Experimental study on performance optimization control of CO₂ refrigeration cycle in the indirect-loop integrated system for electric vehicles.

To address the issue of low efficiency of the vehicle CO2 refrigeration cycle in high-temperature environments, this study proposes an adaptive control strategy for a CO2 indirect loop (IL) heat pump (HP) integrated system, which can sufficiently enhance its cooling efficiency by improving the heat exchange efficiency of the gas cooler (GC). Firstly, the impacts of different electronic expansion valve (EXV1) openings, coolant flow rates, and indoor air volume on the system’s cyclic characteristics were analyzed, and the coupling control characteristics of the system operating parameters under the optimal energy efficiency ratio (EER) were explored. Additionally, the relationship between the optimal discharge pressure and the GC outlet refrigerant temperature was examined, leading to the proposal of an adaptive control strategy aimed at maximizing the EER. Finally, the feasibility of the optimized control strategy was experimentally validated at 40 ◦C. The results indicate that the coolant flow rate has a minimal impact on the system’s refrigeration performance. The GC outlet refrigerant temperature exhibits coupled control characteristics with both the coolant flow rate and the EXV1 opening. When the system reaches the maximum EER, the ratio of the discharge pressure to the GC outlet refrigerant temperature remains nearly constant at 2.5. The proposed adaptive regulation strategy, based on a correlation formula for optimal discharge pressure, effectively increased the EER by 7.5 % at 40 ◦C while simultaneously reducing the discharge temperature by 13.1 ◦C. These research findings provide new insights into the development of CO2 HP systems for vehicles.