A CO2 heat pump air conditioning system with single stage compression secondary throttling and series gas cooler is proposed in this paper for further enhancement of the performance of the CO2 heat pump system. The heating performance of secondary throttling system and conventional throttling system to achieve identical supply air temperatures at different ambient temperature conditions is compared. The results show that the implementation of secondary throttling system can significantly reduce the energy consumption of the CO2 heat pump system

enhance the system COP. Furthermore

the advantage of the secondary throttling system becomes more pronounced with lower ambient temperatures. Under conditions where the system supply air temperature is 40 °C and the ambient temperature is -20 °C

the power consumption of the secondary throttling system is 256.1 W (by 15.4%) lower than that of the conventional throttling system

and its COP is 0.67 (by 18.3%) higher. For the CO2 heat pump air conditioning system utilizing the secondary throttling

when the indoor supply air temperature is 40 ℃

in full fresh air environment at -20 °C

the coefficient of the heating performance can reach 4.33. In a fresh air environment at 43°C

the coefficient of the cooling performance can reach 2.46

with the indoor supply air temperature being 23.6 °C. Additionally

it has been observed that the system's cooling performance obviously deteriorates when the ambient temperature exceeds 35 °C. The results can provide guidance for designing

enhancing performance and developing control strategies for CO2 heat pump systems in electric vehicles.