WindEnergyEngineering FundamentalsofWind FarmAerodynamic LayoutDesign
FarschadTorabi
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Prefacexiii
1.Windenergy
1.1Historyofwindturbines
1.4Windturbinetypes 5
1.4.1Horizontal-axiswindturbine5
1.4.2Vertical-axiswindturbine7
1.5Windturbinecomponents 11
1.5.1Blades,hub,andlow-speedshaft12
1.5.2Generator14
1.5.3Gearboxandhigh-speedshaft15 1.5.4Tower16
1.5.5Yawsystems17
1.5.6Anemometer17 1.5.7Brakes20
1.5.8Fluctuationmonitoringsystems21
1.5.9Lubricationsystem22
1.5.10Foundation22
2.Windpropertiesandpowergeneration
2.1Atmosphericproperties
2.1.1Globalwinddirection27 2.1.2Turbulence29 2.1.3Variationofwindwithheight29
2.2Statisticalstudyofwind 36
2.2.1Meanandvariance39
2.2.2Probability42
2.2.3Windrosechart47
2.3Windpower
2.3.1Powerofawindelement50
2.3.2Powerofanidealturbine,actuatordiscmodel51
2.3.3Betz’slimit54
2.4Efficiencyofwindturbinecomponents
2.4.1Efficiencyofbladesorcoefficientofperformance57
2.4.2Efficiencyofgearbox59
2.4.3Efficiencyofgenerator61 2.4.4Overallefficiency61
2.5Yearlygainedenergyofawindturbine
3.Basicsofaerodynamics
3.1Airfoils
3.2Aerodynamicforcesonanairfoil
3.6Bladeswithdifferentairfoils
3.7Simulationofwindturbines
3.7.1Stall-regulatedwindturbines100
3.7.2Pitch-controlledwindturbines104 3.8Summary
4.Windturbinewakeanditsroleinfarmdesign
4.1Wakegenerationofawindturbine
4.2Conventionalwakemodels
4.2.1Jensen’smodel116
4.2.2Frandsen’smodel120
4.2.3Larsenmodel127
4.2.4Ghadirianmodel136
4.3Gainedenergyofafarm
5.Analyticalmodelbasedonsimilaritysolution
5.1Turbulentfree-shearwake 151
5.2Self-similaritymethod 151
5.3Similaritysolutionforasinglewindturbine 154
5.3.1Initialwakeprofilejustbehindthewindturbine154
5.3.2Calculationofwakeexpansionradiusandvelocitydeficit ataxisline158
5.4Wakeinteraction 161
5.5Simulationofawindfarm 164
5.5.1Windprofilejustbehindawindturbine164
5.5.2Wakeprofileatfardistancesbehindthewindturbine165
5.6Simulationofwindfarms 166
5.6.1Fourwindturbinesinarow166
5.6.2A 4 × 4 windfarm169
5.7Thegainedenergyofawindfarm 176
5.8Summary 182
5.9Problems 182 References 183
6.Numericalsimulationofawindturbine
6.1Basicfluiddynamicsconcepts 188
6.1.1Governingequationsoffluidflow189 6.1.2Turbulence190
6.1.3Wake192
6.2Differenttypesofmodeling 192
6.2.1Fullrotor193
6.2.2Actuatorline194
6.2.3Actuatordisc195
6.3DevelopmentofactuatordiscmethodusingOpenFOAM 196
6.3.1GeometryandmeshgenerationusingBlockMesh196
6.3.2Sourcetermdefinition198
6.3.3Turbulencemodeling199
6.3.4Solverselectionandsettings200
6.4Modifiedactuatordisc 201
6.4.1Radialloaddistribution201
6.4.2Modifiedsourceterm202
6.5Simulationexample 203
6.5.1Numericalmodel203
6.5.2Computationaldomain204
6.5.3Boundaryconditions204
6.5.4Gridsensitivity205
6.5.5Results206
6.6Summary 211
6.7Problems 213 References 213
7.Numericalsimulationofawindfarm
7.1Windfarmlayoutgeneration
7.1.1Automaticlayoutgeneration216
7.1.2Upstreamvelocity218
