Comparison of Theoretical and Experimental Power output of a Small 3-bladed Horizontal-axis Wind Turbine

Transcription

1 Mrslnd Press Journl of Americn Science 9;5(4):79 9 Comprison of Theoreticl nd Experimentl Power output of Smll 3-blded Horizontl-xis Wind Turbine K.R. Ajo Deprtment of Mechnicl Engineering, University of Ilorin, Ilorin, Nigeri E-mil: jomech@unilorin.edu.ng & J.S.O. Adeniyi Formerly of the Deprtment of Mechnicl Engineering, University of Ilorin, Ilorin, Nigeri E-mil: deniyijso@yhoo.com Abstrct A smll three-blded horizontl xis wind turbine ws prmeterized, modelled, developed nd tested for power performnce. The turbine bldes were fbricted from Mnsoni Altissim wood with rotor swept re of 3.65 sq. metres nd blde pitch ngle of 7. The turbine ws instlled on the roof top of University of Ilorin, Fculty of Engineering Centrl Workshop Building t hub height of 14.9 metres from the ground level while the turbine genertor ws sourced loclly. The direct current (d.c.) power output of the test turbine mesured t the bttery bnk terminl by Feigo Power Anlyzer. Instlled t bout 8.4m from the test turbine is meteorologicl tower (MET) crrying n APRS nemometer with dt logger nd mesured the wind speed nd direction over the test period. The cut-in wind speed, tht is, the speed t which the wind turbine strts to produce power ws determined to be 3.5 m/s. One minutes verges of wind speed nd power output hve been used to determine the power curve for the wind turbine in ccordnce with IEC(Interntionl Electrotechnicl Commission) guideline for smll wind turbines. Mesured power increse consistently with incresed wind speed nd the power curve obtined compred firly well with other stndrd power curves. [Journl of Americn Science 9;5(4):79-9]. (ISSN: ). Keywords: Pitch ngle, hub height, meteorologicl tower, nemometer, power curve. 1. Introduction The precise dte when mn first used mchine to ssist him in his dily work would be virtully impossible to determine. However, it seems cler tht the erliest mchines were bsed on the principle of rottion s mens of providing continuous motion for routine tsks such s grinding corn or pumping wter. Thus, there were the mills, driven by niml or mn-power, in which the rotting shft ws verticl nd driven by long horizontl bem, fixed to it, nd pulled or pushed round by the niml wlking round in circulr pth (Golding, 1976). Wind energy hs been used for long time. The first field of ppliction ws to propel bots long the river Nile round 5B.C. By comprison wind 79

2 Mrslnd Press Journl of Americn Science 9;5(4):79 9 turbines technology is firly recent invention. The first simple windmills were used in Persi s erly s the 7 th century for irrigtion purposes nd for milling grins. The erodynmic reserch for wind turbines hs contributed significntly to the success of modern wy of hrnessing wind energy. For most unsolved problems engineering rules hve been developed nd verified. All of these rules hve limited pplicbility, nd the need to replce these rules by physicl understnding nd modeling is incresing. Simplified nlyses of horizontl-xis wind turbine flows imed t overll erodynmic performnce prediction developed for modern rotor theories re vilble in literture. There hve, however, been few thorough tests of the dequcy of such nlyses by direct comprison with ctul mesurements over wide rnge of configurtions nd conditions. The conversion of wind energy to useful energy involves two processes: the primry process of extrcting kinetic energy t the rotor xis, nd the secondry process of the conversion of such mechnicl energy into useful electricl energy depicted in Figure 1. is converted into het s it is bsorbed by lnd res, bodies of wter nd the ir over the Erth surfce (Ajo et l., 9). The ir hs mss, though its density is low, nd when this mss hs velocity, the resulting wind hs kinetic energy which is proportionl to 1/[mss x (velocity) ]. The mss of ir pssing in unit time is ρ AV nd the kinetic energy pssing through the re in unit time (power vilble in the wind) is: 1 1 P. 3 w = ρ AV V = ρav ρ = Air density (pprox.1.5 kg/m 3 t se level) V = Velocity of wind (m/s) A = Are through which the wind psses normlly (m ). This is the totl power vilble in the wind (pprox. 3.6 x1 1 ( 1) kw ) obviously, only frction of this power cn ctully be extrcted. The power extrcted by wind turbine cn therefore be given s: 1 P = k. ρav k = C. N. N p g 3 b ( ) C p = coefficient of performnce or power coefficient (pprox..593) Figure 1: Conversion from wind power to electricl power in wind turbine 1.1 Energy in the wind Wind is merely ir in motion. It is produced by the uneven heting of the Erth s surfce by energy from the Sun. Since the Erth s surfce is mde of different types of lnd nd wter, it bsorbs the Sun s rdint energy t different rtes. Much of this energy N g N b = Genertor efficiency = Gerbox/bering efficiency The torque generted by the wind turbine is: P T = s ω s T s = mechnicl torque t the turbine side (3) 8

