C H A P T E R - I
I N T R O D U C T I O N
T he enormous potential of wind as energy source, mainl y because of its renewable nature and decentralised avai l abi l ity, makes it a promising energy option.
A wind machi ne is essentially a device to convert the kinetic energy available in m o v i ng air into usable f o r m of work such as pumpi ng wat er and generati on of e l e c t r i c i t y . Based on the wind data for Indian conditions, it is found that windmi l l s designed to respond to low wind speeds of 12 km/h are likely to be most appropri ate for w a t e r l i ft i ng f r o m shallow wells on small farms (77, 79). A p a r t f ro m i rr i ga
tion. the wind p o w e r can also be used for grinding, cane crushing, aer at i on of fish ponds and t hreshi ng (70).
T h e r e are t wo basic types of wi ndmi l l s — horizontal axis and v e r t i cal axis.
T he h o r i z o n t al axis machi nes are most co m m o n of all wind turbines. T he wi ndmi l l s w i t h design t i p-speed r ati o upto 3.0 are known as low speed windmi l l s and those w i t h higher values as high speed windmi l l s. F o r running irrigation pumps and other st at i onar y devi ces, l ow speed wi ndmi l l s are suitable as these have b e t t e r torque c h a r ac t er i st i c s . Mo s t recent works on wind energy appears to be rel at ed to the gener ati on of e l e c t r ic a l power and l i t t l e ef f or t has been directed to w a t e r pumpi ng wi n d mi l l s f or i r r i g at i on (38).
Mul t i b l ad e a wi n d mi l l s have been found to generate high st art i ng torque and oper at e wel l even in low winds (70, 84). R e v i e w of di fferent designs of m u l t i - bladed w i n d m i l l s reveal s that the blades of some are tapered o ut wa r d and of some i nwar d . A f e w designs have c o n s t an t - c h o r d blades for ease in m a n u f a c t ur e. T he r e is a v e r y wi de v ar i at i on in design of low speed mul t i bl aded horizont al axis wind tur bine ( H A W T ) r otor s, in respect of t heir blade shape and number of blades. T he n u m b e r of blades in the exi sting designs of l ow speed H A W T rotors has been found
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to vary f r om 6 to 45 (10). T he r e f o r e, different design parameters for a lo.v speed H A W T r otor need to be o p t i m i z e d .
Sails have been found aer o dy na mi c al l y i neff i ci ent for wind turbines (74).
O w i ng to low Reynol ds numbers encountered by low speed windmills, cambered steel plates have been pr efer r ed over airfoils of N A C A or Wor t man series (14).
T he camber ed steel plates have f ai rl y good lift/drag char act eri st i cs and are easy to m a nu f a ct ur e (33). In di f fer ent designs of the wind rotors using camber ed steel plates, it is found t hat the archi ng or ca m b e r , whi ch is the ratio of co n ca vi t y to the chord length, varies wi del y f r o m one design to another. In vi ew of the i mpor t ance of c a mb e r on a er ody nami c per f o r man ce of blades, some studies were conducted in the past to generate l i ft and drag char act eri st i cs. But these were l i mit ed to angle of at t ack range of -10° to +20° (14).
O w i n g to v ari at i on in magnitude of wind speed and rotor load, the rotors' operati ng t i p-speed ratios often devi ate f r o m the design value resulting in wide vari ati on of angle of att ack (24). In a wind r otor , the range of vari ati on of the angle of at t a ck w i t h speed and radius is considerable, far gr eat er than what is n o r ma l l y found over a i r c r a f t wings. T h e r e f o r e , to sel ect an airfoil section, for design of wi nd rotors or to predi ct the p e r f o r man c e of a r otor made w i t h cert ai n airfoi l section, the a er o d y n a mi c cha r act e r i st i cs ( l if t and drag) of the airfoil across a wide range of angle of att ack is necessary. E xc e p t wi t hi n a nar r ow range of angle of a t t ac k , airfoi l c ha r ac t e r i st i cs are highl y n o n - l i n e a r , and results of wind tunnel tests must be used in p r a c t i c e in place of a nal yt i cal predictions ( 37). T he i nf or mat i on on lift and drag pr oper t i es of airfoils such as camber ed steel plates, w hi ch are suitable f or low speed mul ti bl ad ed h o r i z o n t al axis wi n d mi l l s , is very l i mi t ed (25). T h e r e f o r e , it is necessary to generate l i f t and drag char act eri st i cs of c a mbe r ed steel plates across a w i d e range of angle of att ack for o p t i mi z a t i o n of c a m b e r and making available the air foi l data for p e r f o r m a n c e predi cti on.
