Slotted Blades Savonius Wind Turbine Analysis by C

Energies2013,6,6335-6351;doi:10.3390/en6126335OPENACCESSenergiesISSN1996-1073*,AntonioEsposito1,AlbertoMilazzo2,CalogeroOrlando1andFlavioTrentacosti11FacultyofEngineeringandArchitecture,KoreUniversityofEnna,CittadellaUniversitaria,94100Enna,Italy2FacultyofEngineering,UniversityofPalermo,VialedelleScienze,90128Palermo,Italy*Authortowhomcorrespondenceshouldbeaddressed;E-Mail:[e-mail];Tel.:[phone].Received:13November2013;inrevisedform:27November2013/Accepted:28November2013/Published:4December2013Abstract:InthispaperanewbucketconfigurationforaSavoniuswindgeneratorisproposed.NumericalanalysesareperformedtoestimatetheperformancesoftheproposedconfigurationbymeansofthecommercialcodeCOMSOLMultiphysicsrwithrespecttoSavoniuswindturbinewithoverlaponly.Parametricanalysesareperformed,forafixedoverlapratio,byvaryingtheslotposition;theresultsshowthatforslotpositionednearthebladeroot,theSavoniusrotorimprovesperformancesatlowtipspeedratio,evidencingabetterstartingtorque.ThiscircumstanceisconfirmedbystaticanalysesperformedontheslottedbladesinordertoinvestigatethestartingcharacteristicoftheproposedSavoniuswindgeneratorconfiguration.Keywords:windturbine;Savonius;CFDanalysis;slottedblades1.IntroductionTheInternationalEnergyAgencyexpectsthatby2030theprimaryworldenergydemandwillincreaseby45%,andhydrocarbonfuelswillcontinuetohaveaprimaryrole,covering80%ofthedemand[1].Theincreaseoftheworldenergydemand,mainlyfulfilledbyhydrocarbonfuels,hasresultedinanincrementingreenhousegasemissions,withseriousrepercussionsonourenvironment.Reductioningreenhousegasemissionscanbeachievedbyincreasingplantenergyefficiencyandbytakingconcreteactionsonenergysavingandrecoveryintheindustrialandcivilfield[2–4].AlthoughwindenergyisEnergies2013,66336abletoreplacetraditionalenergysourcesincleanways,manyconcernsareassociatedwithitsuseinurbanenvironments,suchasperformancesinurbanenvironmentornoiselevel,aswellasthevisualimpact,concerningintegrationwithexistingbuildings[5].TheverticalaxiswindturbinesofSavoniusandDarrieustypesarethemostinterestingonesfromthefeasibilityandsustainabilitypointofviewintheurbanenvironment.EfficiencyofSavoniusgeneratorsisratherlowcomparedtobothhorizontalaxisandDarrieustypeturbines[6];however,theyshowsomeadvantagesrelatingto:(i)simpleandinexpensivedesign;(ii)self-activationinpresenceofwind;(iii)operatingcapabilitynotreliantonwinddirection.Moreover,theyarecharacterizedbyhighstartingtorque,whichrepresentsanadvantageforurbanapplications.TherearemanystudiesintheliteraturefocusingonimprovementofaerodynamicperformanceofSavoniusturbine.Amongthese,severalauthorshaveanalyzedsavoniusbladebehaviorinthepresenceofvalves,orgaps,inthecenterareaoftheblades,inordertoallowtheflowpassingfromonesideofbladetoanotherandthusreducingthedrugfortheretreatingblade.Plourdeetal.[7]andAbrahametal.[8]haveanalyzedexperimentallythebehaviorofboththeendplatesandtheeffectsoflongitudinalventsalignedwiththeaxisofrotation.Resultshaveshownthatuseofventinggivelessimprovementsofthebladeperformancesifcomparedtoendplate.Sahaetal.[9]haveinvestigatedtheperformancesoftwo-stagerotorsystembyinsertingavalvesontheconcavesideofbladeabletobeopenedintheadvancingbladeandclosedinthereturningone.Thisconfigurationhasallowedtoreducethenegativetorqueoftherotorconsiderably.Guptaetal.[10]haveinvestigatedthebehaviorofacombinedSavonius-DarrieuswindrotorandtheassociatedadvantageswhileAltanetal.[11]designedadeflectorwithdifferentarrangementsplacedinfrontoftherotorwiththeaimofpreventingthenegativetorque.Fujisawa[12]showedthatinordertoincreasethepowercoefficientoftheSavoniusgeneratoritisusefultosuperimposebladesclosetotherotationalaxis.Recently,asystematicstudyonthebehaviorofSavoniuswindturbinewithbladesoverlappingwasperformedbyAkwaetal.[13],whofoundthatthebestperformancecanbeobtainedforaparticularoverlappingratio.Onthisbasis,thispaperpresentsanewslottedbucketconfigurationnumericallyanalyzedbymeansofthecommercialcodeCOMSOLMultiphysicsr.2-DimensionalCFDanalysesareperformedforbothstaticconditions,inordertocharacterizethestartingperformanceofthenewconfigurationproposed,anddynamicconditionsbyvaryingsomegeometriccharacteristicsoftheslottedbladesaswellasthetipspeedratio.Although3-DimensionalstudiesmayprovideamorerealisticbehavioroftheSavoniusturbine,the2-Dnumericalanalysis,firstlyvalidatedbymeansofexperimentalresultsobtainedbyBlackwelletal.[14],hasallowedtopointouttheeffectofthemodifiedbladewithlesscomputationalefforts.2.ComputationalFluidDynamicAnalysesInthissectionthemaingeometricalfeaturesoftheslottedbladesarefirstlydefinedanddescribedincomparisonwiththeclassicalSavoniuswindgenerator[15].ThencontrolvolumesandcorrespondingboundaryconditionsconsideredfortheanalysesareintroducedandabriefdescriptionofthemathematicalmodelusedforCFDanalysesarerecalledforthesakeofcompleteness.Energies2013,663372.1.CharacteristicsoftheSlottedBladeThetestedconfigurationsarecharacterizedbyanoveralldiameterofthebladeDr=1m,whichhasbeenchosen