3 Mrslnd Press Journl of Americn Science 9;5(4):79 9 P = power output of the turbine ω s = rotor s speed of the wind turbine The power coefficient C p is the percentge of power in the wind tht cn be converted into mechnicl power nd the rtio of the blde tip speed to the wind speed is referred to s the tip-speed rtio (TSR). TSR = ω s R V R is rdius of the wind turbine rotor.. Smll Wind Turbines All wind turbines cn be chrcterized s either Horizontl Axis Wind Turbines (HAWT) or Verticl Axis Wind Turbines (VAWT). In HAWT, the rotor spins bout n xis horizontl to the erth s surfce. The rotor of VAWT spins bout n xis perpendiculr to the Erth s surfce. Verticl xis wind turbines re typiclly developed only for built environment. Chnges in wind direction hve fewer negtive effects on this type of turbine becuse it does not need to be positioned into the wind direction. However, the overll efficiency of these turbines in producing electricity is lower thn HAWT. VAWTs re ctegorized s Svonius or Drrieus types, ccording to the principle used to cpture the wind flow. For the Svonius type, the wind pushes the bldes, which implies tht the rottion speed is lwys lower thn the wind speed. Contrry to tht, the shpe of the rotor of the Drrieus type mkes it possible for the rotor to spin fster thn the wind speed. Rotors of HAWT re plced on towers to position them where the wind speed is fstest nd exhibits ( 4) 81 most power (Jonkmn, 3). A ncelle typiclly resides top the tower nd contins the support structure for the rotor, the rotor shft, gerbox nd the electric genertor (lterntor). The gerbox is used to trnsform the low-speed high-torque power of the rotor to high-speed, low-torque power tht cn run the electric genertor. Rotor cn be positioned upwind or downwind of the tower. Downwind rotor configurtions cn trck the wind utomticlly s wind direction chnges. However, the wind must flow round the tower to rech the rotor of downwind turbine. This results in complex flow ptterns nd periodic fluctutions in erodynmic lods, which hve importnt dynmic effects on the turbine structure. Flow pssing through the rotor plne is unobstructed by the tower for upwind rotor configurtions. The hub structure connects the bldes to the drive shft. Hubs re generlly chrcterized s either rigid or teetering. In rigid hub designs, the hub is rigidly ttched to the drive shft. In contrst teetered hubs re connected to the drive shft by mens of teeter pin, bering tht permits the rotor to rock into nd out of the plne of rottion. Teetered hubs hve the benefit tht bending moments brought bout by thrust forces cting on the bldes re not trnsferred to the ncelle nd tower structure. Consequently, the ncelle nd tower structures of turbines with rigid hub must be designed more robustly thn those with teetered hub. Smll wind turbine need to be relible, ffordble nd lmost mintennce free. To meet these criteri, optiml turbine performnce is sometimes scrificed for simplicity in design nd opertion (Andrew, 5). Thus, rther thn using the genertor s motor to strt nd ccelerte the rotor when the wind is strong enough to begin producing power, smll

