Hydrogen Fuel Cell Power System

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~ChapterThreeThehydrogenfuelcellpowersystem~83 Thischapterdiscussestwomajorissues:(i)fuelcelltheoryandengineering,and(ii)providingfuelforthefuelcell.Thefirstsectionthoroughlydiscussesthesciencebehindfuelcellsingeneral.Ofthedifferentkindsoffuelcells,theprotonexchangemembranetypeisidentifiedastheunquestionedbestcandidateforthesmallvehicleapplication.Engineeringissueslikehowtocoolthefuelcellstackandwhethertopressurizethefuelcellarediscussedingeneraltermsandthefeasibilityofcertaindesignoptionsisdetermined.(Ontheotherhand,morecomplete,quantitativeanalysesofcoolingandpressurizationrequiremoredetailedinformationaboutsteady-stateandtransientpowerrequirementsthatarenotcalculateduntilChapter4,andarethusproperlyanalyzedwithinthatchapter).Thesizeandweightofthefuelcellarediscussedwithreferencetopreviousresearchers’results.Thesecondhalfofthischapterconcernsfuelingofhydrogenfuelcells,whichisconsiderablymorecomplexthanfillingatankwithgasoline.Itexplainswhyreformedfossilfuelscannotyetbeusedtopowerafuelcellvehicle,anddescribesvariousoptionsforstoringpurehydrogenonboardthevehicle.Theissueofsafetyisdiscussed.84 3.1.FuelCellScience3.1.1.FundamentalsFuelcellsareelectrochemicalenginesthatproduceelectricityfrompairedoxidation/reductionreactions.Onecanthinkofthemasbatterieswithflowsofreactantsinandproductsout.Incontrast,thebatteryhasafixedsupplyofreactantsthattransformintoproductswithoutbeingsteadilyreplaced.Thedistinctionisaniceoneaszinc-air“batteries”havereplaceablezincelectrodes,makingthemverylikefuelcells.Astandardhighschoolchemistrydemonstrationinvolvespassinganelectriccurrentfromabatterythroughajarofwater(withdissolvedsalts)bywayoftwometalelectrodessuspendedinthewater.Hydrogenevolvesatthecathodeandoxygenattheanodeasthewaterisbrokenintoitsconstituentelementsbyelectrolysis.Essentially,afuelcellusesthereverseprocess:hydrogenandoxygenarecombinedtoformwater,andelectricityisproduced.Technically,thetwochemicalsdonothavetobehydrogenandoxygen;theredoxreactionrequiresonlyareducerandanoxidizer,butsinceoxygeniseasytoobtainfromtheairandhydrogenhassuitablyfastreactionkinetics,thesearethetwomostoftenchosen.Allfurtherexampleswillusehydrogenandoxygenunlessotherwisenoted.Specifically,thehydrogenandoxygendiffuseintotheirrespectiveelectrodes,ionize,andonetypeofionmigratesthroughanelectrolyteandrecombinesattheothersidewiththeotheriontoformwater.Atanygiventemperaturethereisanequilibriumratioofionstomolecules.Coatingsof85 noblemetalcatalystontheelectrodeslowertheactivationenergyoftheionization/recombinationprocess,acceleratingtherestorationofequilibriumastheionsareconsumedbythefuelcell.Theprinciplebehindfuelcellswasdiscoveredasearlyas1839byWelshphysicistandjudgeSirWilliamGrove.However,duetohighcosts,thetechnologywasnotsignificantlyuseduntiltheAmericanGeminispacemissionsofthe1960's.Forthisandsubsequentspacemissions,fuelcellswerethoughttobesaferthannuclearelectricgenerationandcheaperthansolar.Theyhavebeenthrusttotheforefrontofenergytechnologyinthe1990's,however,ashighpowerdensitieshavemadethemfeasibleforbothstationaryandportableapplications.ONSIcorporation,asubsidiaryofUnitedTechnologies,hasproducedover170ofitsPC25stationary200kWfuelcellsystemssincetheirintroductionin1992.Fuelcellshavetheadvantagesofhighefficiency,loworevenzeropollution,quietoperation,andfewermovingparts–onlypumpsandfanstocirculatecoolantandreactantgases,respectively–forgreaterreliabilitythaninternalcombustionengines(oncefuelcellsystemsarewell-developed).3.1.1.1ThermodynamicsAnelectrolytephysicallyseparatesthetworeactantsandalsopreventselectronicconduction,whileallowingionstopassthrough;theelectronstravelthroughanexternallooptosupplytheload.Electrodesareattachedtoeithersideoftheelectrolyte.Attheanode,thehydrogenisoxidized:+-H2(g)2H+2eTheelectronspassthroughtheloadtoprovidethedesiredcurrentandendupatthecathode,wherethematchingreductionreactionoccurs:86 -2-2e+?O2(g)OElectrostaticbalanceisreachedasthehydrogenionsdiffusethroughtheelectrolytetogettothecathode:+2-2H+OH2O(l)Figure3.1Fuelcellschematicelectricalload2H2O2-4H+2--4e2O4e2H2OconductiveanodepolymercathodeconductiveseparatorelectrodeelectrolyteelectrodeseparatorplatemembraneplateThetheoreticalenergyreleaseoftheoverallreactionisdeterminedbytheenthalpychangeHintheoverall(isothermal)reaction,87 H2(g)+?O2(g)H2O(l)(H(=-285.8kJ/mol;G(=-237.2kJ/mol;)GistheGibbsfreeenergyandstandardconditions,asindicatedbythenoughtsuperscript,areT=25(C,partialpressuresof1atmforeachofthegases,andwaterintheliquidstate.Thislastdistinctionisimportant.Forhightemperaturefuelcells,wateremergesinthegaseousstate,sothe“l(fā)owerheatingvalue”wouldbeused.Inthis(non-standard)case,H=-241.8kJ/molandG=-228.6kJ/mol.Thehigherheatingvalueisusedintheremainingcalculationssincemostfuelcellsoperatebelowtheboilingpointofwater.Thenextstepistodeterminefuelcellefficiency.G(=H(-TS(-1-1sothestandardchangeinentropyis-0.163kJ?mol?K.Anenergybalanceonafuelcellshowsthatd/dt(dQ+dWelec)=d/dt(dH+dKE+dPE)Kineticenergy(KE)andpotentialenergy(PE)changesareassumedtobenegligible,andsteadystateoperationisassumed.A“perfect”fuelcelloperatingirreversiblymeansdQ=TdS.Thus,Welec,theelectricalpoweroutput,isWelec=H-TS=G88 Efficiencyatanygivenpointisusuallydefinedherebydividingthemaximumworkoutbytheenthalpyinput,sofuelcellefficiencyis:FC=G/HUsingthestandardfreeenergyandenthalpygivenpreviously(G(=-237.2kJ/mol,H(=-285.8kJ/mol)showsthemaximumthermodynamicefficiencyunderstandardconditionsis83%.Thechangeinstandardfreeenergycanbeusedtocalculatethemaximumreversiblevoltageprovidedbythecell:G(=-nFEr(wherenisthenumberofelectronsinthereactionaswritten,FisFaraday’sconstant(96,487coulombspermoleofelectrons),andEr(isthestandardreversiblepotential.Sincen=2here,Er(=1.229V.ThisreversiblepotentialchangeswithchangingpressureastheNernstequation:12/RT?AA?EE=+oln?HO22÷rrnF?A÷èHO2?Again,n=2,andthe“A”saretheactivitiesofthevariousspecies–thepartialpressuresforhydrogenandoxygen,and1forwatersinceitisintheliquidform.(Activitymeasurestheconcentrationrelativetostandardconditions,soforthegasesthisbecomesthepartialpressurerelativeto1atm,andforsolutesitisrelativetoa1-molalitysolutioninsolvent.Wateristhesolvent,soitsactivityis1)Thevoltageincreasederivedfromanincreaseinpressurebyafactor(assumingthatthehydrogenintakepressureisincreasedbythesamefactorastheoxygenintake),isgivenbelow.89 **12/RT?PPHO22()22()?RT15.RTDEr=ln?12/÷==lnll15.lnnF?**÷nFnFèPPHO22()11()?The“(2)”subscriptsrefertothepressurizedstatesofthereactantgasesandthe“(1)”totheunpressurizedpartialpressures.At80(Cand2molesofelectronsinthereactionaswritten,thetheoreticalvoltagechangeis16mVforadoublingoftheintakepressuresfromthedefault1atmto2atm.Foranincreaseinpressureto300kPa,atypicalfigureusedforfuelcells,avoltageincreaseof25mVcouldberealized.However,asthenextsectiononkineticsshows,theimprovementErisactuallymuchlarger.(Someresearchershaveconsideredvariable-pressuresystemswheregreatercompressionisusedwhenhigherpowerisdemanded.Thisproduceshigherefficienciesathighpower,andlowerparasiticcompressorlossesatlowpower.Ontheotherhand,aconstant-pressurefuelcellischeaperandsimplertomanufacture.)Therelationshipbetweenvoltageandtemperatureisderivedbytakingthefreeenergy,linearizingaboutthestandardconditionof25(C,andassumingthattheenthalpychangeHdoesnotchangewithtemperature:DDDGHTS-Er=-=-nFnFdErDDEr=TdTPdEDSooDEr=-=-()TC25()TC25dTnFP90 Becausethechangeinentropyisnegative,theopen-circuitvoltageoutputdecreaseswithincreasingtemperature;thefuelcellistheoreticallymoreefficientatlowtemperatures.However,othereffectslikemasstransportandionicconductionarefasterathighertemperaturesandthismorethanoffsetsthedropinopen-circuitvoltage.Intheory,thepowergeneratedbyasinglefuelcellissimplythereversiblepotentialtimesthenumberofelectronsgeneratedpersecond(i.e.thecurrent).Thefuelcellsareconnectedina“stack”tomultiplythepower.3.1.1.2KineticsThecellpotential,however,isalsolimitedbythekineticsofthereaction.TheselossesaremostoftenshowninwhatisknownasaTafelplotorpolarizationcurve;cellpotentialinvoltsisgraphedagainstthecellcurrentdensityinamperespersquarecentimeterofcellarea.Thecurrentdensitybasicallyrepresentshowfastthereactionistakingplace(itisthenumberofelectronspersecond,dividedbythesurfaceareaofthefuelcellelectrolyteface);measuredvoltagedividedbyreversiblevoltageisequaltotheefficiencydefinedpreviously.Thepowercurveasafunctionofcurrent(V*iversusi)showsapeakathighcurrentdensity.91 Figure3.2Tafelplot1.010000.9900voltage0.8power8000.77000.66000.55000.4400voltage(V)0.33000.22000.1100powerdensity(mW/cm^2)0.0002004006008001000120014001600currentdensity(mA/cm^2)Atnon-zerocurrentdensities,thereiswhatisknownasan“activationoverpotential”:todrivethedissociationoftheoxygenandhydrogenmoleculesquickly,acertainactivationenergymustbeexceeded.Essentially,theoxygenandhydrogenmoleculesmustdiffuseinthroughporesinthemetalcatalystandadsorb.Thisisa“threephaseinterfaceproblem,”sincegaseousfuel,solidmetalcatalyst,andliquidelectrolytemustallcontact.Thecatalystreducestheheightoftheactivationbarrierbutalossinvoltageremainsduetothestill-slowoxygenreaction.