STUDY ON WIND TURBINE AND ITS AERODYNAMIC

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Int. J. Mech. Eng. & Rob. Res. 2015Yamini Sarathi et al., 2015ISSN 2278 – 0149 www.ijmerr.comVol. 4, No. 1, January 2015 2015 IJMERR. All Rights ReservedResearch PaperSTUDY ON WIND TURBINE AND ITSAERODYNAMIC PERFORMANCEYamini Sarathi1*, Khemraj Patel1, Arti Tirkey1, Prakash Kumar Sen1 and Ritesh Sharma1*Corresponding Author: Yamini Sarathi, [email protected] paper aims to present the Study on wind turbine and its aerodynamic performance. Rapidincrease in global energy requirements has resulted in considerable attention towards energygeneration from the renewable energy sources. In order to meet renewable energy targets,harnessing energy from all available resources including those from urban environment isrequired. This paper represents the study of wind turbine. There are two type of wind turbineVertical axis wind turbine and Horizontal axis wind turbine that had been discussed on thispaper. Vertical Axis Wind Turbines (VAWTs) are seen as a potential way of utilizing urban energysources. Most of the research on the wind turbines constitutes condition monitoring andperformance optimization of VAWTs under a constant velocity of air where the transient effectshave not been accounted. The inconsistent behavior of the wind may change the nature of theflow field around the VAWT which could decrease its life cycle. This study is an attempt to useComputational Fluid Dynamics techniques to study and analyses the performance of a windturbine under accelerating and decelerating air inlet velocity. A Horizontal-Axis Wind Turbine(HAWT) blade with 10,000 Watt power output has been designed by the Blade ElementMomentum (BEM) theory and the modified stall model, and the blade aerodynamics are alsosimulated to investigate its flow structures and aerodynamic characteristics.Keywords: Vertical axis wind turbine, Horizontal-axis wind turbine, Reynold numberINTRODUCTIONby which the wind is used to generatemechanical power or electricity. Wind turbinesconvert the kinetic energy in the wind intomechanical power. This mechanical power canbe used for specific tasks (such as grindinggrain or pumping water) or a generator canconvert this mechanical power into electricityWind turbines operate on a simple principle.The energy in the wind turns two or threepropeller-like blades around a rotor. The rotoris connected to the main shaft, which spins agenerator to create electricity. The terms windenergy or wind power describes the process1Department of Mechanical Engg., Kirodimal Institute of Technology, Raigarh, CG, India.249

Int. J. Mech. Eng. & Rob. Res. 2015Yamini Sarathi et al., 2015(Hsiao et al., 2013). In general, the sectionalshape of the Horizontal-Axis Wind Turbine(HAWT) blade consists of the two dimensional(2D) airfoils, which result the lift and dragforces by virtue of pressure differences acrossthe 2D airfoil. Because of this, the BladeElement Momentum (BEM) theory is widelyused to outline a procedure for theaerodynamic design of a HAWT blade. Theoptimum distributions of the chord length andthe pitch angle in each section can be acquiredaccording to the design parameters, whichinclude the rated wind speed, number ofblades, design tip speed ratio and designangle of attack (Manwell et al., 2009; Bai andHsiao, 2010; and Hsiao and Bai, 2013). Theperformance capabilities of the wind turbinesdepend greatly on the torque output whichfurther depends upon the torque generatingcapability of the rotor. HAWT are more efficientas compared to VAWT but require good qualitywind energy. In urban areas where wind is inconsistent and highly fluctuating, VAWT ismore beneficial due to its low starting torquecharacteristics as well as other advantages likebeing in-expensive to build and of simpledesign (Rohatgi and Barbezier, 1999; Manwellet al., 2009; and Park et al., 2012). Windenergy has become one of the fastest growingrenewable energy sources, because energygenerated by wind power is one of the cleanestenergy resources available. As Horizontal-AxisWind Turbine (HAWT) blades become lighterand more flexible, the system dynamics mustbe analyzed comprehensively in order toevaluate and understand the complexinteraction of the elastic vibrations of the windturbines and the unsteady aerodynamic forcesacting on them. In line with the demand forlighter wind turbine blades, new advancedfabrication methods and composite materialshave been introduced, and this has resulted inreduced structural damping, which is aproperty that until very recently was impossibleto model and enhance (Riziotis et al., 2004).TYPES OF WIND TURBINEHorizontal-Axis Wind Turbine(HAWT)Horizontal-Axis Wind Turbines (HAWT) havethe main rotor shaft and electrical generator atthe top of a tower, and must be pointed intothe wind. Small turbines are pointed by asimple wind vane, while large turbinesgenerally use a wind sensor coupled witha servo motor. Most have a gearbox, whichturns the slow rotation of the blades into aquicker rotation that is more suitable to drivean electrical generator.Figure 1: Horizontal-Axis Wind TurbineVertical Axis Wind Turbines(VAWTs)Vertical axis wind turbines are a type of turbinewhere the main rotor shaft runs vertically. These250

