Comparative thermodynamic performance analysis of a cascade refrigeration system with new refrigerants paired with CO2

Abstract

This paper provides a theoretical comparative analysis for the thermodynamic performance of a cascade refrigeration system using different refrigerants paired with carbon dioxide (CO2). The cascade refrigeration system consists of a low-temperature loop and a high-temperature loop. The former loop uses CO2 as working fluid while the later loop uses four different refrigerants namely HFE7000, HFE7100, NH3 and R134a. HFE7000 and HFE7100 are new refrigerants called hydrofluroethers. A code is developed in engineering equation solver for the basic energy and exergy balance equations of the system. For the reference conditions; 223 K evaporator temperature and 308 K condenser temperature, the coefficient of performance was obtained as 1.398, 1.3363, 1.309, and 1.221 for refrigerant couples NH3/CO2, R134a/CO2, HFE7000/CO2, and HFE7100/CO2 respectively. The exergy efficiencies for the reference conditions were calculated as 47.01%, 45.84%, 44.30%, and 41.06% respectively for NH3, R134a, HFE7000, and HFE7100 refrigerants paired with CO2. The condenser and evaporator temperatures and heat exchanger effectiveness were varied to determine their effect on the cooling coefficient of performance and the exergy efficiency. The evaporator has been identified as the component having the highest rate of exergy destruction, whereas the expansion valve in the low temperature loop has the lowest rate of exergy destruction. The total equivalent warming impact (TEWI) values of the refrigerant pairs of the system are calculated and contrasted for the reference operating conditions for a cooling load of 175 kW. The HFE7000 is considered as a promising refrigerant according to the COP, exergy efficiency, and TEWI results for the proposed cascade refrigeration system, as it can be used as a substitute to R134a.