(Notethatthehydrogenactivationoverpotentialisnegligiblecomparedtotheoxygenoverpotential;theoxygen1reactionisfivetosixordersofmagnitudeslower.)Also,competingreactionsoccurattheoxygenelectrode:oxidationoftheplatinum,corrosionofcarbonsupport,andoxidationoforganicimpuritiesontheelectrode.Thetotaloverpotentialis0.1to0.2V,reducingthemaximumpotential2tolessthan1.0Vevenunderopen-circuitconditions.Thereisalsoacontinuousdropinvoltageascurrentincreases,andthisisduetolinear,ohmiclosses(i.e.resistance)intheionicconductionthroughtheelectrolyte.Thethinnerthemembrane,92 thelowerthisloss.Thinnermembranesarealsoadvantageousbecausetheykeeptheanodeelectrodewetby“back”diffusionofwaterfromthecathode,whereitisgenerated,towardstheanode.Finally,atveryhighcurrentdensities(fastfluidflows),masstransportcausesarapiddrop-offinthevoltage,becauseoxygenandhydrogensimplycannotdiffusethroughtheelectrodeandionizefastenough,andproductscannotbemovedoutquicklyenough.TheTafelplotisoftenmodeledsemi-empiricallywithanequationoftheformniEEbiR=-ln()--imeowherethevoltageEisafunctionofcurrentdensityi.Higherpressureimproveskineticsaswellasthermodynamicsduetothehigherconcentrationofreactants;theNernstequationdoesnottellthewholestory.Foranincreaseinpressureto300kPa,avoltageincreaseof25mVisexpectedfromtheNernstequation.Theadditionalvoltagederived3fromthekineticsofthereactionisestimatedat29mVfor3atmoperationbyoneresearcher.Thisdoublesthevoltageincreaseto54mV.ThegaininvoltageinaBallardMkIVsinglecelloperating-2at400mA?cmwasexperimentallydeterminedtobe2.7timesgreaterthanpredictedbythe4Nernstequationfor-intheexampleofpressurizingto3atm,thisisanincreaseof67.5mV.Intermsoftheoveralleffectofpressurization,significantlylargerincreasesinvoltageareoftenrealizedat300kPa,asthefollowingEnergyPartnerspolarizationcurvesbelowshow.Here,the2-2differenceinvoltageswidenstoasmuchas0.150V(25%)at1400mA?cm.At1000mA?cm,theimprovementis14%.93 Figure3.3Effectsofpressurizationonpolarizationcurves1.21.11.03atmpressurized0.90.80.70.6atmospheric0.5pressurevoltage(V)0.40.30.20.10.00200400600800100012001400currentdensity(mA/cm^2)DatafromBarbirisforasinglecellrunningonhydrogen/air,withaGoreMEAandoperatingtemperatureof60(C.Air-sidestoichiometryis2.5,meaningthat2.5timesasmuchoxygenissuppliedthanthe5minimumneededforstoichiometry.3.1.1.3AnoteonefficiencySinceefficiencywilllaterbeusedtocalculatewasteheatgeneration,themethodhereistodivideelectricalpoweroutputbytherateofenergyconsumed.Asalludedtoearlier,“energyconsumed”ismeasuredintermsofthehigherheatingvalueofthehydrogenused.94 Inotherwords,poweroutnFelectronsVoutput2FVoutputEfficiencyh===powernHDDHinhydrogenHHVHHVwherenareflowratesinmolespersecond,FistheFaradayconstant,Visthevoltageofthecelloutput,andHHHVis-285.8kJ/mol.Inpractice,thehigherheatingvalueenthalpycanbeconvertedtoanequivalentvoltageof1.481V,sothatVoutputh=1481.VThisequivalentvoltageconceptisveryusefulincalculatingefficiencyandwasteheat;withtheefficiencydefinedinthisway,thewasteheatgeneratedissimplyQnH=-D()1hHHVandthemaximumefficiencyisathermodynamically-limited83%.(Iftheassumptionthatwaterstaysintheliquidformisincorrect,thewasteheatthatmustberejecteddecreasesbecausethevaporizationofthewatercoolsthestack).3.1.2TypesoffuelcellsTheclassificationoffuelcellsisgenerallybytypeofelectrolyteused.Theelectrolytecanbea95 solidpolymer,liquidacidorbase,ceramic,ormoltenionicsalt.Forreasonsthatwillbedetailedbelow,protonexchangemembranefuelcells(PEMFCs)arethetypemostsuitedforscooterapplications.3.1.2.1PhosphoricAcidFuelCell:well-developed,lowdensityThefuelcelltechnologythathasbeenincommercial(non-military)useforthelongesttimeusesphosphoricacid,ofteninsiliconcarbideceramicmatrices,astheelectrolyte.Thephosphoricacidfuelcellrunsatover150(C,whichincreasescatalystactivity.Thehighertemperaturesarealsonecessarybecausephosphateanionsadsorbontotheoxygen(reduction)electrodebelow100(C,6reducingcatalyticperformance.Phosphoricacidfuelcellsaccepteitherdirecthydrogenor“reformate”,amixtureofhydrogen,carbondioxide,water,carbonmonoxideandpossiblynitrogenandtracegasesproducedfromtheconversionoffossilfuelsintohydrogenandcarbonmonoxide.Carbonmonoxidemixedwiththefuel(anode)flowisdangerous,becauseitcan“poison”thecatalyst.Basically,thismeansthatactivesitesonthemetalstructurebecomefilledwithcarbonmonoxidemolecules,makingthemunavailableforhydrogencatalysis;thisisaproblemwithhydrogenfuelreformedfromhydrocarbons,wherethehydrogenismixedwithcarbonmonoxide.Carbonmonoxidepollutionintheairreachingthecathodeislessofaproblem,becauseheretheconcentrationofoxygenandtheoxidationpotentialarehigh;theoxidationofcarbonmonoxidetocarbondioxideproceedsrapidlyandthecatalystisnotpoisonedalthoughthereisaslightdropinvoltage.Dependingontemperature,phosphoricacidfuelcells(PAFCs)areabletotolerateconcentrationsofCOinthe7fuelstreamofupto1-3%.Theoutputofasteamreformerisroughlydoublethisvaluewhichiswhyanadditionalcarbondioxidecleanupstepisneeded.96 For5kWapplications,passivecooling(heatsinksandfins)maybesufficienttocoolthesystem.Thehightemperaturesalsohavetheadvantageoftransportingtheproductwaterassteaminsteadofasliquid.Thetemperaturesarelowenoughthatnoblemetalalloycatalystsarerequired,onthe-2-28orderof0.2mg?cmonthehydrogenelectrodeand0.4mg?cmontheoxygenelectrode.PAFCshaveaboutathirdoftheperformanceofmodernpolymerelectrolytemembranefuelcells-2(PEMFCs),intermsofpowerpermembraneareainW?cm,becausePEMFCshavestrongeracidsintheirelectrolytesandbecausethethinnerpolymermembraneshavemuchlowerohmiclosses.PowerdensitiesarelowerinPAFCsforanotherreason:thesiliconcarbidematrixthatholdsthephosphoricacidelectrolytemustbeaminimumof0.1-0.2mmthickformechanicalstability,increasingtotalstacksize.Finally,PAFCsmustbekeptabove45(Cevenwhennotinusebecausebelowthistemperaturetheacidsolidifiesandexpands,riskingdamagetoelectrodesorsilicon9carbidematrix.Thesetworeasonsoflowpowerdensityandfinickytemperatureconditionsexplainwhy,afterafewbusdemonstrations,PAFCshavebeenrelegatedtostationaryapplications.3.1.2.2ProtonExchangeMembraneFuelCell:formobileapplications,thebestThetypeoffuelcellcurrentlyreceivingthemostattentionisthePEMfuelcell;PEMstandsvariouslyfor“protonexchangemembrane”or“polymerelectrolytemembrane”.Themembraneisusuallyaperfluorosulfonicacidpolymer.Thisisapolytetrafluoroethylene(PTFE,tradenameTeflon)chainwithsidechainsterminatinginanSO3Hgroup.Itisthehydrogenonthissulfonategroupthatdissociatesfromthepolymerwhenwetandappearsasprotonsinthesolution;polymer-acidshavetheadvantagethattheanion(–SO3tail)isfixedintheelectrolyteratherthandissolved..97 OnecommonPEMisNafion,apolymerdevelopedbyDuPontinthe1960'sforuseasaseparatorinthechlor-alkaliindustryandnowusedforotherindustrialelectrochemicalpurposes.Itissometimesclassifiedwiththecompoundsknownas“superacids”becausetheyarestrongerthan2puresulfuricacid.Nafionandsimilar“ionomers”arecurrentlyproducedattherateof100,000m210,112ayear,andsellforabout600$/m.Thisispredictedtodecreaseto50$/mifproduction212expandsto1millionmperyear(150,000PEMFCautomobileengineseachyear)Figure3.4NafionchemicalstructureTheratioofntom(i.e.activesitestoinactivechainmonomers)determinestheacidityoftheelectrolyte.Polymerelectrolytemembranescanbemadeextremelythin,lessthan50μm,makingfordenselypackedstacksand,consequently,highpowerdensities.Thethinnessofthepolymerelectrolytealsomeanshighconductanceandlowohmicresistancelosses,foraboutthreetimestheperformance(in98 -2W?cm)ofPAFCs.Themoderateconditionsthatthefuelcellrunsunderarealsoabenefitwhencomparedtothealternatives-highlycorrosiveacids,orhightemperatureceramicsandmoltensalts.Ontheotherhand,PEMFCsareespeciallyvulnerableto“flooding”:themembranebecomesover-wetduetoproductionofwateratthecathodeanddiffusionofreactantsisblocked.Also,platinum-2isrequired,lessthan0.4mg?cmforeachofanodeandcathode,mainlytoresisttheeffectsofcarbonmonoxidepoisoningfromimpurehydrogen.PartofthereasonisthatPEMFCsoperateatarelativelylowtemperature,under100(C,becausehighertemperaturesremovewaterfromthemembraneanddamageit.Attheselowtemperaturecatalystsaresimplynotasactive.Morecatalystisrequiredatthecathodethanattheanodeduetothemuchloweractivityofoxygenionization.Pt/Rualloysareoftenusedattheanodealloyinordertopreventcarbonmonoxidepoisoning,becausethepresenceofRuchangesthelatticeconstantoftheresultingcatalystandmakescarbonmonoxideadsorptionmoredifficult.PEMFCssignificantlydegradeinperformancewhenoperatingonreformateofmorethan50ppm.-2Withmaximumpowerdensitiesofabout600mW?cmandplatinumcostingabout$450perounce,thisisacatalystcostof10.6$/kW.Inthestack,thefuelcellsarearrangedsothations(protons)passthroughthemembrane,whileelectronsareconductedthroughtheseparatinggraphiteplatesintheoppositedirection.99 Figure3.5Stackdiagram100 3.1.2.3AlkalineFuelCells:poisonedbycarbondioxideAlkalinefuelcells,whichworkatatemperatureofabout100(C,areamongthemostefficient,andallowtheuseoflessexpensivecatalystmaterialslikenickelbecausetheuseofanalkalineelectrolyteandconsequenthighpHshiftstheelectrochemicalpotentialtoreduceactivationoverpotential.Theyhavefairlyhighpowerdensities.Theanodereactionis:--H2+2OHú2H2O+2eThecathodereactionis:--?O2+H2O+2eú2OHThenetreactionisthesameasthePEMFC:thecombinationofhydrogenandoxygenmoleculesto+formwater.However,theionicconductorhereisthehydroxideanionratherthanH.Theelectrolyte(typicallypotassiumhydroxide,KOH)iseitherfixedinanasbestosmatrixorpumpedandcirculatedasaliquid.PowerdensityisnotashighasinaPEMfuelcell.Anotherseriousdeficiencyisthatalkalinefuelcellscannotwithstandcarbondioxide;carbondioxideintheairorfuelstreamsreactswiththeelectrolytetoformpotassiumcarbonateswhich13canprecipitateout.ThecontentofCO2inairwas353ppmin1990,butmodernAFCscanonly14tolerate50ppm.Thisisfineforspaceapplicationswhereoxygencanbesuppliedinitspureform,butingroundvehiclespoweredbyAFCs,scrubbersareneededtoreducethecarbondioxideleveltoacceptablelevels.Thisisnotpracticalforscooters,althoughZevco,aBritishcompany,isworkingonataxifleetrunningonalkalinefuelcellsequippedwithchemicalcarbondioxide15scrubbers.101 3.1.2.4SolidOxideandMoltenCarbonateFuelCells:highertemperatureForapplicationsabove600(C,moreexoticmaterialscanandmustbeusedfortheelectrolyte.Withasolidoxidefuelcell(SOFC),theelectrolyteisaceramicoxideandconductiontakesplace2-byoxygenion(O)hoppingthroughtheelectrolyte.Moltencarbonatefuelcellsusemoltenionicsalts(e.g.Li2CO3,K2CO3,andmixturesthereof).Bothofthesefuelcellsfindtheirutilityinstationarypowerapplications,whereefficiencygainscanberealizedbyusingtheexhauststreamanditshighgradewasteheattodriveagasturbinebottomingcycleorprovidecogeneratedheat.Theseadditionaloutflowswouldbewastedinascooterwhichlacktheroomforadditional“bottoming”cycles.Thehightemperaturesofthesefuelcellsofferthepossibilityof“internalreforming,”wherenaturalgasandsteamareintroduceddirectlyintothefuelcellandsteam-reformingandwater-gascleanupoccurautomatically.SOFCscanevenusecarbonmonoxideasfuelwithoutreforming.Thehightemperaturesmeanthatnoblemetalcatalystsarenotneeded,butalsobringwiththemtheirownmaterialsproblems.Neitherofthesefuelcellshasthehighpowerdensityneededforvehiclepower.3.1.2.5DirectMethanolFuelCells:long-termpromiseAdirectmethanolfuelcellisanexceptiontotherulethatfuelcellsarecategorizedbytheirelectrolytes;inthiscase,itisthefuelthatdefinesthefuelcell.