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T he design of H A W T r otor for a given di amet er involves d et ermi nat i on of number of blades, type of airfoil and radial vari ati on of blade chord length and twist (20, 34). T he normal procedure for det e r mi ni n g blade shape for a H A W T r otor is to o p t i m i z e independently each radial element by continuously varying chord and twist to obtain m a x i m u m energy ext r act i o n at design tip-speed ratio.
T he design tip-speed ratio of a wind turbine is that at which all blade elements operate at o p t i mu m angle of att ack resulting in m i n i m u m d r a y - t o - l i f t ratio.
Pr opel l er theory devel opment f ol l owed two independent approaches. One of these has been called m o m e n t u m theory and the ot her, blade el ement theory.
T he basis of the m o me n t u m theory is to d et er mi ne forces act ing on the blade to produce the mot i on of the fluid. T he theory has been useful in predi ct i ng ideal e f f i ci en cy and flow v e l oci t y. It, ho we v er , does not give the i nf o r ma t i o n on the r o t o r shape necessary to generate the fluid mot i on. T h e approach of blade el ement theory is opposite to that of the m o m e n t u m theory in that the concern is wi t h the forces produced by the blades as a result of the motion of fluid. T h e early t heoret ical a er o dy na mi c designs of H A W T rotors were based on assumptions that the blades operated wi t ho u t f ri ctional drag and that there wer e infinite number of blades in the r o t o r (20). These assumptions f aci l i t at ed closed f o r m solution for design of rotors wi t h m a x i m u m e f f i c i en c y . F o r this, the strip t heory, whi ch is co mb i n a t i o n of m o m e n t u m theory and blade el ement theory was used. T h e strip theory also assumes that the flow around a blade el ement is t w o - d i m e n s i o n a l .
F o l l o w i n g the resurgence of wi nd po we r in ear l y 1970 s, the e f f or t s were made to a r r iv e at o p t i m u m design and to p r e d i ct the peak p er fo r ma n c e of H A W T r otors by taking into account the ef f ect s of drag and a finite n u m b e r of blades.
Some approaches considered the e f f ec t of only drag (23, 47, 76, 89), whereas others (73, 81, 85, 86) took into account the ef f ect s of tip-losses t hrough a finite number of blades also. T he o p t i m u m design and peak p e r fo r m a n c e p r edi ct i on m e t h ods do not yield closed f o r m solutions when the e f f ect s of drag and tip-losses
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arc considered. The major c o mp l ex i t y in these methods is the number and t y p e- of i t e r at i v e processes required to satisfy the cont i nui t y and moment um relationships si mul taneousl y wi t h the flow around blade el ement , drag and blade tip effects in order to find the axial and angular speed int erf er ence factors (75). T herefor e, it is necessary to extend the aer odynami c analysis of H A W T rotors to arrive at conditions for m a x i m u m power ext r act i on. T he additional relationship thus obtained m a y f aci l i t at e closed form solution for o p t i m um design and peak performance predi ct i on for H A W T rotors.
O w i n g to var i at i on in wind speeds and m i s - m a t c h i n g of load, the wind rotors operate quite often at tip-speed ratios l ower as well as higher than .the design value. T h e r e f o r e , it is desirable to p redi ct the output power of a H A W T r otor , not only at design val ue of tip-speed r ati o but also at other values.
