Page - 347 - in Emerging Technologies for Electric and Hybrid Vehicles

Energies 2016,9, 240 AlthoughCO2ACsystemshave special benefits, theyhave somedisadvantages, suchas low critical temperatureandhighoperatingpressure [22].Otherproblemsindefrostingofexteriorheat exchangersandperformancedeteriorationundercoldambient temperatureconditionsexist inCO2 automotiveheatpumpsystemsaswell asR134aheatpumpsystems. Byre-arranging the radiator andoutdoorheatexchangersof theCO2heatpumpsysteminelectriccars, theheatingcapacityand COPwere increasedby54%and22%,respectively [23]. Toenhancecoolingperformance,Lee et al. [24] presentedanelectricalACsystemwithCO2 thatusedan inverter-drivencompressor. Thecooling capacityandCOPof this tested systemwere increasedby36.8%up to6.4kWandby30.3%up to 2.5kW,respectively.Maetal. [25]conductedathoroughreviewoftheCO2heatpumpandrefrigeration cycle. Theyconcludedthatsomemodifications, suchasusinganinternalheatexchanger, two-stage compression,andexpansionworkrecoveryaswellasenhancingheat transfer, could improve theCO2 transcritical cycleperformance toa level similar to thatofaconventionalheatpumpsystem. InadditiontousingCO2,otherpossiblerefrigerants intheheatpumpACsystemincludeR1234yf, R152a,R290,R245faandwater [26–29]. Theperformanceof theR1234yf“drop-in”automotiveAC systemwas analyzed and comparedwith that of systemswithCO2 andR32 by experimentation andsimulation[30]. TheCOPandcapacityofR1234yfsystemwereupto2.7%and4.0%lower than those of R134a system, respectively, while the compressor discharge temperature and amount of refrigerant chargewere 6.5 ˝Cand10% lower than thoseofR134a system[31]. Consequently, the R1234yf“drop-in”ACsystemwasthemost feasiblecandidate forautomobiles fromthestandpointof systemperformanceandoperatingconditions.However,moreworkmustbecompletedbefore the R1234yf systemcanbewidelyaccepted inEVs [31–33]. Ghodbane [34,35]presented thesecondary loopsystemusingR152aas theworkingfluid inmobileclimatecontrol systems. Thissystemshowed averygoodcoolingandheatingperformance,buthadaslowerresponse to loadchanges, complex systemconnectionsandahighcost. 2.2.Applicationof InverterTechnology Frequencyvariationtechnology,acommonwaytosaveenergy, isalsowidelyusedinACsystems. AnR134a automotiveAC system capable of operating as an air-to-air heat pump is described in Figure1 [36]. Figure1.Schematicdiagramof theexperimentalautomotiveair conditioning/heatpumpsystem[36]. 347