(Typically,theelectrolyteusedinconjunctionwithDMFCsisaPEM).Dilutemethanolisflowedthroughtheanodeasthefueland102 brokendowntoprotonsandelectronsandwater.Methanolischosenbecauseitisoneofthefewwidely-availablefuelsthatiselectroactiveenoughtouseinafuelcell;ethanolcanalsobeusedbutwithpoorerefficacy,duetoitspoorerelectricalactivity.Muchresearchisbeingdoneondirectmethanolfuelcells,becauseafuelcellrundirectlyonliquidfuelwouldofferdramaticadvantagesinoverallsystemdensitysinceneitherlow-densityhydrogennorbulkyreformerswouldbeneeded.Methanolcanbemaderelativelyeasilyfromgasolineorbiomass,andalthoughitonlyhasafifththeenergydensityofhydrogenbyweight,asaliquiditoffersmorethanfourtimestheenergypervolumewhencomparedtohydrogenat250atmospheres.ForaDMFC,theoverallanodereactionis:+-CH3OH(l)+H2O(l)CO2+6H+6eAtthecathode,+-6H+3/2O2+6e3H2O(l)Thenetreactionistheoxidationofmethanol:CH3OH(l)+3/2O2(g)CO2(g)+2H2O(l)(Er(=1.214V)Forthelowerheatingvaluereaction,Eris1.170V,slightlylessthanhydrogenat1.185V.Foreachmoleofmethanol,sixelectronsaretransferred,andthiswouldseemfarsuperiortothetwoelectronsofahydrogenPEMFC,butthecomparisonisproperlywithnotonemoleofhydrogen,103 butonemoleofmethanolreformedintohydrogen.Inthetheoreticalmaximumcase,threemolesofhydrogenmoleculescanbereformed,sothetotalnumberofelectronsareequalineachcase.Notethatthepresenceofcarbondioxidemeansthatanacidelectrolytemustbeused.Sincetheionbeingexchangedisaproton,theDMFCcanoperatewithapolymerelectrolytemembrane.Onemajorproblemisthattheoxidationofmethanolproducesintermediatehydrocarbonspecieswhichpoisontheelectrode.Theothermajorproblemisthattheexchangecurrentformethanolis16muchlowerthanthatforhydrogen-byasmuchassixordersofmagnitude.Thismeansthattheoxidationofmethanolattheanodebecomesasslowastheoxygenelectrodereaction,andlargeoverpotentialsarerequiredforhighpoweroutput.Also,thereishighcrossoverofthemethanolthroughtheelectrolyte,meaningthatthefuelmoleculesdiffusedirectlythroughtheelectrolytetotheoxygenelectrode.Asmuchas30%ofthemethanolcanbelostthisway,severely16compromisingpower.-2Asaresult,maximumdemonstratedpowerdensitiesin1998wereontheorderof200mW?cmwithairat2.5atmina2"x2"cell,insufficientforcurrentapplicationsexceptverysmallportable17-2fuelcells.Platinumloadingswere4mg?cm,almostafactoroftengreaterthanhydrogen-airfuelcells.Futureimprovementsincatalystandmembranedesignareexpectedtochangethissituation,-2-218withyear2000goalsof300mW?cmat1mg?cm.3.1.3StackcharacteristicsThePEMfuelcellisthebestforthis,andmanyother,vehicleapplications.Thenextstepistocharacterizehowlargeafuelcellwouldbeneededtosupplythedemandedpower.Thisisdone104 firstbyacomparisonwithpublishedoverviewsofautomobilefuelcellpower,andsecondwithamoredetailed,ground-upmodelbasedonaDirectedTechnologies,Inc.(DTI)study.3.1.3.1FuelcellstackspecificationsThefuelcellstackissizedherebydeterminingastandardmotorvoltageandconnectingmanyfuelcellsinseriestocreatethedesiredvoltage.Maximumcurrentdensityisfixedbythepropertiesofthemembrane,butthetotalcurrentcanbeincreasedbychoosingcellswithlargersurfacearea.Foragivennominalpoweroutput,afuelcellthatisoversizedforthatnominalpowermeansthatthepowersystemismoreoftenoperatingatasmallerfractionofmaximumpower-andthusathighervoltagesandhigherefficiency.Ontheotherhand,itisexpensiveandofteninefficientfromanoverallsystemsperspectivetobuildsuchaheavyandexpensivestack,especiallysinceinroadvehiclesthemaximumpowerissoinfrequentlyrequired.Theprocessdescribedaboveiscarriedoutinsection4.4.1,afterdetailedpowerandperformancerequirementsarederivedinsections4.1,4.2,and4.3.However,togiveawaytheending,themaximumgrosspoweroutputis5.9kWforstandardurbandriving(5.6kWnetofparasiticpower).3.1.3.2PublishedresultsforautomobilefuelcellstacksFirst,PEMfuelcellstackcharacteristicsareestimatedbasedonprevioustop-downstudiesforautomobiles.ThePNGV(PartnershipforaNewGenerationofVehiclesgovernment/industrycollaboration)105 goalsforfuelcellstackmassandweightare0.35kW/kgand0.35kW/L,byapproximatelythe19year2000.Thisisexclusiveofauxiliarysystemslikeradiatorsandblowers.Thesamesourcesetsgoalsof0.5kW/Land0.5kW/kgby2004.(Forthe5.9kWsystemstudiedhere,thattranslatesinto12kgand12L).However,itshouldbenotedthatBallardachieved1kW/Lasearly20as1996.ThePNGValsodefinedyear2004goalsof50$/kWnetpowerforthefuelcellsystem(stackand21auxiliaries)andanintermediatepricetargetof150$/kWby2000.Thus,fora5.6kWsystem,infiveyearsthecostcouldbeaslowas$280ifautomotive-sizedsystemcostsscaledowntothescootersize.Thisdependsonsignificantcostreductionsfromcurrentprices,whichareontheorderof$1000/kW.Ogdenetalsurveyedpriceestimatesintheliteratureandfoundapricerangeof$33to$100pernetkWforthefuelcellstack,and$10to$20perkWforthepeakingpowerbattery.Therange22costestimatesisthus$185-$560forthe5.6kWfuelcell-onlyscooter.3.1.3.3DetailedconstructionThefuelcellengineitselfismadeofseveralcellselectricallyandphysicallyconnectedinabox-shaped“stack”.Oxygenandhydrogenmustbebroughttothemembraneswheretheycanreact,whilethemembranesthemselvesmustbekeptwetsothattheycanconductthehydrogenions(protons).Surpluswatermustbepushedoutofthestack,andwasteheatmustberejectedfromthestacktoavoidoverheatingandmembranedamage.Thecells,whicharemadeofelectricallyconductivegraphiteormetal,areconstructedinseriessovoltagesfromeachcelladdup;thesamecurrentflowsthroughtheentirestack.Thehydrogenandoxygenflowinmoldedmanifolds106 typicallybuiltoffthesideofthestack,andaredividedintoparallelfeedsintotheindividualcells;waterandexhaustgasarecollectedinanothermanifoldandrejectedtotheatmosphere(thewatermayberecycledtohumidifytheincominggases).Essentially,eachfuelcellinthestackcontainsanMEA,ormembraneelectrodeassembly,whichconsistsofthepolymerion-conductionmembranewithflatelectrodesheetsattachedtoeitherside.Oxygenandhydrogenarechanneledtothecathodeandanodesidesrespectivelyinflowfieldscarvedorpressedintoplatesthatarenexttotheelectrodes.Thesecanbetwoseparateplates,eachservingasingleelectrode(“unipolar”design),orasingleplatecarvedonbothsideswithflowfieldscanbeusedfortwoadjacentelectrodes(“bipolardesign”).Themembranemustpasshydrogenionsbutnotelectricity;theplatesmustconductelectricity,butnotallowwater,hydrogen,oroxygentopermeatethrough.22TheDTIstudyexaminedanumberofoptionswithmembranesofactivearea116cmto697cm,andcalculatedcostsandweightsforeachofthesubcomponentsusingdesignformanufactureandassembly(“DFMA”)techniques.FourpossiblecelldesignswerestudiedintheDTIreport:unitizedstainlesssteel;three-piecestainlesssteel;amorphouscarbon;andcarbon-polymercomposite.Thethree-piecestainlesssteelcellswerechosenheretoestimatelong-term,mass-producedcostestimatesforthefuelcellstacks.Adetaileddescriptionofthestacksize,weight,andcostanalysisisprovidedinAppendixB.Essentially,themodelwasbuiltfromthebottomup,ratherthansimplyextrapolatingfromtheautomotive-sizedvehicles,althoughsomeofthefiguresarestillbasedontheautomotivemodel.107 TheMEAfromDTI’smodelis70μmthickintotal.Itconsistsofa5μmcompositemembrane,sandwichedbetween28μmthicklayersofelectrode(theseelectrodesconsistofplatinumdepositedoncarbonblackwhichsitsonaninertionomercarrier,andthen25μmofelectricallyconductiveporousbacking,madeofcarbonpaperimpregnatedwithfluoropolymerlikeTeflontorepel23accumulatedwater).Rightnow,typicalmembranethicknessesareontheorderof50μm-127μmforNafion,andaslowas25μmforanewerGoremembrane.Apredictionof5μmisaggressivealthoughitshouldbenotedthat,inDTI’smodel,thethinnessofthemembranemainlyreducescosts,notstacksize.Today’sseparatorplatesaremadefromgraphite,whichhaslowelectricalconductivitybutresiststhecorrosioncausedbytheelectrochemicalpotentialsinthecell.However,itisextremelyexpensivetomachineflowfieldpatternsintothegraphitesurfaces,andgraphiteitselfisnot24inexpensive.