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Exergy Energy, Environment And Sustainable Development Ibrahim Dincer University of Ontario Institute of Technology, 2000 Simcoe Street North, Oshawa, Ontario, L1H 7K4, Canada Marc A. Rosen University of Ontario Institute of Technology, 2000 Simcoe Street North, Oshawa, Ontario, L1H 7K4, Canada AMSTERDAM(cid:1)BOSTON(cid:1)HEIDELBERG(cid:1)LONDON(cid:1)NEWYORK(cid:1)OXFORD PARIS(cid:1)SANDIEGO(cid:1)SANFRANCISCO(cid:1)SINGAPORE(cid:1)SYDNEY(cid:1)TOKYO Elsevier TheBoulevard,LangfordLane,Kidlington,OxfordOX51GB,UK Radarweg29,POBox211,1000AEAmsterdam,TheNetherlands 225WymanStreet,Waltham,MA02451,USA 525BStreet,Suite1900,SanDiego,CA92101-4495,USA Secondedition2013 (cid:1)2013IbrahimDincerandMarcA.Rosen.PublishedbyElsevierLtd.Allrightsreserved Nopartofthispublicationmaybereproduced,storedinaretrievalsystemortransmittedinanyformorbyanymeanselectronic, mechanical,photocopying,recordingorotherwisewithoutthepriorwrittenpermissionofthepublisherPermissionsmaybesought directlyfromElsevier’sScience&TechnologyRightsDepartmentinOxford,UK:phone(+44)(0)1865843830;fax(+44)(0)1865 853333;email:permissions@elsevier.com.AlternativelyyoucansubmityourrequestonlinebyvisitingtheElsevierwebsiteat http://elsevier.com/locate/permissions,andselectingObtainingpermissiontouseElseviermaterial Firstedition (cid:1)2007ElsevierLtd.Allrightsreserved Notice Noresponsibilityisassumedbythepublisherforanyinjuryand/ordamagetopersonsorpropertyasamatterofproductsliability, negligenceorotherwise,orfromanyuseoroperationofanymethods,products,instructionsorideascontainedinthematerialherein. Becauseofrapidadvancesinthemedicalsciences,inparticular,independentverificationofdiagnosesanddrugdosagesshouldbemade BritishLibraryCataloguinginPublicationData AcataloguerecordforthisbookisavailablefromtheBritishLibrary LibraryofCongressCataloging-in-PublicationData AcatalogrecordforthisbookisavailablefromtheLibraryofCongress ForinformationonallElsevierpublications visitourwebsiteatstore.elsevier.com PrintedandboundinGreatBritain 1314151617 10987654321 ISBN:978-0-08-097089-9 Preface Exergy analysis is a method that uses the conservation of a broad manner to furnish the reader with the background mass and conservation of energy principles, together with information necessary for subsequent chapters. Chapter 2 the Second Law of Thermodynamics, for the analysis, provides detailed information on energy and exergy and design,andimprovementofenergyandothersystems.The contrastsanalysisapproachesbasedoneach,whileChapter3 exergymethodisausefultoolforfurtheringthegoalofmore focusesonchemicalexergyanditsimportantroleinexergy efficient energy-resource use, for it enables the locations, analysis for many systems and processes. In Chapter 4, types,andmagnitudesofwastesandlossestobeidentified extensivecoverageisprovidedofenvironmentalconcerns,the andmeaningfulefficienciestobedetermined. impact of energy use on the environment, and linkages During the past few decades, we have witnessed revo- betweenexergyandtheenvironment.Throughoutthischapter, lutionary changes in the way thermodynamics is taught, emphasis is placed on the role of exergy in moving to researched,andpracticed.Themethodsofexergyanalysis, sustainabledevelopment. entropy generation minimization, and thermoeconomics Chapter 5 delves into the use of exergy techniques by are the most visible and established forms of this change. industryforvarioussystemsandprocessesandinactivities Todaythereisamuchstrongeremphasisonexergyaspects such as design and optimization. This chapter lays the ofsystemsand processes.Theemphasisisnowon system foundationforthemanyapplicationspresentedinChapters analysis and thermodynamic optimization, not only in the 6–19, which represent the heart of the book. The applica- mainstream of engineering but also in physics, biology, tionscoveredrangefrompsychrometricprocesses(Chapter economics, and management. As a result of these recent 6),heatpumps(Chapter7),absorptioncooling(Chapter8), changes and advances, exergy has gone beyond thermo- thermal storage (Chapter 9), drying (Chapter 10), renew- dynamicsandbecomeanewdistinctdisciplinebecauseof able energy systems (Chapter 11), steam power plants its interdisciplinary character as the confluence of energy, (Chapter 12), cogeneration and district energy (Chapter environment, and sustainabledevelopment. 