Thermodynamic analysis on a two-stage transcritical CO₂ heat pump cycle with double ejectors

In this study, two ejectors are proposed as expansion devices for a two-stage transcritical CO₂ heat pump cycle to enhance the cycle performance. The two ejectors are arranged at the low- and high-pressure stages, respectively, to recover more available expansion work, and significantly reduce the throttling loss at each stage. The performance of the improved two-stage cycle is evaluated by using the developed mathematical model, and then compared with those of the basic two-stage cycle with a flash tank. The simulation results show that the improved two-stage cycle exhibits higher heating COP and volumetric heating capacity compared to the basic two-stage cycle. By further incorporating an internal heat exchanger, the heating COP can be increased by 10.5-30.6% above that of the baseline cycle when the subcooling degree varied from 0 to 15 °C under given operation conditions of -15 °C evaporating temperature, 10 MPa gas cooler pressure and 35 °C outlet temperature. Additionally, the effects of the gas cooler pressure and intermediate pressure on the maximal heating COP are also discussed.