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【Abstract】The heating efficiency of heat pumps has reached 2-3 times higher than that of the present air conditioning in fuel powered automobile. And the theoretical COP is up to 8 times under the normal winter conditions. The electricity load of air conditioning would decrease by about 50%-70% and the driving mileage would increase about 10% in the heating period once the PTC heaters are replaced by heat pumps. This paper mainly introduces the present research and development of electric vehicle air conditioning.
【Key words】Electric vehicle； air conditioning； heat pump； PTC
【中图分类号】G64 【文献标识码】A 【文章编号】2095-3089（2016）23-0002-02
1. Introduction
Compared with conventional vehicles， electric vehicles have advantages of no air pollution， low noise， high energy-efficient， and more intelligent. In this context， electric vehicles （EVs） such as plug-in and fuel-cell vehicles have been developed to meet environmental regulations[1-5]. Electric vehicles should be equipped with appropriate air-conditioning to yield a comfortable cabin environment as same as fuel vehicles.
Heat pumps have been widely used as the heating devices for commercial and residential buildings because of its high energy efficiency[6-9]. But the heat pump air conditioning on the automobile is not yet used in a large scale.
2.The development trend of the electric vehicle heat pump air conditioner
Currently automotive heat pump air conditioning mainly has R134a heat pump air conditioning， CO2 heat pump air conditioning， solar assisted heat pump air conditioning， etc.
2.1 R134a heat pump air conditioning
R134a heat pump air conditioning is developed as an electric vehicle air conditioning by DENSO[10]. Experimental tests show that the air conditioning has a better performance in the cooling and heating operating conditions. It’s heating efficiency reached 2.3 under the condition of -10℃ outside the car and 25℃inside the car. It can still keep a high efficiency at the ambient temperature of -10℃ to -40℃[10].
M.Hosoz and M.Direk[11] studied the performance of R134a automobile heat pump in air conditioning which uses ambient air as heat source. The system structure is shown in Fig.1. The test results revealed that the heat pump air conditioning can provide sufficient heat capacity in the case of the ambient temperature is not very low， however heating performance of the heat pump system will dramatic decrease as the ambient temperature decreases. 2.2 CO2 heat pump air conditioning
CO2 is an old but natural refrigerant with low global warming potential （GWP， GWP=1） and no ozone depletion potential （ODP，ODP=0）. It operated mostly in transcritical cycle[12] because of its low critical temperature （31.1℃）. In CO2 transcritical cycle， the CO2 vapor is cooled in vapor phase and the condenser is called the gas cooler [13-14].
In 2009， Kim et al.[15] studied the effects of operating parameters on the performance of a CO2 air conditioning for vehicles. They proposed a relation used in an optimum high pressure control algorithm for the trans-critical CO2 cycle to achieve the maximum COP. Within a project a reversible cooling and heating system using the natural refrigerant CO2 as working fluid was designed for an electric city car as the schematic diagram （Fig.2） shows.
heating system in heating mode[16].
2.3 Solar-assisted air conditioning
Back in 1989， Ingersoll-Rand corporation found that it can not only provide some energy but also greatly reduce the peak cooling load when the car is parked under the strong sunlight. Refrigeration of the automotive air conditioning takes a few kilowatts every hour， however it can only produce 200-300W energy per hour when solar cells area is less than 2m2 [17]. So it is still impossible to completely rely on solar batteries to drive the compressor. But it can reduce cab temperature caused by solar radiation. And it can enhance air circulation in the cab through driving the fan to create a comfortable environment before passengers enter the car.
3. Conclusions
Electric vehicles， as a new generation of clean vehicle， have an important significance to alleviate even solve global energy problems， environmental issues， climate issues. The following aspects need further development to improve the performance of the air conditioning.
（1）Establish the simulation model of electric vehicle air conditioner system， and study its dynamic simulation to make a further optimal design.
（2）Develop more efficient compressor， condenser， evaporator and other components， reduce power consumption of the compressor， and increase the efficiency of the heat exchangers.
（3）Solve the matching problem of compressor with heat exchangers， expansion valve and the type of electronic control utilized by the system to regulate the operation of the compressor[18].
References：
[1]Al-Alawi BM， Bradley TH. Total cost of ownership， payback， and consumer preference modeling of plug-in hybrid electric vehicles. Applied Energy 2013；103：488-506. [2]Brouwer AS， Kuramochi T， Broek M， Faaij A. Fullling the electricity demand of electric vehicles in the long term future： an evaluation of centralized and decentralized power supply systems. Applied Energy 2013；107：33-51.
[3]Foley A， Tyther B， Calnan P， Gallachoir BO. Impacts of electric vehicle charging under electricity market operations. Applied Energy 2013；101：93-102.
[4]Kristoffersen TK， Capion K， Meibom P. Optimal charging of electric drive vehicles in a market environment. Applied Energy 2011；88：1940-8.