(Machinedgraphiteseparatorplatescurrentlycost“asmuchas200$/kW”).Futurepricesarepredictedtobeaslowas5$/kW,andcheapermaterialoptionsincludeconductivepolymers,impregnatedamorphouscarbon,andmetaltreatedwithanticorrosivecoatings.Thethree-piececelldesignischosen:ineachactivecell,onemetalseparatorplateismatchedwithtwoseparate,unipolarplatesetchedwithflowfields.51μmseparatorplate76μmanodeflowfield1000μmanodegasket70μmMEA108 76μmcathodeflowfield1000μmcathodegasket[repeatwithnextseparatorplate;totalthickness2.3mm]Figure3.6ActiveCellEachsystemrequirescoolingplatesperiodicallyinterspersedbetweentheactivecellsdescribedabove.Thecoolercellsallowflowofcoolantwithinthestack,andareessentiallyflowfieldsthroughwhichonlywaterflows.Thecellsaremadeofthesameelectrically-conductivestainlesssteeltoallowbulkconductionofcurrent.Thereasonforusingcoolercellsofthesamedesignastheordinary(“active”)cellsistoincreasethenumberofidenticalcomponentsand,again,decreasemanufacturingcosts.Acoolercelltoactivecellratioof1:2isdefinedhere.109 Athree-piecemetalliccoolercellconsistsof:51μmseparatorplate76μmcoolantflowfield1000μmgasket[repeatwithnextseparatorplate;totalthickness1.1mm]Inadditiontotherepeatingcells,thestackrequiresanelectricallyinsulatingplastichousing;tierodstoholdthecellstogether;currentcollectorsattheendsofthestacktoconductelectricitytothepowersystem;andinsulatorsandendplatesoverthecurrentcollectors.3.1.3.4DetailedconstructionresultsTheresultsaresummarizedbelowforthreedifferentfuelcellstacksizes,each56cellslongforatotalvoltagecloseto48V.Table3.1Stacksize,weight,costsummary5.9kW3.3kW1.1kW222membraneactivearea170cm100cm35cmtotalmassofnon-repeatunits5.1kg3.6kg3.3kgtotalmassofrepeatunits2.8kg1.8kg0.8kgtotalstackmass7.6kg5.4kg4.0kgtotalstackvolume7.8L5.3L3.2Lstackpowerdensitybyweight0.780.620.27kW/kgkW/kgkW/kgstackpowerdensitybyvolume0.76kW/L0.62kW/L0.34kW/Ltotalstackcost$244$161$124costofpower42$/kW47$/kW103$/kW110 Thesearelong-termmass-producedprices.Incomparison,aBallard37kWstackavailabletoday25haspowerdensitiesof1.1kW/Land0.8kW/kg,sotheresultsabovearenotoverlyambitious.Forcomparisonpurposes:asimplisticcurve-fittingtotheDTImodel’sresultsforautomotivefuelcellenginesgiveslowercostsof$176,$125,and$96-about20%to30%lower.SeeAppendixBformoredetails.3.1.4GasflowmanagementInadditiontothestackitself,severalsubsystemsareneededinafuelcellpowerplant.Oneofthemostimportantonesisthegasflowsubsystem.Oxygenandhydrogenareintroducedintothefuelcellsystemattheappropriateflowraterequiredforthecurrentatanygivenmoment;thisrequiresavariable-flowsystemifthestoichiometryistobekeptconstant.Eveninan“atmospheric-pressure”system,somepressureoveratmosphericisneededinordertopushthegasesthroughtheoften-serpentinepassagescarvedintotheflowplates,andtoforceliquidwateroutofthesamepassages.Thisadditionalpressureisontheorderof0.1to2.0psi(0.7to13.8kPa)above26,27atmospheric;IFC(InternationalFuelCells)quotes0.8psi.Aminimumflowspeedof0.35m/sisneededtoeliminateproductwater,andaSchatzEnergy28,31,28ResearchCenterpatentestimatesflowvelocitiesatupto7m/s.Duetothefactthatthecathodereactionismuchslowerthantheanodereaction,oxygenisoftensuppliedatahigher-than-stoichiometricflowrate.Theratioofairflowratetotheminimumflowraterequiredforstoichiometricoxygen-hydrogenreactionisoften2.0orhigherinorderthattheconcentrationofoxygenintheairnotdroptoomuchasitpassesthroughtheflowfield,andtheexcessairalsohelpstopushproductwatergeneratedatthecathodeoutofthefuelcell.111 Anairfilterisneededtopreventforeignobjectsfrombeingtakenintothefuelcell.Hydrogenissupplieddead-endedinthedesignproposedhere.Thismeansthatthereisnooutlettotheanodeside;thepressureissimplyallowedtoequilibrateinthestacktomatchthepressureregulatoroutput.Electrochemicalconsumptionismatchedbyreplacementfromthehydrogenstore.Theresultis100%hydrogenutilization.Thistechniquecanonlybeusedforpurehydrogenvehiclesbecausetheanodestreamfromreformedhydrocarbonsorammoniawouldcontaininerts(N2,CO2,H2O)andpoisons(CO)thatwouldbuildupattheanodedeadend.Dead-endedfuelcellsstillneedtoventoccasionallyinordertopurgeimpuritiesthatmayhaveenteredthefuelcell;thisinvolvesopeningtheanodeexhaustvalvesforabriefperiodandallowingthehydrogentoflowdirectlyintotheatmosphereforaverybrieftime.Effectiveutilizationisslightlylessthan100%forthisreason,althoughinthisstudythislossisassumedtobenegligible.(Stacksrunningopen-endedhavehydrogenutilizationsofapproximately85%unlesspurehydrogenisrecycled.)Tomanagetheairflow,blowersarebrieflydiscussedbelow.Then,theutilityofpressurizedoperationisanalyzed.3.1.4.1BlowersBlowersareusedinatmosphericsystemstodrawairintothefuelcell;nocorrespondingdeviceisneededforthehydrogenside,becauseinallcurrentdesignsthehydrogeniskeptunderpressurehigherthantheoperatingpressureofthefuelcell,andexpandsasitisreleasedfromthestoragecontainerorproducedaboveatmosphericpressureinareformer.Thebloweristypicallypoweredelectricallyfromthefuelcelloutput,withabatteryrequiredforstartup.112 Theblowerpowerrequiredis:DPVW=hwhereistheblowerefficiency.Forthepowerrequirementsofa5kWfuelcell(2.0psipressuredropandaflowrateof15.6cubicfeetperminute)anda50%blowerefficiencythepowerconsumptionis200W.Anindustrialheavy-dutyblowerthatcouldbeusedtoprovidetherequiredoutputistheAmetek295.7"BLDCthree-stageblower,model116638-08,withacatalogsalepriceof$430.3.1.4.2CompressorsCompressingtheairinputincreasestheconcentrationofoxygenpervolumepertime–itseffectivepartialpressure–andthusincreasesthefuelcellefficiency.Thismeansthatasmallerandlighterfuelcellstackcanbeused,atthepriceofparasiticpowerrequiredbythecompressorandincreasedcost.Inaddition,thedrop-offinvoltagecausedbymasstransportproblemsisdelayeduntilhighercurrentdensities.Anotherbenefitofhigherpressuresisthat,forthesamemolarflowrate,alowervolumetricflowratecanbeused.Thus,humidificationiseasierbecauselesswaterisneededforsaturation(permoleofair).Finally,flow-fielddesignislessrestrictivebecauselargerpressuredropscanbetoleratedintheflowfield.Sincemostofthehydrogenstoragetechniquesinvolvepressurizedhydrogen,itisnotbedifficulttoobtainamatchingpressurizedhydrogenstream;inanycase,atypicalPEMcantolerateapressure30differenceofabout0.5barforNafion-115.113 Inatightly-integratedsystem,thecompressorcanbepoweredmechanicallyasaturbocharger,usingashaftattachedtoaturbinerunningofftheexhaustfromthefuelcell.Thisallowsrecoveryofsomeoftheexpansionwork.Ontheotherhand,thesystemmaybesimplerifthecompressorispoweredonlybyanelectricmotor,withseparatebatteryforstartuppurposes.31Compressorsontheorderof0.5-10kWaredifficulttofindandgenerallyinefficient.TheDepartmentofEnergyhasagoalof3kg,4L,68%efficiency,and$200at70-80grams/second32,33foraturbocompressor/expanderfora50kWfuelcellsystem.Notethatthisistentimestheflowraterequiredfora5-6kWscooter,andefficiencydecreasesdramaticallywith“turn-down”(operatingbelowthedesignpoint),soactualefficiencywilllikelybelower.Also,forthelowflowratesinvolved,variablespeedpositive-displacementcompressors(ratherthanturbomachines)aretypicallyusedandefficiencymaybelowerforthatreasonaswell.TheDOEtargetsizeandweightarequitelowforthebenefitinvoltage,butcostwouldbesignificantwhencomparedtothecostofthetotalsystem.Thenetbenefitofcompressionforthescootersystemisanalyzedinsection4.6inthesystemmodelingchaoter.3.1.5WatermanagementWaterisvitaltofuelcellsinthattheelectrodesandtheelectrolytemustbekeptwetinordertoallowprotonconductionthroughtheacidmedium.Waterentersthesystemfromexternallyhumidifiedhydrogenand/orairstreamsandgenerationatthecathodebytheelectrochemicalreaction.Duetohydrogenbonding,onaverage1to2.5watermoleculesaredraggedalongwitheachprotonasitmigratesfromtheanodetothecathode;thisisknownas“electro-osmoticdrag.”114 Wateralsoflowsintheotherdirectionduetoback-diffusion,sincetheconcentrationofwaterinthecathodeelectrodeismuchhigherthanattheanodeelectrode.Waterexitsthesystemwiththecathodeexhaustasblown-outliquidorvapour.Thegreatestdangerposedbywateristhatofdryingout.Lossofwatercandryouttheelectrodesorthemembrane,leadingtoarunawayinoverheatingandcurrentlossanddamagetothemembrane.Ontheotherhand,iftoomuchliquidwateraccumulatesatanelectrode,itcanblockthediffusionofgasintothatelectrode,preventingdissociationandslowingdowntheoverallconversiontoelectricity.Dropsincurrentdensityareoftenthesymptomsofflooding.Voltage(efficiency)ishigherwithhumidifiedflowthanwithunhumidifiedreactantstreams.Inteststations,“external”humidificationistypicallyachievedbybubblingthereactantsthroughareservoirofwater.IFChasdemonstrated“internalhumidification”byabsorbingwaterintoporousbipolarseparatorplates,andusingthisreservoirofwatertoreplenishthewaterintheelectrodesandelectrolyte.Variousmethodshavebeenproposedtoremovewaterfromthecathode.Ifthetemperatureandflowratearehighenough,thewarmedoxidantcanvaporizetheproductwaterandcarryitawayaswatervapor.Iftheoxidantpressureandflowratearehighenough,theliquidwaterisphysicallypushedout,althoughflowratesthataretoohighwilldryoutthemembraneandanode.Finally,aseparatepathofhydrophilicpolymercanbeusedto“wick”(drawbycapillaryaction)thewaterfromthecathodebacktotheanodeside,whichtendstodryoutfaster.Fuelcellsrunningwithadead-endedfuelsupplycannothumidifytheanodeflow,becausewaterwouldaccumulateinthehydrogendeadend.So,topreventtheanodefromdryingout,the115 incomingairflowishumidifiedbypassingitoverawettedpolymerwickingwaterfromareservoir,orthroughawaterbottle.Back-diffusionisallowedtocarrythemoisturethroughthemembranebacktotheanode,andthethinnerthemembrane,themorequicklyback-diffusioncanoperate.Iftemperaturesarelow,thisdiffusioncanbeenoughtoremovetheneedforexternalhumidification.Waterinthefuelcellhastobedeionized;thiscanbedonebyforcingthewaterthroughafilterorbysupplyingthevehiclewithdeionizedwateronly.Inthecasewhereallwaterneedsaresuppliedbycondensationandthenrecirculationoftheproductwater,deionizationisnotaconcern.Inthescootersystem,withoutroomforacondenserandsecondcoolingfan,thewatermayjustbeexhausted.3.1.6HeatThedesignedoperatingtemperatureofthefuelcellaffectsvariousfactors.Ahigheroperatingtemperaturemeansthatmoreoftheproductwaterisvaporized,sothatmorewasteheatgoesintothelatentheatofvaporizationandlessliquidwaterislefttobepushedoutofthefuelcell.Highertemperaturesalsomeanfasterkineticsandavoltagegainthatingeneralexceedsthevoltagelossfromthenegativethermodynamicrelationshipbetweenopen-circuitvoltageandtemperature.Heatrejectionisalsoabettedathighertemperaturesduetothelargerdifferencebetweenthefuelcelltemperatureandambienttemperature.Ontheotherhand,lowerstacktemperaturesmeanshorterwarmuptimesforthesystemasawhole,andlowerthermomechanicalstresses.Corrosionandothertime-andtemperature-dependentprocessesareretarded,andmuchlesswaterisrequiredforsaturationofinputgases.116 