Comprehensive Exergy and Exergoeconomic Analyses and Optimization of a Three-Stage Cascade Refrigeration System Using Environmentally Friendly Refrigerants

Ultra-low-temperature (ULT) refrigeration systems are widely required in applications such as biomedical storage, cryogenic processing, and advanced scientific facilities, where both exergy efficiency and economic performance are critical. In this study, a comprehensive analysis and optimization of the exergy and exergoeconomic performance evaluation of a triple-stage cascade refrigeration system under ULT refrigeration is presented using ethylene (R1150), ethane (R170), propylene (R1270), difluoroethane (R152a), propane (R290), and fluoroethane (R161). A detailed exergy/exergoeconomic analysis, along with an optimization procedure, is performed at both the overall system level and the component-wise level in order to determine the key performance indicators. Minimizing the total cost product rate for each evaporator and condenser temperature is achieved by optimizing the condensing temperatures of the low-temperature cycle and the medium-temperature cycle. The component-wise analysis reveals that major thermodynamic irreversibilities occur in the HTC compressor and throttling valves, with maximum relative exergy destruction reaching 24.71% in TV-3 and the highest exergy destruction ratio reaching 14.92% in the HTC compressor. From an exergoeconomic perspective, the evaporator exhibits the largest combined exergy destruction and capital investment, and operational and maintenance cost rate (up to 24.73 $/h), while the condenser shows the highest exergoeconomic factor (up to 14.24%). The system-level results show that R1150/R170/R152a has the best exergetic and exergoeconomic performance compared to the other refrigeration combinations within the temperature ranges. Compared with R1150/R170/R290, this combination increases exergy efficiency by up to 5.05% for evaporator temperature variations and up to 7.84% for condenser temperature variations while reducing exergy destruction by up to 7.50% and 11.14%, respectively. Furthermore, the same combination decreases the total exergy destruction cost rate and product cost rate by up to 11.15% and 7.88%, respectively. In contrast, R1150/R170/R290 generally exhibits the poorest overall performance. The findings demonstrate that the refrigerant choice is crucial in achieving better exergetic and exergoeconomic performance for ULTs under various evaporator and condenser conditions.