Thermodynamic Analysis of an NH3/CO2 Cascade Refrigeration System with Subcooling in the Low-Temperature Circuit Utilizing the Expansion Work

Abstract

NH3/CO2 cascade refrigeration system is recognized one of the most promising technologies in low-temperature application. In this paper, a NH3/CO2 cascade refrigeration system with subcooling in low-temperature circuit driven by recovery expansion work has been proposed. The aim of this study is to investigate the proposed cascade refrigeration system compared with conventional cascade refrigeration system. Mathematical models based on energy conservation and exergy balance are established. The selection of different refrigerants in auxiliary subcooling system is discussed. The effects of operating parameters such as the condensation temperature of the low-temperature circuit, evaporation temperature, and expander efficiency on system performance are evaluated. The results show that the coefficient of performance and exergy efficiency of the proposed system are about 7.56% and 7.98% higher than that of conventional cascade refrigeration system. The discharge temperature of NH3 compressor can be significantly reduced by 18.33%. The isentropic efficiency of the expander has a large impact on the system performance.