The e f f ec t of finite number of blades on power output f ro m a H A W T r otor is taken t hrough P r a n dt l tip-loss f ac t or . The f actor is unity for infinite number of blades. T h e t ip-l oss f actor of uni t y signifies no tip-losses. The e f f ec t of n umb er of blades, as i ncorporated t hrough tip-loss f ac t o r , on peak and off- desi gn p er fo r ma n c e of H A W T r otors obtained through co mp u t er si mulation reveals that a r o t o r w i t h large n umb e r of blades per f o r ms b et t e r (35, 85). H o w e v e r , when the number of blades in a r o t o r is increased for a given solidity ratio ( the ratio of area occupi ed by blades to the r o t o r swept area), the chord length reduces, r educi ng the Reynol ds number of r o t o r blades. A t low Reynol ds number s, the a er od y n a mi c p e r f o r m a n c e of airfoils det eri or at es resul ting in r educti on of power out put f r o m r o t o r . T h e peak and of f - de si gn p e r fo r ma n c e predi cti on methods e m p l o y ing c o m p u t e r si mul a t i on and using airfoi l data, do not take into account the effects of Reynol ds n umb e r on r o t o r p e r f o r m a n c e . Thus, by using these methods alone, the numb er of blades for a H A W T r o t o r cannot be opti mi zed. In v i ew of this, it is necessary to o p t i m i z e the n umb e r of blades in low speed H A W T rotors t hrough wind tunnel studies on model rotors w h i c h are t he o r e t ic a l l y designed for m a x i m u m p o we r e x t r ac t i o n .
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There is wide vari ati on in flow conditions across a rotor model in wind tunnel and those across the full size rotors operating under actual wind conditions. It is. therefore, desirable to scal e- up the o p t i mal l y designed wind rotor model and test under actual wind conditions for obtaining its power and torque characteri sti cs.
In vi ew of the above, the studies on design and per for mance aspects of low speed horizontal axis wind turbine rotors wer e undertaken w i t h the following objectives :
(i) T o study the lift and drag char act eri st i cs of steel plates wi t h dif fer ent camber s over a wide range of angle of att ack in order to o p t i mi z e the camber of blade for appl ication to wind energy conversion systems.
(ii) T o develop a model through aer o dy na mi c analysis for o p t i m u m design, and peak and off- desi gn per for mance predi ct i on of H A W T rotors.
(iii) T o study po we r and torque c har act er i st i cs of aer o dy na mi c al l y designed r otor models t hrough wind tunnel testing, for opt i mi z at i on of number of blades in l ow speed H A W T rotors.
( iv) T o develop a scal ed- up unit of o p t i m a l l y designed r otor model and test for its power and torque char act eri st i cs under act ual wind conditions.
In or der to achi eve the above obj ect i ves, seven blades w i t h c a m be r var y i ng f ro m zer o to 14 per cent wer e tested in wi nd tunnel at Reynol ds n umb e r of 2.23 x 10~\ F r o m the aer odynami cs of H A W T rotor s, a relationship a mong d r a g - t o - lir't r at i o, tip-loss f ac t o r and speed i nt e r f er e nc e f act or s was established. Wi th the help of this relationship, o p t i m u m design, and peak and o f f - desi gn p er fo r ma n c e p redi ct i on methods for H A W T r otors w er e devel oped.
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T he results f r o m the new o p t i m u m design and peak performance predi ct i on method wer e c ompar ed w i t h those f r o m the ot her established methods. T he solidity ratio and blade twist wer e opti mi zed throuqh o p t i m u m design method. The number of blades for low speed H A W T rotors was opti mized through wind tunnel testing of rotor models. A f t e r o p t i m i z at i o n of design par amet er s, a 5 m di amet er rotor was developed and tested under actual wind conditions. The test results of the 5 m d i a me t e r r ot or were compar ed wi t h those of the corresponding model and also wi t h those obtained through the ot'f-design p er f o r man ce predi ct i on method.
