Thermodynamic analysis of a modified two-stage transcritical CO2 refrigeration cycle with an ejector and a subcooler

Abstract

In the application scope of large-scale supermarkets, the practicality of transcritical CO₂ refrigeration device has been confirmed to be quite satisfying. A modified two-stage transcritical CO₂ refrigeration cycle with an ejector and a subcooler (MTC) is proposed in this paper. In the modified cycle, the ejector recovers expansion works and reduces irreversible losses in the throttling process. The subcooler provides subcooling degree for the CO₂ entering the low-temperature (LT) evaporator, thus increasing the refrigeration capacity and improving the COP of the modified cycle. Thermodynamic analysis has shown that the exergy efficiency $\left({\eta }_{ex}\right)$ and COP of MTC under given operating condition has been enhanced by 13.7 % and 14.2 % compared to BTC. The discharge temperature at the outlet of high-pressure compressor in MTC is decreased by 8.3℃. Under typical operating condition, the optimal discharge pressure of MTC is 9.07 MPa, which is lower than that of BTC. The correlation to calculate optimal discharge pressure for single-stage CO₂ refrigeration cycle is also suitable for MTC under given operating conditions. The MTC has also shown better performance under variable operating conditions. For MTC, when the temperature at the outlet of gas cooler increases from 35 – 45℃, the COP and ${\eta }_{ex}$ are enhanced by 14.1 % - 16.1 % and 12.8 % - 14.2 % compared to BTC, respectively. When gas cooler outlet pressure decreases from 11.0 to 7.5 MPa, the COP and ${\eta }_{ex}$ are enhanced by 14.0 % - 22.8 % and 13.4 % - 18.7 %. As the evaporating temperature at the cold side of subcooler increases from -25 to -15℃, the COP and ${\eta }_{ex}$ increase from 1.82 to 1.98 and 27.4 % to 29.7 %. With the ratio of refrigeration capacity ($R_{ec}$) between medium-temperature evaporator and low-temperature evaporator varies from 0.7 to 1.2, the COP and ${\eta }_{ex}$ are improved by 10.7 % - 16.3 % and 10.7 % - 15.4 % compared to BTC. There is the maximum exergy loss at gas cooler in the MTC, whereas that of BTC is located in the expansion valve before LT evaporator. The economic analysis shows the cost per unit of exergy of MTC is decreased by 11.5 % under typical operation condition. According to simulation results, the modified cycle has better performance in severe working conditions such as high gas cooler outlet temperature and low gas cooler outlet pressure in the given range of working conditions compared to BTC.