13), trigeneration and multigeneration (Chapter 14), cryo- This book is a research-oriented textbook; therefore, it genics and liquefaction (Chapter 15), crude oil distillation includes practical features in a usable format often not (Chapter16),hydrogenproduction(Chapter17),fuelcells includedinother,solelyacademictextbooks.Itisessentially (Chapter18), andaircraft systems (Chapter 19), intended for use by advanced undergraduate or graduate Chapter 20 covers the relation between exergy and students in several engineering and nonengineering disci- economicsandtheexploitationofthatlinkthroughanalysis plinesandasanessentialtoolforpractitioners.Theoryand tools such as exergoecomics. Chapter 21 extends exergy analysis are emphasized throughout this comprehensive applications to large-scale systems such as countries, book, reflecting new techniques, models, and applications, regions, and sectors of an economy, focusing on how effi- togetherwithcomplementarymaterialsandrecentinforma- ciently energy resources are utilized in societies. Chapter tion.Coverageofthematerialisextensive,andtheamountof 22 focuses the utilization of exergy within life cycle informationanddatapresentedissufficientforexergy-related assessment and presents various applications. Chapter 23 courses or as a supplement for energy, environment, and discusseshowexergycomplements,andcanbeusedwith, sustainable development courses, if studied in detail. We industrial ecology. Multiobjective optimization, which believe that this book will be of interest to students and draws on much of the earlier information in this book, is practitioners,aswellasindividualsandinstitutions,whoare described in Chapter 24. It concludes by highlighting the interestedinexergyanditsapplicationstovarioussystemsin potential of exergy as a tool in energy policy making and diverse areas. This volume is also a valuable and readable addressingtheneedforexergyeducationandawarenessin referenceforanyonewhowishestolearnaboutexergy. Chapter25.Aclosingsection,includingthoughtsonfuture The introductory chapter addresses general concepts, perspectives,ispresentedinChapter 26. fundamentalprinciples,andbasicaspectsofthermodynamics, Incorporatedthroughoutaremanyillustrativeexamples energy, entropy, and exergy. These topics are covered in andcasestudies,whichprovidethereaderwithasubstantial xiii xiv Preface learning experience, especially in areas of practical Complete references are included to point the truly application. curiousreaderintherightdirection.Informationontopics The appendices contain unit conversion factors and notcoveredfullyinthetextcan,therefore,beeasilyfound. tables and charts of thermophysical properties of various Wehopethisvolumeallowsexergymethodstobemore materialsinthe International Systemofunits. widelyappliedandthebenefitsofsucheffortsmorebroadly derived, so that energy use can be made more efficient, clean,andsustainable. IbrahimDincer and MarcA.Rosen Oshawa, August 2012 COMPANIONWEBSITE http://booksite.elsevier.com/9780080970899/. Acknowledgments Some of the material presented in the previous edition of l Dr. Ahmet D. Sahin, Professor, Istanbul Technical this book was derived from research that we carried out University, Turkey with distinguished individuals who were part of our l Dr. Ahmet Z. Sahin,Professor, KFUPM, Saudi Arabia research group or collaborated with us over the years. We l Dr. David S. Scott, Professor Emeritus, University of again highlyappreciate their efforts: Victoria,Canada l Dr. Iyad Al-Zaharnah, Instructor, KFUPM, Saudi l Dr. Mikhail Granovskiy, Senior Researcher, UOIT, Arabia Canada l Dr. Syed M.Zubair, Professor, KFUPM,Saudi Arabia l Dr. Arif Hepbasli, Professor,EgeUniversity, Turkey l Dr.FrankC.Hooper,ProfessorEmeritus,Universityof Inaddition,thecontributionsofseveralpastundergraduate Toronto, Canada andgraduatestudentswereacknowledged,includingJason l Dr. Mohammed M.Hussain, NRC-IFCI,Canada Etele,LowyGunnewiek,GeraldKresta,MinhLe,Norman l Dr. Mehmet Kanoglu, Professor, University of Gaz- Pedinelli, and RaymondTang. iantep, Turkey In this second edition, we gratefully acknowledge the l Dr. Mehmet Karakilcik, Assistant Professor, Cukurova assistance provided by Ph.D. students Tahir Ratlamwala, University, Turkey