Multi‑objective Optimization of Thermodynamic and Economic Performances of Natural Refrigerants for Cascade Refrigeration

摘要

In this paper, thermo-economic optimization and comparative analysis of a cascade refrigeration system configured with flashgas removal in its high-temperature cycle (HTC) and flash intercooling with indirect subcooling in lower temperature cycle(LTC) using different natural refrigerant pairs is performed. Thermo-economic optimization is carried out to maximize theexergetic efficiency and minimize the overall cost rate. The optimization model involves six design variables which includesubcooling and de-superheating parameters, LTC evaporation and condensation temperatures, HTC condenser temperatureand cascade temperature difference. The comparative analysis of twenty-two natural refrigerant pairs based on the resultsof thermodynamic and economic optimizations reveals that R717-R290 is most efficient pair and R290-R1150 is least efficientrefrigerant pair thermodynamically whereas R717-R1270 is the best and R600a-R290 is the worst pair economically.Seven potential refrigerant pairs are chosen via the thermodynamic and economic optimization results and they are furthercompared based on their performances obtained through multi-objective optimization (maximization of exergetic efficiencyand minimization of total cost rate). Multi-objective genetic algorithm is used for optimization which results in seventy nondominatedPareto optimal solutions where the TOPSIS method is used to select a unique solution for each refrigerant pair.A comparison of refrigerant pairs using these unique solutions shows that R717-R1270 is the best refrigerant pair for thecascade system under consideration. It is also found that R717-R1270 results in 7.77% rise in COP and 5.32% reduction inoverall cost when compared with NH_3–CO_2 refrigerant pair working under identical operating conditions.