Int. J. Mech. Eng. & Rob. Res. 2015Yamini Sarathi et al., 2015turbines can rotate unidirectional even with bidirectional fluid flow. VAWT is mainly due tothe advantages of this kind of machine overthe horizontal axis type, such as their simpleconstruction, the lack of necessity of overspeed control, the acceptance of wind fromany direction of the mechanical designlimitations due to the control systems and theelectric generators are set up statically on theground. Generally, there have been two distincttypes of vertical axis wind turbine that is theDarrieus and savonius types. For the Darrieus,there are three common blades that areSquirrel Cage Darrieus, H-Darrieus and EggBeater Darrieus (David and Spera, 1998).EFFECT OF REYNOLD NUMBERFigure 2: Vertical Axis Wind TurbineFor axial-flow (commonly referred to ashorizontal-axis) turbines, small changes in angleof attack a of the local relative wind w.r.t. bladechord occur throughout blade rotation, and aredue to mean shear in the boundary layer (whichcan also cause varying deformation) orturbulence. This means it’s relatively easy topredict turbine performance with models thatemploy static foil section data, e.g., BladeElement Momentum (BEM) methods.Reynolds Number Effects on WindTurbines1. The maximum power coefficient CP.max isincreased. This is caused by a lower dragFigure 3: Local Reynolds Number on the Reference Rotor Blade EU100 at VariousOperating Conditions. The Local Reynolds Number Only Depends on the SectionVelocity, Vrel and the Respective Chord Length, c at Constant Kinematic Viscosity, 251

Int. J. Mech. Eng. & Rob. Res. 2015Yamini Sarathi et al., 2015in the low drag bucket, which leads to lessprofile losses and thus a power coefficientwhich is closer to the theoretical maximumof CP 0.59.by increasing the blade set angle (AbbottIra et al., 1945).Reynolds Number Effect ofContaminated Profile2. The power coefficient at low tip speedratios is increased. The reason thereforeis that more lift can be generated due tothe higher maximum lift caused by theReynolds number effect. Also less bladesections operate in stalled conditionswhich results in less drag. Hence profilelosses are decreased at low tip speedratios as well.Figure 4 shows the Reynolds number effecton the lift coefficient. With increased ReynoldsFigure 4: The Effect of Carborundum 60on the Lift Curve at Various ReynoldsNumbers3. The shape of the power coefficient curvechanges, its saddle becomes wider. Thisis a consequence of 1. And 2: less drag atthe Best lift to drag ratio and higher stallangle of attack. This is Advantageous foroperating conditions at non optimum tipspeed Ratios, i.e., for operation at maximumtip speed below rated power.4. The optimum blade set angle is increased.The reason therefore is that the best lift todrag ratio is shifted to smaller angles ofattack and thus smaller lift coefficients.Hence the optimum performance isreached at smaller section in flow angleswhich is achieved by increasing the pitch.Figure 5: The Effect of Carborundum 60on the Pressure Distribution at TwoReynolds Numbers5. The optimum tip speed ratio is increased.The reason therefore is that the optimumlift to drag ratio occurs at smaller angles ofattack and hence at smaller lift coefficients.The decrease in lift coefficient iscompensated by increasing the sectionvelocity, which shifts the optimum tip speedratio to higher values.6. The thrust coefficient is increased at low tipspeed ratios, but can be partly decreased252

