Physics of Energy Conversion

216 Pages·2015·3.5 MB·English

by  Katharina Krischer, Konrad Schönleber

DeGruyterTextbook
Krischer,Schönleber∙PhysicsofEnergyConversion
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Katharina Krischer and Konrad Schönleber
Physics of Energy
Conversion
|
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Preface
Amongthecurrentcentralchallengesofmankindareproblemsrelatedtoasustain-
ableenergysupply,thegrowingglobalenergydemand,andtheclimatechangedue
toanincreasinglevelofCO intheatmosphere.Discussionsonsuchenergy-related
2
questionshavereachedthepublicatthelatestsincethebeginningofthe21stcentury,
andarticlesonenergyandclimatetopicsareomnipresentinthenews.However,for
laypersonsitisoftendifficulttodrawcorrectconclusionsfromtheinformationorto
assessitcritically.Termsarefrequentlyusedinanimprecisewayornotdefined,dif-
ferentaspectsofaproblemaremixed,oronlypartofthepictureisgiven.Foranopen
societaldiscourse,bettereducationofthepublicingeneralisnecessary,anditisthe
taskofuniversitiestocontributetothis.
Studentsofthenaturalsciences,inparticularphysicistsandchemists,andofengi-
neeringshouldthusbeprovidedwithagoodunderstandingofenergysciences,i.e.
withthephysicalfoundationsofenergyconversionprocesses.Suchabasisisneces-
saryforacompetentdialog,andhastobeincorporatedinotheraspectsoftheenergy
discussion,suchaseconomic,socialorpoliticalpointsofview.Yet,evenwhenisolat-
ingthescientificsideofthistransdisciplinarytopic,alecturerfacestheproblemthat
thetopicseemstobetoobroadtobecoveredwithinonelectureinaconsistentway.
Forexample,powerplants,usuallytaughtinengineering,shouldbecoveredaswell
assolarcells,atopicnativetosemiconductorphysicsorfuelcells,whicharetypically
taughtinclassesofelectrochemistryorphysicalchemistry.
Itistheaimofthistextbooktoprovideaunifiedviewonthedifferentenergyconver-
sionprocesses.Theapproachisdrivenbythermodynamics:eachenergyconversion
devicecanbedescribedasanopenthermodynamicsystem,andefficienciesforthe
idealconverteraswellastherealisticconvertercanbeassigned.Inthiscontext,itis
necessarytobeawareoftheactualworkintowhichacertainamountofenergyinput
canbeconverted.Thisbecomesmostapparentwhentryingtointerpretthecolloquial
expression“energyconsumption”,whichcontradictstheprincipleofconservationof
energyandthusalsothefirstlawofthermodynamics.Itturnsoutthatthemaximal
amountofworkthatcanbeextractedfromagivenamountofenergyisagoodmea-
sureforitsvalue.Thisphysicalquantityiscalledexergyoravailability,withthefirst
termbeingadoptedinthisbook.Theexergyquantifiesthequalityofacertainamount
ofinputenergy,anditisthispropertywhichis,infact,referredtowhentalkingabout
“energyconsumption”.Thus,theconceptofexergyallowsforthecomparisonofthe
qualityofdifferentformsofenergysuchasheat,chemicalenergy,orelectricalenergy.
Accordingly,thecentralthemeisthetreatmentofenergyconvertersasopensystems
andtheperformanceofefficiencyanalyses,basedontheconceptofexergy.Thisnat-
urallyleadstothechosenoutlineofthebook:Theintroductionofexergyinclosed
systemsinChapter2,followedbyanintroductiontoopensystemsandthetransfer
vi | Preface
oftheexergyconcepttothisclassofsystemsinChapter3.Equippedwiththisknowl-
edge,weproceedtodiscussthetwomostimportantheatengines,steampowerplants,
andgasturbinepowerplantsinChapter4.Thisdiscussionstartswiththethermody-
namicallyideal,i.e.reversible,processes,andcontinueswithsomeaspectsofthede-
signcriteriaofrealpowerplants.However,hereaswellasinthefollowingchapters,
weconcentrateonthefundamentalphysicaldescriptionratherthantheconcreteim-
plementationandstateoftheartrealizations.Hence,forexample,materialsscience
aspectsaretothelargestpartleftaside.
Chapters5and6treatthethermodynamicsofelectricityandchemicalreactions,re-
spectively. The former connects solid state physics and thermodynamics, the latter
summarizeschemicalthermodynamics.These conceptsare thenused inChapter7,
whichisdevotedtoelectrochemicalenergyconversion.Aselectrochemistryisoften
nottaughtintheregularcurriculum,thechapterstartswithanintroductiontoimpor-
tantelectrochemicalconceptsbeforeagainevaluatingidealandrealisticconditions
ofconversionofchemicalintoelectricalenergy.Again,itisourgoaltoelaborateona
unifiedthermodynamicdescriptionofgalvaniccells,whichshouldbringthestudent
inapositiontounderstandmorespecializedworksonfuelcellsorbatteries,butthe
differentrealizationsoftheseclassesofdevicesarenotthetopicofthisbook.
Chapters8,9,and10arethendevotedtosolarenergy.Thepropertiesofsolarradia-
tion,includingitsthermodynamicdescription,arediscussedinChapter8.Chapters9
and10arethendevotedtothetwocommonsolarenergyconversionroutes,namely
solarthermalandphotovoltaicenergyconversion.Asbefore,thethermodynamicprin-
ciplesoperativeinthesedeviceswhenviewedasopensystemsarethethreadofthe
argumentation.Incontrast,differenttypesofrealizations,suchasthediversetypesof
solarcellsandtheirmaterialpropertiesplayaminorrole.EspeciallyinChapter10the
unifyingdescriptionoftheenergyconversionprocessbecomesapparent.Thephysics
ofsolarcellscombinesconceptsdevelopedinChapters3,5,7,and9.Thefinalchapter,
Chapter11,doesnotdevelopnovelconcepts,butbrieflysummarizesrelevantpossi-
bilitiesforexergystorage,applyingtheprinciplesdevelopedthroughoutthebook.
Thisbookiswrittenforstudentsinphysics,chemistry,engineering,orrelateddisci-
plinesintheirlastyearofbachelorstudies,butitmightalsoserveasatextbookfor
anintroductorymasterlevelcourseonenergysciences,whichwouldthenbeideally
complementedbyoneormoreadvancedcoursesonstateoftheartdevicesandcurrent
researchanddevelopmentproblemsinaparticulartypeofsystem,suchassolarcells,
fuelcells,orpowerplants.Itisexpectedthatthestudentshaveabasicknowledge
ofphenomenologicalthermodynamicsofclosedsystemsandofsolidstatephysics.
ThefundamentalsofthesetwosubjectsaresummarizedinAppendicesAandB,re-
spectively.Dependingontheactualcurriculum(ordisciplines)ofthestudentsinthe
course,itmightbenecessarytoreviewthesetopicswithinthelecturetoagreateror
lesserextent.
Preface | vii
Thebookisbasedonalecturegivenseveraltimesforphysicsstudentsintheirlastyear
ofbachelorstudiesattheTechnischeUniversitätMünchen(TUM).Wehadseveraldis-
cussionswiththetutorsoftheaccompanyingexercisesandseveralothercolleagues,
andwewouldliketothankthemfortheirinput.Inparticular,wethankQiLi,Simon
Filser,andKatrinBickelfortheircriticalreadingofthemanuscript,helpfulcomments,
andadvice.Specialthanksalso toHelen Shiells for herlinguisticadviceandproof
reading of the manuscript. Furthermore, we acknowledge funding by TUM.solar in
theframeworkoftheBavarianCollaborativeResearchProject“Solartechnologiesgo
hybrid”(SolTec).
Munich,April2015 KatharinaKrischerandKonradSchönleber