4 Mrslnd Press Journl of Americn Science 9;5(4):79 9 wind turbines rely solely on the torque produced by the wind cting on the bldes. Furthermore, smll wind turbines re often locted where the generted power is required, which is not necessrily where the wind resource is best. In low or unstedy wind conditions slow strting potentilly reduces the totl energy generted. Also, sttionry wind turbine fuels the perception of wind energy s n unrelible energy source. The min technicl chllenge in the design of smll wind turbine is to come up with system configurtion nd control lgorithm tht mximizes wind energy production from the turbine nd lso provide fvourble chrging conditions for btteries. This tsk is complex becuse of the vribility of the wind, which results in vrying wind turbine power output. Idelly, the system configurtion nd its control should optimize the mtch between the wind turbine rotor nd lod, thereby llowing the mximum vilble power from the wind to be used, while t the sme time chrging the btteries with n optimum chrge profile (Corbus et l., 1999). The genertors of smll turbines often cuse significnt resistive torque tht must be overcome erodynmiclly before the bldes will strt turning. Furthermore, pitch control is rrely used on smll wind turbine becuse of cost. Thus, it is not possible to djust the turbine blde s ngle of ttck to the previling wind conditions. This problem is prticulrly cute during strting. A further mjor difference is tht smll turbines usully operte with vrying shft speed in n ttempt to mintin mximum performnce s the wind speed vries. Mny lrge turbines run t constnt speed s this llows the genertor to mintin synchronicity with the utility grid. The IEC 614- (Interntionl Electrotechnicl Commission) defines smll turbine s hving swept re less thn m, which correspond to power output of bout 1kW. In ddition, there is further subdivision in tht turbine of swept re less thn m (bout 1.kW) do not need to hve their tower included in the certifiction process (Introduction to Wind Turbine Technology: ccessed t on 1 th Februry, 7 ). Clusen & Wood (1999) hve mde further subdivision s shown Tble1.1. Tble 1. Operting prmeters of smll wind turbines Ctegory Power Turbine Mximum Genertor type(s) (kw) blde rdius(m) rotor speed (rpm) Micro Permnent mgnet (PM) Midrnge Permnent mgnet or induction Mini Permnent mgnet or induction.1 Wind Turbine Bldes All forms of wind turbine re designed to extrct power from moving ir strem. The bldes hve n irfoil cross-section nd extrct wind by lift force cused by pressure difference between blde sides. When ir psses over n irfoil section; it trvels fster over the top of the blde thn it does below it. This mkes the ir pressure bove the blde lower thn it is below. Due to the unequl pressures the blde experiences lifting force (BWEA). For mximum efficiency, the bldes often incorporte twist nd tper. The mechnicl power produced by rotor is purely function of the blde geometry nd the incident velocity. The design prmeters tht ffect 8

5 Mrslnd Press Journl of Americn Science 9;5(4):79 9 erodynmic performnce include blde pitch (ngle of ttck), tper, nd twist distribution. For given blde, its geometric shpe is usully fixed, i.e. the erodynmic shpe, tper nd twist distribution do not chnge. The torque produced by the rotor cn be controlled in two wys: chnging the geometry by vrying the blde pitch ngle, or by chnging the rotor s rottionl speed so tht the rotor opertes t the optiml blde tip speed rtio. In the beginning most wind turbine bldes were dpttions of irfoil developed for ircrft nd were optimized for wind turbine uses. In recent yers development of improved irfoil sections for wind turbines hs been on going. The previling tendency is to use NACA 63 (Ntionl Advisory Council on Aeronutics) nd NREL S89 (Ntionl Renewble Energy Lbortory) (Ajo, 8) irfoil cross-section tht my hve modifictions in order to improve performnce for specil pplictions nd wind conditions.. The Gerbox The gerbox is required to speed up the slow rottionl speed of the low speed shft before connection to the genertor. The speed of the blde is limited by efficiency nd lso by limittions in the mechnicl properties of the turbine nd supporting structure. The gerbox rtio depends on the number of poles nd the type of genertor. A direct driven genertor (without gerbox) would require genertor with 6 poles to generte electricity t 5Hz..3 Turbine genertor The genertor used in gered wind turbines re more or less stndrd off-the-shelf electricl mchines, so 83 tht mjor development steps were not necessry. There re different types of genertors depending on their configurtion nd res of ppliction. The wind turbine genertor converts mechnicl energy to electricl energy. The efficiency of n electricl genertor usully flls off rpidly below its rted output. The phenomenon of inducing current by chnging the mgnetic field in coil of wire is known s electromgnetic induction which underpins the design of most electric genertors (Cotrell, ). With the importnt exception of electrosttic genertors such s the Vn de Grf mchine, ll importnt schemes for converting mechnicl motion into electricl energy depends on Frdy s Lw of electromgnetic induction. 3. Aerodynmic theory of wind turbines Accurte models of the erodynmic spects of wind turbines re essentil to successfully design nd nlyze wind energy systems. Wind turbine erodynmic models re used to relte wind inflow conditions to lods pplied to the turbine (Jonkmn, 3). The true fluid flow pssing round nd through wind turbine is governed by the Nvier-Stokes equtions. Unfortuntely, these equtions re so complex tht nlyticl solutions hve been found for only few simple cses. The subsequent nlysis develops the most common erodynmic theory employed in the wind turbine design nd nlysis environment, the blde element nd momentum theory (BEM). To id the understnding of combined blde element nd momentum theory it is useful initilly to consider the rotor s n ctutor disc. Although this model is very

