Energy-exergy-entropy coupling analysis of an air source heat pump with two typical vapor-injection configurations

The vapor-injection (VI) as an effective technology can improve heating performance of air-source heat pump (ASHP) at low ambient temperature. It is necessary to evaluate the efficiency of energy utilization that is related to VI configurations. This study employs the energy-exergy-entropy coupling analysis to find the correlation and difference, establishing the mathematical model of two-stage compressor air source heat pump with a flash tank (TCAHP-FT) and two-stage compressor air source heat pump with an intermediate heat exchanger (TCAHP-IHX). The results indicate that the TCAHP-FT unit exhibits a 4.9%–10.9% and 2.0%- 3.0% improvement over the TCAHP-IHX unit for heating capacity and COP respectively at the ambient temperature ranged from -4°C to -20°C. The good heating performance has a higher exergetic efficiency. However, the total entropy generation of the TCAHP-FT unit exhibits an 8.2%-19.3% higher than that of the TCAHP-IHX unit. It shows that the better heating performance with VI does not mean the lower total entropy generation for the unit. By the entropy analysis, VI reduces the entropy reduction of the compressor outlet rather than entropy generation to improve heating performance. Moreover, as the isentropic efficiency is between 0.8 and 0.95, this paper states the superiority of entropy reduction of the compressor outlet to evaluate VI effectiveness. Regarding the TACHP-IHX unit, the VI effectiveness is related to the sub-cooling degree (SCD) before throttle on the main loop that affects VI flow rate. When the SCD before throttle on the main loop rises by 5 K for TCAHP-IHX unit, the entropy reduction of compressor outlet increases by 0.0017 kJ/(kg·K), and the heating capacity and COP improve by 4.0% and 1%, respectively.