Optimization of a cascade refrigeration system using refrigerant C₃H₈ in high temperature circuits (HTC) and a mixture of C₂H₆/CO₂ in low temperature circuits (LTC)

This paper discusses the multi-objectives optimization of a cascade refrigeration system using refrigerant C₃H₈ in high temperature circuits (HTC) and a mixture of C₂H₆/CO₂ in low temperature circuits (LTC). The evaporator temperature, condenser temperature, C₂H₆/CO₂ mixture condensation temperature, cascade temperature differences, and the CO₂ mass fraction are chosen as the decision variables. Whereas cooling capacity, cold space temperature, and ambient temperature are taken as the constraints. The purpose of this research is to design a cascade refrigeration system whose optimum performance are defined in terms of economics and thermodynamics. Accordingly, there are two objective functions that should be simultaneously optimized including the total annual cost which consists of the capital and operational cost and the total exergy destruction of the system. To this aim, the optimum operating temperature of the system and CO₂ fraction should be determined so that the system has minimum exergy destruction and annual cost. Results show that, the optimum value of the decision variables for this system can be determined by trade-off between annual cost and exergy destruction.