7.2Simulationexample:HornsRevoffshorewindfarm
7.2.1Numericalsettings220
7.2.2Boundaryconditions220
7.2.3Computationaldomainandmeshstudy221
7.2.4Results222
8.Optimizationforwindfarmlayoutdesign 8.1Optimizationalgorithms
8.4.1Optimizationforaconstant-directionwind238 8.4.2Optimizationforarealcase241 8.4.3Optimizationofa
8.5Applyingadditionalconstraints
C.Somewindturbinespecifications
C.4NordtankNTK200
C.5VestasV27
C.6VestasV29
C.7MiconM530
C.8EnerconE-30
C.9NordtankNTK400
C.10VestasV39
C.11NordtankNTK500/41
C.12VestasV44
C.13EnerconE-40/6.44
C.14WinconW755/48
C.15VergnetGEVHP1000/62
C.16SiemensSWT-1.3-62
C.17VestasV802MW
C.18VestasV902MW
C.19EnoEnergyEno100
C.20SiemensSWT-2.3-93Offshore
C.21MapnaMWT2.5-103-I
C.22SiemensSWT-4.0-130
C.23SiemensSWT-6.0-154
C.24Aerodyn-8.0MW
C.25AMSCwt10000ddSeaTitan
C.26Summary
D.Samplewindfarms
D.1A4-in-a-rowwindfarm
D.2A 4 × 4 windfarm
D.3HornsRevwindfarm
D.4AghkandwindfarminIran
E.Optimizationmethods
MehrzadAlizadeh
E.1Crowsearchalgorithm
E.2Whaleoptimizationalgorithm
E.3Teaching–learning-basedoptimizationalgorithm
E.4Particleswarmoptimizationalgorithm
E.5Geneticalgorithm
E.6Summary
F.ImplementingoptimizationmethodsinC++
F.1Geneticalgorithm(GA)
G.ImplementingbladeelementmomentummethodinC
Preface
Humansrequireenergyfortheirlives.Withoutsustainableenergy,thelifeofhumansisnotguaranteed.Thenewgenerationsandpopulationgrowthaddmore importancetoenergygeneration.However,anydevelopmentinenergygenerationmustmeetthestandardsofsustainability.Energygenerationfromfossil fuelscausesglobalwarmingandwillgraduallyendangerhumanlivesifitisnot stopped.Forthisreason,peoplearetryinghardtofindacarbon-freesolutionfor energygeneration.
Windenergyisoneoftheoldestsourcesofenergyharnessedbyhumanbeings.Ancientpeoplehaveusedthepowerofwindfordrivingtheirsailingships andboats.Inadditiontohumanbeings,windplaysavitalroleinthelifeof alllivingspecies.Thefactthatwinddoesnotcontaminatetheairandisinexhaustiblemakeswindenergyquiteattractiveforenergygeneration.Inaddition totheabovecharacteristics,windenergycanbefoundalmostanywhere.These characteristicshavemadewindenergyveryattractiveforthescientistsandindustrialsectors.
Installingwindturbinesinaspecificlandrequireslotsofinitialplanning. Thesiteselection,installingprocess,powerregulationandtransmission,civil works,andmanyotherissuesmustbecheckedbeforehand.Eachofthementionedproblemsrequiresspecialattentionandcalculations.Oneoftheinvolved issuesisthelayoutdesignofthefarm.Sincewindturbinesaffectthewindspeed, theyhaveanaerodynamiceffectontheirdownstreamneighbors.Accordingto thiseffect,thedownstreamturbinesoperatewithalowerpower.Thus,ifaproper designisnotchosen,thewindturbineswillnotworkintheiroptimumstate.The resultisasubstantialreductioninnetannualenergyproduction.Theenergyreductionmaymakethewindfarmdesigneconomicallyimpractical.Therefore, wehavetoobtainapracticallyeconomiclayoutforthefarm.
