Optimization and thermal performance of U-type evacuated tube solar collector filled with phase change material

U-type evacuated tube collector (ETC) is one of the most popular types of solar collectors for solar thermal applications. However, ETCs suffer from drawbacks caused by the intermittency of solar radiation. The addition of phase change materials (PCMs) inside ETC can reduce the energy fluctuations by storing the excess thermal energy during the daytime and releasing it to meet the supply deficit in the absence of sunlight. Therefore, a numerical model was conducted to assess the thermal performance of a PCM-filled U-type ETC with fins and experimentally validated. The design of different energy storage solutions, including the melting point of the PCM, the flow rate of heat transfer fluid (HTF) and the diameter of the inner glass tube were optimized. A two-fold beneficial effect of extending the hot water supply period and a reduction of the peak outlet temperature of HTF is achieved by filling PCM inside U-type ETC with fins. When the melting temperature of the PCM is 323 K, the reduction of the HTF peak outlet temperature is 7.4 K and the liquid fraction of the PCM approaches to a relatively high value, while the thermal efficiency and the thermal storage efficiency of the ETC are 50.72% and 19.20%, respectively. Meanwhile, the hot water supply time over 308 K extends by 160 min, compared to the case without PCM. It is also found that a high rate of the HTF inlet flow can shorten the hot water supply time with a smaller liquid fraction of the PCM. Furthermore, enlarging the glass tube leads to a higher thermal efficiency and thermal storage efficiency. This work shows that filling PCM with appropriate designs in the ETC with fins can promote the performance of ETC .