Recent developments

In the decade since 1973, many countries have initiatedsubstantial wind-energy research and development pro-grammes, and the continuing application of modern designand manufacturing methods has led to rapid progress. TheAmerican programme has been particularly noteworthy. Itcommenced in 1973, and by September 1975 the firstmedium-scale test-bed machine, the Mod 0, was in oper-ation near Cleveland, Ohio. This horizontal-axis wind

510 IEE PROCEEDINGS, Vol. 130, Pt. A, No. 9, DECEMBER 1983

turbine had a two-bladed rotor, located on the downwindside of the tower, a diameter of 38 m, and delivered itsoutput of up to 100 kW, via a synchronous generator, intothe local utility grid system. This first prototype was suc-ceeded by four similar machines, known as Mod 0A, whichwere installed and operated in locations ranging fromHawaii to Puerto Rico between 1977 and 1983; Fig. 5

Fig. 5 Mod 0A on Block Island, USA 38 m diameter, 200 kW

shows the Mod 0A installed on Block Island, off theRhode Island coast. Although all the Mod 0A machineshad two-bladed downwind rotors each with a diameter of38 m, the same as the Mod 0, their installation in higher-wind-speed locations justified uprating their design outputto 200 kW. Three of these machines used laminatedwooden blades, while the fourth had glass-fibre reinforcedcomposite blades. All were connected to their local gridsystems via synchronous generators and operated at a con-stant rotor speed of 40 rev/min. A 45:1 ratio, speedincreasing gearbox which was located, like the generator,braking system etc., in a nacelle on top of the tower (as inthe Boeing Mod 2 depicted in Fig. 3) then gave therequired 1800 rev/min input to the 60 Hz generator. Powercontrol in high wind speeds was effected by changing theblade pitch, i.e. by rotating each blade about its lengthwiseaxis. These four experimental wind turbines accumulatedtens of thousands of hours operation, and very successfullydemonstrated that, with modern microprocessor-basedcontrollers, operation can be completely unattended, withautomatic start-up and synchronisation with the gridwhen the wind speed is high enough to give a useful poweroutput, automatic orientation of the rotor to face the wind(via a wind-direction sensor located on the nacelle and thehydraulically actuated yaw-orientation system) and auto-matic shut down when the wind speed is too low, too highor if a fault occurs. As would be expected, the economicbenefits of being able to operate unattended are very con-siderable.

The next major milestone in the American programme

was the completion, in 1979, of the 61m diameter, 2 MWrated, General Electric Mod 1, which also had a two-bladed downwind rotor. This was followed, in 1980-81, bythe construction of five Boeing Mod 2 wind turbines. This2.5 MW rated, 91 m diameter machine, shown in Fig. 3,incorporates a number of features intended to reduce theoverall weight and cost. The heavy, lattice steel tower ofearlier designs is replaced by a slender and relatively light,tubular steel tower; the blades (fabricated in steel andlocated upwind of the tower) are rigid over the central partof their span, and only the tips are rotatable so as toprovide the necessary power limitation and control in highwind speeds; the rotor also incorporates a teetered hub,which allows the plane of blade rotation to rock back-wards and forwards by a few degrees, relative to the verti-cal plane, and so alleviates the high blade root stressesassociated with wind shear and turbulence. These andother features allowed substantial weight reductions, perunit rotor swept area. Although some mechanical prob-lems have been experienced, as might be expected withsuch a major and innovative project, these are being reme-died and this design represents a substantial step forwardon the way to providing wind energy economically.

Several other multimegawatt wind turbines have alsorecently been completed, including the WTS 3 and WTS 4machines built by Hamilton-Standard (USA) in co-operation with Wind Turbine Systems, a subsidiary of theSwedish Swedyards group. Each of these machines has adiameter of 78 m, with a two-bladed horizontal-axis rotorlocated downwind from a tubular steel tower and bladeswhich are filament-wound, glass-fibre-reinforced compos-ite. Both were completed and installed in 1982, the 3 MW-rated WTS 3 in southern Sweden, and the 4 MW-ratedWTS 4 in the USA. A second Swedish multimegawattwind turbine, built by the Swedish company KaMeWa incollaboration with the German company ERNO, has atwo-bladed horizontal-axis rotor which is upwind of itsconcrete tower, a rotor diameter of 75 m, steel blades anda rated output of 2 MW; this machine was installed onGotland, an island to the east of Sweden, in the autumn of1982. Then, in early 1983, the 3 MW-rated 100 m-diameterGrowian wind turbine was completed in northernGermany. MAN were the main contractors for this two-bladed horizontal-axis wind turbine, which has a down-wind rotor and blades which have steel spars for strength,surrounded by glass-fibre-reinforced composite fairings togive the required aerofoil shape.

