Experimental Study on the Performance of Secondary Throttling CO₂ Heat Pump System for Electric Vehicles

A CO₂ heat pump air-conditioning system with single-stage compression secondary throttling and a series gas cooler is proposed to enhance the performance of a CO₂ heat pump system. The heating performances of secondary and conventional throttling systems to achieve identical supply air temperatures under different ambient temperature conditions were compared experimentally, and the cycle characteristics when the cooling performance of the system was optimal at different ambient temperatures were investigated. The results showed that the implementation of a secondary throttling system could significantly reduce the energy consumption of the CO₂ heat pump system, thereby enhancing the system coefficient of performance (COP). Furthermore, the advantages of the secondary throttling system become more pronounced at lower ambient temperatures. Under conditions where the system supply air temperature was 40 ℃ and the ambient temperature was -20 ℃, the power consumption of the secondary throttling system was 256.1 W and 15.4% lower than that of the conventional throttling system, and its COP was 0.67 and 18.3% higher. Under a fresh air environment at -20 ℃, the CO₂ heat pump air conditioning system utilizing the secondary throttling could achieve a heating COP of up to 4.33 when the supply air temperature was 40℃. Under a fresh air environment at 43 ℃, the system could reach a cooling COP of 2.46 when the supply air temperature was 23.6 ℃. Additionally, it was observed that the cooling performance of the system significantly deteriorated when the ambient temperature exceeded 35 ℃.