Performance Evaluation of Three Types of Air-to-Water Cascade Heat Pumps for Cold Climate Applications Using Different Refrigerant Pairs

Abstract

Abstract. The performance of conventional air-to-water heat pumps significantly deteriorates at low ambient temperatures due to high temperature lift and compressor limitations. This study investigates the thermodynamic performance of three cascade air-to-water heat pump configurations for cold climate applications using sixteen refrigerant pairs. A numerical model was developed in Python using CoolProp for thermophysical properties, considering ambient temperatures down to −50 °C and a heating supply temperature of +60 °C. The results indicate that R32/R134a and R410A/R134a achieve the highest COP, reaching up to 2.56 at +10 °C and approximately 1.98 at −30 °C; however, their operation is limited by environmental constraints and operating range. R744-based systems enable operation at extreme low temperatures down to −41 °C, with COP values of 1.62 (R744/R134a) and 1.60 (R744/R290). Among the configurations, the first and third show the highest efficiency (COP ≈ 2.0), while the second exhibits a ~4.6% lower COP but offers operational flexibility, including single-stage operation above −10 °C. Considering both performance and environmental impact, R744/R290 is identified as the most promising refrigerant pair for future applications.

Keywords: Cascade heat pump; cold climate; COP; pressure ratio; environmentally friendly refrigerant