As the above descriptions indicate, a considerablevariety of options are still being explored by wind-turbinedesigners, e.g. whether to locate the rotor upwind or down-wind of the tower, whether or not to use a teetered hub,what material to use for the blades and for the tower,whether to grid connect using a synchronous generator oran induction generator or some form of variable speed butfixed frequency generation etc. And, although the windturbines just described all have two-bladed rotors, a goodcase can also be made for using three-bladed rotors. Thetwo 630 kW-rated 40 m-diameter machines built inDenmark, and in operation since 1980, see Fig. 6, are boththree bladed, and the proposed 60 m-diameter Danishmegawatt-scale machine also has three blades. MAN havealso designed a successor to their 100 m-diameter, two-bladed Growian, which will have three blades and a diam-eter of 56 m. Single-bladed wind turbines are alsopracticable, and a number have been built including, mostrecently, the Monopteros shown in Fig. 7, which was com-pleted by MBB in North Germany, in late 1981, and isnow undergoing test. The single blade of this machine is

IEE PROCEEDINGS, Vol. 130, Pt. A, No. 9, DECEMBER 1983 511

24 m long, and carbon-fibre composite provides its mainstructural strength.

The development of large horizontal-axis wind turbinesis continuing, and in the UK the Wind Energy Group(comprising Taylor Woodrow, British Aerospace and GEC)

6 40 m diameter, 630 kW wind turbines at Nibe, Denmark

is being funded by the Department of Energy and theNorth of Scotland Hydro-Electric Board (NSHEB) todesign and build a 60 m-diameter 2-bladed horizontal-axiswind turbine. This steel-bladed machine will, like theBoeing Mod 2 shown in Fig. 3, have its rotor upwind of itstower and use a teetered hub to reduce blade stresses. Itwill be sited on Burgar Hill, Orkney, one of the windiestlocations in the UK where the hub-height average windspeed is about 12 m/s, and its rated output will be 3 MW;

completion is due in the winter of 1985-86. In the mean-time, the Wind Energy Group has designed and con-structed the 20 m-diameter 250 kW-rated MS-1 windturbine shown in Fig. 8. This has its rotor upwind of thetower, and its blades have load-carrying steel spars, eachwith a surrounding aerofoil-shaped skin made from glass-fibre-rcinforccd composite; power limitation and control in

Fig. 7 MBB Monopteros, Northern Germany, with its single 24 m blade

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Fig. 8 Wind Energy Group 20 m diameter, 250 kW wind turbine,Orkney, UKhigh wind speeds is achieved by rotating the blade tips.The MS-1, which models many of the features of the WindEnergy Group's 60 m design, was completed and installedin July 1983 on Burgar Hill, Orkney, on a site close to thatdesignated for the 60 m machine. July 1983 also saw thecompletion, on Burgar Hill, of the 22 m-diameter 300 kW-rated HWP-300, designed and constructed by JamesHowden Ltd. This is a three-bladed horizontal-axis windturbine, and has blades made from epoxy-impregnatedlaminated wood; grid-connection is via a synchronousgenerator. Howden's were also responsible for supplyingand installing the American-designed 24 m-diameter 3-bladed horizontal-axis wind turbine, purchased by theCEGB and which commenced operation on a site adjacentto Carmarthen Bay power station in November 1982. Thismachine will provide the CEGB with initial experience inoperating a grid-connected wind turbine, and the CEGBhave recently announced their intention of supplementingthis by ordering a proven megawatt-scale wind turbine, tobe operational on a site adjacent to Richborough powerstation, in Kent, by the end of 1985. If initial operatingexperience is satisfactory, the CEGB than plan to install agroup of up to 10 megawatt-scale machines, to be oper-ational by about 1990. The CEGB also have considerableinterest in the potential of offshore wind-energy systems,because studies such as that reported in Reference 3 haveshown that the UK has a very large offshore wind-energypotential: large enough, in principle, to provide an outputcomparable with the present total UK electricity consump-tion. Offshore wind-turbine arrays avoid many of the sitingconstraints that would be experienced on land, and initialeconomic assessments, such as that by the Energy Tech-nology Support Unit [3], are encouraging. However, muchfurther work is needed, and the construction of offshorewind-turbine arrays is unlikely to commence before the1990s. The CEGB are contributing to the ongoing studiesin this area, in the UK and internationally, and haveoffered to host the offshore prototype machine which isbeing discussed by the International Energy Agency.

