Heating performance analysis and operation optimization of CO₂ secondary throttle heat pump system for electric vehicles.

To address the challenges of high energy consumption during winter heating and limited driving range in electric vehicles, a transcritical CO2 secondary throttling heat pump system, which integrates an internal heat exchanger and dual expansion valves, is proposed in this study. By optimizing the dual indoor heat exchangers and a staged throttling, the refrigerant heat exchange efficiency of the system within the heating cycle is enhanced. A 1D thermal management simulation model incorporating both cabin and battery is first developed, whose accuracy is demonstrated to be quite satisfactory by comparing numerical results with experimental data. Subsequently, a comparative analysis between the secondary throttling system and a conventional heat pump reveals a heating capacity increase of 50.88 % at 0 ◦C. Further parametric studies are conducted under varying valve openings, indoor/outdoor airflow rates, and compressor speeds, demonstrating that the coordinated control of dual EXVs effectively regulates the intermediate pressure. Additionally, airflow parameters and compressor speed significantly influence overall system performance. Under optimized operating conditions, the system achieves significant improvements in both heating capacity and coefficient of performance, demonstrating the effectiveness of the proposed design. This investigation provides a viable technical pathway for optimizing CO2 heat pump air conditioning systems.