Thepresentbookfocusesontheaerodynamicoptimizationofwindfarms. Althoughthisisavasttopicandthephysicalphenomenaarequitecomplex,we triedtoleadthereaderstepbystepthroughthefield’sconcepts.Itstartswith simpleissuesandgraduallyexplainsthedetailsofthelayoutdesignprocess. Thepresentbookassumesthatthereadersmaybenewtothefield.Therefore, thebasicsofeachtopicareexplainedbydifferentexamples.Thus,thebookis suitableforallpeopleiftheyareeithernewtothetopicorexperienced.Different
practicalexamplessupportallthechapters,andalsosomecodesaregivenas appendixestothebook.Therefore,industrialwindexpertscanbenefitfromthe preparedmaterial.
Thisbookconsistsofeightchapters.Chapter 1 talksaboutwindenergyitself,theroleofwindenergyinhumanlife,differenttypesofwindturbines,and thewindturbinecomponents.Chapter 2 dealswiththewind,whattheorigin ofwindis,anditsstatisticalstudies.Chapter 3 focusesonthebasicsofaerodynamics.Thischapterisfundamental,andItriedtocollectonlythebasics ofaerodynamicsnecessaryforsupportingthenextchapters.Obviously,many differenttopicsshouldbestudiedtohaveageneraloverviewoftheaerodynamics.Inthischapter,thebladeelementmomentummethodisintroduced, whichcanbeusedforthesimulationofawindturbine.Chapter 4 summarizes someconventionalmethodsthatareusedforthesimulationofthewakeofwind turbines.Thesemodelsarewidelyusedbyresearchersallovertheworld,and someofthemareimplementedinapplications.Chapter 5 explainsthewake modeldevelopedbytheauthor.Thismodelhasbeenusedandverifiedseveral times.Theresultsshowagoodagreementwiththenumericalmodels.Chapter 6 givestheguidelinesthatarenecessaryforthesimulationofasinglewind turbineusingCFDcodes.Differentscenariosthatleadtotheimprovementof themodelarediscussedinthischapter.Chapter 7 extendsthemodeldeveloped inChapter 6 tothesimulationofawindfarm.Itisshownthatthesimulation ofawindfarmrequiressomeconsiderations.Themainpointsarediscussedin detailandexplainedthroughexamples.Finally,Chapter 8 dealswiththeoptimizationofwindfarm,usingdifferentoptimizationalgorithmsandthecontents developedinallthepreviouschapters.Inthisview,Chapter 8 isthemaingoal ofthepresentbook,buttheknowledgeofthepreviouschaptersarenecessary.It istriedtomakethechaptersbeself-contained.Infact,allthematerialsthatare requiredforthelastchapter,arediscussedinotherones.However,sincewind turbineaerodynamicisacomplexphenomenon,readersareencouragedtorefer tootherreferences.
Inadditiontothechapters,thebookconsistsof7appendixes.Appendix A givesanintroductiontotheancientPersianwindmillsthatareconsideredthe oldestoperationalwindmillsintheworld.Appendix B collectssomepractical airfoilsthatareusedinthewindturbineindustry.Thepropertiesoftheairfoils aregiveninfiguresandequations.Appendix C representssomeoperational windturbinesandgivestheirpropertiesasfiguresandtables.Tosupportthe windfarmsimulations,Appendix D presentssomewindfarmsandtheircharacteristics.Appendix E,writtenbymystudentMr.MehrzadAlizadeh,talksabout differentoptimizationmethodssincetheyarefrequentlyusedinwindfarmoptimizationprocesses.Tosupportthecontextofthebook,thegeneticalgorithm, andtheparticleswarmoptimizationmethodsareimplementedinC++language andarepresentedinAppendix F.Finally,theimplementationofthebladeelementmomentummethodisgiveninAppendix G.Thedatacollectedinthese
Preface xv appendixesareusedintheexamples,problems,andexplanationsofdifferent chapters.
Ihopethatthepresentbookishelpfulforallthepeoplelookingforacleaner environment,andIhopethatthesestepswillmaketheworldabrighterplace forallhumanbeingsandotherlivingspecies.
