The effectiveness of vapor-injection for an inverter air-source heat pump compared with vapor-return

The vapor-injection assisted with air-source heat pump (ASHP) has become one of the focuses in the last decade. It can improve the heating performance of the ASHP system in low ambient temperatures by experiments. However, the vapor-injection is ineffective in moderate ambient temperatures owing to the high pressure of the compressor intermediate chamber. On the other side, the effectiveness of vapor-injection is rarely discussed. Based on experiments and the thermodynamic analysis method, this paper proposes an inverter air-source heat pump (IASHP) with vapor-injection and vapor-return. With the scope from −20 °C to7.0 °C for ambient temperature, it focuses on the heating performance of the prototype with different compressor frequencies for both vapor-injection and vapor-return modes. Moreover, the effectiveness of vapor-injection is discussed. The results show that the vapor-injection is superior to vapor-return for improving the heating performance of the prototype. Not only does the vapor-injection significantly affect refrigerant mass flow rate, but it also makes the relative entropy reduction (RER) of the compressor outlet decrease by 0.025–0.03 kJ/(kg·K). In the case of ambient temperature at −4.0 °C, the vapor-injection improves by 6.6 % and 3.4 % for heating capacity and COP of the prototype. Besides, the effectiveness of vapor-injection lies in the RER of the compressor outlet, which is suitable for vapor-injection using different refrigerants.