In the USA, the next major step in their large wind-turbine programme will probably be the construction ofthe General Electric Mod 5, on a site in Hawii. This 2-

IEE PROCEEDINGS, Vol. 130, Pt. A, No. 9, DECEMBER 1983

bladed horizontal-axis machine will be the world's largest,with a diameter of 122 m and a rated output of 7.3 MW. Anotable feature is that the blades will be made from epoxy-impregnated laminated wood, with relatively small rotat-able blade tips to provide power limitation in high windspeeds. Canada, which has pioneered the development ofmodern Darrieus wind turbines, like that shown in Fig. 4,has now commenced the design and construction of thefirst megawatt-scale vertical-axis wind turbine. This will bea two-bladed machine with a rotor height of 94 m and adiameter of 64 m (so as to give a swept area of 4000 m2,equivalent to a horizontal-axis wind turbine with a diam-eter of 71 m) and it will be rated at 4 MW. As withall large wind turbines, the rotational speed is low,14.5 rev/min for this machine, but the vertical-axis configu-ration gives a ground-level shaft power output whichpermits the use of a direct driven synchronous alternator,and so avoids the necessity for a speed increasing gearbox.Static frequency-conversion equipment will ensure that theoutput frequency is correctly matched to the utility. Oper-ation is due to commence in late 1985.

The development of Darrieus wind turbines is also con-tinuing in the USA, and Fig. 4 depicts the 17 m-diametertest-bed machine at Sandia Laboratories, in Albuquerque,New Mexico. The basic Darrieus configuration does,however, have the disadvantage that one cannot readilycontrol the power output in high wind speeds, e.g. bychanging the blade pitch, as in a horizontal-axis windturbine. For this to be effective in a Darrieus wind turbine,the blade pitch would have to be varied cyclically everyrevolution, which would introduce considerable mechani-cal complexity. An alternative and simpler method of con-trolling the power output of a vertical-axis wind turbine isto change the blade inclination to the vertical, see Mus-grove [10], which reduces the angle of attack between theblades and the air flow (analogous to changing the pitchon the blades of a horizontal-axis wind turbine), as well asreducing the effective rotor-swept area. The developmentof this variable-geometry vertical-axis wind turbine is oneof the two principal activities within the UK Departmentof Energy's wind-energy programme. The design of the130 kW-rated 25 m-diameter prototype shown in Fig. 9has been completed, by a consortium led by Sir RobertMcAlpine and Sons Ltd., and construction is now inprogress. Operation will commence in early 1985, at aSouth Wales coastal site adjacent to Carmarthen Baypower station, just a few hundred metres from the CEGB'sexisting 24 m-diameter horizontal-axis wind turbine. This25 m-diameter prototype Musgrove wind turbine modelsmany of the features of a 100 m-diameter 4.4 MW versionintended for subsequent use offshore.