PouriaAhmadi,andAhmet Ozbilen atUOITinreviewing l Dr. Xianguo Li, Professor, University of Waterloo, and revising several chapters; checking for consistency; Canada and preparingfigures, tables,and questions/problems. l Dr. AdnanMidilli, Professor, Rize University, Turkey Last butnotleast, wewarmlythank our wives,Gu¨ls¸en l Mr.HusainAl-Muslim,Engineer,ARAMCOandPh.D. DinçerandMargotRosen,andourchildrenMeliha,Miray, _ _ Student,KFUPM,SaudiArabia Ibrahim Eren, Zeynep, and Ibrahim Emir Dinçer, and l Dr.MehmetFatihOrhan,AssistantProfessor,American Allison and Cassandra Rosen. They have been a great University of Sharjah, UAE source of support and motivation, and their patience and l Dr. Leyla Ozgener, Associate Professor, Celal Bayar understanding throughout this project have been most University, Turkey appreciated. l Dr. Onder Ozgener, Solar Energy Institute, Ege IbrahimDincer and MarcA. Rosen University, Turkey Oshawa,August2012 xv About the Authors Ibrahim Dinçer _ Ibrahim Dinçer is a full professor of Mechanical Engi- Marc A. Rosen neeringandprogramsdirectorinthefacultyofEngineering and Applied Science at University of Ontario Institute of Technology. Renowned for his pioneering works, he has Marc A. Rosen is a professor of Mechanical Engi- authored and co authored many books and book chapters, neering at the University of Ontario Institute of Tech- over 800 refereed journal and conference papers, and nology in Oshawa, Canada, where he served as founding numerous technical reports. He has chaired many national Dean of Engineering and Applied Science. Dr. Rosen is and international conferences, symposia, workshops, and an active teacher and researcher in thermodynamics, technical meetings. He is the founding chair/co chair of energy technology, sustainable energy, and the environ- various well-established international conferences, mental impact of energy and industrial systems. He is including the International Exergy, Energy, and Environ- a registered Professional Engineer in Ontario and has ment Symposium. He has delivered over 200 keynote and served in many professional capacities, including found- invitedlectures.Heisanactivememberofvariousinterna- ing Editor-in-Chief of the journal Sustainability and tional scientific organizations and societies, and serves as Editor-in-Chief of the International Journal of Energy Editor-In-Chief for International Journal of Energy and Environment Engineering, and a member of the Research,InternationalJournalofExergy,andInternational Board of Directors of Oshawa Power and Utilities Journal of Global Warming, as well as associate editor, Corporation. A Past-President of the Engineering Institute regional editor, and editorial board member on various of Canada and the Canadian Society for Mechanical prestigiousinternationaljournals.Heisarecipientofseveral Engineering, Dr. Rosen received an Award of Excellence research, teaching, and service awards, including the in Research and Technology Development from the Premier’sResearchExcellenceawardinOntario,Canada,in Ontario Ministry of Environment and Energy. He is also 2004. He has made innovative contributions to the under- a Fellow of the Engineering Institute of Canada, the standing and development of exergy analysis of advanced American Society of Mechanical Engineers, the Canadian energysystemsforhisso-calledfivemainpillars:(1)better Society for Mechanical Engineering, the Canadian efficiency, (2)better cost-effectiveness, (3)better environ- Academy of Engineering, and the International Energy ment,(4)bettersustainability,and(5)betterenergysecurity. Foundation.HehasworkedforImatraPowerCompanyin He was the chair of a new technical group in ASHRAE Finland, Argonne National Laboratory, and the Institute namedExergyAnalysisforSustainableBuildings. for Hydrogen Systems, near Toronto. xvii Chapter 1 Thermodynamic Fundamentals Chapter Outline 1.1 Introduction 1 1.3.5 TheSLT 9 1.2 Energy 2 1.3.6 SLTStatements 9 1.2.1 ApplicationsofEnergy 2 1.3.7 TheClausiusInequality 10 1.2.2 ConceptofEnergy 2 1.3.8 UsefulRelationships 10 1.2.3 FormsofEnergy 3 1.4 Exergy 10 1.2.3.1 Macroscopic 3 1.4.1 TheQuantityExergy 10 1.2.3.2 Microscopic 3 1.4.2 ExergyAnalysis 10 1.2.4 TheFirstLawofThermodynamics 4 1.4.3 CharacteristicsofExergy 11 1.2.5 EnergyandtheFLT 4 1.4.4 TheReferenceEnvironment 