Optimization and comparative assessment of variable-circuitry heat exchanger for air-source heat pumps using R32, R290 and R454B

Faced with strictly greenhouse gas regulations and diverse refrigerant alternatives, traditional fixed-circuitry heat exchangers in air-source heat pumps (ASHPs) struggle to achieve optimal performance in both cooling and heating modes. Although variable-circuitry heat exchangers (VCHXs) offer a feasible technical solution to this problem, existing designs are mostly based on commonly used refrigerants like R32 and fail to fully consider the significant differences in thermophysical properties of low-GWP alternative refrigerants such as R290 and R454B. This limits the broader application of the VCHX technique in high-efficiency and low-carbon ASHP systems. Therefore, this paper conducted numerical studies on the outdoor fin-and-tube heat exchangers (FTHXs) of ASHPs using R32, R290, and R454B. The simulation revealed the optimal circuitry configurations for each refrigerant under both condenser and evaporator, and corresponding VCHX schemes were designed accordingly. Comparative experiments show that after adopting VCHXs, the APF of R32, R290, and R454B systems increased by 4.1%, 5.6% and 4.7%, confirming the effectiveness of dynamically matching the circuitry with the operating mode to enhance the annual energy efficiency. Further evaluation based on life cycle climate performance (LCCP) demonstrated that combining VCHX with low-GWP refrigerants can yield significant environmental benefits. The total life-cycle carbon emissions of the R32, R290, and R454B systems were reduced by 3.8%, 5.1%, and 4.4%, respectively. Particularly, the R290 system showed the best life-cycle environmental performance due to its extremely low GWP and small charge. The APF value of the R454B system adopting VCHX surpassed that of the R32 prototype system, highlighting the potential of VCHX technique to mitigate the performance limitations of non-azeotropic mixture. This paper clarifies the key impact of refrigerant properties on VCHX design, and validates the effectiveness of VCHX in improving the annual energy efficiency of ASHPs and promoting the application of low-GWP alternative refrigerants. It provides a technical reference and design basis for the development of heat pump applications that meet next-generation environmental standards.