On the thermal performance of evacuated tube solar collector integrated with phase change material

The aim of present work is to analyze a U-type of evacuated tube solar collector in which phase change material (PCM) is employed to store surplus solar energy in daytime and reuse it at nighttime when the solar irradiation vanishes. Both charging and discharging processes are considered and modeled. Two critical points should be considered in designing the proposed collector; the stable requirement outlet temperature of the domestic solar collector during daytime (about 40 degrees C) and assurance of a complete melting/solidification process to reach the maximum energy storage and release. Since the absorbed energy from solar radiation by heat transfer fluid and PCM in both day and nighttime strongly depends on the U-type tube's diameter, three values of which (namely, 6, 8, and 10 mm) are selected to attain the best case. The results show that the case with 6 mm diameter yields a 25% and 13.5% improvement in the liquid fraction and fluid outlet temperature, respectively, at 3:00 PM in the charging process, compared to other cases. Also, this case has achieved a 20% delay in the thermal energy release process and has increased the fluid outlet temperature by 24%, compared to other cases, at 9:00 PM in the discharging process.