Energy and Exergy Analyses of Different Transcritical CO2 Refrigeration Cycles

Carbon dioxide has received increasing attention owing to its zero ODP and negligible GWP. Furthermore, carbon dioxide also has desirable thermodynamic properties, such as large specific heat, low viscosity, and large heat conductivity. Carbon dioxide CO 2 has low critical pressure and temperature which are 7.36 MPa and 31.1°C, respectively. The low critical temperature causes the heat rejection process to occur above the critical point and heat absorption process to happen below the critical point. However, due to the high throttling loss, the energy efficiency of the basic transcritical CO 2 cycle is lower than that of the conventional low pressure refrigeration cycle. In the present study three different kinds of transcritical carbon dioxide cycles that are the transcritical cycle with expansion valve (called also the conventional transcritical cycle), the transcritical cycle with expander and the transcritical cycle with ejector are analyzed. The effect of operating parameters on the maximum performance and exergy efficiency of the three cycles is investigated. Results reveal that replacing the expansion valve by an expander or an ejector does not only improve the maximum COP and the exregy efficiency but also reduces the optimal heat rejection pressure.