6 Mrslnd Press Journl of Americn Science 9;5(4):79 9 simple, it does provide vluble insight into the erodynmics of the rotor (Bossnyi, 1). 3.1 Actutor Disc Theory nd the Betz limit In the one-dimensionl Rnkine-Froude ctutor disc model, the rotor is represented by n ctutor disc through which the sttic pressure hs jump discontinuity (Jonkmn, 3). Consider control volume fixed in spce whose externl boundry is the surfce of strem tube. The fluid psses through the rotor disc, cross-section of the strem tube upwind of the rotor, nd cross-section of the strem tube downwind of the rotor s shown in Figure. The following ssumptions re mde in the ctutor disc model: (1) Purely One-dimensionl nlysis () Infinitely thin disc which offer no resistnce to ir pssing through it. (3) Wind is stedy, homogenous nd fixed in direction. (4) Air is incompressible, inviscid nd irrottionl. (5) Both the flow nd the thrust re uniform cross the disc. The flow is uniform t the upwind (sttion ) nd downwind (sttion 3) boundries of the control volume. (6) The upwind nd downwind boundries re fr enough removed from the rotor tht the sttic pressure t these points is equl to the unobstructed mbient sttic pressure. The sttic pressure on the strem tube portion of the boundry is lso equl to the unobstructed mbient sttic pressure. Figure. Control volume of ctutor disc model (source: Jonkmn, 3) For wind turbine rotor to ct s n ctutor disc, the rotor would hve to be composed of n infinite number of very thin, drgless bldes tht rotte with tip speed much higher thn tht of the incoming wind. Also, sttion 1 designted to be slightly upwind nd sttion slightly downwind of the rotor. Since ir does not pss through the strem tube porting of the control volume boundry by definition of strem tube, pplying the conservtion of mss (which sserts tht the instntneous rte of chnge of mss within control volume must equl the net flux of mss out of the control volume) to the control volume gives: V A = V A = V A = V 1 1 3A 3 Vi is the wind speed t sttion i nd is the crosssection re of sttion i. Since ccording to ssumption (5) nd since Ai ( 5) is equl to equl by necessity, the component of the wind speed nd cross sectionl re of the plne of the disc re designted without subscripts for the rest of this nlysis (i.e. V = V = V nd A= = ). V 1 A 1 1 A 1 A The thrust t the rotor disc, T, cn be found by pplying the conservtion of liner momentum to the control volume in the xil direction. This results in: V A 84

7 Mrslnd Press Journl of Americn Science 9;5(4):79 9 T = ρa V ρa3v 3 Substituting eqution (3.1) into eqution (3.) gives T = ρav V V 3 ( 6) ( ) ( 7) ρ is the density of the ir. The thrust t the rotor disct is lso the differentil pressure between sttion 1 nd multiplied by the disc re. T = P P 8 ( ) ( ) 1 A Since no work is done on either side of the turbine rotor, Bernoulli s eqution cn be pplied to obtin the pressure incorported into eqution (3.4) 1 1 P + ρ V = P1 + ρv () 9 nd, 1 1 P3 + ρ V3 = P + ρv ( 1) Pressure P o nd P 3 P1 P re identicl by ssumption (6) so pressure nd cn be eliminted from eqution (3.4) with the help of equtions (3.5) nd (3.6) to obtin, ( ) ( 11) 1 T = ρ A V V3 By equting equtions (3.3) nd (3.7), the velocity of the flow through the rotor disc is the verge of the upwind nd downwind velocities V + V V = 3 ( 1) An xil induction (sometimes clled interference) fctor is defined s the frctionl decrese in wind velocity between the freestrem (upwind) nd the rotor plne: V V = ( 13) V or, V = V ( 1 ) ( 14) Using eqution (3.8) V ( 1 ) ( 15) 3 = V The velocity lost t the rotor plne, V V is known s the induced velocity. As increses from zero, the downwind flow speed stedily decreses until, = ½, mening tht the rotor hs completely stopped nd the simple theory is no longer pplicble. By substituting for V 3 (3.7) cn be re-written s: T 1 from eqution (3.11), eqution = ρ AV 4( 1 ) (16) The power extrcted from the wind by the rotor, P, is the product of the thrust, T, nd the wind velocity t the rotor plne, V,from eqution (3.1) P 1 3 = ρ AV 4( 1 ) (17) The non-dimensionl power coefficient C p representing the frction of vilble power in the wind tht is extrcted by the turbine, is defined s: P C P = 1 ( 18) ρav 3 Substituting eqution (3.13) into eqution (3.14) yields; C P = 4 ( 1 ) ( 19) The dimensionless thrust coefficient is therefore given s, T C T = ρav 1 = 4(1 ) C T ( ) 85

