Thermodynamic analysis of a transcritical CO2/propylene cascade system with split unit in HT cycle

A natural refrigerant-based transcritical cascaded system used for simultaneous cooling and heating has been analyzed. Propylene has been employed in low temperature cycle (bottoming cycle) and carbon dioxide in the high temperature cycle (topping cycle). Being a low critical temperature fluid, CO2 transcritical cascade systems offer low energy utilization factor (EUF) for a given application. The topping cycle of the system is provided with a split unit and the bottoming cycle is a subcritical cycle. EUF improvement of 8 % is observed by the use of split unit. The cascaded refrigeration-heat pump system is numerically optimized for overall EUF, intermediate temperature, and mass flow ratio with the help of multilinear regression analysis. The developed model is subsequently used to examine the influences of the main parameters on the system configurations under the maximum EUF condition. The effects of various operating variables on the optimum design quantities are discussed. Results of thermal performance analysis for CO2-propylene cascaded cycle is compared to that of conventional refrigerants.