Study on expansion power recovery in CO2 trans-critical cycle

Due to the ozone depletion potential and global warming potential of CFCs and HCFCs, CO2 is considered as most potential alternative refrigerant. However, there are serious throttle losses and low system efficiency to CO2 trans-critical cycle because of its low critical temperature and high operating pressure. The aim of this paper is to design an expander to recover expansion power in CO2 trans-critical cycle. The theoretical analysis and calculation show that 14-23% of input power of compressor can be recovered. A prototype of rolling piston expander is designed and manufactured and its test facility is established. The test facility consists of CO2 trans-critical cycle, the expander, the chilling water system and the cooling water system. The experimental results show that the recovery ratio and expander efficiency are affected by rotational speed, inlet temperature and mass flow of expander. The highest recovery ratio can reach to 0.145, which means 14.5% of input power of compressor can be recovered. The expander efficiency can reach to 45%.