Study on the effect of ejector on the performance of low temperature transcritical CO2 two-stage compression refrigeration system

This paper presents a refrigeration experimental platform that can switch between a two-stage transcritical CO₂ compression system with and without an ejector. Through theoretical simulation and experimental methods, the effects of ejector on system performance were investigated under the conditions of indoor temperatures ranging from -34∼-20 °C, gas cooler pressures ranging from 8.2∼9.7 MPa, and low-pressure electronic expansion valve (EEV) openings ranging from 50 %∼100 %. The study reveals that the primary reason for the discrepancy between theoretical simulation and experimental results is that the theoretical model of compressor volumetric efficiency is insensitive to changes in low-pressure compression ratio, while the actual piston compressor is highly sensitive to such changes. The results also demonstrate that the application of ejector significantly enhances system’s refrigeration capacity and coefficient of performance (COP), as well as reduces energy consumption and carbon emissions. The variation of low-pressure EEV opening has a more significant impact on the ejector’s enhancement of refrigeration capacity and COP than indoor air temperature and gas cooler pressure. The ability of ejector to reduce system power consumption is relatively stable under different conditions. Further analysis indicates that the entrainment capability and efficiency of the ejector, as well as the volumetric and isentropic efficiencies of low-pressure compressor, are most sensitive to changes in low-pressure EEV opening. The adaptive regulation of ejector can maintain the stability of system’s refrigeration capacity per unit mass and volume. Only by adjusting low-pressure EEV opening to the optimal level can the ejector fully replace the function of high-pressure EEV.