Advantages of pressure recovery in the regeneration nearly-isothermal compression CO2 refrigeration Cycle: A theoretical study

The cooling performance remains the primary bottleneck in CO2 refrigeration cycles. Oil injection has been proposed as an effective method to achieve near-isothermal compression and enhance the efficiency of regenerative CO2 systems. However, the optimal amount of injected oil and the appropriate discharge pressure for transcritical CO2 cycles remain unclear. This paper develops a theoretical model of the regeneration nearly-isothermal compression (RNIC) CO2 cycle equipped with an oil pressure energy recovery device. A thermodynamic analysis is conducted to evaluate key system characteristics, including the coefficient of performance, discharge temperature, specific work of refrigerant and oil, and discharge density. Optimization of discharge pressure and oil injection ratio is performed to reveal the influence of operating temperature on these parameters. Results show that the maximum COP improvement, compared with the conventional regeneration cycle, can reach 51 % when oil injection is applied. The discharge temperature can be significantly reduced from 125 °C to 50 °C due to the cooling effect of injected cold oil. However, the discharge density of CO2 exceeds 600 kg/m3, and the volumetric ratio is much higher than that of the conventional cycle. The RNIC cycle therefore introduces new technological challenges, including efficient oil separation, geometric design of two-phase CO2 compressors, and the development of high-performance pressure energy recovery equipment.