TheupperlimitofoperationforPEMFCsisabout90(Cbecausewaterevaporatesfromthemembraneandperformancedropsquickly;permanentdamagecanoccurtothemembrane.AtPrincetonandvariousotherplaces,membranesarebeingdevelopedthatcanhandlehighertemperatureswhileretainingthehighpowerdensityofPEMFCs.ThisisbeingattemptedbymakingnovelelectrolytethatarecompositesofNafionwithmaterialsthatholdwatermoretightlyandpreventevaporation:proton-conductingglassesandhydratedoxidesofsilicon.Another34techniqueistoreplacewaterinNafionwithdifferentchemicalsthathavehigherboilingpoints.TheprimaryobjectiveishigherCOtolerance(sincethecatalystsaremoreeffectiveathighertemperatures)withoutdrasticlossofperformance.Anotherheat-relatedissueispreheatingoftheinletairandhydrogen.Thisisadvantageoustopreventfloodingattheinletatthepartofthestackclosesttotheairentry.Thisisthepointatwhichtheairiscoolest,havingnotyetwarmeduptothefullstacktemperature,andhasthehighestconcentrationofoxygensincetheairhasnotyetbeendepletedofoxidant.Cathodewater35productionishighesthere,andthecoolairsaturateswithwatereasilyifnotpreheated.Preventingoverheatingisdescribedingreaterdetail.Underthemoststrenuousconditions,a5.9kWfuelcelloperatingat50%efficiencyproduces5.9kWofwasteheat-asignificantinstantaneousloadtomanage.However,aswillbeshownlater,theaveragefuelcellloadforascooterinacitydrivingcycleisone-tenthofthemaximum,andatthispowerlevelefficienciesarehigherthanatmaximumload.Thethermalmassinthesystemdampsoutthepeakswhenthetransientheatgenerationsareconvertedtotemperaturerises,asanalysisinsection4.4.4.3shows.Coolingcanbeachievedthroughanumberofmeans.First,theevaporationofsomeoftheproductwateratthecathodeabsorbssomeheat.Second,activecoolingwithairorliquidcoolantscanbe117 usedtotransferheattoradiators.Third,passivecoolingofthefuelcellcanbeperformedwithcoolingfinsandheatsinks.Finally,thefuelcellmightbecoupledwithsubsystemsthatabsorbheatliketurbinereheatersandmetalhydridecontainers.3.1.6.1ActivecoolingPumpingacoolantfluid(eitherairorwater)throughcoolingpassageswithinthefuelcellstackwouldallowmuchheattoberemoved.Thisheatwouldhavetobedissipatedataradiator,soinadditiontothepumpingenergy,afanwouldbeneededtoincreaseconvectionovertheradiator.Requirementsincludenoncorrosivecoolantfluid;apumptocirculatethefluid;seals;aradiatorandradiatorcoolingfan;adeionizingfilter;andasurgetank.ResearchersattheInstituteofIntegratedEnergySystemsattheUniversityofVictoriaestimatedthepowerrequiredbythefan36blowingovertheradiatorat83Wfora5kWBallardMarkVstack.Radiatorandfansize,weight,andcostaredescribedinmoredetailinsection4.4.4.4,aftertheradiatorcoolingcapabilityrequirementsarecalculated.3.1.6.2PassivecoolingCoolingusingfinswithoutapoweredfanblowingoverthemisvirtuallyimpossibletoachieveatsizesgreaterthan50Wbecauseofthelimitedsurfaceareaandthelowtemperaturedifferencebetweenthemaximum~80(CtemperatureofPEMFCsandtheambienttemperature,whichinTaiwancanbeashighas40(C.However,forcedairventilationoverfinsattachedtothefuelcellstackispossibleforcellsofthepropernarrowaspectratio,asHPowerfuelcellstackshave118 demonstrated.Lesstotalcoolingairflowisrequiredthanforwatercoolingbecausetheintermediate(watercirculation)stepiseliminated,andstackweightandvolumearesavedbynothavingtointerposecoolercellsbetweenactivecells.Ontheotherhand,somekindofheatconductionpathisneededtogetfromthestacktotheenvironment.Also,theheatcapacityofairisfarlowerthanwater,andwhencoupledwiththesmallchangeintemperaturefromfuelcelltoambientenvironment,theresultispoorheattransfer.3.1.6.3BoilingrefrigerantUsingaclosed-looprefrigerantthatboilsatthefuelcelloperatingtemperature(50(C-80(C)wouldallowagreatdealofthermalenergytobewithdrawnfromthestack.Inaddition,thepressureofthevaporizedcoolant(inconjunctionwithacheckvalve)couldbeusedtodrivethecoolantcyclewithoutrequiringapump,minimizingparasiticlosses.Afanwouldlikelystillbeneededonthecondensation(radiator)side,butotherwisethesystemwouldfeatureautomaticcontrolwithoutelectronics.Activecoolingwithaliquidcoolantloopischosenhere,tobecertainofsufficientcooling.Aircoolingisunlikelytobesuccessfulinastackaslargeas5.9kW,astheavailablesurfaceareaislow.Heatissuesarediscussedinmoredetailinsection4.4.4.119 3.2.FuelforthefuelcellThePEMFCmustbesuppliedwithhydrogen.Thishydrogencanbestoredashydrogen,butitcanalsobe“reformed”fromotherchemicalsusinganonboardchemicalprocessor.Theadvantagesofthelatterarethatliquidfuels,withtheirhighenergydensity(asafunctionofvolume)andeasierdistribution,canbeused.However,reformingrequiresadditionalequipmentwhichcomesatapremiumonasmallvehicle.Also,reformersusuallyproducesomepollutionontheirownbutthisistrivialcomparedtocombustionengines.Modelingresultsfromsections4.5.1and4.4.1.4willshowthatthefuelrequirementforthescootertotravel200kmat30km/his250gramsofhydrogen,andamaximumflowrateofapproximately-10.05molesH2?sisneededattheupperlimitof5.9kWofgrosspower.3.2.1.ReformedfuelsThestandardtechnologypredictedforthefirstcommercialfuelcellcarsishydrogenreformedonboardfromliquidfossilfuelslikemethanolorgasoline,becausethesefuelsarewidelyavailableandhaveexcellentenergydensities.Someinterestingothertechnologies,likeproducinghydrogenfromdissolvingcertainchemicalsinwater,andreformingammoniaintohydrogen,arementionedherebecausetheymightfinduseinthesmallerscooterapplication.3.2.1.1.Hydrocarbonreforming.Liquidhydrocarbonfuelscontainagreatdealofenergyperunitvolume,farmorethanhydrogen,120 andarecheaplyavailable.Thetwomajorcandidatesforfuelcellvehiclesaremethanolandgasoline.Methanolproduceslesscarbondioxideandreduceslong-termoildependence,andiseasiertoreform;gasolinehasextensiveexistingproductionanddistributioninfrastructure.Naturalgasisanotherpossibility.Table3.2Fuelgravimetricandvolumetricenergydensities,lowerheatingvaluebasisbymassbyvolumeHydrogengas,atmosphericpressure120MJ/kg11kJ/LatSTPCompressedhydrogengas,3600psi120MJ/kg2,700kJ/Lat3600psiGasoline(approximate)44MJ/kg31,800kJ/LliquidMethanol20MJ/kg15,900kJ/LliquidNaturalgas(puremethane)50MJ/kg36kJ/LatSTPCompressedmethane,3600psi50MJ/kg8,700kJ/Lat3600psi37,38DatafromElectricVehicleTechnology,CARBreportTherearetwomajormethodsofreforminghydrocarbonfuels:partialoxidation(“POX”)andsteamreforming.Partialoxidationispartialburningofthefueltoproducecarbonmonoxideandhydrogen.Itisexothermic.Topickasamplefuel,ifmethaneispartiallyoxidized:CH4+?O2úCO+2H2(H=-35.7kJ/mol)Gasolinepartialoxidationproceedsinessentiallythesameway.Partialoxidationrequireshightemperatures,900(C-1500(C,butcanhandleawidervarietyoffuelsthansteamreforming.121 Steamreformingcombinesthefuelwithsteamtoproducethesameproducts,andisendothermic.ThesteamreformingequivalentofthereactionabovewouldbeCH4+H2OúCO+3H2(H=+206.2kJ/mol)Asteamreformingsystemismoreefficientbecausewasteheatfromthelaterprocessescanberecycledasinputintotheendothermicsteamreformingprocess(forexample,ifunusedhydrogenintheanodeexhaustisburnt).Steamreformersalsoproducemorehydrogenbecausesomecomesfromthewater.Ingeneral,partialoxidationisthesimplersystembecauseofthefewerheatintegrationandwatermanagementissues;ithaslowercapitalcostsforthisreason.Partialoxidationreformersalsohavesuperiorstartuptimes,fuelflexibility,andmayhavefasterresponsetimes.However,partialoxidizersforvehiclesrunonairsomuchintertnitrogenflowsthrough;flowratesarehigherandthusthedownstreamreactorslikethewater-gasshiftandthePROXmustbelarger;also,theconcentrationofhydrogeninthereformateislowerinPOXreactorsthanitisforsteamreformers.Table3.3SteamreformingversuspartialoxidationPartialOxidationSteamReformingPROS&Simplersystem&Theoreticallymoreefficient&Fastresponsetime&Morehydrogeninreformate&Greaterfuelflexibility(higherfuelcellefficiencies)CONS&Airintakemeans&Slowresponsetime(severalseconds)greaterflowrates,&Catalystsneededlargercomponents&Cannothandlegasoline&Lessefficient&Capital-intensiveheatexchangersnecessary&H2burners,ifneeded,addcomplexity122 Thenextstepineitherprocessisthewater-gasshiftreaction.Here,mostoftheremainingcarbonmonoxideisreactedwithwatertoproduceadditionalhydrogen.Atypicalconversionisfrom7.1%COinasteamreformer’soutput(or46.1%infromapartialoxidationreactor),to0.5%coming39outofthewater-gasshiftreactor.Essentially,CO+H2OúCO2+H2(H=-41.2kJ/mol)Thereisoneotherimportantissue:thefuelcellmustbeoptimizedtoacceptananodefuelstreamthatcanbeaslowas40%hydrogenforPOX,or75%hydrogenforsteamreforming.Theloweramountofhydrogenineithercasemeansthatfuelcellefficiencyislowered.Also,watermustbeeithercarriedtosupplythewater-gasshiftreaction(andanysteamreformer),orrecirculatedfromtheexhaust.Thetotalwaterneededisontheorderof3gramspergramofH2forpartialoxidation,and4.5gramspergramofH2forsteamreforming.COpoisoningisanimportantissueforpolymerelectrolytemembranefuelcells,becausetheCOpoisonstheplatinum(orother)catalystontheelectrode,reducingvoltageatagivencurrentdensity.Thus,forthesamepoweroutput,afuelcellrunningoffreformedhydrogenmustbesizedlarger.ItisdifficulttocompletelyeliminateCOfromthereformerexhaust,andfuelcellscanonlytolerateatmost50ppmCObeforeefficiencydropsdramatically,soafinal“cleanup”stepisrequiredevenafterthewater-gasshift.Apreferentialoxidizer(“PROX”)orsimilarcleanupdeviceisneededtoperformCOremoval.