8 Mrslnd Press Journl of Americn Science 9;5(4):79 9 The mximum power coefficient is 1/3. C p Substituting for in eqution (3.15) gives: C p 16 = This is known s Betz limit. C T occurs when ( 1) The thrust coefficient hs mximum vlue of 1 when = ½ Therefore the mximum possible efficiency for n idelized wind turbine is roughly 59.3%. 4. Model Turbine Cpturing the chrcteristics nd nunces of threedimensionl flow seprtion in stochstic inflow environment on lrge piece of rotting mchinery poses s significnt chllenge s understnding the underlying fluid mechnics. A number of design codes hve been used over the lst decde to model the wind turbine s dynmic behviour. They crry out design clcultions or develop dynmic nd stedy stte models for ll components within wind turbine. In longer term, they cn be used in complete optimiztion of wind turbine system including models for mechnicl prt (wind, drive trin, stll nd vrible pitch control),model for genertors (induction genertor, synchronous genertor, permnent mgnet genertor), nd models for power converters, trnsformers, nd the grid (Florin et l., 4). The Visul Bsic progrm ws written becuse of its flexibility nd ese of its ccessibility in formulting the design to suit the desired objective. Stndrd shft nd gers equtions in conjunction with wind energy equtions were used to develop the Visul Bsic progrm hereby referred to s WINMECH. The input prmeters to the progrm nd the output results t 86 the rted wind speed of 1 m/s nd operting time of six (6) hours per dy, gives power output of bout 59 Wtts nd 3.56 kw/hr of energy 4.1 Weibull Distribution nd Annul Energy Production (AEP) The expected nnul energy production (AEP) for the model turbine cn be estimted t rted wind speed nd this will lter be compred with tht of the test wind turbine. The Visul Bsic-modelling progrm is run t different wind speed nd the expected power output is serves s input dt to the WINMECH turbine performnce model progrm. In the WINMECH progrm, shown in Figure 5, the wind speed probbility is clculted s Weibull curve defined by the verge wind speed nd shpe fctor k. For ech wind speed, instntneous wind turbine power (W) is multiplied by the Weibull wind speed probbility (f). This cross product (net W) is the contribution of wind speeds to verge turbine power output. The sum of these contributions is the verge power output of the turbine on continuous 4 hours bsis. Air density fctor is the reduction from se level performnce. Figure 3. Model turbine nnul energy output estimtion using WINNMECH

9 Mrslnd Press Journl of Americn Science 9;5(4): The Test Turbine The wind turbine under test is instlled on the roof top of University of Ilorin, Fculty of Engineering Centrl Workshop t bout 14.9m from the ground. Ilorin, Nigeri is on ltitude 8.5 N nd the site verge temperture nd ir density re 7 C nd 1.1kg/m 3 respectively (Lsode, 4). The test turbine shown in Figure 4, hs rotor dimeter of.15m nd rted power of 11wtts t 1m/s. It is three-blded, upwind vrible speed, horizontl xis hving blde pitch ngle (ngle of ttck) of 7. It is permnent wind fcing t 85 of compss North. The turbine uses n utomobile lterntor (genertor) modified to higher rting to produce d.c. power output mesured by FEIGAO Power Anlyzer.The output power is stored in two 1V d.c. btteries connected in prllel. Instlled t bout 8.4m from the test turbine is meteorologicl tower (MET). This is more thn three rotor dimeters from the test turbine in the mesurement sector s required by the IEC stndrd. The MET tower crries n APRS nemometer t hub height trnsmitting wind dt to dt logger housed inside the Dt Centre. The dt logger supports Secure Digitl (SD TM ) crd up to 18MB where ll the wind dt re logged nd lter trnsferred to comptible personl computer (PC) for nlysis. Figure 4. Test Turbine showing nemometers nd Meteorologicl Tower (MET) Figure 5. Test turbine nnul energy output determintion using WINMECH 6. Power Curves Anlysis The mount of power tht wind turbine produces depends on the wind speed t the time. The power curve describes the reltionship between the wind speed nd the power tht the turbine genertes Mesured (d.c.) Power Tble below shows the mesured direct current (d.c.) power t site verge ir density (1.1 kg/m 3 ) nd normlized to the power t se level ir density

