Performance enhancement and control strategy of a CO₂ integrated direct cooling thermal management system using a coaxial scroll expander-compressor unit for electric vehicles

To address the significant throttling losses and limited efficiency of transcritical CO2 cycles in high-temperature environments, this study comprehensively investigates a CO₂ heat pump system with a novel coaxial corotating scroll expander-compressor unit (CSECU) for electric vehicle thermal management. Two integration architectures for cabin and battery cooling, Expansion-throttling integration (ETI) and Single Expansion Integration (SEI), are proposed and comparatively analyzed. For cabin cooling, the study reveals that the cooling capacity is primarily governed by the coaxial speed, and the coaxial effect restrains the discharge pressure increase, helping decouple control. Consequently, the CSECU performs better than the traditional system with a maximum coefficient of performance of 4.41, 3.80, and 3.45 at 35, 37.5, and 40 °C. For the cabin and battery cooling, SEI demonstrates superior performance, achieving a maximum COP of 4.26, 3.77, and 3.52, higher than 3.51, 3.03, and 2.72 for ETI. This performance enhancement is attributed to the optimized utilization of latent heat at the battery cooling plate (due to lower inlet quality) and higher work recovery from the main refrigerant flow. Dynamic simulations under the World Light Vehicle Test Cycle (WLTC) confirm that the SEI strategy not only cools the battery to the target 25 °C(1064.2 s) but also controls the cell temperature difference less than 2.06 °C(14.2% lower) and operates consistently within a high-efficiency COP range of 4.10–4.30(19.1% higher), resulting in significant energy savings for the vehicle.