Exergetic Performance Assessment of a Two–Stage Compression Transcritical CO2 Refrigeration Cycle

A two-stage compression transcritical refrigeration cycle (TCRC) with an internal heat exchanger using R744 (CO2) as a refrigerant is theoretically investigated in this study. The cycle consists of two compressors, a mixing chamber, a gas cooler, an internal heat exchanger (IHX), two expansion valves, a flash chamber (separator), and an evaporator. In the TCRC with a cooling capacity of 20 kW at an evaporation temperature of −10 °C, the superheating value was taken at 8 °C at the evaporator outlet by aid of the IHX. The aim of this study is to evaluate the energetic and exergetic performances of the TCRC for different gas cooler pressures and gas cooler exit temperatures. For this purpose, exergy destructions, relative irreversibilities, and exergy efficiencies were determined for all components and the whole system. The second-law efficiency of this cycle was compared with that of the conventional transcritical CO2 refrigeration cycle (TRC). The optimum gas cooler exit temperatures for 8, 9, 10, 11, and 12 MPa pressures were recorded as approximately 34, 39, 46, 54, and 59 °C, respectively.