11 1.2.6 EconomicAspectsofEnergy 4 1.4.5 ExergyversusEnergy 12 1.2.7 EnergyAuditMethods 5 1.4.6 ExergyEfficiencies 13 1.2.8 EnergyManagement 5 1.4.7 SolarExergyandtheEarth 13 1.2.8.1 MaintenanceOpportunities 5 1.5 IllustrativeExamples 14 1.2.8.2 Low-CostOpportunities 6 1.5.1 IllustrativeExample1 14 1.2.8.3 RetrofitOpportunities 6 1.5.2 IllustrativeExample2 14 1.3 Entropy 6 1.5.3 IllustrativeExample3 15 1.3.1 OrderandDisorderandReversibility 1.5.3.1 FurtherDiscussiononEntropyGeneration andIrreversibility 6 AssociatedwithHeatTransfer 16 1.3.2 CharacteristicsofEntropy 7 1.5.4 IllustrativeExample4 16 1.3.3 SignificanceofEntropy 8 1.6 ClosingRemarks 19 1.3.4 Carnot’sContribution 8 Problems 19 understanding these concepts, as well as basic principles, ABSTRACT generaldefinitions,practicalapplications,andimplications. This chapter provides background for understanding energy, Illustrativeexamplesareprovidedtohighlighttheimportant entropy,andexergyconcepts.Also,basicthermodynamicprin- aspectsofenergy,entropy,andexergy. ciples and general definitions are explained, and various practical applications and implications of these thermodynamic The scope of this chapter is partly illustrated in quantities are discussed. The first and second laws of thermo- Figure 1.1, where the domains of energy, entropy, and dynamics are described, as are energy audits and energy exergy are shown. This chapter focuses on the portion of management.Thechapterfocusesontheportionofthefieldof the field of thermodynamics at the intersection of the thermodynamics at the intersection of the energy, entropy, and energy, entropy, and exergy fields. Note that entropy and exergyfields. exergy are also used in other fields (such as statistics KEYWORDS and information theory); therefore they are not subsets Energy; Entropy; Exergy; First law of thermodynamics; Second of energy. Also, some forms of energy (such as shaft lawofthermodynamics;Energyaudit;Energymanagement. work) are entropy free, and thus entropy subtends only part of the energy field. Likewise, exergy subtends only part of the energy field because some systems (such as 1.1 INTRODUCTION air at atmospheric conditions) possess energy but no Energy,entropy,andexergyconceptsstemfromthermody- exergy. Most thermodynamic systems (such as steam in namicsandareapplicabletoallfieldsofscienceandengi- a power plant) possess energy, entropy, and exergy, and neering.Thischapterprovidesthenecessarybackgroundfor thus appear at the intersection of these three fields. Exergy.http://dx.doi.org/10.1016/B978-0-08-097089-9.00001-2 (cid:1)2013IbrahimDincerandMarcA.Rosen.PublishedbyElsevierLtd.Allrightsreserved 1 2 Exergy EXERGY electric power. The steam leaving the turbine is then condensed,andthecondensateispumpedbacktotheboiler tocompletethecycle.Breederreactorsuseuranium-235as a fuel source and can produce more fuel in the process. A solarpowerplantusessolarconcentrators(parabolicorflat mirrors) to heat a working fluid in a receiver located on atowerwhereaheatedfluidexpandsinaturbogenerator,as in a conventional power plant. In a spark-ignition internal combustion engine, chemical energy of fuel is converted into mechanical work. An air–fuel mixture is compressed and combustion is initiated by a spark device. The expan- ENERGY ENTROPY sionofthecombustiongasespushesagainstapiston,which FIGURE1.1 Interactionsbetweenthedomainsofenergy,entropy,and resultsintherotationofacrankshaft.Gasturbineengines, exergy. commonly used for aircraft propulsion, convert the chemical energy of fuel into thermal energy that is used to run theturbine.Theturbineisdirectlycoupledtoacompressor 1.2 ENERGY that supplies the air required for combustion. The exhaust Energycomesinmanyforms.Thermodynamicsplayakey gases,uponexpandinginanozzle,createthrust.Forpower role in the analysis of processes, systems, and devices in generation, the turbine is coupled to an electric generator which energy transfers and energy transformations occur. and drives both the compressor and the generator. In The implications of thermodynamics are far reaching and a liquid-fuel rocket, a fuel and an oxidizer are combined, applications span the range of the human enterprise. and combustion gases expand in a nozzle, creating Throughout our technological history, our ability to a propulsive