10 Mrslnd Press Journl of Americn Science 9;5(4):79 9 (1.5 kg/m 3 ). Normliztion to se level ir density is done by multiplying mesured power by the rtio of se level ir density to site verge ir density. Mesured wind speed is lso binned nd normlized. The IEC stndrd requires t lest three one-minute points per bin. This condition ws met except for normlized wind speed of 18 m/s, 19 m/s nd m/s. Tble. Direct current (d.c.) power t site verge ir density nd normlized to se level ir density. Wind Speed Normlized Wind Speed Mesured Power (d.c.) Power (d.c.) normlized to se level (m/s) (m/s) t site verge ir density ir density (W) (W) The power curves for the mesured d.c. power t site verge ir density nd normlized to se level ir density re shown in Figure 6 nd Figure 7. Mesured Power (W) Wind Speed (m/s) Figure 6. Direct current (d.c.) Power Curve t site verge ir density, 1.1kg/m 3 Power (W) Wind Speed (m/s) Figure 7. Direct current (d.c.) Power Curve t se level ir density, 1.5kg/m 3 6. Mesured Versus Model Power Curve To determine the performnce of the test turbine, mesured power output i.e. the ctul power produced by the turbine in opertion is compred to the power of the modelled turbine. Figure 8 below shows the comprison of the modeled turbine nd the test turbine power. 88

11 Mrslnd Press Journl of Americn Science 9;5(4): References 5 Test Turbine 45 Model Turbine 4 Poly. (Test Turbine) Poly. (Model Turbine) 35 3 Power (W) y =.44x x x x y =.55x x x x Wind Speed (m/s) Figure 8. Model vs. Mesured power curve. 7. Discussion The result of the wind speed mesurements indicted tht the dily verge wind speed is lower thn the cut-in wind speed nd for most prt of the dy the wind turbine is idling. Normliztion to se level ir density hs no significnt effect on the result. Mesured power increses consistently with incresed wind speed. The resulting power curve showed some discrepncies t certin wind speed but compred fvourbly with stndrd power curves. The curve fit of the test turbine nd tht of the model turbine indicted tht the projected power output of the test turbine t the cut-out wind speed of 5m/s is pproximtely 57.8W. Using the WINMECH modeling progrm, the verge output power of the test turbine is 8W nd the nnul energy output is 698kW.These results when compred to the verge output power of 536W nd nnul energy output of 4698kWh of the model turbine yields cpcity fctor of pproximtely fifteen percent (15%). 1. Ajo K.R, Mhmood M.R nd Iynd M.O (My, 9). Interfce for modeling the power output of smll wind turbine. Indin Journl of Science nd Technology Vol. No 5. Andrew K.W (September, 5). Aspects of the Aerodynmics nd Opertion of Smll Horizontl Axis Wind Turbine. Ph.D. Thesis, Deprtment of Mechnicl Engineering, University of Newcstle, Austrli 3. Bossnyi E (1). Blded for Windows Theory Mnul, 8/BR/9 ISSUE: 1 Grrd Hssn nd Prtners Limited 4. Bossnyi E (1). Blded for Windows Theory Mnul, 8/BR/9 ISSUE: 1 Grrd Hssn nd Prtners Limited 5. Clusen P.D nd Wood D.H (1999). Reserch nd Development issues for Smll Wind Turbines. Renewble Energy 16, pp Corbus D, Bring-Gould I, Drouilhet S, Gervorgin V, Jimenez T, Newcomb C nd Flower L (1999). Smll Wind Turbine Testing nd Applictions Development. NREL/CP Cotrell J (Jnury, ). A preliminry Evlution of Multiple-Genertor Drive trin Configurtion for Wind Turbines. NREL/CP Florin I, Anc D.H, Clemens J, Poul S nd Frede B (Mrch, 4). Advnced Tools for Modelling, Design nd Optimiztion of Wind Turbine System. Conference proceedings, Nordic wind power conference, 89

12 Mrslnd Press Journl of Americn Science 9;5(4):79 9 Chmlers University of Technology, Finlnd Mrch, pp Golding E.W (1976). The genertion of electricity by wind power. E&F.N SPON Ltd, London 1. Jonkmn J.M (3). Modelling of the UAE wind turbine for refinement of FAST- AD. NREL/TP , Ntionl Renewble Energy Lbortory, Golden Colordo, U.S.A 11. Lsode O.A (4). An Improved Solr Cbinet Dryer with Convective het Trnsfer. Journl of Applied Science, Engineering nd Technology, Vol.4 No.; pp Introduction to Wind Turbine Technology, Accessed on 1 th Februry, 7 9