Experimental study on optimal heating cycle characteristics of CO₂ secondary throttling heat pump system for electric vehicles

As HFC refrigerants widely employed in electric vehicle heat pump systems face stringent restrictions due to environmental concerns, CO2 has attracted significant attention as a highly promising alternative refrigerant. This paper presents a novel CO2 heat pump air conditioning system with a single-stage compression secondary throttling series gas cooler for electric vehicles. The heating cycle characteristics of the system at the maximum coefficient of performance (COP) are investigated for different combinations of dual electronic expansion valves (EEV 1 and EEV 2) openings, ambient temperature, indoor air volume, and compressor speed. Additionally, the relationship between the intermediate pressure and the discharge pressure is discussed. The results indicate that utilizing dual EEVs allows for graded regulation of system pressure, enabling effective control of the compression ratio, and achieving the objectives of reducing the system energy consumption and improving the performance. When the system achieves its maximum COP, a certain proportional relationship exists between the intermediate pressure and discharge pressure, and this proportionality is almost independent of factors such as ambient temperature, indoor air volume, and compressor speed. The outlet refrigerant temperature of the indoor gas cooler (IGC) exhibits a negative correlation with the COP. For each 1 degrees C reduction in the temperature difference between the outlet refrigerant of the IGC and the ambient air, there is an average increase of 4% in the COP. Based on the experimental data, a linear correlation between the optimal intermediate pressure and discharge pressure is established with a high degree of confidence. Additionally, a corresponding control logic is proposed and validated. The results can provide guidance for enhancing performance and developing control strategies for CO2 heat pump systems in electric vehicles.