An efficient optimization and comparative analysis of cascade refrigeration system using NH₃ /CO₂ and C₃H₈/CO₂ refrigerant pairs

A cascade refrigeration system operating with CO₂ in the low temperature circuit and NH3 as well as C₃H₈ in the high temperature circuit are investigated for the thermo-economic optimization. Optimization results are used for the comparative analysis of both the refrigerant pairs (NH₃/CO₂ vs. C₃H₈/CO₂). Optimization problem is formulated to minimize the total annual cost and exergy destruction of the system, and solved using a heat transfer search algorithm. Four operating parameters such as evaporator temperature, condenser temperature, condensing temperature of the low temperature circuit, and cascade temperature difference are investigated for the optimization. An application example of cascade refrigeration system is presented. Results are obtained in the form of Pareto-optimal points. Comparative results reveal that C₃H₈/CO₂ pair offered 5.33% lower cost and 6.42% higher exergy destruction compared to NH₃/CO₂ pair. The effect of design variables and its sensitivity to the performance of cascade system are also presented and discussed.