Design Space Exploration of Small-Scale PEM Fuel C

AmericanInstituteofAeronauticsandAstronautics1DesignSpaceExplorationofSmall-ScalePEMFuelCellLongEnduranceAircraftBlakeA.Moffitt*andThomasH.Bradley†GeorgiaInstituteofTechnology,Atlanta,Georgia,30332-0150DimitriMavris‡GeorgiaInstituteofTechnology,Atlanta,Georgia,30332-0150andDavidE.Parekh§GeorgiaTechResearchInstitute,Atlanta,Georgia,30332-0801Duetotheirhighenergydensity,protonexchangemembrane(PEM)fuelcellsystemsarebecomingincreasinglyattractiveastheprimarypowerplantforlow-power,long-enduranceaircraftapplications.AlthoughPEMfuelcelltechnologyhasbeenappliedforautomotiveandstationaryuse,limiteddesignandexperimentalworkhasbeenperformedanddocumentedforactualaircraftapplications.InordertobetterunderstandthedesignandperformancetradeoffsforPEMfuelcellpoweredaircraft,ahigh-levelconceptualdesignstudyofsmall-scalelong-enduranceaircraftisperformed.ThisstudybuildsupondesignlessonslearnedthroughthedevelopmentandflighttestingofaPEM-powereddemonstratoraircraftdesignedandbuiltbytheGeorgiaInstituteofTechnology.Thestudyfocusesonidentifyingandexploringtheconceptdesignspaceappropriateforsmallunmannedairvehicleswithrangesofupto5000kmflyingatlowaltitudeswithendurancesofupto64hours.AQualityFunctionDeploymentisusedinconjunctionwithaMatrixofAlternativestodefinemultiplecompetingaircraftconfigurationsbasedoncurrentadvancedtechnologiesinPEMfuelcells,hydrogenstorage,electricpropulsion,aircraftdesign,andstructuralmaterials.AbaselinepropulsionsystemconsistingofaliquidcooledPEMfuelcellwithcompressedhydrogenstoragepoweringmultipleelectrictractorpropellermotorswaschosen.Thecorrespondingbaselineaerodynamicconfigurationconsistedofahigh-aspectratiotaperedwingwithmultipletractorpropellers.Elevendesignvariablesgoverningthepowerplant,propulsion,andaircraftdesignwerechosenandusedasinputstoacombinationofsurrogateandphysicsbasedmodelsthatweresolvedusingfixedpointiteration.Usingrange,endurance,climbrate,andaircraftmassasmetrics,theproblemwasoptimizedusingasequentialunconstrainedminimizationtechnique(SUMT)withanextendedinteriorpenaltyfunctionusingasimplexoptimizationsearchalgorithm.Severaldesignconstraintswereactiveattheoptimalsolutionsforbothrangeandendurance.Resultsshowedthatthedesignwasprimarilydrivenbydesignvariablesgoverninghydrogenstorage.Theanalysisalsoshowedthatoptimizingadesignforenergydensitydidnotproducethebestaircraftdesignforeitherlongrangeorlongendurance.Withthesamepayload,aircraftoptimizedforrangeandenduranceweremuchsmallerandhadbetterrange,endurance,andclimbperformancethanaircraftoptimizedforenergydensity.*GraduateStudent,TheDanielGuggenheimSchoolofAerospaceEngineering,270FerstDriveN.W.Atlanta,Georgia30332-0150,AIAAStudentMember.†GraduateStudent,TheGeorgeW.WoodruffSchoolofMechanicalEngineering,801FerstDriveN.W.Atlanta,Georgia30332-0405,AIAAStudentMember.‡BoeingProfessorofAdvancedAerospaceSystemsAnalysis,TheDanielGuggenheimSchoolofAerospaceEngineering,270FerstDriveN.W.Atlanta,Georgia30332-0150,AssociateFellowofAIAA.§DeputyDirector,GeorgiaTechResearchInstitute,andProfessor,TheGeorgeW.WoodruffSchoolofMechanicalEngineering,801FerstDriveN.W.Atlanta,Georgia30332-0405,AssociateFellowofAIAA.AmericanInstituteofAeronauticsandAstronautics2I.IntroductionONG-ENDURANCEunmannedaerialvehicles(UAVs)arethesubjectofconsiderableinterestamongtheaerospacecommunityfortheirpotentialtoperformreconnaissanceandsensingmissionsthatcannotjustifythecostofspace-basedsatellitesbecauseoflowerpriority,missiondurationorvalue.Inaddition,long-enduranceaircraftcanexhibitlowercapitolcost,bettermissionadaptability,fastermissioncycletimeandtheyrequirelessoftheradiationandtemperaturehardeningofspacecraft.Atpresent,amajorityofthelong-enduranceaircraftavailablearepoweredbyconventionalgasturbinepowerplants1.Polymerelectrolytemembrane(PEM)fuelcellpowerplantspoweredbycompressedhydrogenhaveparticularadvantagesoverothertechnologiesavailableforlong-enduranceaircraftbecausePEMfuelcellsystemshavehighspecificenergy,highefficiency,improvedenvironmentalperformanceandcanbeincorporatedintorechargeableenergystoragesystems2.Whereadvancedbatteriescanreachelectricaloutputenergydensitiesof200Wh/kgatthemodulelevel3,fuelcellscanachieve800Wh/kgatthesystemlevel4,5.Whereadvancedbatterieshavechargeanddischargeefficiencyofnearly100%6,afuelcell/electrolyzerrechargeablefuelcellsystemcanhaveachargeefficiencyof80%andadischargeefficiencyof50%7.The2.5xefficiencyadvantageofbatteriesdoesnotmakeupforthe4xenergydensityadvantageofthefuelcellsystem.Forcomparison,aconventionalgasoline-fueledinternalcombustionpowerplant(SFC=600g/kWh,11kgengine8,10hrsendurance,3kWcruise,zerofueltankweight)hasaspecificmechanicalenergyofapproximately1030Wh/kg,butisendurance-limitedbecauseitcannoteasilyberechargedinflight.Althoughtheaviationcommunity’sinterestinfuelcellaircraftisnowwellestablished,thereexistsaneedforacomprehensive,documentedanalysisoftheperformanceofpracticallong-endurancefuelcellaircraft.TheGeorgiaTechResearchInstitute,theAerospaceSystemsDesignLaboratory,theDanielGuggenheimSchoolofAerospaceEngineering,andtheGeorgeWoodruff