Thermal modeling of air-type double-pass solar collector with PCM-rod embedded in vacuum tube

Highlights: • An air ETSC-PCM was developed and exploited. • A thermal model of the collector was established to predict the outlet temperature. • The equivalent thermal efficiency of the phase change material is also proposed. • The integrated PCM-rod solves the problem of intermittent and instability in solar thermal utilization. • The heat loss of the collector was evaluated. Based on an evacuated tube collector, we designed an air–type double–pass solar collector with a PCM–rod embedded in a vacuum tube. In this study, the basic parameters of the proposed collector are discussed comprehensively and the spatial layout of each component is illustrated. Simultaneously, a thermal model of the collector has been established to predict the outlet temperature of the collector. The equivalent thermal efficiency of the phase change material is also proposed to explain the change in the thermal efficiency of the collector caused by the PCM. Finally, the thermal performance of the proposed collector was tested and compared with its theoretical value. The results show that the comparison between the simulated and experimentally measured outlet temperatures is in good agreement; the root mean square error and mean bias difference on the outlet temperature are 1.56 K and 1.33 K, respectively. Additionally, even if the irradiation intensity reduces by 39% within 30 min, the outlet temperature decreases by only 7.5 K. The thermal model is also used to analyze the influence of airflow, radiation intensity, ambient temperature and surrounding air speed on the outlet temperature. It was observed that the first three have a greater impact, but the temperature fluctuation is only 0.36% when ambient wind speed is 0.2–2 m/s. In contrast, the heat loss of the collector was evaluated and it was found that the heat loss of the vacuum tube accounted for 0.4%–2.8% of the heat energy of the collector, while the heat loss of the manifold to the ambient accounted for only 0.04%. The phase change material stored 0.247 MJ of heat, of which 0.245 MJ was provided to air, and the conversion rate of heat absorption and release was 99.2%, and the working time of the collector is extended by 8 h. In terms of thermal efficiency, the instantaneous thermal efficiency continued to increase from 13% to 397%, and the equivalent thermal efficiency of the phase change material reached a maximum of 484%.