Although the emphasis in most national wind-energyprogrammes has been on the development of megawatt-scale wind turbines, considerable effort has also gone intosmaller machines. The development in Denmark of windturbines in the size range 10-15 m diameters has been par-ticularly notable, and Fig. 10 shows a typical Danishmachine, the 15 m-diameter 55 kW-rated Nordtank. Thisis designed for use on farms, connected to the grid via aninduction generator. When the wind blows, the farmer usesthe electricity from his wind turbine to reduce his pur-chases of electricity from the utility. And, when the windturbine's power output is in excess of the farmer's ownneeds, the surplus is purchased by the utility at a buy-backprice, per kilowatt-hour, which is about half the utility'sselling price. When there is no wind, the farmer buys all hiselectricity from the utility in the usual way. Encouraged bya 30% government subsidy on the initial cost of the wind

turbine, more than a dozen manufacturers in Denmarknow offer wind turbines similar to the Nordtank, and totalsales now exceed 200 per year. With an installed cost [8]of approximately 600/kW, these small wind turbines yielda simple payback period of approximately 5 years (afterallowing for the 30% subsidy), for locations where the

Fig. 9 Musgrove 25 m diameter vertical-axis wind turbine, UK

annual average wind speed is about 5.5 m/s, provided thatthe annual costs of operation and maintenance (O & M)are no more than 2% of the capital cost: if annual O & Mcosts are 5% of the capital cost, the payback period wouldincrease to about 6 | years. Data on actual O & M costsare still being accumulated, but the evidence so far indi-cates that the range from 2% to 5% per annum is a rea-sonable expectation for the better machine designs. Similarwind turbines are now being manufactured in a number ofcountries, including the Netherlands and the USA, and the

Fig. 10 Nordtank 15 m diameter, 55 kW rated, Denmark

IEE PROCEEDINGS, Vol. 130, Pt. A, No. 9, DECEMBER 1983 513

market for such machines can be expected to grow sub-stantially, as increased production rates lead to lowercapital costs. The UK has so far seen little progress in thedevelopment of wind turbines in this size range, partly dueto lack of the government support which is provided inDenmark and elsewhere, and partly due to uncertainties asto the tariffs which would be applied by area electricityboards to individually owned, grid-connected wind turb-ines. This second problem has now been resolved with thepublication in October 1983 of the relevant tariffs, asrequired by the May 1983 Energy Act.

Grid-connected small wind turbines (where 'small'denotes diameters up to about 20 m, and ratings up toabout 100 kW) have applications in rural areas in manydeveloped countries, as well as in some developing coun-tries. They also have considerable potential in remote loca-tions, world wide, operating in parallel with diesel enginesas fuel savers. The problems in optimising and engineeringa satisfactory wind/diesel system are substantial, butprogress is being made and a number of wind/dieselinstallations are now being field tested.

In the USA, many manufacturers are now producingsmall wind turbines for a wide variety of applicationsincluding water pumping and battery charging in remotelocations, although the greatest market is for grid-connected wind turbines. Although many of these grid-connected machines are individual installations, as inDenmark, many more are being installed in arrays, i.e.wind farms. The wind-farm concept was pioneered by USWindpower and their installation in the Altamont Pass, tothe east of San Francisco, contains several hundred windturbines, each with a diameter of 17 m and rated at 50 kW.Fig. 11 shows part of this wind farm. The concept has been

Fig. 11 Windfarming in the Altamont Pass, USA

emulated by many other entrepreneurs in the past twoyears, which has seen a total (so far) of about 2000 windturbines installed in wind farms, mostly in California.Almost all of these have had diameters less than 20 m,mainly because larger machines are not readily availablecommercially.

The financial arrangements have been crucial to thesuccess and rapid growth of wind farms. Typically, thewind-farm developer puts together a package for investorswhich provides for the purchase, installation and main-tenance of an array of wind turbines, on a leased site in alocation where average wind speeds are high. An essentialpart of the package is the connection of the wind farm tothe local utility grid (or some large user of electricity) plus

their purchase, at an agreed rate, of all the electricity gen-erated. Individual investors in a wind farm contributeseveral thousand dollars, and several hundred such inves-tors are formed into a partnership which owns and oper-ates the wind farm. Federal and State tax credits andallowances are such that high-rate taxpayers (paying tax ata top rate of about 50%) can recoup most of their invest-ment in the first year of operation, and almost all of theremainder over the next four years, virtually regardless ofhow much energy the wind turbines actually produce.Given good, reliable machines on a well chosen site, thepotential return on the investment is a very attractive one,and as indicated the financial risk is minimal; hence therecent rapid growth of wind-farm developments. However,concern has been expressed that the tax concessions aretoo generous, and are encouraging some manufacturers toproceed from prototype construction to volume pro-duction without adequate field testing; there is already evi-dence that the machines installed in some wind farms arenot very reliable and, consequently, have a low availability.The more responsible wind-farm developers are proceedingmore cautiously and accumulating much valuable experi-ence, which will undoubtedly have applications in manyother parts of of the world.