force (thrust) to propel the rocket. A typical harnessenergyanduseitforsociety’sneedshasimproved. nuclear rocket propulsion engine offers a higher specific The industrial revolution was fueled by the discovery of impulse when compared to chemical rockets. A fuel cell howtoexploitenergyonalargescaleandhowtoconvert converts chemical energy into electric energy, directly heat into work. Nature allows the conversion of work making use of an ion-exchange membrane. When a fuel completelyintoheat,butheatcannotbeentirelyconverted such as hydrogen is ionized, it flows from the anode into work, and doing so requires a device (e.g., a cyclic through the membrane toward the cathode. The released engine). Engines attempt to optimize the conversion of electrons at the anode flow through an external load. In heat towork. a magnetohydrodynamic generator, electricity is produced by movinga high-temperature plasma through a magnetic field. A refrigeration system utilizes work supplied by an 1.2.1 Applications of Energy electric motor to transfer heat from a refrigerated space. Most of our daily activities involve energy transfer and Low-temperature boiling fluids such as ammonia and energy change. The human body is a familiar example of refrigerant-12absorbthermalenergyastheyvaporizeinthe abiologicalsysteminwhichthechemicalenergyoffoodor evaporator, causing a cooling effect in the region being bodyfatistransformedintootherformsofenergysuchas cooled. heat and work. Engineering applications of energy These are only a few of the numerous engineering processes are wide ranging and include power plants to applications.Thermodynamicsisrelevanttoamuchwider generate electricity, engines to run automobiles and rangeofprocessesandapplicationsnotonlyinengineering, aircraft, refrigeration and air conditioning systems, and so but also in science. A good understanding of this topic is forth. requiredtoimprovethedesignandperformanceofenergy- Many examples of such systems are discussed here. In transfer systems. ahydroelectricpowersystem,thepotentialenergyofwater is converted into mechanical energy through the use of 1.2.2 Concept of Energy a hydraulic turbine. The mechanical energy is then con- vertedintoelectricenergybyanelectricgeneratorcoupled Theconceptofenergywasfirstintroducedinmechanicsby to the shaft of the turbine. In a steam power-generating Newton when he hypothesized about kinetic and potential plant,chemicalornuclearenergyisconvertedintothermal energies. However, the emergence of energyas a unifying energy in a boiler or a reactor. The energy is imparted to conceptinphysicswasnotadopteduntilthemiddleofthe water,whichvaporizesintosteam.Theenergyofthesteam nineteenth century and is considered one of the major is used to drive a steam turbine, and the resulting scientific achievements in that century. The concept of mechanical energy is used to drivea generator to produce energy is so familiar to us today that it seems intuitively Chapter | 1 ThermodynamicFundamentals 3 obvioustounderstand,yetweoftenhavedifficultydefining energy of a system depends on the choice of a zero level. itprecisely. For example, if ground level is considered to be at zero Energy is a scalar quantity that cannot be observed potential energy, then the potential energy of the mass directly, but can be recorded and evaluated by indirect 100 m above the ground has a positive potential energy measurements. The absolute value of the energy of equal to the mass (1 kg) multiplied by the gravitational asystemisdifficulttomeasure,whereastheenergychange constant(g¼9.807m/s2)andtheheightabovetheground isrelativelyeasy toevaluate. (100 m). Its potential energy will be 980.7 (kgm2)/s2 (or Examplesofenergyuseinlifeexperiencesareendless. 980.7 Newton-meters (Nm), or 980.7 J). The datum plane The sun is the major source of the earth’s energy. It emits forpotentialenergycanbechosenarbitrarily.Ifithadbeen a spectrum of energy that travels across space as electro- chosen at 100 m above the ground level, the potential magnetic radiation. Energy is also associated with the energy of the mass would have been zero. Of course, the structure of matter and can be released by chemical and difference in potential energy between the mass at 100 m atomic reactions. Throughout history, the emergence of and the mass at ground level is independent of the datum civilizations has been characterized by the discovery and plane. effectiveapplicationofenergytohelpmeetsociety’sneeds. 