Experimental study on performance of the trans-critical CO2 heat pump with flash tank vapor injection at variable revolution ratio conditions

The trans-critical CO2 heat pump has a wider application prospect since its advantages of more friendly envi-ronment, higher heating temperature and more compact equipment volume. In this study, performance of a trans-critical CO2 heat pump with flash tank vapor injection technique was experimentally studied. The effect laws of revolution ratio and expansion valve opening on the operating parameters were acquired to guide the design of an effective control strategy. Then influences of the discharge pressure, intermediate pressure and injection ratio on heating capacity, COP and heating temperature were analyzed at varied revolution ratio conditions. The results show that compared with revolution ratio = 1, varied revolution ratio can improve the COP by 7%, and decrease discharge pressure by 0.85 MPa. When revolution ratio <1, near revolution ratio approximate to 0.89, the COP reaches a peak value of 1.98. Compared with the simple two-stage compression cycle, im-provements of the heating temperature, heating capacity and COP of the flash tank vapor injection cycle are 16.5%, 26.9% and 8.4%, respectively.