[5]Doucette RT， McCulloch MD. Modeling the prospects of plug-in hybrid electric vehicles to reduce CO2 emissions. Applied Energy 2011；88：2315-23.
[6]Jang JY， Bae HH， Lee SJ， Ha MY. Continuous heating of an air-source heat pump during defrosting and improvement of energy efficiency. Applied Energy 2013；110：9-16.
[7]Yang SH， Rhee JY. Utilization and performance evaluation of a surplus air heat pump system for greenhouse cooling and heating. Applied Energy 2013；105：244-51.
[8]Byrne P， Miriel J， Lent Y. Experimental study of an air-source heat pump for simultaneous heating and cooling—Part 1： Basic concepts and performance verification. Applied Energy 2011；88：1841-7.
[9]Byrne P， Miriel J， Lent Y. Experimental study of an air-source heat pump for simultaneous heating and cooling—Part 2： Dynamic behavior and two-phase thermosiphon defrosting technique. Applied Energy 2011；88：3072-8.
[10]Takahisa Suzuki， Katsuya Ishii. Air Conditioning System for Electric Vehicle， SAE Technical Papers.960688.1996.
[11]M. Hosoz， M. Direk. Performance evaluation of an integrated automotive air conditioning and heat pump system. Energy Conversion and Management 2006；47：545-559.
[12]Wang W， Ma Z， Jiang Y， Yang Y， Xu S， Yang Z. Field test investigation of a double-stage coupled heat pumps heating system for cold regions. International Journal of Refrigeration 2005；28：672-9.
[13]Lorentzen G， Pettersen J. A new， efficient and environmentally benign system for car air-conditioning. International Journal of Refrigeration 1993；16（1）：4-12.
[14]Lorentzen G. Revival of carbon dioxide as a refrigerant. International Journal of Refrigeration 1994；17（5）：292-301.
[15]S.C. Kim， J.P. Won， M.S. Kim. Effects of operating parameters on the performance of a CO2 air conditioning system for vehicles. Applied Thermal Engineering 2009；29：2408-2416.
[16]Alois Steiner， René Rieberer. Simulation based identification of the ideal defrost start time for a heat pump system for electric vehicles. International Journal of Refrigeration，2015，57：87-93.
[17]Ma，G.Y， Shi，B.X， Wu，L.Z， Chen，G.S， Study on solar-assisted heat pump system for electric vehicle air conditioning. Taiyangneng Xuebao 2001，22（2）：176-180.
[18]Anon， Electric vehicle air conditioning， Automotive Engineering International 1996，104（9）：113-117.
【Key words】Electric vehicle； air conditioning； heat pump； PTC
【中图分类号】G64 【文献标识码】A 【文章编号】2095-3089（2016）23-0002-02
1. Introduction
Compared with conventional vehicles， electric vehicles have advantages of no air pollution， low noise， high energy-efficient， and more intelligent. In this context， electric vehicles （EVs） such as plug-in and fuel-cell vehicles have been developed to meet environmental regulations[1-5]. Electric vehicles should be equipped with appropriate air-conditioning to yield a comfortable cabin environment as same as fuel vehicles.
Heat pumps have been widely used as the heating devices for commercial and residential buildings because of its high energy efficiency[6-9]. But the heat pump air conditioning on the automobile is not yet used in a large scale.
2.The development trend of the electric vehicle heat pump air conditioner
Currently automotive heat pump air conditioning mainly has R134a heat pump air conditioning， CO2 heat pump air conditioning， solar assisted heat pump air conditioning， etc.
2.1 R134a heat pump air conditioning
R134a heat pump air conditioning is developed as an electric vehicle air conditioning by DENSO[10]. Experimental tests show that the air conditioning has a better performance in the cooling and heating operating conditions. It’s heating efficiency reached 2.3 under the condition of -10℃ outside the car and 25℃inside the car. It can still keep a high efficiency at the ambient temperature of -10℃ to -40℃[10].
M.Hosoz and M.Direk[11] studied the performance of R134a automobile heat pump in air conditioning which uses ambient air as heat source. The system structure is shown in Fig.1. The test results revealed that the heat pump air conditioning can provide sufficient heat capacity in the case of the ambient temperature is not very low， however heating performance of the heat pump system will dramatic decrease as the ambient temperature decreases. 2.2 CO2 heat pump air conditioning
CO2 is an old but natural refrigerant with low global warming potential （GWP， GWP=1） and no ozone depletion potential （ODP，ODP=0）. It operated mostly in transcritical cycle[12] because of its low critical temperature （31.1℃）. In CO2 transcritical cycle， the CO2 vapor is cooled in vapor phase and the condenser is called the gas cooler [13-14].
