Experimental study of an R515B/CO₂ cascade refrigeration system for commercial applications

CO₂ has been recognized as an ideal refrigerant for the low-temperature cycle (LTC) in a commercial cascade refrigeration system (CRS). Against the backdrop of global warming, low-GWP refrigerant substitution for the high-temperature cycle (HTC) is becoming increasingly critical, and R515B is regarded as a promising solution. In this paper, an R515B/CO₂ CRS experimental setup is constructed to comprehensively evaluate the system performance under various operating conditions. Moreover, performance variations of the refrigeration compressors in the LTC and HTC are also tested and analyzed. According to the results, the LTC performance is primarily governed by the intermediate and evaporating temperatures, remaining relatively insensitive to the condensing temperature. In contrast, the HTC performance exhibits high sensitivity to the condensing temperature. Consequently, increasing the condensing temperature substantially raises the power consumption of the HTC compressor and leads to a marked decline in the system coefficient of performance (COP _sys). The analysis also confirms the existence of an optimal intermediate temperature. Under typical operating conditions with a condensing temperature of 40°C and an evaporating temperature of −23°C, the system achieves a maximum COP _sys of 1.74. Furthermore, the R515B/CO₂ system demonstrates superior thermodynamic performance compared to similar CRSs utilizing high-GWP refrigerants as reported in the literature. Specifically, at an evaporating temperature of -30°C and a condensing temperature of 40°C, the system achieves a COP _sys of 1.48, representing an improvement of approximately 4.2% over the best-reported R134a/CO₂ CRS under identical conditions. From a life-cycle environmental perspective, the total equivalent warming impact of the R515B/CO₂ CRS is also reduced by approximately 15.0% compared to the R134a/CO₂ CRS. Coupled with its A1 safety classification and low GWP, R515B proves to be a highly promising HTC refrigerant for commercial CRSs.