1.2.3.2 Microscopic 1.2.3 Forms of Energy Microscopic forms of energy are those related to the molecularstructureofasystemandthedegreeofmolecular Energy manifests itself in many forms, which are either activity, and are independent of outside reference frames. internal or transient. Energy can be converted from one Thesumofallthemicroscopicformsofenergyofasystem form to another. In thermodynamic analysis, the forms of is its internal energy. The internal energy of a system energycanbeclassifiedintotwogroups:macroscopicand depends on the inherent qualities, or properties, of the microscopic. materials in the system, such as composition and physical 1.2.3.1 Macroscopic form,aswell astheenvironmentalvariables (temperature, pressure,electricfield,magneticfield,etc.).Internalenergy Macroscopic forms of energy are those which an overall can have many forms, including mechanical, chemical, system possesses with respect to a reference frame, for electrical, magnetic, surface,and thermal. Some examples example, kinetic and potential energies. The macroscopic are considered inthe following: energy of a rising object changes with velocity and eleva- l Aspringthatiscompressedhasahigherinternalenergy tion. The macroscopic energy of a system is related to (mechanical energy) than a spring that is not motionandtheinfluenceofexternaleffectssuchasgravity, compressed,becausethecompressedspringcandowork magnetism, electricity, andsurface tension. onchanging(expanding)totheuncompressedstate. The energy that a system possesses as a result of its l Two identical vessels, each containing hydrogen and motion relative to some reference frame is called kinetic oxygen, are considered that have different chemical energy. Kinetic energy refers to the energy of the system energies. In the first, the gases are contained in the becauseofits“overall”motiondtranslationalorrotational. elemental form, pure hydrogen and pure oxygen, in Thewordoverallisusedheretospecifythatwerefertothe aratioof2:1.Thesecondcontainsanidenticalnumberof kineticenergyoftheentiresystem,notthekineticenergyof atoms,butintheformofwater.Theinternalenergiesof the molecules in the system. If the system is a gas, for thesesystemsdiffer.Asparkmaysetoffaviolentrelease example, the kinetic energy is the energy due to the ofenergyinthefirstcontainer,butnotinthesecond. macroscopic flow of the gas, not the motion of individual molecules. Thestructureofthermodynamicsinvolvestheconceptof The potential energy of a system is the sum of the equilibriumstatesandpostulatesthatthechangeinthevalue gravitational,centrifugal,electrical,andmagneticpotential of thermodynamic quantities, such as internal energy, energies.Theenergythatasystempossessesasaresultof betweentwoequilibriumstatesofasystemdoesnotdepend its elevation in a gravitational field is called gravitational onthethermodynamicpaththesystemtakestogetfromone potential energy (or commonly just potential energy). For state to the other. The change is defined by the final and example,a1kgmass100mabovethegroundhasagreater initial equilibrium states of the system. Consequently, the potential energy than the same mass on the ground. internal energy change of a system is determined by the Potential energy can be converted into other forms of parameters that specify the system in its final and initial energy,suchaskineticenergy,ifthemassisallowedtofall. states. The parameters include pressure, temperature, Kineticandpotentialenergydependontheenvironment magneticfield,surfacearea,mass,andsoforth.Ifasystem in which the system exists. In particular, the potential changesfromstate1tostate2,thechangeininternalenergy 4 Exergy DUis(U2(cid:2)U1),theinternalenergyinthefinalstatelessthat andW1e2istheworkdonebythecontrolvolumeduringthe intheinitialstate.Thedifferencedoesnotdependonhow process fromstate 1to state 2. thesystemgetsfromstate1to2.Theinternalenergythusis The energy E may include internal energy U, kinetic referred to as a state function, or a point function, that is, energyKE,and potential energyPE terms asfollows: afunctionofthestateofthesystemonly,andnotitshistory. E ¼ UþKEþPE (1.3) Thethermalenergyofasystemistheinternalenergyof