FarschadTorabi
Tehran,Iran
June2021
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Windenergy
1.1Historyofwindturbines
Humanhasharnessedandusedwindenergyforaverylongtime.Itisoneof themostancientenergysourcesforthehumanbeing.Thefirstknownusage ofwindenergydatesbacktowhenEgyptiansusedsailboatsandshipsonthe Nileriverover5000yearsago.Othersimilarboatsandvesselscanbeseen inhistoryallovertheworld,includingEuropean,Persian,Chinese,andother ancienthistoricallygreatnations.
Otherthansailingpurposes,thefirstknownwindmillswereincorporatedin Persia(Iran),wherepeopleusedpaddle-typewindmillsforgrindingflourfrom cerealssuchaswheatandbarley(Manwelletal., 2010).Thewindmillconstructiondatesbackto1000yearsago,andthewholestructurewasmadeofclayand wood.ThecitywheretheverticalwindmillsareinstallediscalledNashtifan,a placeneartheborderofIranandAfghanistanwithagoodpotentialforwind. DetailsoftheancientwindmillsareexplainedinmoredetailinAppendix A Centurieslater,windmillsbegantoappearinEuropebutindifferentshapes. MerchantsandcrusadersbroughtthetechnologyfromPersiatoEurope,and theDutchdevelopedthefirstwindpumpstodrainthelakesandmarshesinthe RhineRiverDelta.AlthoughtheconceptofwindmillswasobtainedfromPersia,incontrasttothevertical-axisdesigns,theearlyEuropeanwindmillswere horizontal-axistypewherethepowershaftisplacedhorizontallyandtheobtainedpoweristransmittedbymechanicalmechanismssuchasgears.
Duringthe18thand19thcenturies,thetechnologyofwindmillsreacheda highlevel.ManywindmillsandwindpumpswereinstalledalloverEurope,especiallyinthenorthernEuropeancountries.IntheNetherlands,about6000to 8000windmillsandwindpumpswereinstalled,anditisestimatedthatEuropean countriesinstalledmorethan23000windmills.Duringthesecenturies,technologywasdevelopedandenhancedthroughtrialanderror,andmanufacturing wasstate-of-the-art.Duringthesameperiodandlater,Americancolonistsmade thousandsofwindmills,windpumps,andwindsawmillsallovertheUnited States.Inthelate1800sandearly1900s,thefirstwindturbineswereintroduced bycouplinganelectricalgeneratortothesamewindmills.
Withtheappearanceofthermalpowerplantsandcentralgrids,theroleof windturbinesdeclined,andtheworldstartedusinggridpower.However,after theoilcrisisinthe1970s,theinterestindevelopingalternativesourcesofenergy,includingwindenergy,started.Fromthenon,theresearchonenhancing
windturbinesincreased.Inmanycountries,suchastheUnitedStates,Germany, Denmark,andmanyothers,universitiesandindustriesstartedproducingmore efficientwindturbines.
1.2Prosandconsofwindenergy
Likeanysystemorsourceofenergy,windenergyhasitsownprosandcons. Themainadvantagesofwindenergyare:
CleanorGreenEnergy Itisprettyevidentthatwindisapartofnature,and itblowsduetothethermaleffectsontheearth.Hence,itsproducesno emissions,noheat,andnoothersourceofpollution.Consequently,wind energyiscategorizedasacleanorgreenenergysource.Incomparisonwith commonthermalpowerplants,windturbinescausenoacidrain,smog,or greenhousegasses.
TheEnergyAmount Thereismuchmorewindpowerthanhumansneedeven ifweconsidertheenergyconsumptiongrowth.Windpowerisabundant andinexhaustible.Ifweharnessonlyasmallpercentageoftheglobalwind energy,itwouldbemorethanenoughforallourneeds.
RenewabilityandSustainability Windblowsduetothethermalenergyobtainedfromthesun.Thismeansthataslongasthesunandearthexist, windwillblow.Forthisreason,windenergyisconsideredasoneofthe renewablesourceswhichwecantrust.
