Control of Fuel Cells - Federico Zenith.pdf

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Control of Fuel Cells
FedericoZenith
Controloffuelcells
Doctoralthesis
forthedegreeofphilosophiædoctor
Trondheim,June2007
NorwegianUniversityofScienceandTechnology
FacultyofNaturalSciencesandTechnology
DepartmentofChemicalEngineering
716548348.001.png
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NTNU
NorwegianUniversityofScienceandTechnology
Doctoralthesis
forthedegreeofphilosophiædoctor
FacultyofNaturalSciencesandTechnology
DepartmentofChemicalEngineering
©2007FedericoZenith.Somerightsreserved.Thisthesisisdistributedaccordingtotothetermsofthe Attribution-ShareAlike3.0licencebyCreativeCommons. You
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(butnotinanywaythatsuggeststhatheendorsesyouoryouruseofthework),andthatifyoualter,transform,orbuilduponthiswork,youmaydistributethe
resultingworkonlyunderthesame,similaroracompatiblelicence.
ThisthesiswastypesetinPalatinoand AMS EulerbytheauthorusingL A T E X2 " .
ISBN978-82-471-1433-9(printedversion)
ISBN978-82-471-1447-6(electronicversion)
ISSN1503-8181
DoctoralthesesatNTNU,2007:65
PrintedbyNTNU-trykk
Abstract
Thisthesisdealswithcontroloffuelcells,focusingonhigh-temperatureproton-exchange-membranefuelcells.
Fuelcellsaredevicesthatconvertthechemicalenergyofhydrogen,methanolorotherchemicalcompoundsdirectly
intoelectricity,withoutcombustionorthermalcycles.Theyareefficient,scalableandsilentdevicesthatcanprovide
powertoawidevarietyofutilities,fromportableelectronicstovehicles,tonation-wideelectricgrids.
Whereasstudiesaboutthedesignoffuelcellsystemsandtheelectrochemicalpropertiesoftheircomponentsabound
intheopenliterature,therehasbeenonlyaminorinterest,albeitgrowing,indynamicsandcontroloffuelcells.
Intherelativelysmallbodyofavailableliterature,therearesomeapparentlycontradictorystatements:sometimes
theslowdynamicsoffuelcellsisclaimedtopresentacontrolproblem,whereasinotherarticlesfuelcellsareclaimed
tobeeasytocontrolandabletofollowreferencesthatchangeveryrapidly.Thesecontradictionsaremainlycaused
bydifferencesinthesetsofphenomenaanddynamicsthattheauthorsdecidedtoinvestigate,andalsobyhowthey
formulatedthecontrolproblem.Forinstance,thereislittledoubtthatthetemperaturedynamicsofafuelcellcanbeslow,
butusersarenotconcernedwiththecell’stemperature:poweroutputisamuchmoreimportantmeasureofperformance.
Fuelcellsareverymultidisciplinarysystems,whereelectricalengineering,electrochemistry,chemicalengineeringand
materialsscienceareallinvolvedatvariouslevels;itisthereforeunsurprisingthatfewresearcherscanmasterallofthese
branches,andthatmostofthemwillneglectormisinterpretphenomenatheyareunfamiliarwith.
Theambitionofthisthesisistoconsiderthemainphenomenainfluencingthedynamicsoffuelcells,toproperly
definethecontrolproblemandsuggestpossibleapproachesandsolutionstoit.
Thisthesiswillfocusonaparticulartypeoffuelcell,avariationofproton-exchange-membranefuelcellswitha
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membraneofpolybenzimidazoleinsteadoftheusual,commerciallyavailableNafion.Theadvantagesofthisparticular
typeoffuelcellsforcontrolareparticularlyinteresting,andstemfromtheiroperationattemperatureshigherthanthose
typicalofNafion-basedcells:thesenewcellsdonothaveanywater-managementissues,canremovemoreheatwiththeir
exhaustgases,andhavebettertolerancetopoisonssuchascarbonmonoxide.