Int. J. Mech. Eng. & Rob. Res. 2015Yamini Sarathi et al., 2015numbers the boundary layer becomes thinner,but more powerful. The impact of carborundumthus decreases. The stall occurs at higherangle of attack, so climax increases.Figure 7: Reynolds Number Effecton Drag for Clean ProfileReynolds Number Effect of CleanProfileFor the clean thin profile the Reynolds numbereffect is typical, just like the description in theclassical literature. As shown in Figures 6 and7, with increased Reynolds number,Clmaxincreases, lift curve is more linear, itsslope goes up slightly, Cdmindecreases andthe laminar bucket becomes smaller. Figure10a is a schematic of a straight-bladed-fixedpitch VAWT which is the simplest, but typicalform, of the Darrieus type VAWTs. Figure 10billustrates typical flow velocities around arotating VAWT blade at a given azimuthalangle , as well as the aerodynamic forcesperceived by the blade. The azimuthal angle is set to be zero when the blade is at thetop of the flight path and is increases in acounter-clockwise direction.Reynolds Number Effect of FlowControl DevicesFigures 8 and 9 show the effect of a Gurneyflap at two Reynolds numbers. It is clear thatthe selected Gurney flap suitable for the lowReynolds number is not optimal for the highReynolds number.Figure 8: The Effect of Gurney Flap on theChange of Lift Referred to the CleanProfile at Two Reynolds NumbersFigure 6: Reynolds Number Effect on Liftfor Clean Profile253

Int. J. Mech. Eng. & Rob. Res. 2015Yamini Sarathi et al., 2015Figure 10: Basics of VAWT: a) Sketchof a Fixed-Pitch Straight-Bladed VAWT;b) Typical Flow Velocities in DarrieusMotionFigure 9: The Effect of Gurney Flap on theChange of Lift Drag Ratio Referred to theClean Profile at Two Reynolds Numbers(a)AERODYNAMICS OF A VAWTThe movement of the blades in a VAWTentails a large range of flow regimes from restto the operating condition. In order tounderstand the starting behavior, it is usefulto consider the flow conditions experiencedby the turbine blade when it rotates aroundits vertical axis.(b)Figure 10a is a schematic of a straightbladed-fixed-pitch VAWT which is the simplest,but typical form, of the Darrieus type VAWTs.Despite the simplicity, its aerodynamicanalysis is still quite complex. One feature isthat the relative velocities perceived by theblade always change as the blade moves atdifferent azimuthal positions. Figure 10billustrates typical flow velocities around arotating VAWT blade at a given azimuthal angle , as well as the aerodynamic forces perceivedby the blade. The azimuthal angle is set tobe zero when the blade is at the top of the flightpath and is increases in a counter-clockwisedirection.254