(APROXissonamedbecauseit,duetothecatalystmicrodesign,it“prefers”tooxidizecarbonmonoxideratherthanhydrogen;PROXefficienciesareoftenmeasuredintermsofhowmuchhydrogenismustbesacrificedtoreduceCOdowntoacceptablelevels.)123 Theamountofhydrogenthatcanbeproducedbyreformingvarioushydrocarbonsislistedbelowforbothsteamreformingandpartialoxidation;asmodeled,bothprocessesincludeawatergasshiftreactiontocreatemorehydrogenfromshiftingcarbonmonoxide.Theresultsarelistedaseffectivehydrogenconcentrationsinthefuel;duetoinefficienciesnotallofthehydrogencanberecovered,butontheotherhand,someoftheweightfractionsaregreaterthanthefractionofhydrogeninthefuelmoleculesthemselvesbecausemuchhydrogencomesfromthewater.Table3.4HydrogenoutputfromreformedhydrocarbonfuelsSteamreformingPartialoxidationFuelFormulawt%H2gH2permolsH2Owt%H2gH2permolsH2OLfuelpermolfuelLfuelpermolfuelMethanolCH3OH19%1501.013%1000.0EthanolC2H5OH26%2093.022%1682.0MethaneCH450%2052.038%1511.0(LNG)GasolineC8H15.443%30116.328%2008.1(approx.)DieselfuelC14H25.542%35728.328%23114.2(approx.)40DatafromCARBstudy.Excellentoverallweightfractionsandhydrogendensitiescanbeachievedinthefuel,butnotethatthisdoesnotincludetheadditionalweightofreformerequipmentrequired,northeextrawaterthatisneeded.Reformersforautomobilesarepredictedtocost16-25$/kWfor50kWsizedstacks,but41thereislittledataonpricingsofar.Thefuelcellindustryissplitonwhetherreformedmethanolorgasolinewillsucceedfirst;methanolreformswithgreaterefficiency,butgasolineisperhapseasiertodistribute(althoughitshouldbe124 notedthatonlygasolinefreeofdetergents,sulfur,andotheradditivesshouldberunthroughreformers.Sulfurwouldpoisonthereformer,PROX,orfuelcellcatalystsandtheeffectsofelementsotherthancarbon,hydrogen,nitrogen,andoxygenpassingthroughthereformermaybenegative.Gasolineislikelytobereformulatedforfuelcellcarsrunningonreformedgasoline.)Thescooterdesignisextremelyvolume-andweight-sensitive,andbulkyandcomplexheatexchangersaretobeavoided,soifreformersmakesenseatall,alessefficientPOXissuperior.3.2.1.2MethanolreformingexampleMethanolreformingisdiscussedasanexampleofhydrocarbonreformingforthescooter.Thismostsimplealcohol(CH3OH)isoftencitedasaleadingcandidateforfuelcellvehicles,becauseasaliquidithashighenergydensity,andbecauseitiseasiertoreformthangasoline.Itcontains12.5%hydrogenbyweight.ThefiguresforaHotSpotPOXreformeraregivenbelow.JohnsonMattheypredictsanadditional20%volumeforaPROXontopofthatrequiredbythePOX,and95%efficiencyforthissecond42stage,withoverallefficienciesof89%.Fuelcellsrunningonreformatecannotbeoperateddead-ended,sohydrogenutilizationattheanodedecreasestoabout85%,foratotalefficiencyof76%.Thereducedhydrogencontentinthereformateoutput(whencomparedtopurehydrogen)reduces-2thevoltageofthefuelcellbyapproximately0.128voltsperA?cm,orroughly20%atmaximum43poweroutput.125 Table3.5Reformerperformancereformervolume7.2Lreformerweight9.5kg+cleanupunitweighthydrogenoutputaftercleanup1.36L/s-1hydrogenoutput0.061mol?sfractionofhydrogeninoutput43%fractionofCOinoutput8±5ppmoverallefficiency76%outputpermoleofmethanol2.3molH2Atthisproductionrate,55molesofmethanolwouldbeneededfortherequired250gramsofhydrogengasoutput.Thiscorrespondsto1.8kgofmethanol,andavolumeof2.2L,muchlessthanacompressedgascylinderorametalhydridehydrogenadsorptiondevice.Thetotalvolumeisonly9.4L,forPOXandPROXandfueltank;thetotalweightofthesystemis11.3kgplustheweightofthePROX.Assumingan8kgpreferentialoxidizer,andconsideringthereformerandPROXandfueltankasasingle“hydrogenstoragesystem”producesdensitiesof1.3wt%and26g/L,measuredintermsofhydrogenoutput.3.2.1.3.AmmoniaAmmoniareformingisaninterestingoptionthatissomewhatdifferentfromthestandardfossil-fuelreformingideas,sinceitofferscleancombustionfromachemicalfeedstockthatiscommerciallyavailableasafertilizer.FuelcellcompanieslikeAnalyticPowerandH-Powerhavedevelopedprototypesrunningonliquefiedammonia.Ammoniacontains17.6%hydrogenatomsbyweight.ThisisasgoodasPOX-reformedmethanol126 andwiththeadvantagethattheonlyproductsofammoniareformingarehydrogenandnitrogengas,ammoniawouldseemtobeanexcellentcarrierforhydrogen.Also,ammoniaiseasilyliquefiedunderpressure,andataliquiddensityof601g/Lat300K,equivalenttoanH2volumetricdensityof55g/L.Thisliquefactionrequiresapressureofonly10barat300K.Inprinciple,theammoniacrackingreactionisNH3?N2+3/2H2(H=+46.4kJ/molNH3)Thereactiontakesplaceatover400(C,whichrequiresanexternalheatsourcesincetheexhaustfromatypicalprotonexchangemembraneexitsatonly80(C.Traditionally,someofthehydrogeninthereformer’soutputstreamisburnedtoprovidetheworkingtemperatureforthereformerandtoprovidetheheatneededforcracking,althoughitispossibletotunetheanodeutilizationofthefuelcellsothattheexhauststreamfromthefuelcellhasenoughheatingvaluefromtheunconsumedhydrogentosupplytherequiredheatifburned.Thereformerefficiency,measuredasthefractionofproducthydrogenthatdoesnotneedtobeburned,isatmost70%andthesystem44cannotberundead-ended,sothatthereisa15%anodeutilizationpenalty.FuelcellcompanyAnalyticPowerhascreatedanammoniareformingsystemcalledthe“A-Cracker”.Thesystemweighs6lbs,andhasdimensionsof4"x2.5"x12",andoperatesat60%efficiency(overallanodeutilizationefficiencyof51%duetoincompleteconsumptionofhydrogenattheanode).Thesystemconsistsofadissociator,hydrogenburner,andregeneratorforrecyclingheat.Themaximumflowrateis6standardlitersofammoniaperminute,whichtranslatesto450.0040netmoles/sofhydrogen(enoughhydrogenfora500Wealkalinefuelcell).127 For5.9kW,andassumingalackofeconomiesofscale,thereformerwouldrequire33kgofequipmentwithavolumeof22Linadditiontostorageoftheammonia.Theammoniaitselfwouldtakeupabout2.2kgand3.7Lnotincludingthetankitself.Thesystemmasswouldbe35kgand26L,forquitepoorsystemhydrogendensitiesof0.57wt%and7.7g/L.Amajorproblemistheundissociatedammoniaconcentrationintheproductgas.Althoughtheconcentrationislessthan50ppm,thisisstillenoughtodamagefuelcellswithacidelectrolytes,soanacidscrubberisneededtoremovethefinaltracesofammoniagasfromthecracker.Also,ammoniaistoxic;spillsandevaporativeemissionscouldbedangerousinadifferentwaythangasolineorhydrogenspills,asinhaledammoniadamagesthelungs.Ammoniainfrastructureexistsforagricultural(fertilizer)useinmanyareasoftheworld.46Ammoniasellsforroughly$1perpound.Asafuel,thismakesisextremelyexpensive:at120MJ/kgofhydrogenlowerheatingvalueand50%reformingefficiency,thisisequivalenttoacostof205$/GJofhydrogenlowerheatingvalue,abouttentimesthecostpredictedforhydrogenreformedfromhydrocarbonsinlargecentralizedplants.Thisexpensemeansthatammoniacannotbeconsideredaviableoptionforscooters.3.2.1.4ChemicalhydrideenergystorageHydrogenfuelcanalsobeproducedbychemicalreactionwithsolid“chemicalhydrides”.Thistechniqueliessomewherebetweenthemetalhydridesandreforming,butisincludedhereinthereformingsection.Chemicalslikelithiumhydride,lithiumaluminumhydride,andsodiumborohydridecanbe128 combinedwithwatertoevolvehydrogengas(exothermically):LiH+H2OúLiOH+H2-1(H=-312kJ?molLiH)LiAlH4+4H2OúLiOH+Al(OH)3+4H2-1(H=-727kJ?molLiAlH4)NaBH4+4H2OúNaOH+H3BO3+4H2Thesecompoundsarelighterthanthereversiblemetalhydrides,andreleasemorehydrogenbecausehydrogenisliberatedfromthewaterreactant.Thethreeexamplechemicalhydridesandtheirhydrogenstoragecapabilitiesarereproducedbelow.Table3.6ChemicalhydridecomparisonLiHLiAlHNaBH44hydrogen-to-hydrideratio(wt%)25.2%21.1%21.3%hydrogen-to-hydrideplus7.7%7.3%7.3%stoichiometricwaterratio(wt%)hydrogen-to-systemratio(wt%),6.4%6.1%6.1%assuming20%additionalweightmassofhydridepowderneeded980g1177g1173gfor250ghydrogenoutputcostformassofhydridegivenabove$268$503$178(laboratory-scalepricing)47DataisfromBrowningetal,withtheexceptionofcostinformationwhichisfromlaboratorycatalogs:AldrichfortheLiAlH4andAlfa48,49Aesarfortheothers.129 Theweightfractionismuchhigher,andthesystemrequirementsforcontainmentaremuchlesssincethepartialpressureofhydrogenoverthehydrideislow(1-10atm).Browningetalestimateanextra20%“fortheweightofthehydrogenandwatercylinders,mixingdeviceandcontrolvalves.”Forexample,fortheLiHtoproduce250gramsofhydrogen,0.98kgofpowderedhydrideanda2.2kgtankofwaterwouldbeneeded.Anadditional200gofcontrolandmixingsystemswouldbeneeded.Packagingthechemicalhydridesinsmallsubunitswouldmakefueldistributioneasy;ifproperlyandsafelycontained,theycouldevenbesoldinconveniencestores.Containmentisnotatrivialproblem;thehydridesmustbeprotectedfromwaterinliquidorvapourformthatmightcauseanextremelyexothermicreactionandignitethereleasedhydrogen.Oneotherissuesisthatthescooteraccumulateshydroxideswhichmustbecollectedandsafelydisposedof,orbetter,reprocessed.Finally,thelessexothermicofthetworeactions,thatforLiAlH4,produces182kJofheatpermoleofH2released.Atmaximumfuelcellpower,when0.05molesofhydrogenareconsumedeverysecond,over9kilowattsofheataregenerated!Thisisanenormousamountwhichexceedsthatproducedbythefuelcellitself,andmakesforalargeheatrejectionproblem.Asforcost,thepricesarecurrentlyforsmallquantitiesofchemicals(2kgmaximum)producedathigh(>95%)purityforexperimentalpurposes.Ifchemicalhydrideswereactuallytobeusedforvehicles,thispricewouldhavetodecreasesignificantly.Also,thewastesolutionleftafterthereactionwouldbereprocessedandthecostwouldbeexpectedtobelowerthanthatformanufacturingafreshbatchofchemicalhydridepowder.Thistypeoftechnologyisnotwelldevelopedandtallbarriersofcostandsafetymustbeovercomeifchemicalhydridesaretobeseriouslyconsideredforhydrogenvehicles.Theexothermicityofthe130 reactionappearstobewastedinthescooter,wheretheprimarypurposeofheatmanagementistogetridofheat.Theydoofferthetemptingpossibilitiesofextremelyhighenergydensitiesandeasydistributioninconvenientunits.3.2.2DirecthydrogenstorageAnalternativetoreformersandchemicalhydridesistostorefueldirectlyintheformofhydrogen.Thisiseasierintermsofsystemcomplexity,andtheexpenseofproducingthehydrogenisoffloadedtocentralprocessingplantswherehydrogencanbeproducedfromchemicalreformingofnaturalgasorotherfossilfuels.3.2.2.1SafetyHydrogenisoftenthoughtofasdangerousfuel.Thisispartlytrue;unlikegasolineandmosthydrocarbons,whichonlyigniteoveranarrowrangeoffuel-to-airratios(forexample,about1.3-507.1%forgasoline),hydrogencanigniteoverawiderangeofconcentrations(4%-75%inair).Also,hydrogenhasarelativelylowignitionenergy;alow-energysparkcanbeginanalmostinvisibleflame(0.2mJatstoichiometricconditionsinair,lessthan10%oftheignitionenergyof51methane,propane,orisooctane.)131 Figure3.7Ignitionenergyofhydrogen52DatafromFischerOntheotherhand,hydrogenisaverylightatom,andleakstendtodispersequickly.Beinglighterthanair,hydrogenalsotendstodiffuseupwardsratherthanaccumulateneartheground.