Experimental study of R744 heat pump system for electric vehicle application

•A new R744 heat pump system for electric vehicles is proposed in this manuscript.•Experiments have been carried out to study the systems thoroughly.•Comparison of cooling/heating performance of R744 and R134a systems is made.•The performance merits of CO2 is re-evaluated.•R744 heat pump system is highly recommended for electric vehicle application. In this paper, two types of refrigerant, R134a and R744, are studied with different heat pump (HP) structures. Gas cooler, evaporator and indoor gas cooler (IGC) are all micro channel heat exchangers. One four-way valve and a combination of accumulator with internal heat exchanger (ACCU/IHX) are applied in R744 HP system, which can make the heat pump system structure simpler. In addition, one bidirectional throttle valve is applied before the evaporator to obtain the cooling and heating functions for R744 HP system. And the indoor gas cooler and evaporator are in series structure not only for the heating mode but also for the cooling mode in order to increase the heat transfer surface in the R744 system. The system charging test is carried out first to determine the optimal charge amount for the whole system, since it is essential for the stability of the system. Then the R744 and R134a heat pump system cooling and heating performance are studied by the experiment at different ambient working conditions. Test results show that the R744 system proposed in this paper can achieve similar cooling capacity and COP to that of the R134a heat pump. For R744 system, under 40℃ ambient working condition with external air circulation, the cooling capacity can reach 7524 W with COP of 1.36. And the R744 heat pump system can provide higher heating capacity and higher COP than the R134a heat pump system. Especially in the very cold weather, such as the −20℃ and −25℃, the R744 heat pump still can provide enough heating capacity with higher COP. Compared to the traditional R134a heat pump system, the heating capacity of R744 system increases by 83%, reaching 7378 W (R134a, 3994 W) at 6000 revolutions per minute (RPM) under −10℃ ambient temperature conditions. Under −20℃ ambient temperature with external air circulation, the maximum heating capacity is 7502 W with COP of 2.16.