Thefirstpartofthisthesiswillbeconcernedwithdefiningandmodellingthedynamicphenomenaofinterest.Indeed,
acommonmistakeistoassumethatfuelcellshaveasingledynamics:instead,manyphenomenawithradicallydifferent
timescalesconcurtodefineafuel-cellstack’soverallbehaviour.Thedynamicsofinterestarethoseofchemicalengi-
neering(heatandmassbalances),ofelectrochemistry(diffusioninelectrodes,electrochemicalcatalysis)andofelectrical
engineering(converters,invertersandelectricmotors).Thefirstpartofthethesiswillfirstpresentsomeexperimental
resultsofimportancefortheelectrochemicaltransient,andwillthendeveloptheequationsrequiredtomodelthefour
dynamicmodeschosentorepresentafuel-cellsystemrunningonhydrogenandairatatmosphericpressure:cathodic
overvoltage,hydrogenpressureintheanode,oxygenfractioninthecathodeandstacktemperature.
Thesecondpartwillexploresomeofthepossibleapproachestocontrolthepoweroutputfromafuel-cellstack.Ithas
beenattemptedtoproduceamodularisedsetofcontrollers,oneforeachdynamicstocontrol.Itisamajorpointofthe
thesis,however,thatthetaskofcontrollingafuelcellistobejudgedexclusivelybyitsfinalresult,thatispowerdelivery:
allothercontrolloops,howeverindependent,willhavetobedesignedbearingthatgoalinmind.
Theovervoltage,whichcorrespondsnonlinearlytotherateofreaction,iscontrolledbyoperatingabuck-boost DC / DC
converter,whichinturnismodelledandcontrolledwithswitchingrules.Hydrogenpressure,beingdescribedbyanun-
stabledynamicequation,requiresfeedbacktobecontrolled.Acontrollerwith PI feedbackandafeedforwardpartto
improveperformanceissuggested.Theoxygenfractioninthecathodicstreamcannotbeeasilymeasuredwithasatis-
factorybandwidth,butitsdynamicsisstableanddisturbancescanbemeasuredquiteprecisely:itisthereforesuggested
touseafeedforwardcontroller.ContrarytothemostcommonapproachforNafion-basedfuelcells,temperatureisnot
controlledwithaseparatecoolingloop:instead,theairflowisusedtocoolthefuel-cellstack.Thissignificantlysimplifies
thestackdesign,operationandproductioncost.Tocontroltemperature,itissuggestedtousea P controller,possiblywith
afeedforwardcomponent.Simulationsshowthatthisapproachtostackcoolingisfeasibleandposesnoorfewadditional
requirementsontheairflowactuatorthatisnecessarytocontrolaircompositioninthecathode.
Acknowledgements
Iwouldliketothankmysupervisor,professorSigurdSkogestad,forhavingofferedmetheopportunityofwritingthis
workandforhissupportandpatienceinthesefouryears.
Thisworkwouldnothavebeenpossiblewithoutthecollaborationofseveralprofessors,researchersandPhDstu-
dentsfromtheDepartmentofMaterialsTechnologyat NTNU :IwouldliketothankprofessorsReidarTunoldandBørre
BørresenandresearchersOle-EdvardKongsteinandFrodeSeland,whowereco-authorsinmyfirstpaper,fortheirdata,
suggestions,andpatience.BørreandFrodedeserveadditionalthanksforhelpingmeinobtainingtheexperimentaldata
presentedinchapter2.
MythanksgotoHelgeWeydahl,alsofromtheDepartmentofMaterialsTechnologyandwithwhomIsharedmyoffice
forthreeyears,formanyfruitfuldiscussions,inparticularabouttheelectrochemicaltransient.
IwouldliketothankSteffenMøller-HolstfromS INTEF forhisinvaluableeffortsincoordinatingvariousprojects
relatedtohydrogenandfuelcells.
IamthankfultoallthecolleaguesImetduringmystayattheDepartmentofChemicalEngineeringforcreatinga
greatworkenvironment.
Finally,fundingfromStatoil AS andtheNorwegianResearchCouncilisgratefullyacknowledged.
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