All-weather characteristic studies of a direct expansion solar integrated air source heat pump system based on PCMs

Solar integrated air source heat pump possesses competitive advantages of improving heating performance and capacity, particularly in cold region. However, the instability of solar energy is an obstacle for ensuring reliable operation of heat pump system. Selecting CaCl₂·6H₂O/Expanded Graphite (EG) composite phase change materials (PCMs) as the heating source of heat pump, improving it's evaporation temperature, is an effective approach ensuring high-performance operation of the system in cold region. In this paper, a solar regenerator filled with PCMs and array heat pipe was developed, it is a compact device and behaves superior thermal efficiency. The all-weather characteristics of a direct expansion solar integrated air compound heat pump was studied. Based on different solar irradiation, the system operates in four patterns by combining or switching different evaporators: air source heat pump (ASHP); solar energy heat pump (SEHP); composite heat pump with combined (CCHP) or switched (CSHP) evaporators. The investigation results indicate that the performance of the other three patterns were improved variously compared to ASHP. The average COPs and exergy efficiency of SEHP were improved 70% and 67% respectively, a reduction of the time for heating water with temperature from 15 °C to 55 °C of about 50% was obtained. The economy of different heating schemes were investigated. The conclusions show that although the initial cost of the composite heat pump heating (CHPH) is high, CHPH effectively improve environmental protection and has short dynamic investment payback period, indicating that it is a feasible and alternative heating technology for cold and sunny region.