:::5----------------------,---I1.8 - - - - - - - - - - --:-- - - =---o '__~Mk~-\-t>~v.~~S~<,~-c,t~suoSection 1.47 days1-Energy Fundamentals15FIGURE1.8 The contaminantconcentration profile for Example 1.7.1.4ENERGYFUNDAMENTALSJust as we are able to use the law of conservation of mass to write mass balance equa-tions that are fundamental to understanding and analyzing the flow of materials, wecan in a similar fashion use thefirst lawofthermodynamics to write energy balanceequations thatwill help us analyze energyflows.One definition of energy-is that it is the capacity for doing work, where work canbe described by the product of force and the displacement of an object caused by thatforce.A simple interpretation of thesecond lawofthermodynamics suggests that whenwork is done therewill always be some inefficiency;that is,some portion of the energyput into the process willend up as waste heat. How that waste heat affects the environ-ment is an important consideration in the study of environmental engineering and sci-ence.Another important term to be familiar with ispower. Power is the rate of doingwork, so it has units of energy per unit of time. In SI units power is given in joules persecond(J/s) or kilojoules per second(kl/s).In honor of the Scottish engineer JamesWatt, who developed the reciprocating steam engine, the joule per second has beennamed the watt (1Jls = 1 W = 3.412Btu/hr).The FirstLaw of ThermodynamicsThe first law of thermodynamics says, simply,that energy can be neither created nordestroyed. Energy may change forms in any given process, as when chemical energy ina fuel is converted to heat and electricity in a power plant, or when the potentialenergy of water behind a damis converted to mechanical energy as it spins a turbine ina hydroelectric plant. No matter whatis happening,the first law says we should be ableto account for every bit of energy as it takes part in the process under study,so that inthe end we have just as much as we had in the beginning. With proper accounting, evennuclear reactions involving conversion of mass to energy can be treated.To apply the firstlaw itis necessary to define the system beingstudied, much aswasdone in the analysisof massflows.Realize that the system can be anything that we wantto draw an imaginary boundaryaround-itcan be an automobile engine, or a nuclearpower plant, or a volume of gas emitted from a smokestack. Later, when we explore thetopic of global temperature equilibrium, the system will be the earth itself. Once aboundary has been defined, the rest of the universe becomes thesurroundings. Just"<,16 Chapter 1Massand Energy TransferSection 1.4EnergyFundamentals17because aboundaryhasbeendefined, however,does notmeanthat energyand/ormate-rialscannotflow acrossthatboundary. Systems in whichbothenergy andmattercan flowacross theboundaryare referred to as open systems, while those in which energy isallowed to flow across the boundary, butmatteris not, are called closedsystems.Sinceenergyis conserved, we can write the following forwhateversystem wehave defined:(Total energy )(Totalenergy )(Totalenergy)( Net change )crossingboundary+ of mass - of mass = of energy in (1.30)asheatand work entering system leaving system thesjstemForclosed systems,thereis nomovementof mass across theboundarysothesec-ondandthirdtermdropoutof the equation.The accumulation of energyrepresentedbythe right side of (1.30)maycause changes in the observable, macroscopic forms ofenergy, such as kinetic and potential energies,or microscopic formsrelatedto the atomicandmolecularstructureofthesystem. Those microscopic forms of energy includethekinetic energies of molecules and the energies associated with the forces actingbetweenmolecules,betweenatoms within molecules, and within atoms.Thesum of those micro-scopic forms ofenergyis called the system's internal energyandisrepresentedby thesymbolU.Thetotal energy Ethata substance possesses can be describedthenas thesum of its internal energyU,its kinetic energy KE, and its potential energy PE:The analogous equation for changesthatoccurunderconstant pressure involves enthalpyTheaddedcomplications associated with gasesthatchangepressureandvolumearemosteasilyhandledby introducinganotherthermodynamicpropertyof a sub-stance calledenthalpy.TheenthalpyH of a substance is defined as(1.34)(1.33)(1.32)H =U+PVIiU = mcvIiTIiH= m cpIiTFormanyenvironmentalsystemsthesubstances beingheatedare solids or liq-uids for which Cv= cp= candIiU = IiH. We canthenwrite the followingequationfor thechangeinenergystoredin asystemwhenthetemperatureof mass m changesby anamountIiT:whereU is itsinternalenergy, P is its pressure,andV is its volume.Theenthalpyof aunitmassof asubstancedependsonly on itstemperature.Ithasenergyunits(kJorBtu)andhistorically it wasreferredto as a system's"heatcontent."Sinceheatis cor-rectly defined only intermsof energytransferacross 1t_system'sboundaries,heatcon-tentis asomewhatmisleadingdescriptorandisnotusedmuchanymore.Whena process occurs without a changeofvolume, the relationshipbetweeninternalenergyandtemperaturechangeis given by(1.31)E = U +KE+PETABLE1.3SpecificHeat Capacitycof SelectedSubstancesTable 1.3providessomeexamplesof specificheatfor several selected substances.Itisworthnotingthatwaterhas by farthehighest specificheatofthesubstances listed;in fact, it ishigherthanalmostallcommonsubstances. As will benotedinChapter5,this isoneofwater'sveryunusualpropertiesandis in largepartresponsible for themajoreffect theoceanshaveonmoderatingtemperaturevariations of coastal areas.Changeinstoredenergy= m c IiTInmanyapplications of (1.30) thenetenergyaddedto a system will cause anincrease intemperature.Wasteheatfrom apowerplant, for example, will raisethetem-peratureof coolingwaterdrawninto its condenser. Theamountof energyneededtoraise thetemperatureof aunitmass of a substance by 1degreeis calledthespecific heat.Thespecificheatofwateris the basis for twoimportantunits of energy,namelytheBritish thermal unit, orBtu,which is defined to be the energyrequiredto raise 1 lb ofwaterby 1°F, andthekilocalorie, which is the energyrequiredto raise 1 kg ofwaterby1"C, In the definitionsjustgiven, the assumedtemperatureofthewateris15°C(59 OF).Since kilocaloriesarenolongerapreferredenergy unit, values of specificheatintheSIsystemaregiven inkJ/kgDC,where 1 kcal/kg °C = 1 Btu/Ib OF=4.184kJ/kg"C,Formostapplications,thespecificheatof a liquidorsolidcanbetreatedas a sim-plequantitythatvaries slightly withtemperature.Forgases