Int. J. Mech. Eng. & Rob. Res. 2015Yamini Sarathi et al., 2015From the vectorial description of velocities,Figure 10b, we can obtain the followingexpression that establishes the relationshipbetween the angle of attack D the Tip SpeedRatio (TSR) and the azimuthal angle of ablade in Darrieus motion (without velocityinduction)ratios to extract as much power out of the windas possible.Optimum Tip Speed RatioThe optimum tip speed ratio depends on thenumber of blades in the wind turbine rotor. Thefewer the number of blades, the faster the windturbine rotor needs to turn to extract maximumpower from the wind. A two-bladed rotor hasan optimum tip speed ratio of around 6, athree-bladed rotor around 5, and a four-bladedrotor around 3. Different types of turbine havecompletely different optimal TSR values-forexample a Darrieus wind turbine is a verticalaxis (VAWT) design with aerofoil blades whichgenerate aerodynamic lift and therefore theTSR can be high, but a Savonius wind turbinewhich is also a VAWT is a drag design and sothe TSR will always be less than 1-i.e., itcannot spin faster than the wind hitting it.tan D U sin /( R U cos ) sin /( cos )or arc tan(sin / cos )Another important parameter is thereduced frequency which governs the level ofunsteadiness.The reduced frequency k, defined as k c/2Ueff where is the angular frequency ofthe unsteadiness, c is the blade chord and Ueffis the velocity of the blade, can be expressedin terms of TSR as:k (c/d) / ( 2 2 cos 1)Figure 11: Tip Speed RatioTIP SPEED RATIO OF WINDTURBINEThe Tip Speed Ratio (TSR) is an extremelyimportant factor in wind turbine design. TSRrefers to the ratio between the wind speed andthe speed of the tips of the wind turbine blades.TSR Tip Speed of BladeWind SpeedThe Tip Speed Ratio (often known asthe TSR) is of vital importance in the designof wind turbine generators. If the rotor of thewind turbine turns too slowly, most of the windwill pass undisturbed through the gap betweenthe rotor blades. Alternatively if the rotor turnstoo quickly, the blurring blades will appear likea solid wall to the wind. Therefore, windturbines are designed with optimal tip speedREFERENCES1. Abbott Ira H Doenhoff, Albert E von andStivers Lois S Jr (1945), “Summary ofAirfoil Data”, NACA Report No. 824,Memorial Aeronautical.255

Int. J. Mech. Eng. & Rob. Res. 2015Yamini Sarathi et al., 2015Theory”, Design and Application, JohnWiley & Sons Ltd., WS, UK.2. Bai C J and Hsiao F B (2010), “CodeDevelopment for Predicting theAerodynamic Performance of a HAWTBlade with Variable-Speed Operation andVerification by Numerical Simulation”, 17thNational Computational Fluid Dynamics(CFD) Conference, Taoyuan, Taiwan.7. Manwell J F, Mcgowan J G and Rogers AL (2009b), Wind Energy Explained:Theory, Design and Application, 2 ndEdition, John W iley & Sons Ltd.,Chichester, UK.3. David A and Spera P H D (1998), WindTurbine Technology, ASME Press.8. Park K S, Asim T and Mishra R (2012),“Computational Fluid Dynamics BasedFault Simulations of a Vertical Axis WindTurbine”, Journal of Physics: ConferenceSeries, Vol. 364.4. Hsiao F B and Bai C J (2013), “Designand Power Curve Prediction of HAWTBlade by Improved BEM Theory”, Journalof Chinese Society of MechanicalEngineers.9. Riziotis V A, Voutsinas S G, Politis E Sand Chaviaropoulos P K (2004),“Aeroelastic Stability of Wind Turbines:The Problems, the Methods and theIssues”, Wind Energy, Vol. 7, pp. 373-392.5. Hsiao F B, Bai C J and Chong W T(2013), “The Performance Test of ThreeDifferent Horizontal Axis Wind Turbine(HAW T) Blade Shapes UsingExperimental and Numerical Methods”,Energies, Vol. 6, pp. 2784-2803.10. Rohatgi J and Barbezier G (1999), “WindTurbulence and Atmospheric Stability—Their Effect on Wind Turbine Output”,Journal of Renewable Energy, Vol. 16,pp. 908-911.6. Manwell J F, McGowan J G and Rogers AL (2009a), “Wind Energy Explained –256

Horizontal-Axis Wind Turbines (HAWT) have the main rotor shaft and electrical generator at the top of a tower, and must be pointed into the wind. Small turbines are pointed by a simple wind vane, while large turbines generally use a wind sensor coupled with a servo motor. Most have a ge