Comprehensive study on EEVs regulation characteristics of vapor-injection CO2 heat pump for electric vehicles

The regulation characteristics of electronic expansion valves (EEVs) are critical to improving the comprehensive energy efficiency of the heat pump system for electric vehicles. In this paper, a vapor-injection transcritical CO2 heat pump system for electric vehicles was first established to experimentally study the impacts of EEV1 and EEV2 openings on the overall system performance. The results show that the cooling capacities of the vapor-injection system are respectively increased by 3.13% / 5.75% with the EEV1's opening of 15% / 25% and in-jection ratio of 0.0975 / 0.8305, compared against these of the basic system without vapor injection. As the injection ratio rises, the cooling capacity increment tends to increase while the cooling coefficient of performance (COP) declines from 1.7 to 1.51. Then, the regulation characteristics of EEV1 and EEV2 were quantitively analyzed, indicating that the evaporation pressure is mainly determined by EEV2's openings. By adjusting EEV1's opening, the COP of the CO2 vapor-injection heat pump can be maximized at 1.9, with EEV1's opening at 20% and outdoor/indoor ambient temperature at 0 degrees C / 20 degrees C. When adjusting EEV2's opening, the maximum injection pressure corresponds to the maximum COP of 1.84, with EEV2's opening at 65% under 35 degrees C/ 27 degrees C. When the COP reaches the maximum, the refrigerant at the outlet of EEV1 is just saturated liquid under cooling conditions, while it is located in the two-phase region under heating conditions. Besides, the optimal COP regulation strategy of the CO2 vapor-injection heat pump was further formulated based on two modes, as the strategy in the cooling mode differs from that in the heating mode.