ADomesticSource Windenergyisadomesticsource,andwecaninstallwind turbinesalmostinanycountry.Wecanuselocalgridstodistributethe harnessedenergywithminimalloss.
RequiresSmallLand Installingawindturbinerequiresaminimalareafor constructingthefoundation.Incontrasttosolarenergy,windturbinescan beinstalledincurrentlyactivefieldswithoutchangingthelandintoa powerplant.Forinstallingsolarpanels,thelandshouldbededicatedtothe plant,andyoucannotusethelandforagricultureanymoresincethepanels covertheground.However,windturbinesrequireonlyasmallfoundation, andtherequiredspacebetweentheturbinesshouldbeabouthundredsof meters.Thedifferencebetweensolarandwindenergyisthatsolarenergy isharnessedoveraspecificareawhilethewindenergyisharnessedinthe verticalspace.
Thefarmersmayalsowelcometheinstallationofwindturbinesontheir landsbecausetheyrentsmallpiecesoftheirlandwithoutsignificantlylosingtheirusualfarmingproductions.
CreatingJob Installingawindturbineandharnessingwindenergycreates alotofjobopportunities.Sincewindenergyisamultidisciplinaryfield, manydifferentexpertsshouldaccompanyinstallingandmaintaininga windturbine.Theinstallationrequirescivilengineerstoconstructthefoundation,mechanicalengineersformakingtheturbines,electricalengineers
forconnectingthepowertothegrid.Also,itrequireslotsoftechniciansfor wiring,supporting,maintaining,monitoring,andotherrelatedactivities.
LowMaintenance Comparingwithotherpowerplantssuchasnuclear,thermal,hydropower,etc.,windturbinesrequirelessmaintenance.Awind turbinemayworkwithoutfailureforaverylongperiodoftime.TheinstalledwindmillsandwindpumpsinEuropestillworkwithoutfailureafter 50yearsorevenmore.Also,theancientPersianwindmillsareoperational todayafterhundredsofyears.
CostEffective
Oneofthemostimportantfactorsorbenefitsofwindenergyis thatitisquitecost-effective(Muyeenetal., 2008).Inparticular,inplaces wherewindalwaysblows,thecostofwindpowercanrangebetweentwo andsixcentsperkilowatt-hour.Itshouldbenotedthatawindturbineis designedtooperateforover20years.Inthisperiod,itdoesnotconsume anyfuel,and,asitwasstatedbefore,itisalow-maintenancedevice.These factorscontributetothelowpriceoftheproducedenergy.
Sofar,themainadvantagesofwindenergyandwindturbinesarediscussed. But,ithasitsowndisadvantages,too.Thefollowingaresomeofthemaindisadvantagesofwindenergy:
Noise Windturbinesgeneratenoisethatinsomecasescausesproblemsfor people.However,insomedesigns,thebladedesignissuchthatthegeneratednoiseisnotinthehearingrange.Theseturbines,althoughapparently silent,producenoiseforotherlivinganimals.Ingeneral,noiseisoneof thepollutionsourcesofwindturbines.
AestheticPollution Windturbinesaremadebig.Therefore,theymakeaestheticpollutionwhentheyareinstalledandmadeoperational.Especially, offshorewindturbinesmaymakethebeachesuncomfortableduetothe lackofaesthetics.
DamagingLocalWildlife
Therotationofturbinebladesmayharmtheflyingbirds.Moreover,thenoiseoftheturbinemayhavesomeundesirable influenceonlocalwildlife.
Recycling Thewindturbinecomponentsaremadeofmetals,composites,nonmetallicparts,etc.Therefore,asanyotherobject,theyhavemanyparts that,iftheycannotberecycled,willproducewastewhichisharmfulfor theenvironment.
GenerationinRemoteLands Aswementionedbefore,usingwindturbines aslocalgeneratorsmayreducethewiringandpowerlosscomparedto thegridloss.However,inmanycases,thelocalwindisnotverystrong.In otherwords,inmanycases,thewindispowerfulinplaceswhereelectricity isnotrequired.Forexample,offshorewindhasmorepowerthanonshore wind(NgandRan, 2016).Hence,windpowerplantsshouldbeinstalled whereelectricityisnotneeded.
