Evacuated tube solar thermal collector with enhanced phase change material thermal storage: An experimental study

•Pure PCM is not a proper storage solution for evacuated tube solar water heater.•A PCM-metal foam composite shows a far superior performance compared to pure PCM.•Presence of metal foam mitigates the temperature fluctuations of water flow.•Water flowrate has opposite effects on storage and release thermal energy. The present study experimentally investigates the performance of a conventional evacuated tube solar collector (ETSC) equipped with a small-scale storage unit. A thermally enhanced paraffin-based phase change material (PCM) is employed as the thermal storage medium. The thermal enhancement of the PCM is done by introducing copper foam into its structure (PCM-MF). The obtained experimental data suggest that pure PCM is very slow in storing the thermal energy generated by the ETSC and later it cannot be effective in supplying enough hot water at a desired rate using the thermal energy stored inside it at a desired rate. The PCM-MF composite on the other hand is 2–3 times faster than pure PCM in storing the thermal energy supplied by the collector. As for releasing the stored thermal energy, when PCM-MF composite is employed, the experimental setup can generate hot water with temperatures 10–77% higher than the input water for about 2 h2 hours in the absence of solar energy input. By testing three water flowrates of 1 l/min, 2 l/min, and 3 l/min, the effects of flowrate through the collector manifold and the heat storage unit are investigated as well. It was observed that the higher values enhance the heat storage inside the unit but in the meantime it has negative impact on the release of thermal energy from the storage unit to the water flow.