asystem,whichincreasesastemperatureisincreased. For Forachangeofstatefromstate1tostate2withaconstant instance, we have to add energy to an iron bar to raise its gravitational acceleration g, Equation 1.3 becomes the temperature.Thethermalenergyofasystemisnotreferred following: toas heat, asheat isenergyintransit between systems. E (cid:2)E ¼ U (cid:2)U þmðV2(cid:2)V2Þ=2þmgðZ (cid:2)Z Þ 2 1 2 1 2 1 2 1 (1.4) 1.2.4 The First Law of Thermodynamics wheremdenotesthefixedamountofmasscontainedinthe TheFirstLawofThermodynamics(FLT)isthelawofthe system, V thevelocity, and Zthe elevation. conservation of energy, which states that, although The quantities dQ and dW can be specified in terms of energy can change form, it can be neither created nor the rate laws for heat transfer and work. For a control destroyed. The FLT defines internal energy as a state volume, an additional term appears from the fluid flowing function and provides a formal statement of the conser- acrossthecontrolsurface(enteringatstateiandexitingat vation of energy. statee).TheFLTforacontrolvolumecanbewrittenasthe However,itprovidesnoinformationaboutthedirection following: in which processes can spontaneously occur, that is, the X X reversibility aspects of thermodynamic processes. For Q_cv ¼ E_cvþW_cvþXm_ehê(cid:2)X m_ihî or (1.5) example, the FLT cannot indicate how cells can perform E_ ¼ Q_ (cid:2)W_ þ m_ h^ (cid:2) m_ h^ cv cv cv i i e e workwhileexistinginanisothermalenvironment.TheFLT provides no information about the inability of any ther- where m_ is mass flow rate per unit time, ^h is total specific modynamic process to convert heat fully into mechanical energyequaltothesumofspecificenthalpy,kineticenergy, work, or any insight into why mixtures cannot spontane- andpotential energy,that is, ^h ¼h þ V2/2þgZ. ouslyseparateorunmixthemselves.Aprincipletoexplain these phenomena and to characterize the availability of 1.2.6 Economic Aspects of Energy energyisrequiredtodothis.Thatprincipleisembodiedin theSecondLawofThermodynamics(SLT),whichwewill Although all forms of energy are expressed in the same explainlater inthis chapter. units (joules, megajoules, gigajoules, etc.), the financial valueofenergyvariesenormouslywithitsgradeorquality. 1.2.5 Energy and the FLT Typically, electrical and mechanical energy are the most costly,followedbyhigh-gradethermalenergy.Attheother For a control mass, the energy interactions for a system extreme,thermalenergy,whichisonlyafewdegreesfrom maybedividedintotwoparts:dQ,theamountofheat,and ambient, has virtually no commercial value. These exam- dW, the amount of work. Unlike the total internal energy ples highlight theweakness of trying to equate the energy dE, the quantities dQ and dW are not independent of the containedinsteamortheheatcontentofgeothermalfluids manneroftransformation,sowecannotspecifydQanddW with the high-grade energy obtainable from fossil fuels or simply by knowing the initial and final states. Hence, it is nuclearpower.Economicsusuallysuggeststhatoneshould not possible to define a function Q, which depends on the avoid using energy at a significantly higher grade than initial and final states, that is, heat is not a state function. neededforatask.Forexample,electricalenergy,whichhas The FLTfor acontrol masscan bewrittenas follows: a high energy grade, should be used for such purposes as dQ ¼ dEþdW (1.1) mechanical energy generation and production of light, sound, and very high temperatures in electrical furnaces. When Equation 1.1 is integrated from an initial state 1 to Electric space heating, on the other hand, in which elec- a final state 2, it results inthe following: tricity is used for raising the temperature of ambient air (cid:3) Q1(cid:2)2 ¼ E2(cid:2)E1þW1(cid:2)2 or only to about 20 C, is an extremely wasteful use of elec- (1.2) tricity. This observation applies in both domestic and E2(cid:2)E1 ¼ Q1(cid:2)2(cid:2)W1(cid:2)2 industrial contexts. In many jurisdictions, there is excess where E and E denote the initial and final values of the electricity generation capacity at night; therefore, some of 1 2 energyEofthecontrolmass,Q1e2istheheattransferredto thenighttimeelectricityissoldatreducedpricesforspace thecontrolmassduringtheprocessfromstate1tostate2, heating purposes, even though this is inherently wasteful.