Performance research on a CO2 heat pump system with novel control strategy in electric vehicle

The efficiency of the thermal management system is crucial for electric vehicles (EVs). This study proposes a novel dual electronic expansion valve (EXV1 and EXV2) sub-area control strategy to improve the heating performance of the CO₂ heat pump (HP) system in low temperatures. The study analyzed the impact of the EXV1 opening on the system’s operating parameters. A comparison assessment was subsequently conducted on the heating capacity, COP, and gas cooler (GC) outlet air temperature before and after optimizing the control strategy at −20 °C. A method for evaluating performance under heating and refrigerating conditions was proposed and validated using a real vehicle in the environmental chamber. Additionally, the study compared the energy consumption differences between the CO₂ and R134a systems at −20 °C. The effects of these two systems on vehicle range at various temperatures were also compared using the WLTC (World-Light-Vehicle-Test-Cycle). The results show that when the EXV1 opening is exceeds 300 steps (30 %), it has less influence on the operating parameters of the system components. Furthermore, at −20 °C, the optimized control strategy improved the average heating capacity, and COP by 11.6 %, and 9.8 %, respectively, and the GC average outlet air temperature by 8.2 °C. The target thermal management system can meet the heating and refrigerating demands. The minimum temperature of the outlet air can be 5.24 °C at 40 °C and the maximum temperature of the outlet air can be 57.16 °C at −15 °C. The CO₂ HP system saves about 34.7 % of energy consumption compared with the R134a system at −20 °C. Under the WLTC, the range of the CO₂ HP system is improved by 9.3 % and 16.6 % compared to the R134a system at −20 °C and −10 °C, respectively.