BeingDiscontinuous
Likeanyothernaturalsourceofenergy,thegenerated powerisdiscontinuous.Forexample,windmaystopblowing,oritsvelocityanddirectionmayvary.Hence,asophisticateddesignshouldbedone
FIGURE1.1 Currentstateandforecastingofenergygainedfromonshorewindfarms(datataken from(IEA, 2019a)).
toobtainsustainableenergy.Inmanycases,thegeneratedpowershould bestoredinlarge-scalestoragedevices.Oritshouldbecoupledwithother sourcestoobtainareliableandsustainablesourceofenergy.
VariationofPower Thewindpowerisproportionaltothecubeofitsvelocity(Burtonetal., 2001).Thus,ifthespeedofwindbecomestwotimes faster,thepowerbecomeseighttimeslarger.Consequently,theturbinedesignshouldbesuchthatitoperatesinlow-speedwindsandcanwithstand high-speedwindpower.Thiscreatesmanydesignandmechanicalproblems,whichinturnaddsmorecosttotheproject.
1.3Trendofwindenergyintheworld
Theenergyobtainedfromwindhasagrowingtrendbothforonshoreandoffshorefarms.Itmeansthattheenergycostwilldecreaseinthefuture,andwind energywillbecomemoreeconomical.
Mostinstalledpowerplantsareonshoresincetheirconstructionismoreeconomical,althoughtheavailableonshorepowerislessthanoffshore.Thedatain Fig. 1.1 showsthattheenergygainedfromonshorefarmshasanexponential trendsince2000.However,toreachthegoalin2030,moreeffortisneeded.
Constructinganoffshorewindfarmismuchmorechallengingthananonshoreone,despitethefactthatoffshorewindenergyismorepowerful.AccordingtoIEAreport(IEA, 2019a),newoffshorewindprojectsareachieving 40–50%capacityfactor.Thislargefactorbecomespossiblebyinventinglarge windturbinesthatcancapturethemostavailableresources.Suchcapacityfactorisnowavailableforgas-firedpowerplantsandismorethanforonshore windturbines,beingaboutdoublethoseofsolarpanels.Theseadvantagesmake offshorewindturbinesquiteattractive.
FIGURE1.2 Currentandforecastcapacityofoffshorewindcapacity(datatakenfrom(IEA, 2019b)).
Europeistheleaderininstallingoffshoreplants.AsthedatainFig. 1.2 indicates,offshorewindfarmsareofgreatinterestinmanycountriesandregions. Until2040,Europewillstillbetheleaderinharnessingoffshoreenergy,followedbyChina.Meanwhile,othercountries,suchastheUnitedStates,Korea, India,andJapan,havesomeplansforaccessingoffshorewindenergy.
1.4Windturbinetypes
Ingeneral,windturbinesarecategorizedas horizontal-axiswindturbines, or HAWTs,and vertical-axiswindturbines, orVAWTs(Johnson, 1985).Thedifferencebetweenthetwoisrelatedtothepositionoftherotatingshaftandthe directionofthewind.Iftherotatingshaftoftheturbineisalignedwithwind direction,itiscalledanHAWT,andiftherotatingshaftoftheturbineisperpendiculartothewinddirection,itiscalledaVAWT.Bythisdefinition,forbeing aVAWT,therotatingshaftshouldnotnecessarilybeinstalledperpendicularto theground.Therotatingshaftmaybeinstalledparalleltotheground,butitis stillperpendiculartothewinddirection.
1.4.1Horizontal-axiswindturbine
Suchturbinesarethemostcommonlyusedturbines.Theymayhaveone,two, orthreebladesthatconvertwindenergyintomechanicalenergyintheshape ofarotatingshaft,knowasa low-speedshaft.Themechanicalenergyofthe low-speedshaftistransmittedtoageneratorviaagearboxtoincreasetheshaft velocity.Thegearboxincreasestheshaftangularvelocitytomakeitsuitablefor generatingpowerbythegenerator.Insomecases,thegeneratorisdesigned sothatthelow-speedshaft’srotationalspeedwouldbeenoughformaking electricity.Suchwindgenerators,madeusingapermanentmagnet,arecalled direct-drivegenerators.Allthecomponents,includingtheblades,low-speed
FIGURE1.3 DifferentconfigurationofHAWT.
shaft,gearbox,high-speedshaft,andthegeneratorareinstalledontopofatower inacasethatiscalled nacelle.
