The effect of annular phase-change material layer on the thermal behavior of solar collector tube

Abstract

The temperature of the Heat Transfer Fluid (HTF) in the solar collector tube is greatly affected by the instantaneous solar radiation and which usually causes the HTF temperature large fluctuations. To improve this performance, a novel solar collector tube integrated by annular phase-change material (PCM) layer and solar collector tube was proposed in this study, and the thermal behavior of the tube with 630 mm in length and 20/70 mm in diameter inside and outside has been studied. Solar Salt (60 wt% NaNO3 and 40 wt% KNO3) was applied as the PCM. Air was selected as HTF. The thermal characteristics of the novel and traditional solar collector tubes were experimentally and numerically investigated under different solar radiations and air flow rates. The results show that the temperature in the solar collector tube with PCM were 65.45–128.75 °C higher than that without PCM after cooling for 4 h in different solar radiations. The temperature in the solar collector tube decreased as the air flow rate increased. The average temperature falling rate with PCM were 35.6–76.4 °C/m·s−1 slower than that without PCM during the heat storage process, the solar collector tube with PCM shows more advantages at high flow rates. Within the measuring time of 300 min, the total thermal efficiency of the collector tube with PCM layer is higher than that of without PCM layer. The solar collector tube with PCM is better suited to create a stable mid-temperature and high-temperature environment in continuous solar radiation and heat extraction.