There are, so far, no wind farms outside the USA,although the Netherlands is proceeding with plans toinstal a 10 MW wind farm at Sexebierum, which willcontain about forty wind turbines, each about 25 m diam-eter and rated at approximately 250 kW; this is due forcompletion in 1985.

5 Economics

Cost data for virtually all commercially available windturbines having diameters larger than 10 m were collatedin mid-1982 and are presented in References 8 and 11. Thedata, mostly obtained directly from manufacturers, isreproduced in Fig. 12 which shows how the normalisedcost per unit rotor area varies with rotor diameter. Costper unit rotor area is a more useful parameter than costper rated kilowatt, and indeed the latter can be very mis-leading; e.g. as the rated power is proportional to the cubeof the rated wind speed, a relatively small increase in thelatter will significantly increase the rated power and give acorresponding reduction in the nominal cost per kilowatt,but without giving any significant increase in the windturbine's energy output. Too great an increase in the ratedwind speed may even lead to a reduction in a windturbine's annual energy output, due to reduced gearboxand generator efficiencies at the lower power levels associ-ated with the below-rated wind speeds, which may well

400

C,o300

200

100

+E I0* A

A "oE + prototype

& production 1'so production 10'sE denotes

estinyite ,0 10 20 30 40 50 60 70 80 90 100 110 120

diameter.mFig. 12 Normalised cost per unit area, variation with wind-turbinediameter

514 IEE PROCEEDINGS, Vol. 130, Pt. A, No. 9, DECEMBER 1983

contain most of the annual available energy. Because good,modern wind turbines have comparable efficiencies, it ismore meaningful to compare costs per unit rotor area.However, allowance must be made for the fact that largewind turbines have their rotors at greater heights abovethe ground, where wind speeds are higher, see eqn. 3.

Given the usual assumption that the velocity variationwith height can be represented by a one-seventh power law(i.e. n equals one-seventh in eqn. 3), the power in the windwill be proportional to (//1/7)3 = H 43, where H is the hubheight of the rotor. Increasing the height of a rotor by afactor of 5, e.g. from 15 m to 75 m, will consequentlyincrease the power available at the rotor by a factor50.43 _ 20. This means that, for comparable energy cost,one can afford to pay twice as much per square metre, fora turbine with a 75 m hub height, by comparison with awind turbine having a 15 m hub height. The variation ofwind power density with height may be allowed for by thesimple expedient of defining a normalised cost per unitrotor area C1 0 , as follows:

an annual charge rate c is a very useful one, and c isrelated to r by the equation

10costarea

.43

(7)

where H, as previously stated, is the wind turbine's hubheight. This parameter can be interpreted as the equivalentcost, per unit rotor area, relative to the wind speeds experi-enced at a height of 10 m (the normal meteorological mea-suring height). The cost data in Fig. 12 have beennormalised in this way.

The costs presented in Fig. 12 for each wind turbine areinclusive of the tower and onsite erection, but generallyexclude the foundation costs and grid connection(although these costs are included for the larger Americanmachines). Although foundation and connection costs canbe significant (of the order 15-20% of the total) their con-tribution is small by comparison with the scatter evident inthe Figure. It should be noted that the costs quoted are formid-1982, with currency conversions at the rates prevailingin October 1982, and do not include capitalised O & Mcosts. As indicated by the key, most of the wind turbineshave only been made, so far, in relatively small numbers.Some of the wind turbines now being installed in largenumbers in American wind farms are not included in Fig.12, as they are not yet available for sale individually, andthe manufacturers do not quote prices.

The data in Fig. 12 shows considerable scatter, as isonly to be expected given the present transitional state ofwind-turbine technology. Although many national wind-energy programmes have assumed, explicitly or implicitly,that large wind turbines will give superior economics, itcan be seen from the Figure (even after allowing for foun-dation and grid connection costs) that there is, as yet, noevidence to support this assumption. In fact, small- andmedium-sized wind turbines appear, at present, to offerslightly superior economics, and, since their developmenttime scale is shorter and volume production benefits canbe achieved more speedily, it seems probable that thesmall- to medium-size range (e.g. 1550 m diameter) willcontinue to offer superior economics, at least for the nearfuture. (Except, of course, for applications such as offshorewind systems, where the high costs of offshore constructionand operation encourage the use of wind turbines withdiameters of 100 m or more.)