Currentcommercialwindturbinesusedforthegenerationofelectricalpower arethree-bladedinwhichthebladesizemaybeupto100metersandmore.The technologypreferstoincreasethesizeofthebladesaslargeaspossiblesothat morepowercanbeobtainedwithasingleturbine.Windturbineswith100-meter bladescanproduceupto10MWpowerandhavebeenusedinoffshorefarms. However,themanufacturershaveplanstobuildgiantturbineswithbladesup to120meterstoproduceover15MWpower.Thethree-bladedturbinesalso havelowertorquerippleswhichmakesthemsuperioroverone-andtwo-bladed designs.
Inordertogetthemostbenefitfromwindenergy,HAWTshouldpointtowardthewind.Hence,propermechanismsshouldbeinstalledonHAWTsto detectandrotatetheturbine.Insmallturbines,asimplevaneisenough,but inlargeronesnecessarysensorsandyawsystemsshouldbeincluded.Based onthis,HAWTsaredividedintoupwind,downwind,anddoublerotorconfigurations(WagnerandMathur, 2013).Upwindtypeturbinesrotatetowardsthe windwhiletheirbladeshittheundisturbedwindfirst.Fig. 1.3ashowssucha configuration,and,asitcanbeseen,thewindpassesthenacelleafterpassing theblades.Indownwindtypeturbines,windfirstpassesoverthenacelleand onlythenthebladesasshowninFig. 1.3b.Inadoublerotordesign,bothsides havearotor,asshowninFig. 1.3c.
Theupwindtypeturbineismorecommonamongtheabovethreeconfigurationsbecauseundisturbedwindfirsttouchestheblades.Itmeansthatthewind islessturbulentandexertslessdynamicstressontheblades.Indownwindturbines,thewindtouchesthebladesafterpassingoverthenacelle.Consequently, theflowbecomesmoreturbulent.However,ascanbeseeninFig. 1.3,downwindconfigurationshavetheadvantagethatthebladesdonothitthetowerdue totheirdeflection.Inupwindsystems,thisfactshouldbeconsideredindesigningthewholesetup.
Doublerotortypeturbinesarenotverycommonandcanbeseeninsome smallinstances.Havingtwocounterrotatingbladesmakestheconfiguration
morecomplicatedthanupwindordownwindones.However,thecountertorquesgeneratedfrombothrotorssimultaneouslycancelthehorizontalforces andmayleadtoamorestableturbinewithlowervibrations.Atthesametime, havingtworotorsmeansthattheturbinegeneratesmorepower.
1.4.2Vertical-axiswindturbine
IncontrasttoanHAWT,therotatingshaftofaVAWTisperpendiculartothe winddirection(Mathew, 2006).Beingperpendiculartothewinddirectiondoes notmeanthattheshaftisperpendiculartotheground.Therotatingshaftcanbe installedhorizontally,butitsrotationisstillperpendiculartothewinddirection. ThetwodifferentconfigurationsareillustratedinFig. 1.4.
Theverticalarrangementmeansthatsuchturbinesarenotsensitivetothe winddirection.Therefore,theydonotrequireanyyawingsystem.Inaddition, VAWTshavelowerrotationalspeedandhenceproducelessnoise.Ingeneral, VAWTsarelesscostlythanHAWTs,buttheyarecomparablylesseconomical. ThereasonisthatVAWTshavelowerefficiencythanHAWTs.Forthisreason, intheconstructionoflargewindfarms,HAWTsareusedinsteadofVAWTs.
