Wind-Turbine-Technology

WindEnergyTechnologyWhatworks&whatdoesn’tOrientationTurbinescanbecategorizedintotwooverarchingclassesbasedontheorientationoftherotorVerticalAxisHorizontalAxisVerticalAxisTurbinesAdvantages•Omnidirectional–Acceptswindfromanyangle•Componentscanbemountedatgroundlevel–Easeofservice–Lighterweighttowers•CantheoreticallyuselessmaterialstocapturethesameamountofwindDisadvantages•Rotorsgenerallyneargroundwherewindpoorer•Centrifugalforcestressesblades•Poorself-startingcapabilities•Requiressupportattopofturbinerotor•Requiresentirerotortoberemovedtoreplacebearings•Overallpoorperformanceandreliability•HaveneverbeencommerciallysuccessfulLiftvsDragVAWTsLiftDevice“Darrieus”–Lowsolidity,aerofoilblades–MoreefficientthandragdeviceDragDevice“Savonius”–Highsolidity,cupshapesarepushedbythewind–Atbestcancaptureonly15%ofwindenergyVAWT’shavenotbeencommerciallysuccessful,yet…Everyfewyearsanewcompanycomesalongpromisingarevolutionarybreakthroughinwindturbinedesignthatislowcost,outperformsanythingelseonthemarket,andovercomesallofthepreviousproblemswithVAWT’s.Theycanalsousuallybeinstalledonarooforinacitywherewindispoor.WindStorMag-WindWindTreeWindWandlerTipSpeedRatioCapacityFactorHorizontalAxisWindTurbines•RotorsareusuallyUp-windoftower•Somemachineshavedown-windrotors,butonlycommerciallyavailableonesaresmallturbinesActivevs.PassiveYaw•ActiveYaw(allmedium&largeturbinesproducedtoday,&somesmallturbinesfromEurope)–Anemometeronnacelletellscontrollerwhichwaytopointrotorintothewind–Yawdriveturnsgearstopointrotorintowind•PassiveYaw(Mostsmallturbines)–Windforcesalonedirectrotor•Tailvanes•DownwindturbinesAirfoilNomenclaturewindturbinesusethesameaerodynamicprincipalsasaircraftLift&DragForces•TheLiftForceisperpendiculartothedirectionofmotion.WewanttomakethisforceBIG.•TheDragForceisparalleltothedirectionofmotion.Wewanttomakethisforcesmall.α=lowα=medium10degreesα=HighStall!!αVR=RelativeWindVΩRΩrVα=angleofattack=anglebetweenthechordlineandthedirectionoftherelativewind,VR.VR=windspeedseenbytheairfoil–vectorsumofV(freestreamwind)andΩR(tipspeed).ApparentWind&AngleofAttackTip-SpeedRatioTip-speedratioistheratioofthespeedoftherotatingbladetiptothespeedofthefreestreamwind.Thereisanoptimumangleofattackwhichcreatesthehighestlifttodragratio.Becauseangleofattackisdependantonwindspeed,thereisanoptimumtip-speedratioΩRVTSR=ΩRRWhere,Ω=rotationalspeedinradians/secR=RotorRadiusV=Wind“FreeStream”VelocityPerformanceOverRangeofTipSpeedRatios•PowerCoefficientVarieswithTipSpeedRatio•CharacterizedbyCpvsTipSpeedRatioCurve0.40.30.20.10.0Cp121086420TipSpeedRatioTwist&Taper•Speedthroughtheairofapointonthebladechangeswithdistancefromhub•Therefore,tipspeedratiovariesaswell•Tooptimizeangleofattackallalongblade,itmusttwistfromroottotipPitchControlvs.StallControl•PitchControl–Bladesrotateoutofthewindwhenwindspeedbecomestoogreat•StallControl–Bladesareatafixedpitchthatstartstostallwhenwindspeedistoogreat–Pitchcanbeadjustedforparticularlocation’swindregime•ActiveStallControl–ManylargerturbinestodayhaveactivepitchcontrolthatturnsthebladestowardsstallwhenwindspeedsaretoogreatAirfoilinstall•Stallarisesduetoseparationofflowfromairfoil•Stallresultsindecreasingliftcoefficientwithincreasingangleofattack•StallbehaviorcomplicatedduetobladerotationRotorSoliditySolidityistheratiooftotalrotorplanformareatototalsweptareaLowsolidity(0.10)=highspeed,lowtorqueHighsolidity(0.80)=lowspeed,hightorqueRAaSolidity=3a/ABetzLimitBetzLimit5926.2716Cmax,pRotorWakeRotorDiscAllwindpowercannotbecapturedbyrotororairwouldbecompletelystillbehindrotorandnotallowmorewindtopassthrough.Theoreticallimitofrotorefficiencyis59%NumberofBlades–One•Rotormustmovemorerapidlytocapturesameamountofwind–Gearboxratioreduced–Addedweightofcounterbalancenegatessomebenefitsoflighterdesign–Higherspeedmeansmorenoise,visual,andwildlifeimpacts•Bladeseasiertoinstallbecauseentirerotorcanbeassembledonground•Captures10%lessenergythantwobladedesign•UltimatelyprovidenocostsavingsNumberofBlades-Two•Advantages&disadvantagessimilartooneblade•Needteeteringhubandorshockabsorbersbecauseofgyroscopicimbalances•Capture5%lessenergythanthreebladedesignsNumberofBlades-Three•Balanceofgyroscopicforces•Slowerrotation–increasesgearbox&transmissioncosts–Moreaesthetic,lessnoise,fewerbirdstrikesBladeCompositionWoodWood–Strong,lightweight,cheap,abundant,flexible–Popularondo-ityourselfturbines•Solidplank•Laminates•Veneers•CompositesBladeCompositionMetal•Steel–Heavy&expensive•Aluminum–Lighter-weightandeasytoworkwith–Expensive–SubjecttometalfatigueBladeConstructionFiberglass•Lightweight,strong,inexpensive,goodfatiguecharacteristics•Varietyofmanufacturingprocesses–Clothoverframe–Pultrusion–Filamentwindingtoproducespars•MostmodernlargeturbinesusefiberglassHubsThehubholdstherotortogetherandtransmitsmotiontonacelleThreeimportantaspects•Howbladesareattached–Nearlyallhavecantileveredhubs(supportedonlyathub)–Struts&Stayshaven’tprovedworthwhile•FixedorVariablePitch?•FlexibleorRigidAttachment–Mostarerigid–SometwobladeddesignsuseteeteringhubsDriveTrainsDriveTrainstransferpowerf