Effect of drag on the performance for an efficient wind turbine blade design

Transcription

1 Available online at Enegy Pocedia 18 (01 ) Abstact Effect of dag on the pefomance fo an efficient wind tubine blade design D. Eng. Ali H. Almukhta Univesity of Technology - jana96004@yahoo.com The utilization of wind enegy fo powe geneation puposes is becoming inceasingly attactive and gaining a geat shae in the electical powe poduction maket woldwide. In the pesent study, a mathematical model is developed to study the paametes that affect the electical powe geneated by the wind tubines. The consideed paametes ae tubine swept aea, ai density, wind speed, lift/dag atio and powe coefficient as a function of blade tip speed. The study shows that the opeational aeodynamic paametes has a diect effect on the geneated powe which will lead the developes and eseaches to focus on the highest aeodynamics pioity that should be consideed fo manufactuing and suitable the new geneations of wind tubines. List of symbol A oto swept aea (m ) a, a Axial and tangential intefeence facto B Numbe of blade BET Blade Element Theoy C Pofile chod fo blade (m) C A, C T Axial and tangential foce factos C D, C L Dag and lift coefficients C P Powe Coefficient C q Moment coefficient D Dag foce (N) d oto diamete (m) E,G Dimensionless factos fo axial and tangential intefeence factos m Local momentum coefficient fo blade station n Numbe of evolution of oto pe minute(pm) adius of tubine oto (m) local adius of tubine oto (m) V wind speed (m/s) V ated wind speed (m/s) angula velocity (ad/s) Angle of attack (deg.) Aeofoil elative angle (deg.) Ai density (kg/m 3 ) b Pandtal efficiency Tip speed atio Published by Elsevie Ltd. Selection and/o pee eview unde esponsibility of The TeaGeen Society. Open access unde CC BY-NC-ND license. doi: /j.egypo

2 Ali H. Almukhta / Enegy Pocedia 18 ( 01 ) d Design tip speed atio local tip speed atio Solidity 1. Intoduction The wind is natual phenomenon elated to the movement of ai passes caused pimay by diffeential sola heating of the eath suface. Most wind tubine blades whee adaptations of aifoils developed fo aicaft and wee not optimized fo wind tubine uses. In ecent yeas developments of impoved aifoil sections fo wind tubines have been ongoing. That may have modifications in ode to impove pefomance fo special applications and wind conditions. To gain efficiency, the blade is both tapeed and twisted. The tape, twist and aifoil chaacteistic should all be combined in ode to give the best possible enegy captue fo the oto speed and site conditions [1]. Milboow and Ainslie [] used a steamline cuvatue technique fo calculation of flow patten pefomance pediction. The powe coefficients obtained though this method at high oto loadings wee highe than those obtained though momentum consideations. Lanzafame [3] and Messina used the blade element momentum theoy to obtain maximum electical enegy output fo a oto with two blades, with a 10 m diamete, in a given wind site. Yukio Watanabe [4] used the bounday element method (BEM) to evaluate the maximum the powe coefficient Cp of the hoizontal axis wind tubine (HAWT) blade opeating in low eynolds numbe ange less than In this pape, the suitable design was obtain fo fast unning wind tubine oto by using blade element theoy and momentum theoy to compute the aea unde the cuve (Cp-. The design paametes Aiflow ove a stationay aifoil poduces two foces, a lift foce pependicula to the aifoil and a dag foce in the diection of aifoil, the existence of the lift foce depends on lamina flow ove aifoil. When the aifoil is move in the diection of the lift this tanslation will combine with the motion of the ai to poduce a elative wind diection (W), the aifoil has been eoiented t maintain a good lift to dag atio. The lift and dag foces can be split into components paallel and pependicula to the diection of the undistubed wind. The lift is pependicula to the elative wind but is not in the diection of aifoil tanslation, and these components combined to fom the tangential foce and axial foce as shown in Fig.(1). The tangential foce acting on the tubine oto in the diection of tanslation which is available to do useful wok, and allow fo the blades to otate aound to hoizontal axis and causes a toque that dive some load connected to the tubine oto as shown in Fig.(1). The othe foce is axial foce on the diection of the undistibuted wind which must to be used in the design of aifoil suppots to assue stuctual integity, and the towe must be stong enough to withstand this foce. 3. Powe coefficient The faction of powe extacted fom the powe in the wind by a pactical wind tubine is usually given the symbol Cp standing fo the coefficient of pefomance which is not a constant, but vaies with the wind speed, the otational speed of the tubine, and tubine blade paametes such as angle of attack and twist angle. The HAWT have vaiable twist angle, the twist angle is vaied to hold powe coefficient at it lagest possible value up to the ated speed of the tubine, designing the blades to have a maximum coefficient of pefomance bellow the ated wind speed helps to maximize the enegy poduction of the tubine.

3 406 Ali H. Almukhta / Enegy Pocedia 18 ( 01 ) The axial and tangential intefeence facto (a and a ) can be calculated fom the following elationships Whee C C sin C cos, C C cos C sin T Fom Fig.() can get L D A L D Whee the solidity atio is Substitute equations and 3 in equation 1 can get a 1 a (1 4cos a )C V(1 T B fom equation and 4 can get a 1 a (1 4cos a )C V(1 T V(1 sin (1 cos a ) Afte simplify can get E a 1 a C 4sin T cos Fo Fig.() can get and 8 V (1 a W C CB A W V(1 sin Substitute equation 8 and 3 in 7 we get G a 1 a CA 4sin