SinceVAWTsdonotrequiretoberotatedtowardthewind,theyaremore efficientingustywindsbecausetheyarealreadyfacingthegust.AnotheradvantageofVAWTsoverHAWTsistheirlargersurfacearea.Incomparisonto HAWTs,theVAWTshaveagreatersurfaceareaforenergycapture.Thus,their energyinputcanbemanytimesgreatercomparedtoHAWTs.TheabovecharacteristicsresultinthefactthatVAWTscanbeinstalledinmorelocations,for example,onroofs,alonghighways,inparkinglots.
Forinstance,installingaVAWTontheroofofabuildingwillcausethe turbinetoexperienceahigherwindspeedbecausethebuildingpushesthewind topassoveritsroof.Thusthewindspeedonabuildingroofishigherthanthe far-field.ThiseffectcanbeseeninFig. 1.5
FIGURE1.4 IllustrationofVAWT.
FIGURE1.5 Effectofbuildingsonwindspeed.
SinceVAWTsrequirefewercomponents,suchasyawsystems,theyarealwayslesscomplicatedthanHAWTsandthusarelessexpensive.Inaddition, theycanbemadetoproducefrommilliwattstomegawattsofpower.Oneof theadvantagesofVAWTsisthatthegeneratorcanbeinstalledontheground, makingitrelativelysimplerformaintenance,meaningthattheirmaintenanceis alsolessexpensive.
AnotheradvantageofVAWTsisthattheirbladesrotateslowly,whichisvery importantindifferentways.Firstofall,VAWTsarecommonlyconsideredless dangerousforbirds,otheranimals,andhumans.Secondly,thefactthatthey producelessnoiseissignificantfromtheenvironmentalpointofviewsincethe noiseofwindturbinesmayharmthewildlifeandshouldbeavoidedasfaras possible.Finally,thelow-speedrotationalsomakesthemproducelessmechanicalwearing.
Inadditiontotheirbenefits,VAWTshavesomedrawbacksthathavemade themunsuitableforlargewindfarms.Firstofall,theyarenotasefficientas HAWTs.HAWTshaveacapacitycoefficientaslargeas50%,whilethisvalue issignificantlylessforVAWTs.Theirlowercapacityfactorisbecauseateach timeinstant,onlyonebladeisatitsbestperformance,andtheotherbladesare notfacingthewind;hencearefarfromtheiroptimalposition.Moreover,the bladerotationcausesadditionaldragforce,whichinturnreducestheoutput power.
AnotherdisadvantageofVAWTsisthattheyusuallyrequireaprerotationor pushforstartingtowork.Fromthescientificpointofview,itisbecausetheir efficiencyatzerorotationalspeediszero.Itmeansthat,whateverthespeedof thewind,theoutputpower,whichistranslatedintoshaftrotation,iszero,orin simplerwords,itdoesnotrotate.
TherearemanydifferenttypesofVAWTs,andeveryyearnewdesigns emerge.Someofthemostfamousareasfollows:
Cupanemometer
Theveryfirstanemometers,suchasthatshowninFig. 1.6, areconsideredvertical,inwhichthegeneratedpowerisaboutafewmilliwatts.Thegeneratedoutputiscalibratedwithwindspeed;hencewecan usethemforspeedmeasurements.Theadvantageofthistypeisthatitis notsensitivetowinddirectionandrotatesevenataverysmallwindspeed. Savonius Whilecupanemometergeneratorsarenotusedforpowergeneration, Savoniustypesareusedforthispurpose.Theturbineisnamedafterits FinnishengineerinventorSigurdJohannesSavonius,whointroduceditin 1922.Inthisturbine,showninFig. 1.7,somecurlybladesareinstalledon averticalaxisinsuchawaythatwhenthewindblows,itexertssignificant forceintheconcavesurfaceandlessontheconvexones.Theimbalanced forcescausetheturbinetorotate.
Asitisclear,thewinddirectiondoesnotcontributetotherotationdirection,buttheexertedpowerontheverticalshaftoscillatesastheblades rotateandmakedifferentangleswiththewinddirection.
FIGURE1.6 Cupanemometer.
FIGURE1.7 SavoniusVAWT.
