Thermodynamic investigation of a direct-expansion solar assisted heat pump with evacuated tube collector-evaporator

Abstract

Direct-expansion solar-assisted heat pump (DX-SAHP) system has proven to be an effective energy-saving application. In view of the low output temperature and large radiant heat loss in the conventional flat plate type DX-SAHP, this paper proposed an evacuated tube type DX-SAHP system to realize performance enhancement. Then, an experimental prototype of the proposed system was originally designed and constructed in laboratory. Moreover, the thermodynamic performance of the system was thoroughly investigated and analyzed under different operating conditions. The results indicate that the thermodynamic performance of the proposed system is sensitive to the solar radiation intensity and collector area. About every 100 W m−2 increase of solar radiation intensity, the COP increases nearly 1.70% and the heat collection efficiency decreases nearly 8.44%. Increasing collector area (from 1.5 m2 to 3 m2) results in 8.81% increase of COP and 22.47% reduction of the system exergy efficiency. A higher circulating water temperature can effectively improve its exergy performance and higher ambient temperature can enhance its energetic performance. Additionally, the optimization of the evacuated tube collector-evaporator should be preferential to improve the system due to large exergy destruction and low exergy efficiency.