The delivered cost of energy from a wind turbinedepends not only on its capital cost but also on its life-time, its operating and maintenance costs, and the requiredrate of return on one's capital investment. The concept of

(8)where r is the required real rate of return (i.e. after allow-ing for inflation) and y is the lifetime in years; y equalannual repayments of cl will repay an initial invesment /and provide a real rate of return r on the capital invested.

A 5% rate of return, in real money terms (as is specifiedin the UK for public sector investments) together with alifetime of 20 years, i.e. r = 0.05, y = 20, corresponds to anannual charge rate of 8%: to this must be added theannual costs associated with operating and maintenance(O & M).

Preliminary assessment of wind-turbine operating expe-rience in the USA indicates that, for large- and medium-sized machines, annual O & M costs can be expected to beabout 2% of the capital cost; for smaller wind turbines, theO & M costs can be expected to be higher, but there is, asyet, no reliable data on O & M costs for small machines.Adding a 2% allowance for O & M costs to the initial 8%charge rate discussed above gives an overall charge rate,inclusive of O & M costs, of 10%.

Fig. 12 indicates that most of the more competitivewind turbines have normalised costs in the range 125/m2to 250/m2; with a 10% overall charge rate, the corre-sponding annual costs are in the range 12.5/m2 to 25/m2.For a location where the annual average wind speed Vm =5.5 m/s (at 10 m height above the ground), eqn. 6 indicatesthat a good, modern, wind turbine will deliver about450 kWh/m2 annually, so giving an overall cost of energyin the range 2.8 p/kWh to 5.6 p/kWh; correspondingly lessin locations where wind speeds are higher. Electricity inthis price range is competitive with electricity from thefossil fuels, oil, gas and coal, in many parts of the world,although it must be noted that further work is necessary todemonstrate that long life and a low level of O & M costscan be achieved in conjunction with the indicated costrange.

Fig. 12 also shows that the lowest wind-turbine costs atpresent correspond to wind turbines of about 15 m diam-eter. However, O & M costs for these smaller machinescan be expected to be higher than for medium- and larger-scale machines, which have fewer components per unitenergy output. And foundation and grid connection costscan also be expected to be higher, per installed kilowatt,on smaller machines. However, medium-sized wind turb-ines have the advantage, by comparison with megawatt-scale machines, that they can be developed and theirdesign subsequently refined much more rapidly and eco-nomically; and their production and purchase involves sig-nificantly lower levels of financial risk, to manufacturersand purchasers, respectively. It therefore seems probablethat wind turbines in the size range of about 20-40 mdiameter will be able to offer superior economics, and it issignificant that many manufacturers are now developingcommercial wind turbines in this size range.

The economic criteria used in the preceding text in cal-culating electricity output costs in the range 2.8 p/kWh to5.6 p/kWh, i.e. a required real rate of return of 5% and a20 year lifetime, are more appropriate to governments andto utilities than to individual purchasers. Such private pur-chasers require an earlier return on their investment. InDenmark, as has already been noted, wind turbines ofabout 15 m diameter give simple payback periods of theorder 5 to 6 years, and total sales are about 200 per year.Because even the most successful Danish machines are at

IEE PROCEEDINGS, Vol. 130, Pt. A, No. 9, DECEMBER 1983 515

present manufactured in relatively small numbers (lessthan one per week), there is considerable scope forreducing costs, and so reducing the payback period as themarket expands and the production volume increases.

As these cost reductions are achieved, in Denmark andelsewhere, wind farming will become progressively moreattractive. In the UK, for example, cereal crops grown onseveral million hectares of land provide farmers with agross income of about 500/ha. In locations where windspeeds average 5.5 m/s, and large areas of the UK do expe-rience wind speeds of this magnitude, wind turbines spaceda nominal 10 diameters apart would given an annual elec-tricity output of about 50000 kWh/a, worth approximately1500/ha. And this income is, of course, additional to thefarmer's continuing income from his crops. Given potentialrevenues of this magnitude, one can expect the use of windturbines in rural areas to expand rapidly, once the eco-nomic viability of such installations has been successfullydemonstrated.