4 Ali H. Almukhta / Enegy Pocedia 18 ( 01 ) Tip speed atio is the dimensionless atio of the tip speed to the upsteam wind speed(v) n 60V Then Fom equation 10 and Fig.() we get cot ( )(1 1 a a ) The toque coefficient is Q AV C q 1) But the moment coefficient can get fom C q a (1 d Substitute equation 11 and 6 in 13 we get C q cot (1 E d Let the local toque coefficient (m ) m 4(1 E cot Accoding to Pandtal, the eduction of efficiency which esults is given fo wind machine having (B)blades, and the wind oto is unning in the neighbohood of optimal condition the Pandtal elation follows that b 1 B Then equation 1 becomes is

5 408 Ali H. Almukhta / Enegy Pocedia 18 ( 01 ) C q 1 0 m b d The diffeent values of the coefficient (C q ain by measuing the aea situated between the vaious cuves gaduated tip speed atio values using numeical integation. Then the powe coefficient C p is elated to (C q )by elation C C P q 4. Aeofoil dag chaacteistics The definition of the dag coefficient fo wind tubine blade is based not on the fontal aea but on the plan aea, fo easons that will become clea late. The flow past a body which has a lage span nomal to the flow diection is basically two-dimensional and in such cases the dag coefficient can be based upon the dag foce pe unit span using the steam-wise chod length fo the definition Fo a wing of lage span the value of C D is oughly 0.01, at modeate eynolds numbe. The dag coefficient of an aeofoil also vaies with angle of attack. Fig.() shows that on the uppe suface pessue is ising as the flow moves towads the tailing edge, this is called an advese pessue gadient and seeks to slow the ai down. If the ai is slowed to a standstill stall will occu and the pessue dag will ise shaply. The stength of the advese pessue gadient inceases with angle of attack and so it can be expected that the dag will ise with angle of attack. Fig.(3) shows the vaiation of C D symmetical NACA001 aeofoil. The lift/dag atio ( shown in Fig. (4)) has a significant affect upon the efficiency of a wind tubine and it is desiable that a tubine blade opeates at the maximum atio. The natue of the flow patten aound an aeofoil is detemined by the eynolds numbe and this significantly affects the values of the lift and dag coefficient. The geneal level of the dag coefficient incease with deceasing eynolds numbe and below a citical eynolds numbe of about the bounday laye emains lamina causing a shap ise in the coefficient. The affect on the lift coefficient is lagely concened with the angle of attack at which stall occus. As the eynolds numbe ises so does the stall angle and because the lift coefficient inceases linealy with angle of attack below the stall, the maximum value of the lift coefficient also ises. Chaacteistic fo the NACA001 aeofoil ae shown in Fig.(5)

6 Ali H. Almukhta / Enegy Pocedia 18 ( 01 ) Compute Pogam A simulation pogam in matlab was designed, and blade element and momentum theoies ae used to get the fomulas that used in this pogam and because the fast unning HWAT has chosen theefoe the blade numbe has taken between 1 and 4 and aeofoil section NACA esult and Discussion Wind-C 1. Fig.(6) the C P - P is only 0.47, achieved at a tip speed atio of 7, which is much less than Betz limit. The discepancy is caused in this case by dag and tip losses but the stall also educes the C P at low values of the tip speed. Even with no losses included in the analysis the Betz limit is not eached because the blade design is not pefact.. The othe pinciple paamete to conside is the solidity. Fo the thee-blade machine above the solidity is but this can be alteed eadily by changing its numbe of blades. The main effect to obseve of changing solidity ae as follows, see Fig.(7). Low solidity poduce a boad, flat cuve which means that the C P will change vey little ove a wide tip speed atio ange but the maximum C P because the dag losses ai high. High solidity poduced a naow pefomance cuve with a shap peak making the tubine vey sensitive to tip speed atio changes and if solidity is too high, has a elatively low maximum C P. the eduction in C Pmax is caused by stall losses. An suitable solidity appeas to be achieved with thee blades, but two blades might be an acceptable altenative because although the maximum C P is a little lowe

7 410 Ali H. Almukhta / Enegy Pocedia 18 ( 01 ) the spead of the peak is wide and that might esult in a lage enegy captue. It might be agued that a good solution would be to have a lage numbe of blades of small individual solidity but this geatly inceases poduction costs and esults in blades which ae stuctually weak and vey flexible. 3. Fig.(8) shows how the toque developed by a tubine ises with inceasing solidity. Fo highspeed tubines designed fo electicity geneation as low a toque as possible is desiable in ode to educe geabox costs. The peak of the toque cuve occus at a lowe tip speed atio than peak of the powe cuve. The peak of cuve occus while the blade is stalled. 4. Fig.(9) the effect of solidity on thust, the thust foce on the oto is diectly applied to the towe on which the oto is suppoted and so consideation influences the stuctue design of the towe. The thust on the oto ae inceases with inceasing solidity. Fig.(1) Foce Diagam Fig.() Velocity diagam

8 Ali H. Almukhta / Enegy Pocedia 18 ( 01 ) Fig.(3) The pessue distibution aound aeofoil NACA001 Fig.(4) Lift/Dag atio vaiation

9 41 Ali H. Almukhta / Enegy Pocedia 18 ( 01 ) Fig.(5) Vaiation of the dag and lift coefficient with eynolds Numbe in the stall Fig.(6) Pefomance cuve fo thee blade tubine

10 Ali H. Almukhta / Enegy Pocedia 18 ( 01 ) Fig.(7) Effect of changing Solidity Fig.(8) the effect of solidity on toque

11 414 Ali H. Almukhta / Enegy Pocedia 18 ( 01 ) Fig.(9) The effect of solidity on thust efeences 1. institute of Technology Depatment of Mechanics, Sweeden Apil es and Pefomance of wind Canfield IMechE vol.3 Pat A: J.Powe and Enegy, Italy

12 Ali H. Almukhta / Enegy Pocedia 18 ( 01 ) Handbook, New Yok 001