Thermodynamic analysis of a pressure exchanger and ejector enhanced transcritical CO2 refrigeration cycle

The natural refrigerant CO₂ has been used in refrigeration, heat pumps, and automotive air conditioning due to its environmental friendliness. However, significant throttling loss limits the performance of transcritical CO₂ refrigeration cycles. So, energy recovery is significant in improving performance of system. To achieve the function of energy recovery, this paper presents a novel energy recovery device named pressure exchanger (PX). PX can recover energy by the direct contact between a high-pressure and low-pressure fluids and present higher expansion work recovery efficiency compared with ejector. To explore its energy-saving potential in CO₂ refrigeration systems, this paper proposes a modified transcritical CO₂ refrigeration cycle with PX. The theoretical model based on first and second laws of thermodynamics are established to discuss the effects of PX efficiency (expansion and compression), gas cooler pressure and outlet temperature, and evaporation temperature on the cycle performances. The particle swarm optimization algorithm is employed to determine the optimal discharge pressure and intermediate pressure. The results show that the modified cycle outperforms the basic vapor injection cycle with subcooler. Compared to the basic cycle, the COP of the modified cycle is improved by 10.5 %∼18.8 %, and the compressor discharge volume is on average reduced by about 39 %. The performance improvement of the modified cycle is more significant at higher evaporation temperatures and higher gas cooler outlet temperatures. Therefore, using PX to improve the performance of CO₂ refrigeration systems shows significant application potential.