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BladeDesignBladedesignAsuccessfulbladedesignmustsatisfyawiderangeofobjectives,someofwhichareinconflict.Theseobjectivescanbesummarizedasfollows:(1)maximizeannualenergyyieldforthespecifiedwindspeeddistribution;(2)limitmaximumpoweroutput(inthecaseofstallregulatedmachines);(3)resistextremeandfatigueloads;(4)restricttipdeflectionstoavoidblade/towercollisions(inthecaseofupwindmachines);(5)avoidresonances;(6)minimizeweightandcost.Designprocess•Thedesignprocesscanbedividedintotwostages:theaerodynamicdesign,inwhichobjectives(1)and(2)aresatisfied,andthestructuraldesign.•Theaerodynamicdesignaddressestheselectionoftheoptimumgeometryofthebladeexternalsurface–normallysimplyreferredtoasthebladegeometry–whichisdefinedbytheaerofoilfamilyandthechord,twistandthicknessdistributions.•Thestructuraldesignconsistsofbladematerialselectionandthedeterminationofastructuralcrosssectionorsparwithintheexternalenvelopethatmeetsobjectives(4)to(6).•Inevitablythereisinteractionbetweenthetwostages,asthebladethicknessneedstobelargeenoughtoaccommodateasparwhichisstructurallyefficient.AerodynamicdesignTheprimaryaerodynamicfactorsaffectingthebladedesignare:•Designratedpowerandratedwindspeed•Designtipspeedratio•Solidity•Airfoil•Numberofblades•Rotorpowercontrol(stallorvariablepitch)•Rotororientation(upwindordownwindofthetower)Primaryaerodynamicfactorsaffectingthebladedesign•Theoverallsizeoftherotorsweptarea,andhencethelengthoftheblades,isdirectlyrelatedtodesignratedpowerandratedwindspeed.•Otherthingsbeingequalitisusuallyadvantageoustohaveahighdesigntipspeedratio.Αhightipspeedratioresultsinalowsolidity,whichinturnresultsinlesstotalbladearea.Thisinturnshouldresultinlighter,lessexpensiveblades.Theaccompanyinghigherrotationalspeedisalsoofbenefitintherestofthedrivetrain.•Ontheotherhand,hightipspeedratiosresultinmoreaerodynamicnoisefromtheturbine.•Becausethebladesarethinner,theflapwisestressestendtobehigher.Thinnerbladesarealsomoreflexible.Thiscansometimesbeanadvantage,butthinnerbladesmayalsoexperiencevibrationproblems,andextremedeflectionscanresultinblade-towerimpacts.•Thetipspeedratioalsohasadirecteffectonthechordandtwistdistributionoftheblade.Primaryaerodynamicfactorsaffectingthebladedesign•Asdesigntipspeedratiosincrease,selectionoftheproperairfoilbecomesprogressivelymoreimportant.•Ιnparticularitisnecessarytokeepthelift-to-dragratiohighiftherotoristohaveahighpowercoefficient.•Itisalsoofnotethattheliftcoefficientwillhaveaneffectontherotorsolidityandhencetheblade'schord:thehighertheliftcoefficient,thesmallerthechord.•Ιnaddition,thechoiceofairfoilistoasignificantextentaffectedbythemethodofaerodynamiccontrolusedontherotor.Forexample,anairfoilsuitableforapitchregulatedrotormaynotbeappropriateforastall-controlledturbine.•Oneconcernisfouling:certainairfoils,particularlyonstall-regulatedturbines,arequitesusceptibletofouling(due,forexample,toabuildupofinsectsontheleadingedge).Thiscanresultinasubstantialdecreaseinpowerproduction.Primaryaerodynamicfactorsaffectingthebladedesign•Windturbinebladesfrequentlydonothavejustoneairfoilshapealongtheentirelength.Morecommonly(butnotalways),theairfoilsareallofthesamefamily,buttherelativethicknessvaries.Thickerairfoilsneartherootprovidegreaterstrengthandcandosowithoutseriouslydegradingtheoverallperformanceoftheblade.Primaryaerodynamicfactorsaffectingthebladedesign•Withpresentmanufacturingtechniquesitisgenerallyadvantageoustοhaveasfewbladesaspossible.•Thisisprimarilybecauseofthefixedcostsinfabricatingtheblades.•Ιnaddition,whentherearemoreblades(foragivensolidity)theywillbelessstiffandmayhavehigherstressesattheroots.•Αtthepresenttimeallcommercialwindturbineshaveeithertwoorthreeblades.•Two-bladedwindturbineshavehistoricallyhadalowersoliditythanthree-bladedmachines.Thiskeepsthebladecostlow,whichisoneofthepresumedadvantagesoftwobladesoverthreeblades.Primaryaerodynamicfactorsaffectingthebladedesign•Themethodofpowercontrol(stallorvariablepitch)hasasignificanteffectοnthedesignoftheblades,particularlyinregardtothechoiceoftheairfoil.•Αstall-controlledturbinedependsοnthelossofliftwhichaccompaniesstalltoreducethepoweroutputinhighwinds.Itishighlydesirablethatthebladeshavegoodstallcharacteristics.Theyshouldstallgraduallyasthewindspeedincreases,andtheyshouldberelativelyfreeoftransienteffects,suchasarecausedbydynamicstall.•Ιnpitch-controlledturbines,stallcharacteristicsaregenerallymuchlessimportant.However,itisimportanttοknowthatthebladesperformacceptablywhenbeingpitchedinhighwinds.Itisalsoworthnotingthatbladescanbepitchedtowardseitherfeather(decreasingangleofattack)orstall(increasingangleofattack).Primaryaerodynamicfactorsaffectingthebladedesign•Therotororientationwithrespecttothetowerhassomeeffectonthegeometryoftheblades,butmostlyinasecondarymannerrelatedtothepreconingoftheblades.•Thispreconingisatiltingofthebladesawayfromaplaneofrotationasdefinedbythebladeroots.Aspreviouslynoted,mostdownwindturbinesoperatewithfreeyaw.Thebladesmustbeconedawayfromtheplaneofrotationtοenabletherotorstοtrackthewindandmaintainsomeyawstability.•Someupwindrotors