Contents
Preface|v
1 Introduction|1
1.1 Termsanddefinitions|1
1.1.1 Units|5
1.1.2 Example:Energyconsumptionandproduction|6
1.2 Energyconversionprocesses|7
1.2.1 Exergy|11
1.2.2 Conversionefficiency|12
1.2.3 Example:heatingofaroom|13
2 Exergyinclosedsystems|15
2.1 Thermodynamicbasics|15
2.1.1 Thefirstlawofthermodynamics|16
2.1.2 Thesecondlawofthermodynamics|17
2.2 Exergyinclosedsystems|18
2.3 Exergytransfers|21
2.3.1 Exergytransferviaworktransfer|22
2.3.2 ExergytransferviaheattransferatT =T |22
h
2.3.3 Exergyandanergycontentsoftransferredenergy|24
2.3.4 Thelawsofthermodynamicsintermsofexergy|24
2.4 Exergysources|25
2.5 Heatpumps|26
2.5.1 Workingprinciple|27
3 Exergyinopensystems|31
3.1 Thermodynamicsofopensystems|31
3.1.1 Thefirstlawofthermodynamicsforopensystems|32
3.1.2 Steadyflowconditions|34
3.1.3 Technicalwork|35
3.2 Exergyofopensystems|37
3.3 Importantexamplecomponents|38
3.3.1 Heatexchangers|38
3.3.2 Waterturbines|41
3.3.3 Windturbines|41
4 Thermalpowerplants|45
4.1 Steampowerplants|45
4.1.1 Rankinecycle|46
x | Contents
4.1.2 Efficiencyofasteampowerplant|51
4.1.3 Examplecalculation|55
4.1.4 Modificationsinarealsteampowerplant|57
4.2 Gasturbinepowerplants|60
4.2.1 Joule–Braytoncycle|62
4.2.2 Optimizationcriteria|64
4.2.3 Efficiencyofagasturbinepowerplant|66
4.2.4 Examplecalculation|67
4.2.5 Intercooling|69
4.2.6 Combinedcyclepowerplants(CCPP)|71
5 Electricalexergy|73
5.1 Theelectrochemicalpotential|73
5.1.1 Interfaces|75
5.1.2 Electricalcurrents|77
5.2 Voltagesources|78
5.3 Generators|79
5.3.1 Electricalpoweroutput|80
5.3.2 Mechanicalpowerinput|81
5.4 Thermoelectrics|82
5.4.1 Seebeckcoefficients|83
5.4.2 Thermoelectricenergyconversion|84
6 Chemicalexergy|87
6.1 Basicconcepts|87
6.1.1 Examplecalculation|89
6.2 Thedrivingforceofachemicalreaction|90
6.2.1 Chemicalactivity|91
6.2.2 Thedrivingforceofachemicalreactionatagivenstate|94
6.3 Theexergyoffuels|94
6.4 Efficiencyofthecombustionprocess|97
7 Electrochemicalenergyconversion|99
7.1 Electrochemistry|99
7.1.1 Thestandardhydrogenscale|100
7.1.2 Originoftheelectrodepotential|101
7.1.3 Electrodepotentialandcellvoltage|103
7.1.4 TheNernstequation|105
7.1.5 Electrochemicalvoltagesources|106
7.2 Electrochemicalenergyconversion|108
7.2.1 Maximalefficiency|108
7.2.2 Efficiencyofarealisticgalvaniccell|110