Theoretical analysis of a transcritical double-stage nitrous oxide refrigeration cycle with an internal heat exchanger

Thermodynamic analysis and optimization studies are performed on three transcritical nitrous oxide refrigeration cycles including a single-stage compression (SSC) cycle, a double-stage compression (DSC) cycle and a double-stage compression with an internal heat exchanger (DSCI) cycle. The thermodynamic performance is compared with that of similar cycles using CO2. The results show that the N2O SSC and DSC cycles exhibit a larger optimum COP and a lower discharge pressure compared with the CO2 cycles at given conditions. For the DSC cycle, there exists an optimum COP at certain gas cooler high pressure and intermediate pressure for both working fluids. The double-stage compression and the internal heat exchange have a significant effect on the optimum COP enhancement for N2O transcritical refrigeration cycles. Further, the introduction of the internal heat exchange can also reduce the optimum pressure for the N2O DSC cycle. The comparison between the three cycles shows that the N2O DSCI cycle has the highest optimum COP. Correlations of the optimum gas cooler high pressure and the intermediate pressure are proposed in terms of the gas cooler exit temperature and the evaporating temperature for the three cycles.