Optimization of Two-Stage Cascade Refrigeration Systems Using Hfo Refrigerants in The High-Temperature Circuit and Hfc-134a in Low-Temperature Circuit

Abstract

The eco-friendly refrigeration technologies which are receiving more and more attention in the days by day of for solving energy and environmental problems. In this paper, it is suggested to phase out presently most used refrigerant R-134a considering global warming and to use natural refrigerants such as ammonia, carbon dioxide and hydro carbons in two stage cascade vapour compression refrigeration system for sustainable environment. Thermal model was developed for single stage and also two stage cascade refrigeration systems using HFO-1234yf for low temperature applications in the range of 100°C to -51.5°C and HFO-1234ze for high temperature applications in the range of 100°C to -30.5°C. Numerical computation was carried out by using R1234ze in the temperature range of 60°Cto -30°Cin the high temperature circuit and R1234yf in the range of 20°Cto -50°Cfor low temperature range. The developed model can also predicts the thermal performances of using nine eco-friendly refrigerants in low temperature circuit and R1234ze n low temperature applications. The ecofriendly refrigerants HFO can replace R134a without minor modifications. However up to temperature -50°Cin the low temperature evaporator circuit with temperature overlapping (Approach=10oC) and condenser temperature of 55°Cwith cascade evaporator temperature of 0°C , the combination of R1234ze in high temperature circuit and R1234yf in low temperature circuit gives best results for replacing R134a and for -100°Cof low temperature evaporator circuit and 55°Cof high temperature condenser and -30°C cascade evaporator with temperature overlapping (Approach=10oC) the combination of R1234ze – and R245fa gives better thermodynamic performances as compared to R-1234yf and R245fa. The optimum values for system-6 (using R1234ze in high temperature circuit and HFC-134a in low temperature circuit , the optimum second law efficiency (exergetic efficiency) obtained at low temperature evaporator is at -78°Cto -80°Crespectively and no optimum was found using HFO-1234yf in high temperature circuit and HFC-134a in low temperature circuit.