Seite - 346 - in Emerging Technologies for Electric and Hybrid Vehicles

Energies 2016,9, 240 thermal environment than theACsystemused in conventional vehicles undervery coldweather conditions. Although thePTCheater couldprovide sufficient heat energy towarmup the cabin, its energywas derived frombattery electricity. It resulted in 24% losses of the driving range for fullyEVsdueto the lowenergyefficiencyofPTCheaters [7].Afuelfiredheaterwasanotheroption proposedforheatingwithoutelectricityconsumption,but itdidnotmeetenvironmentaldemands. Muchresearchhas focusedontheheatpumpACsystem. It isbasedonthevaporcompression cycle,whichprovidesbothcoolingandheatingcapacitybyadoptinga4-wayvalve that reverses the directionof refrigerantflow.Lee [8]declaredthat thepowerconsumptionofaheatpumpsystemwas aboutonethirdthatof theelectricPTCheatingsystemfor thesameheatingcapacity.Moreover, the coefficientofperformance (COP)ofaheatpumpACsystemis larger than1, so theheatpumpAC systemseemstobeamorereasonablesolutionthanotherclimatecontrolsystemsproposedforEVs[9]. Variousstudieshavebeenperformedtoenhance theheatpumpACsystemefficiency,especially theheatingperformancewhenfacedwith lowoutdoor temperatures. Besides thesingleair source heatpumpACsystem,multiplesourceheatpumpACsystemshavebeendevelopedforEVs. These systemscansupplysufficientcoolingorheatingcapacitywhileminimizingthe influenceof theAC systemondrivingranges. It is the intent of this paper to review themost recent progress concerning heat pumpAC technologies forEVs. This reviewisbroadlydividedinto twokeycategoriesandwillbesystematically organized. First, single sourceheatpumpACsystems forEVapplications are introduced. In this section, several advanced technologies and strategies concerning single air sourceheatpumpAC systemsarecomprehensivelyreviewed. Second,multiplesourceheatpumpACsystemsareanalyzed. Thesesystemsareappliedforallpossibleheatsources inEVstoenhance theheatingcapacityunder very low outdoor temperature conditions, as well as to achieve high energy efficiency. Finally, conclusionsaredrawnbasedonthevariousreviewsandanalyses. 2. SingleSourceHeatPumpACSystems Considering itsconvenient replacement, lowcostandeasymaintenance, thesinglesourceheat pumpACsystemisstilldominantinEVs,especiallyinmildclimateareas.However, theheatpumpAC system,whichonly involves thenecessarymodificationsbasedonconventionalvehicleACsystems, has lowsystemefficiency[10]. Thereforemanyscholarshavepresented innovative technologies in variousaspects. 2.1.AlternativeRefrigerants Atpresent, therefrigerantR134a,whichhasaglobalwarmingpotential (GWP)of1300, is still dominant in automotiveACsystems, but for future environmental considerations, theKyotoand Montrealprotocolshavebannedorlimitedtheuseofchemicalrefrigerants[11]. Similarly, theEuropean Unionhaspassedregulations torestrict theuseof refrigerantswithaGWPhigher than150 inmobile ACsystems[12].Adirective for thegradualphase-outofhighGWPrefrigerants inmobileACsystems was ratified in 2007 andwent into effect at the beginning of 2008 [13]. In light of this situation, automotiveACsystemsusingotherpotentialsubstituterefrigerantshavebeenstudied[14].CO2 isone of themoststudiedoptionssince ithasadequate thermophysicalpropertieswithnoozonedepletion potential (ODP)andaGWP=1[15,16].Asaresult,moreandmoreauthorsaredevotedtoinvestigating automotiveACsystemsusingCO2 as a refrigerant. PrototypeCO2 automotiveACsystemswere presented in[17,18]. Theyconcludedthat, in theheatpumpmode,highcapacityandCOPcanalso be achieved at low ambient temperature andwith high air supply temperature to the passenger compartment [19]. Furthermore, thesystemperformancewasequalorsuperior to thatof thecurrent R134asystem[20]. ThecoolingCOPratio toR134asystemwas1,while theheating/dehumidifying COPratiowas1.31.Kim et al. [21] studiedtheeffectsofoperatingparametersontheperformanceofa CO2ACsystemforvehicleswithvariousoperatingconditions,which includedifferentgascooler inlet pressures, compressorspeedsandfrontalair temperatures/flowratespassing throughtheevaporator and the gas cooler. They also proposed an algorithm for optimumhigh pressure control for the transcriticalCO2 cycle toachieveamaximumCOP. 346