Experimental investigations on the performance of a rectangular thermal energy storage unit for poor solar thermal heating

A novel rectangular energy storage unit (RTESU) is proposed to effectively harvest low-radiant solar energy in poor solar areas for the application of preheating cold outdoor air. The RTESU consists of tube bundles to charge phase change material (PCM), paraffin wax PCM panels and air channels to discharge heat to cold outdoor air. Experiments on the charging and discharging processes with heat transfer fluid (HTF) temperature (varying from 28 to 36℃) and flow rate (varying from 0.96 to 2.04 m3/h) are performed to investigate the thermal performance of the device for poor solar areas. PCM temperature variations, heat transfer characteristics, heating capacity, and melting and solidification dynamics are analyzed. The results reveal that solar energy can be harvested and stored in PCM through melting process even under extremely poor solar radiation (about 138.68 W/m2) within 3.6 h. During the charging process, the natural convection plays a dominant role, while conduction is dominating throughout the discharging process. Further analysis of the thermal performance of discharging process shows that the heat extraction effectiveness can be improved as the air inlet temperature decreases. The highest heat extraction effectiveness can approach 100% in 10.2 h when the air inlet temperature ranges from 11.5 to 13 °C.