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HeatEnginesTopicsinthislecture:Cyclicprocesses2ndlawofthermodynamicsHeatengineHeatpumpRefrigerator/AirconditioningEfficiency/CoefficientofPerformanceCarnotcycleHeatEngineRefrigerator+AirConditioningHeatPumpTakesHEATfromcoldtohot,butneedsmotortodoWORKtomakeithappen.ConvertsHEATtousefulWORK,butalwayswastessomeHEAT.23CompletecyclesPVABCDThepathABCDAisaclosedloopPVABPVCDTheworkdonebygasfromA–Bisredarea(andispositive).TheworkdonebygasfromC–Disbluearea(andisnegative).PVABCDThenetworkdonebygasinwholecycle=areaenclosedbypath.4Inacompletecycle

U=0W=Qnetworkdone=areaenclosedSincewestartandendatthesamepointFrom1stlawCompletecycles52ndLawofThermodynamicsItisnotpossibletocompletelytransferheattowork.Thereisalwayswasteheat.2ndlawAheatenginetakesHEATfromthefuelandconvertsittousefulWORK.ButthereisalwayssomewasteHEATtotheenvironment.(HeatEngine)6HeatEnginesHOTsourceatThCOLDsinkatTcUsefulworkQhQc

Conservationofenergy

Efficiency

Forheatengine….Usuallyexpressedas%

WQuestion1Acoal-firedpowerstationisahugeheatengine.Itusesheattransferfromburningcoaltodoworktoturnturbines,whichgenerateelectricity.Inonedayalargecoalpowerstationhas2.50x1014Jofheattransferfromcoaland1.48x1014Jofheattransferintotheenvironment.(a)Whatistheworkdonebythepowerstationinoneday?(b)Whatistheefficiencyofthepowerstation?7NowPAUSEthevideoandtrytosolveQ1Question1Acoal-firedpowerstationisahugeheatengine.Itusesheattransferfromburningcoaltodoworktoturnturbines,whichgenerateelectricity.Inonedayalargecoalpowerstationhas2.50x1014Jofheattransferfromcoaland1.48x1014Jofheattransferintotheenvironment.(a)Whatistheworkdonebythepowerstationinoneday?(b)Whatistheefficiencyofthepowerstation?89InternalcombustionenginePistonconnectedtorotatingcrankshaftAirandfueltakeninAdiabatic

compression(rapid

temperaturerise)Fuelignited.

Turnstogas.

Increaseinpressure

Gasremoved.

PistoncomesupreadytostartagainPressuredoesworktopushpistondown10InternalcombustionenginePistonconnectedtorotatingcrankshaftAirandfueltakeninAdiabatic

compression(rapid

temperaturerise)Fuelignited.

Turnstogas.

Increaseinpressure

Gasremoved.

PistoncomesupreadytostartagainPressuredoesworktopushpistondownABBCCDDAOttoCycle11Fuelignites.Internalenergyconvertstoheatinthegas.Workdoneongastocompressadiabatically.RapidtemperatureriseGasesreleased,soheatleavesthesystem.isochoricisochoricAdiabatic

NetresultPressurecausesadiabaticexpansion(workdonebygas).

Gascools12MaximumpossibleefficiencyRealsystemsalwayshaveenergylossesduetofriction.Butevenofwecouldeliminatefriction,thermodynamicsstilllimitsefficiency.TheCARNOT

cyclehasthehighestpossibleefficiency.CarnotcycleAcarnotcyclehas2isothermalstepsand2adiabaticsteps.HeatentersorleavesthegasintheisothermalstepsTemperaturefallsorrisesinadiabaticsteps.13Workisdoneinallsteps.

Networkdone=netheatgainCarnotcycle14Networkdone=areaenclosedonPVgraphThisismaximumtheoreticalefficiencyforengineoperatingbetweentemperaturesofTcandTh.EfficiencyofrealengineForCarnotcycle15Question2Acoal-firedelectricpowerstationhasanefficiencyof38%.Thetemperatureofthesteamintheboileris550oCandtheenvironmentisat20oC.Whatfractionofthemaximumpossibleefficiencydoesthispowerstationachieve?16NowPAUSEthevideoandtrytosolveQ2Question2Acoal-firedelectricpowerstationhasanefficiencyof38%.Thetemperatureofthesteamintheboileris550oCandtheenvironmentisat20oC.Whatfractionofthemaximumpossibleefficiencydoesthispowerstationachieve?17Question3Wouldyoubelieveaninventorwhoclaimedtohavecreatedadevicethatcando12kJofworkbytakingin22kJofheatat600K,andexpellingwasteheattotheenvironmentat300K?18NowPAUSEthevideoandtrytosolveQ3Question3Wouldyoubelieveaninventorwhoclaimedtohavecreatedadevicethatcando12kJofworkbytakingin22kJofheatat600K,andexpellingwasteheattotheenvironmentat300K?1920HeatPumpAheatpumptransfersheatfromacoldplacetoahotplace.Thisisoppositetonaturalprocesses.Needtodosomeworktomakeithappen.HeatabsorbedfromthegroundevaporatesthefluidTemperaturerisesaboveroomtemperatureasgascompressedadiabatically.HeatradiatestoroomandgascoolsAdiabaticexpansionmakesgascooltobelowtheoutsidetemperatureHeatPumpHeatPump

Conservationofenergy

WeuseCoefficientofperformance(COP)ratherthanefficiencyForheatpump….

232ndLawofThermodynamicsItisnotpossibletotransferheatfromacoolerobjecttoahotterobjectwithoutdoingwork.HeatnaturallyflowsfromHOT

COLD.ButtomakeitflowfromCOLD

HOTwehavetodoWORK.2ndlaw(heatpump)Refrigerator/AirconditioningLikeaheatpump,arefrigeratortransfersheatfromacoldplacetoahotplace.Theonlydifferencefromheatpumpisthat‘whatyouget’isQcandNOTQhForrefrigeratorandairconditioning….

Question4(a)AnairconditioningsystemhasaCOPof2.5andusesamotorratedat1600W.Atwhatrateisheatremovedfromtheroom?Atwhatrateisheatexhaustedtotheoutside?Ifthisunitis‘turnedround’andusedasaheatpumpinthewinter,deliveringheatatthesamerateaspartiiabove,whatistheCOPoftheheatpump?NowPAUSEthevideoandtrytosolveQ4Question4(a)Anairconditioningsystemhasa

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