Effect of subcooling using expander-compressor unit in transcritical CO2 systems

Various studies have shown that the subcooling of the refrigerant after the gas cooler enhances the performance in transcritical CO2 refrigeration systems. The integrated and dedicated mechanical subcooling approaches have been proved to be effective and energy efficient. In the present study, the former approach using an expander-compressor is analysed. The work done by the expander is used to operate the compressor compressing a part of the gas back to the gas cooler pressure while subcooling the main gas stream. Three different configurations, S1, S2 and S3 are studied for various gas cooler exit temperatures ranging from 30 to 50 degrees C and are compared with the simple transcritical CO2 system. System S1 shows the highest subcooling compared to the other two systems. It achieves enhancements in cooling capacity and COP up to 36 and 33%, respectively, compared to the simple system.