6 Conclusions

There has been rapid progress in the development of windturbines over the last decade, and a wide variety of designshas been built and tested, in sizes up to 100 m diameter,and with rated power outputs up to several megawatts.However, the data now becoming available suggests thatmedium-sized machines, i.e. less than 50 m diameter, withtheir lower development costs, will give lower energy costs,at least for the near future.

For utility-scale applications, the cost of energy fromthe wind must be compared with the value of the fuel thatthe use of wind energy saves. In more localised applica-tions, the cost of wind energy is judged relative to thedelivered price of energy from other sources, e.g. electricityfrom a utility grid system, or diesel fuel delivered, at con-siderable added expense, to a remote location. Presentenergy prices vary widely from country to country, andeven within individual countries there may be significantprice variations. The average wind speed, and hence thewind-power density, is also subject to considerable geo-graphic variation. Generalisations about the cost of windenergy relative to other energy sources must, therefore, beregarded with caution. However, it is clear that, for manyapplications, in many locations, wind energy is becomingcompetitive, and the transition from research and develop-ment activity to demonstration and deployment has

already commenced, with the installation of approximately2500 wind turbines over the last three years, and a totalinstalled capacity of about 150 MW.

Although wind turbines cannot provide firm power theydo have important energy saving roles, operating in paral-lel with electricity grid systems or, in more remote loca-tions, alongside diesel engines. And system integrationstudies in several countries, including the UK, have shownthat grid-connected wind turbines could provide 20% (andpotentially even more) of national electricity needs. Contri-butions to the total installed capacity will be made byutility operated wind systems, by individually purchasedmachines in rural areas (as in Denmark) and by entrepre-neurially funded wind farms (as in California), and it is notclear which of these will be the most significant. However,because the supplementary revenue from wind farming canexceed, by a substantial factor, the revenues obtainablefrom most other agricultural activities, it seems probablethat wind farms will feature prominently in future develop-ments in Europe, as well as in the USA.

7 References1 SMEATON, J.: 'On the construction and effects of windmill sails',

Proc. Roy. Soc. {London), 17592 GOLDING, E.W.: The generation of electricity by wind power' (E.

& F.N. Spon Ltd., London 1955)3 'Strategic review of the renewable energy technologiesan economic

assessment'. ETSU report R13 (HMSO, London, 1982)4 'Meteorological aspects of the utilisation of wind as an energy source'.

WMO technical note 175, Report 575 (WMO, Geneva, 1981)5 'Solar energy R & D in the European communityseries G, Vol. 1'.

Wind energy proceedings of the EC contractors meeting, Brussels,Nov. 1982 (D. Reidel, 1983)

6 LIPMAN, N.H., MUSGROVE, P.J., and PONTIN, G.W. (Eds.):'Wind energy for the eighties' (Peter Peregrinus Ltd., Stevenage, 1982),Chap. 2

7 'Horizontal-axis wind system rotor performance model comparison'.Report RFP-3508, UC-60, NTIS, Springfield, VA, USA, 1983

8 MUSGROVE, P.J.: 'The economics of existing wind turbines in thesize range 10 to 100 metres diameter'. Wind Energy Conversion 1983,Proceedings of the 5th BWEA wind energy conference, Reading,CUP, 1983

9 WRIGHT, J.K.: 'Alternative methods of electricity generation'.CEGB report P3, in 'Proof of evidence to Sizewell 'B' power stationpublic enquiry', 1982

10 MUSGROVE, P.J., and MAYS, I.D.: 'Recent progress in the devel-opment of the Musgrove vertical axis wind turbine'. Proceedings ofthe 5th Washington wind energy conference, Washington DC, SERI,1981-

11 MUSGROVE, P.J.: 'Wind energy. An evaluation for the CEC,Report XVI1/349/82-EN, 1983 (to be published by the CEC)

516 IEE PROCEEDINGS, Vol. 130, Pt. A, No. 9, DECEMBER 1983