(Thisbenefitcouldbeaprobleminindoorsituationswheretheceilingtrapshydrogen).Thelowerlimitofflammabilityishigherforhydrogenthanitisforgasoline,sogreaterconcentrationsofhydrogenhavetobuildupbeforeignitionisreached.132 Slowleaksinenclosedareasweredefinedasthegreatestriskbyathoroughhydrogensafetystudy53donebyDTIforFordandtheDepartmentofEnergy.Odorantscouldbeaddedtohydrogeninthesamewaythattheyareaddedtonaturalgas,exceptthatmostsulfur-containingcompoundspoisonplatinumcatalysts;also,onlypurehydrogencanbeusedindead-endedsystems,becauseothersubstanceswouldaccumulateintheblockedsupplychannel.ScootersinAsiaareoftendriveninafashionthatwouldbeconsideredrecklessinNorthAmerica.Aswithgasolinescooters,thereissomeriskoffireorexplosionfromacollision;neithergasolinenorhydrogenscooterswouldbeassafeasbattery-poweredscooters.Safetydevicescanbedesignedtoshutdownpowertothebatteryandcutoffhydrogenflowinthecaseofacollision.Fortunately,therisksarefarlowerthanforautomobileswhich,duetoacombinationoftheirlowerfueleconomyandgreaterrange,carryaboutfifteentotwentytimesasmuchfuel.Also,aslatermodelingwillshow,fuelcellscootereconomyisthreeorfourtimesthatoftheequivalentgasoline-poweredscooter,sothetotalenergycarriedonboardisreducedby66%-75%.Thefollowingsectionsdescribemetalhydridestorageandcompressedgascylinders,twoformsofeasilyrefillabledirecthydrogenstorage.3.2.3MetalhydrideenergystorageMetalhydrides,whichareformedwhenmetalatomsbondwithhydrogentoformstablecompoundsareagoodoption.Althoughmetalsareratherheavyforhydrogenstorage,theycanuptakealargeamountofhydrogenperunitvolumesostoragedensityisgood.Theyaretypicallyusedaspowdersinordertomaximizethesurfaceareatomassratio;thesemetalpowdersarestoredin(low)pressurevessels.Metalhydridessufferfromtheproblemsofhighalloycost,133 sensitivitytogaseousimpurities,andlowgravimetrichydrogendensity.Hydrideshavetheinherentadvantageofbeingendothermicwhenreleasinghydrogen,increasingsafety;inaddition,thehydrogeniskeptatarelativelylowpressureof1-10atmwithinthemetalhydridecontainmentcylinder.Ifthecontainmentvesselisnotproperlydesigned,hydrogenembrittlementofthevesselitselfisafactor,though,andcertainmetalhydridesarepyrophoric:theycanburn.3.2.3.1ThermodynamicsThemetalhydrideadsorptionreactionis:M+(x/2)H2úMHx(exothermic;H<0)wherethenumberofhydrogenatoms“x”permetalatom“M”isafunctionofthechemistryofthemetal.Theexothermicityofthereactionmeansthatmoreheatcausestheequilibriumtoshifttowardsfreehydrogengas,andhigherpartialpressureofH2causesashifttowardsadsorptionand(-metalhydrideformation.ThelawofmassactionshowsthattheequilibriumconstantisKeq=[H2]x/2),andsubstitutingthisintothefreeenergyequationillustrateshowhydridesareclassified.G=-RTlnKeq=(x/2)RTln(PH2).134 AhydridewithahighheatofreactionGhasalowerequilibriumpressureofhydrogenoverthemetal/metalhydridesystematagiventemperature,andastrongermetal-hydrogenbond.Forametaltobeausefulstoragemediumforhydrogen,itmusteasily(strongly)bondtohydrogensothatitcanbechargedup,butthebondcannotbetoostrongorelsethemetalwillnotgiveupitshydrogenunderdepressurization/heating.Hydrogenuptakeshowsacharacteristicplateauedcurvewhenplottedforisothermalconditions.Atlowpressures,hydrogenfillsthemetalstructureininterstitialsitesasasolidsolution.Asmorehydrogenisinjected,themetalbeginsaphasetransitionfromthe(metal)phasetothe(metalhydridecompound)phase.Thisistheconstant-pressureplateauphaseoftheisotherm.Abovetheplateau,thehydrogenconcentrationcanstillincreaseasothercompoundsbegintoformwithmorehydrogenthanthenominalratio;also,moreandmorehydrogenmoleculescompressintothemacropores.135 Figure3.8MetalhydrideadsorptioncurveTypicalmetalhydrideisothermsforasinglemetalhydridecompound(LaNi4.7Al0.3)atvarioustemperatures:(W)at30(C,(s)at50(C,(x)54at80(C.DatafromPercheron-GuéganandWelter.Sincethedesorptionprocessisendothermic,raisingagivenhydridetoahightemperatureatafixedpressureshiftstheequilibriumtowardsmoregaseousH2,whilealowtemperaturemeansthatmoregaseousH2isadsorbed.Ataconstanttemperature,theremovalofgaseoushydrogenwillcausestheequilibriumtoshifttoreleasemorehydrogenfromhydrideform.Thislattersituationessentiallydescribeshowmetalhydridesareusedinpractice.Hydridesarealsosensitivetocontaminants(somearepoisonedbyoxygenorwatervapour)socaremustbetakentoonlyintroducepurehydrogentothehydride.136 3.2.3.2KineticsTheuseofmetalhydridesasastoragemediumforhydrogenisdependentnotonlyonthermodynamicsbutonkinetics.Fortunately,theintrinsickineticsofhydrogendissociationarefast;theratedeterminingstepingeneralisheattransportintothepowder.Powdersgenerallydonotconductheatwell,andmetalhydridepowdersarenoexceptionwithathermalconductivityin-1-155therangeof1-2W?m?K.Forcomparison,copper,oneofthebestthermalconductors,hasa-1-1-1-1conductivityof401W?m?K,windowglassisat1.0W?m?K;andfiberglass,athermal-1-156insulator,hasathermalconductivityof0.05W?m?K.Heattypicallyneedstobetransferredbetweenthewallsofthepressurevesselandthepowder.Whilehighsurfaceareameansfasthydrogenadsorptionanddesorption,italsomeanssmallerpowderparticles,whichconductheatmorepoorly.Schemesthoughtoftoimproveconductionincludeembeddingthemetalhydrideinaluminumfoamwithhigh-porosityarteries,orrunningchannelsholdinghotliquidthroughthepowder.Thiscanincreasenetthermalconductivitytoas-1-157muchas7-9W?m?K.Also,hydrogentendstoembrittletheparticlesandcausethemtocrackintosmallerpieces.Thisincreasesthetotalsurfaceareaofthepowder,increasingthehydrogendesorption/adsorptionrate,butthesmallerhydrideparticlescanbeentrainedinthegasflow,requiringfilteringtokeeptheparticlesoutofthehydrogenoutput.Thisleadstoconcernsaboutmetalhydrides’longtermusage.137 Itmightbeusefultohavesomekindofhydrogenstoragereservoirbetweenthemetalhydrideandthefuelcell,inordertodampoutpeaksandvalleysinthehydrogenconsumptionratethatarelaggedbyheattransferintothemetalhydride.Fortunately,thegasabovethemetalhydrideinthecontainercanservethispurpose.Transientresponsetimeisthusfast,althoughextendedflowratewilldependonthehydrogendesorptionratewhichinturniscontrolledbyheatinputintothepowder.3.2.3.3.ClassificationMetalhydridesareclassifiedbythetemperatureatwhichtheplateaupartialpressureofhydrogen(H2)abovethemetalhydrideisgreaterthan1atm.Thetwobroaddivisionsarelow-temperaturemetalhydrides,whichhavepartialpressuresofmorethan1barbelow100(C,andhigh-temperaturemetalhydrides,whichgenerallyrequiretemperaturesofover200(Ctoexceed1barofpartialpressure.Somesamplespecificationsarepresentedbelow.Thedataweretakenfrom5859Browningetal,andaDTIreportonhydrogenstorage.Table3.7:Theoreticalperformanceofvariousmetalhydrideslow-temperaturehigh-temperatureFeTiH1.9LaNi5H6.7MgH2H2storagebyweight1.75%1.43%7.60%---3Densityofmetalhydride5.47g·cm6.59g·cm1.40g·cm33H2storagebyvolume101g/L93g/L106g/L-1HeatofdesorptionH(kJ·molH2)-28.0-8.9-74.0Dissociationtemperature50(C50(C290(CDesorptionpressureatgiventemp.10atm4atm1atmDTIestimatesofcost8.80$/kg–6.60$/kg138 Thechartdemonstrateshydrides’excellentvolumetricdensitiesbutonlyaverageweightdensities.Forvehicleapplications,thelowtemperaturehydridesarefavouredbecausethehotexhaustgasfromafuelcell(80(C)canbecoupledtothefuelstoragesystemtosupplytheheatinputrequiredforhydrogenevolution;alternately,thefuelcellcoolantcanbecirculatedthroughthemetalhydridetodissociatehydrogenfromthehydride.Hightemperaturehydrideshaveperhapsfourtimesthehydrogenweightfractionsoflowtemperaturehydrides,butwouldrequiretheuseofburnersorotherheatingdevicestoreachthe300(neededfor1atmofhydrogenpartialpressure.Forscooters,weight,volume,andsafetyarethemostimportantconcerns.NotethatBrowninget?alestimatethatthehydridestoragecontainerandheatexchangeequipmentwillhalvethestoragedensityofmetalhydrides;MichaelLeofNASAestimatestheextraequipmentatanextra50%60weight.Thelatterassumptionisusedhere.3.2.3.4.MetalhydrideperformanceTocomparepracticallyachievablesystems,threeestimatesaremadebasedonexistinghydridesystemsandprojectionsoffutureperformance.First,Mazda’s20kWfuelcellelectricvehicle,theDemio,runsonanunspecifiedmetalhydridedividedintoeightmodularunits.Itcarriesapproximately1.1%hydrogenbyweightinthemetalhydride(1.4%inthehydrideitself,butanextrafactorof1.25isneededtoaccountforthepressurevesselandattachments),withavolumetricdensityof40g/L.Thedischargerateof0.48gramspersecondisenabledbyheattransferfromfuelcellcoolingwater.At120MJ/kgLHVforhydrogen,thisisaLHVpowerrateof57.2kW,andwithanestimated50%fuelcellefficiencythemaximum139 electricityoutputrateis28.5kW:thequoted20kWplussomemargin.Totalstorageis1.3kgof61,62,63,64hydrogen,forarangeof170km.Thesecondestimateismadefromcharacteristicsofthemetalhydrideandcostpredictions.IftherawalloyhasthestoragepropertiesofTiFeaslistedinTable3.7,thentocarry250gramsofhydrogenwouldrequire14.3kgofhydrideand2.5L.ForrawTiFealloy,DTIestimatesamass-65producedcostof9$/kg,foratotalalloycostof$100.Assumingthatthecontainerandsubsystemsadd50%tocost,weight,andvolumeyieldsfinalresultsof3.7L,21.4kg,and$190.For“retail”pricingthisisdoubledto$380.Forthethirdcomparison,ErgenicsofferstodayametalhydridehydrogensupplycalledtheST-90.Bydefault,thissystemcansupplyhydrogenat30psigatroomtemperatureatarateof28standardliters/minute(0.019g/s).Thesystemuses“Hy-Stor208”alloy,withformulaMmNi4.5Al0.5(Mmstandsformischmetal,aslightlyvariablemixtureofanumberofrareearthelements:approximately,50%cerium;32-34%lanthanum;13-14%neodymium;4-5%66praesodymium;1.5%otherrareearths.)Thesystemismadeofstainlesssteelandisgenerally67soldasone-offunitsfortestandresearchpurposesatacostofapproximately$10,000.