Experimental thermal performance investigation of air-type solar collector with PCM-rod embedded in vacuum tube

Based on an evacuated tube collector, this study designs a direct flow channel, a spiral tube structure, and a PCM-rod, which are combined to form an air-type direct flow evacuated tube collector integrated with a phase change material. In this study, the thermal performance of the proposed collector is tested and a new method of calculating the instantaneous thermal efficiency (η) by the equivalent thermal efficiency of the PCM (ηpcm) was proposed. Meanwhile, the heat storage effect of the PCM-rod and the influence of series or parallel connections and air supply mode on the thermal efficiency are analysed. The results demonstrate that the PCM-rod causes a more gradual temperature change in the collector, and the maximum temperature difference between the inlet and outlet of the collector reached 129.1 °C. When the radiation intensity is weak at 18:00, the outlet temperature of the collector can still reach 70.2 °C, which is 21.8 °C higher than that of the collector without a PCM-rod. At the same time, the use of PCM-rods can effectively improve the η of the collector and exhibit more gradual changes in η, and the maximum η is 285 %. In addition, the ηpcm can reach 225 %, which transfers heat efficiently and plays the role of peak cutting and valley filling. The η of the series-connected collector is lower than that of the parallel-connected collector during the day, but the opposite is true when the radiation intensity is weak. On the other hand, we should choose the upper air supply for the collectors connected in series, and then the lower air supply is more suitable when connected in parallel.