In 2009， Kim et al.[15] studied the effects of operating parameters on the performance of a CO2 air conditioning for vehicles. They proposed a relation used in an optimum high pressure control algorithm for the trans-critical CO2 cycle to achieve the maximum COP. Within a project a reversible cooling and heating system using the natural refrigerant CO2 as working fluid was designed for an electric city car as the schematic diagram （Fig.2） shows.
heating system in heating mode[16].
2.3 Solar-assisted air conditioning
Back in 1989， Ingersoll-Rand corporation found that it can not only provide some energy but also greatly reduce the peak cooling load when the car is parked under the strong sunlight. Refrigeration of the automotive air conditioning takes a few kilowatts every hour， however it can only produce 200-300W energy per hour when solar cells area is less than 2m2 [17]. So it is still impossible to completely rely on solar batteries to drive the compressor. But it can reduce cab temperature caused by solar radiation. And it can enhance air circulation in the cab through driving the fan to create a comfortable environment before passengers enter the car.
3. Conclusions
Electric vehicles， as a new generation of clean vehicle， have an important significance to alleviate even solve global energy problems， environmental issues， climate issues. The following aspects need further development to improve the performance of the air conditioning.
（1）Establish the simulation model of electric vehicle air conditioner system， and study its dynamic simulation to make a further optimal design.
（2）Develop more efficient compressor， condenser， evaporator and other components， reduce power consumption of the compressor， and increase the efficiency of the heat exchangers.
（3）Solve the matching problem of compressor with heat exchangers， expansion valve and the type of electronic control utilized by the system to regulate the operation of the compressor[18].
References：
[1]Al-Alawi BM， Bradley TH. Total cost of ownership， payback， and consumer preference modeling of plug-in hybrid electric vehicles. Applied Energy 2013；103：488-506. [2]Brouwer AS， Kuramochi T， Broek M， Faaij A. Fullling the electricity demand of electric vehicles in the long term future： an evaluation of centralized and decentralized power supply systems. Applied Energy 2013；107：33-51.
[3]Foley A， Tyther B， Calnan P， Gallachoir BO. Impacts of electric vehicle charging under electricity market operations. Applied Energy 2013；101：93-102.
[4]Kristoffersen TK， Capion K， Meibom P. Optimal charging of electric drive vehicles in a market environment. Applied Energy 2011；88：1940-8.
[5]Doucette RT， McCulloch MD. Modeling the prospects of plug-in hybrid electric vehicles to reduce CO2 emissions. Applied Energy 2011；88：2315-23.
[6]Jang JY， Bae HH， Lee SJ， Ha MY. Continuous heating of an air-source heat pump during defrosting and improvement of energy efficiency. Applied Energy 2013；110：9-16.
[7]Yang SH， Rhee JY. Utilization and performance evaluation of a surplus air heat pump system for greenhouse cooling and heating. Applied Energy 2013；105：244-51.
[8]Byrne P， Miriel J， Lent Y. Experimental study of an air-source heat pump for simultaneous heating and cooling—Part 1： Basic concepts and performance verification. Applied Energy 2011；88：1841-7.
[9]Byrne P， Miriel J， Lent Y. Experimental study of an air-source heat pump for simultaneous heating and cooling—Part 2： Dynamic behavior and two-phase thermosiphon defrosting technique. Applied Energy 2011；88：3072-8.
[10]Takahisa Suzuki， Katsuya Ishii. Air Conditioning System for Electric Vehicle， SAE Technical Papers.960688.1996.
[11]M. Hosoz， M. Direk. Performance evaluation of an integrated automotive air conditioning and heat pump system. Energy Conversion and Management 2006；47：545-559.
[12]Wang W， Ma Z， Jiang Y， Yang Y， Xu S， Yang Z. Field test investigation of a double-stage coupled heat pumps heating system for cold regions. International Journal of Refrigeration 2005；28：672-9.
[13]Lorentzen G， Pettersen J. A new， efficient and environmentally benign system for car air-conditioning. International Journal of Refrigeration 1993；16（1）：4-12.
[14]Lorentzen G. Revival of carbon dioxide as a refrigerant. International Journal of Refrigeration 1994；17（5）：292-301.
[15]S.C. Kim， J.P. Won， M.S. Kim. Effects of operating parameters on the performance of a CO2 air conditioning system for vehicles. Applied Thermal Engineering 2009；29：2408-2416.
[16]Alois Steiner， René Rieberer. Simulation based identification of the ideal defrost start time for a heat pump system for electric vehicles. International Journal of Refrigeration，2015，57：87-93.
[17]Ma，G.Y， Shi，B.X， Wu，L.Z， Chen，G.S， Study on solar-assisted heat pump system for electric vehicle air conditioning. Taiyangneng Xuebao 2001，22（2）：176-180.
[18]Anon， Electric vehicle air conditioning， Automotive Engineering International 1996，104（9）：113-117.