Thermodynamic and environment analysis of a modified transcritical CO2 refrigeration cycle integrated with ejector and subcooler

Abstract

The traditional transcritical CO₂ two-stage compression refrigeration cycle (TTRC) has a great advantage in supermarket refrigeration applications. Currently, however, the performance of the TTRC still has the potential to be improved. In this paper, an ejector-enhanced transcritical two-stage compression CO₂ cycle is presented for supermarket refrigeration application. Based on the basic TTRC with a subcooler, a flash tank, an ejector and are introduced. On the one hand, part of the expansion work can be recovered by the ejector. Moreover, the subcooler is employed to increase the subcooling degree of refrigerant entering the expansion valve, which could increase the evaporator's cooling capacity. The cycle performances of the cycles are theoretically studied by energy, exergy and carbon footprint evaluation. Meanwhile, the intermediate pressures of the two cycles are also optimized. Under optimum intermediate pressure, the energy analyses show that the coefficient of performance is improved by 9.6–11.0% and the volume cooling capacity is enhanced by 14.5%-18.4% with the modified cycle. Moreover, the exergy analysis indicates that the expansion valves account for 27.1% of the exergy destruction of the basic cycle, while it is just 7.51% for the modified cycle. The carbon footprint analysis shows that the modified system with CO₂ refrigerant could reduce carbon emissions by 17.95% compared to the conventional refrigerant R404A. It shows the feasibility of using CO₂ to replace R404A refrigerant in commercial supermarket refrigeration, and has significant eco-friendly benefits.