Transcritical CO₂ heat pump air-conditioning systems have gained prominence in new energy vehicle thermal management due to their energy-saving and environmentally friendly characteristics. However, the relatively low coefficient of performance (COP) in cooling mode remains a significant obstacle to developing transcritical CO₂ heat pump air conditioning systems. To enhance system performance, five technical approaches are proposed: internal heat exchangers (IHX), expanders, vortex tubes, ejectors, and combined multiple evaporation steps with vapor injection. The performances of these methods were evaluated through one-dimensional theoretical calculations under vehicle operating conditions. Results indicate that optimizing discharge pressure is critical for all methods, with varying degrees of COP improvement. Expanders provide the most comprehensive benefits, ejectors perform well under specific design conditions, IHX shows notable enhancements in cooling mode, and vortex tubes and combined multiple evaporation steps with vapor injection exhibit broad adaptability across working conditions. These findings offer valuable insights for practical engineering applications and support the adoption of transcritical CO₂ heat pump systems in new energy vehicles.

To further study the influence of heat exchanger structures on system performance and overall power consumption in transcritical CO₂ heat pump air conditioning systems for high-speed trains to improve performance and reduce power consumption, this study builds a numerical simulation model based on the AMEsim simulation platform. The simulation results show that, in the high-speed train heat pump air conditioning system, the influence of the gas cooler structure on system performance is greater than that of the evaporator structure. In terms of the selection of heat exchanger structure, the optimal structure is that during refrigeration, the outdoor heat exchanger adopts the countercurrent arrangement and the indoor heat exchanger adopts the concurrent arrangement (vice versa for heating). At the rated cooling condition of 35 ℃ ambient temperature, the COP is increased by 20.38% compared to the concurrent arrangement in outdoor heat exchangers, and at a rated heating condition of 7 ℃ ambient temperature, the COP is increased by 68.04% compared to the concurrent arrangement of indoor heat exchangers. Considering both the degree of backflow and fan power consumption, in the "fully heat exchange state", system COP increases with backflow degree, while system COP decreases with backflow degree when the air flow rate is insufficient.