Refrigerant charge optimization and thermal performance evaluation of an R290-based secondary loop system for electric vehicles

R290 is regarded as one of the most promising alternative refrigerants for electric vehicles owing to its excellent properties. However, its flammability necessitates stringent safety measures, particularly the implementation of a low-charge strategy and a secondary loop design. Despite its advantages, limited research has been conducted regarding the optimal charge of R290 systems. This study presents the design and implementation of an advanced R290-based secondary loop thermal management system tailored for automotive applications. A combined approach of theoretical modeling and experimental validation was employed to investigate the refrigerant charge characteristics in detail. The system's operational feasibility and high efficiency under realistic conditions were confirmed through experimental evaluation. The analysis determined the optimal R290 charge to be 275 g, representing a 63.3 % reduction compared to an equivalent R134a-based system. This value was substantiated by experimental results and showed strong correlation with predictions derived from three established void fraction models. Further investigation revealed that the gas-liquid heat exchanger held the largest share of refrigerant mass, accounting for approximately 52.55 % to 54.01 % of the total system charge. Comparative assessments between R290 and R134a systems highlighted R290's superior thermal performance. While R290 exhibited a slightly lower coefficient of performance (COP) in cooling mode—an effect more pronounced at elevated ambient temperatures—it significantly outperformed R134a in heating mode. Notably, R290's COP advantage increased as ambient temperatures decreased, underscoring its excellent suitability for low-temperature heat pump applications.