(ItshouldbenotedthatErgenicssellstherawHy-Stor208alloyatacostof204$/kg,accountingfora68significantportionoftheST-90cost.Infact,aErgenicsrepresentativeestimatedthatwithaswitchfromsmall-scaleprocessingforresearchpurposestofactorymanufacturing,costswouldbe69aslowas30$/kg.)Ifdesignedforcustomapplications,thetransferratecanbeincreasedto0.04g/sbyraisingthedesorptiontemperatureto45(C(byawarmwaterbath,forexample),andtheweightcanbereducedto27kgbyswitchingtoanaluminumdesign.Costsontheorderof$3,500-70$4,000forproductioninthehundredsisestimatedbyErgenicsrepresentatives.Fortruemassproduction,thecostswoulddropdramatically,butpredictionofcostsbecomesdifficult.140 Thethreepredictionsarecomparedbelow,forpredictedsystemcapableofcarrying250gramsofhydrogenandacurrently-availablehydridesystemsizedat204grams.Table3.8MetalhydridesystemscomparisonErgenicsscaled-downDTIprediction:ST-90(aluminum)Demiohydridescaled-upFeTiStatusavailableforsaleforvehicleprojectionofanduseinlabsdemonstrationfutureperformanceH2storagecapacity204g250g250gsystemweight27kg22.6kg21.4kgsystemvolume14L6.2L3.7L(2'x1'x3")systemH2storagebyweight0.76%1.11%1.17%systemH2storagebyvolume15g/L40g/L67g/Lcost$3,500datanot$380(shortterm)released(longterm)ExtrapolatingtheDemiohydridedownto250gramsshouldnotbringupnonlinearscalingissues,sincethesystemisalreadydesignedineightmodularcontainers.TheDTIpredictionestimateslongtermcost,whiletheErgenicsST-90representscurrentlyavailabletechnologyatlabbenchscaleprices.Metalhydrideswereusedinsomedemonstrationfuelcellvehiclesbutduetotheirweightandexpensearenotbeingconsideredforthefirstwaveoffuelcellvehicles.Ontheotherhand,small,highlyfuel-efficientvehiclelikeascootermaybeabletotakeadvantageofthesafetyandlow-volumebenefitsofmetalhydrides.141 3.2.4CompressedgasstorageOneofthelessexoticbutmostpracticalofthemethodsofstoringhydrogenistosimplycompressitasagas.Thisincreasesitsdensity.Themajorconcernsarethelargevolumerequiredtostorethegasevenwhencompressed,andtheabilityofthecontainertoresistimpact.Storageconditionsaresetat3600psi(standardfornaturalgascylinders)atambienttemperature(300(K).5000psicylindershavebeensuggestedforgreaterstoragedensity,butamoreconservativeoptionwaschosenhere.TheRedlich-Kwongequationofstatebelowpredictsamolarvolumeof0.117L/molundertheseconditions,16%worsethanthe0.101L/molcalculatedbytheidealgaslaw.???a÷?P+1÷()VbRm-=T?÷èVVbT()+2?mmVmisthevolumepermole,Ristheidealgasconstant,andPisthepressure.aandbareempiricalconstantsthatcanbeestimatedbytheformulas22.5a=0.42748?R?Tc/Pcb=0.08664?R?Tc/PcwhereTcisthecriticaltemperature,Pcisthecriticalpressure,andRistheidealgasconstant.Theamountofworkrequiredtocompressthehydrogengasintothecylindermeansthatthereisanenergypenaltyofapproximately5-10%.Thetemperatureincreaseswhenthehydrogencylinderis142 filledwithcompressedhydrogen,andthepressureishigherthanthenominaloperatingpressureuntilthecylinderhasachancetocool;caremustbetakennottooverpressurizethecylinder.Thedecreaseintemperatureduringusageduetoexpansionofhydrogenisnotasgreataconcern,becausethereleaserateismuchslower.Currenthydrogengasstoragecontainersaremadefromsteelalloysthatareresistanttohydrogenembrittlement;moreadvancedcylindersmadefromaluminumandwrappedwithcarbonfibrelaminateforstiffnessarelighterandcurrentlyusedtocontainbothnaturalgasandhydrogen.Lesswelldevelopedarefully-compositecylindersmadesolelyfromcarbonfibreimpregnatedwithresinorsomeotherbinder;thesecanhavehydrogengravimetricdensitiesofasmuchas9.5%duetotheirlightweight.However,theyaremorefragileandcurrentlyexpensive.3.2.4.1CylinderperformanceDynetekofCalgarycurrentlyproducesarangeofaluminum/carboncylindersforhydrogenstorage.Althoughthesmallestsizetheymanufactureholds50L“watervolume”(i.e.internalvolume),arepresentativefromDynetekestimatedthatacylinder380mmlong,with325mmoutsidediameterandaninternalvolumeof20LcouldbemadeSuchasystemwouldweigh11kgandhaveanexternalvolumeof31.5L.PricewouldbeverydependentonquantitiesbutaDynetek70representativeestimated18-20$/L.Thissystemwouldcontain344gramsofH2atthe0.117L/moldiscussedpreviously.Theresultsareamassfractionof3.1%,volumetricdensityof10.9gramsH2/Lexternal,andacostof$360.Thisisagoodhydrogenmassfraction,andlow-pricedforsomethingavailabletoday,butpoorintermsofvolume.Anaerospace-qualitygasspheremadebyLincolnCompositesweighs2.4kgandtakesupabout143 15L,andholds8.0Lofinternalvolumeat3600psiforastoragedensityof5.4wt%.Twosphereswouldhold270gramsofhydrogenatatotalweightof4.8kgandavolumedensityof14.0gH2/L.Thisisanexceptionallylowweight,but30Lisrelativelybulkyandfittingtwoten-inchspheresinascooterchassismightbedifficult;worse,costsforthisaerospace-standardpressurevesselbegin71,72at$5,500persphere.Wherethisinformationismostimportantisinshowingwhattechnicalperformanceispossible.ThesmallestnaturalgasvehicletanksuppliedbyLincolnCompositesislistedinthetablebelow;it73haspoorergravimetricdensitybutismuchcheaperatanestimatedcostof$900.AlsoconsideredisapairofcarboncompositecylindersfromLuxferComposites.Afinalcomparisonproductisanall-metalcylinder.AirProducts’size“C”aluminumcylinderweighs15kgandholds15.8Lofinternalvolume.Thecylinderis84cmlongand18cmin74diameter(21.4Lexternalvolume)-ratherlongforascooter.Atits(low)maximumdesignpressureof2216psi,177gramsofhydrogencanbestored.Hydrogenstorageisthusat1.2wt%andvolumetricdensityis8.3g/Lexternal.75Thetechnologiesaresummarizedbelow.NotethatD.Browninget.alcalculatethatpressureregulatorsaddanadditional200gramstothelistedweights.144 Table3.9CompressedgasoptionsmanufacturerAirProductsDynetekLincolnLincolnLuxferCompositesCompositesCompositesmodelnumber“C”modelcustom#220088-1naturalgasL58C23Ltankmaterialaluminumcarbonw/carbonw/carbon-fullcarbonaluminualuminumpolymerwrapmlinerlinerstoragepressure2213psi3600psi3600psi3600psi3600psiradiusperunit9cm16cm13.1cm11.7cm7.8cmlengthperunit84cm38cm(sphere)88.9cm47.0cmexternalvolume21.4L31.5L15L38.1L9.1Linternalvolume15.8L20L8.0L23.0L6.0Lnumberofunits11212totalhydrogenstorage177g345g280g395g206gtotalfilledweight15.2kg11.3kg5.3kg17.2kg3.7kgwt%1.2%3.1%5.5%2.3%2.7%volumetricdensity8.3g/L11.0g/L9.4g/L10.4g/L11.4g/L(externalvolume)currentpriceperunit$250-$300$360$5,500$900unknownIntermsoftechnologicalfeasibility,5.4%storageispossible,althoughnotpracticalfromacostperspective.Volumetricdensitiesontheorderof9-12g/Ldefinetherangeofpossiblecylinders,whiletheDynetekcylinderisanexampleofthecurrentcommerciallyavailable,affordablestate-of-the-art.Metalcylinderswilllikelybetooheavy.3.2.4.2.CylindersafetyNaturalgascylindersaretypicallydesignedwithpressurereleasedevices(PRDs)todischargethecontentsofthecylinderincaseoffire.Becausefailureoccursbycompositematerialdegradation145 ratherthanbypressureincrease,mostareeutecticswitchesdesignedtoreleasewhenacertaintemperatureisreached.Theconceptofarapiddischargeofhydrogen(underfiveminutes)issomewhatdisturbing,especiallyconsideringthefactthatthesedevicesaredesignedtooperatewhenengulfedinflame.Ontheotherhand,acontrolledreleaseofflammablehydrogencouldverywellbebetterthananabruptcylinderfailureandexplosion.Amaximumcylinderlifetimeof15,000cycleswasdefinedinthestandardproposedtothe76CanadiangovernmentbyEDOCanada.Thiswouldbeenoughforover120yearsofusageat45kmperdayandonerecycleeverythreedays.Thus,theproblemofinvisiblefatigueflawsandmicrocracksisthusnotasmuchofaconcernasabruptfailureduetoacollision.Hydrogencylindersarelikelyusefulforshorttermfuelcellscooters,butmetalhydridesofferadvantagesofcompactvolumeandheatremovalthatareextremelyvaluableifmetalhydridecostscanbereduced.3.2.5.LiquidhydrogenstorageCryogenictechnologyandexpensivewell-insulatedcylindersarerequiredifthehighvolumetricdensityofliquidhydrogenistobeused.At20Kand0.1MPavapourpressure,5.3wt%H2isachievable.Evenbetter,atthistemperatureliquidhydrogendensityis70g/L,socarrying240gramsofhydrogenwouldonlyrequire3.4Lofliquidhydrogen.However,maintainingthehydrogenatsuchlowtemperaturesisextremelydifficult,withverygoodinsulation,vacuumgaps,andliquid-nitrogen-cooledheatshieldstypicallyrequired.Aswell,theenergyofreducinghydrogento20Kandthenliquefyingitisanimportantfactorwhenconsideringliquidhydrogenstorage.77Thisenergycanamounttoanextra33-40%ofthetotalenergycontentofthehydrogen.146 Anotherproblemisthatasheatleaksslowlywarmuptheliquidhydrogen,moreandmoreisconvertedintogasovertheliquid.Unlessthisgasisallowedtoescape,hydrogenbuildupwouldeventuallycreateleaksinthetank,soaminimumboiloffrateisrequiredandtodothisapressurereleasevalveisneeded.Acarleftunattendedforalongperiodoftimewouldeventuallyloseallitshydrogentothissafetyrequirement.Cryogenicstorageforasmallscaleapplicationlikefuelingascooterisnotfeasible3.2.6SelectionThefewpracticaloptionsforstoringhydrogenaresummarizedandconsideredbelow.Firstofall,liquidhydrogenstorageiseliminatedasbeingtooexpensive,difficulttohandle,andinefficientforthelowstoragerequirementsofscooters,andchemicalhydridestorageispostponeduntilfuturedevelopmentsdemonstratetheirpracticality.Thisleavesmethanolreforming,metalhydrides,andcompressedgascylinders.Thefollowingtableisacomparisonofdimensionsandweightofstoragesystemrequiredtocarry250gramsofhydrogen.Forcomparison,agasolinetankinascooterisabout5L,containsapproximately3.7kgofgasoline,andallowsarangeof240kmat30km/h,whilethebatteryusedintheZES2000scooteris3.7Landweighs38kg,butonlyprovides65kmofrangeat30km/h.147 Table3.10StoragetechnologycomparisonHotSpotDTIhydridereformerDynetek(includes50%+methanolgascylindersystemfactor)dimensions–16cmradius,38–cmlongcylinderexternalvolume9.4L31.5L3.7L(includingtank)totalfilledweight11.1kg11.4kg21.4kg+PROXweightoffuel1.75kgCH3OH400gH2250gH2(250gH2)wt%ofhydrogeninsystem<1.8%3.6%1.17%systemvolumetricdensity22g/L12.7g/L67g/Lestimatedpriceunknown$360(asof1999)$190(long-term)Duetotheirinherentsafety,decenthydrogengravimetricdensity,andexcellentvolumetricdensity,metalhydridesareagoodchoiceforelectricscooters.Theyofferanimportantsidebenefit,thatofactingasaheatsinkforwastefuelcellheat.Onedifficultyisrefueling;sincemetalhydridetanksarelikelytocostoverahundreddollars,swappingfreshpacksforoldisnotlikelytobeaviabledistributionmodelunlessmodularunitscanbemadethatsatisfyafractionoftherefuelingneed.Refuelingathydrogenpumpingstations,aninferiordistributionoption,islikelynecessarybutnotimpossible.Ontheorderoffivetofifteenminuteswouldberequiredtofillasmallmetalhydridecontainer,withthefillratedependentuponpressureandtherateatwhichatwhichtheadsorption78heatcanberemoved.Compressedgascylindersat3600psiareamorewell-establishedtechnology,buttheyhavethedrawbacksoflowersafetyandpoorperceptionofsafety.31.5Lislikelyextremelybulkyforascooter.Refuelingwouldbedonefromhydrogenfillingstationsatmuchhigherpressurethanfor148 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