Analysis of a hybrid transcritical CO2 vapor compression and vapor ejector refrigeration system

Abstract

The paper presents analysis of a hybrid transcritical CO2 (T-CO2) refrigeration cycle coupled with a heat-driven vapor ejector refrigeration cycle. The heat generated by compression of CO2 in the transcritical vapor compression refrigeration system (VCRS) is utilized to drive the secondary vapor ejector refrigeration system (VERS). The performance of the hybrid system is evaluated by maintaining identical evaporator temperatures for both T-CO2 VCRS and VERS. Cooling capacity and COP of the VERS is influenced by heat recovered from T-CO2 system, thus, limiting the choice of working fluids for the VERS. Analytical model based on conservation of mass, momentum and energy, incorporating real gas behavior is used for arriving at an optimum ejector geometry used in the VERS. Amongst the five fluids analyzed, R32 is found to be most suitable for the VERS used in the hybrid system. R32 provides the highest coefficient of performance (COP) and cooling capacity by recovering maximum heat from the generator, while, other fluids yield nearly identical performance. The proposed hybrid system using R32 provides significant improvement in system performance ranging from 10 to 50% in cooling capacity over the baseline T-CO2 system for the range of evaporating temperatures from −2.5 °C to 12.5 °C.