An experimental evaluation of direct flow evacuated tube solar collector integrated with phase change material

The current study presents an experimental analysis for integrating a phase change material (PCM) in a typical direct flow evacuated tube solar water heater with a U-tube heat exchanger (HX). Each evacuated tube is filled with 0.8 kg of paraffin wax to store the absorbed incident solar energy. As water flows through the U-shape copper tube inside the PCM, the stored energy is transferred to the water through a combination of conduction and convection. The proposed system is investigated under two configurations; un-finned and finned HX to investigate the effectiveness of adding the fin. Outdoor experiments are carried out to demonstrate the thermal performance of the purposed systems under various scenarios including the charging phase and the overnight heat loss as proposed by Chinese National Standard CNS 7277-12558 [1]. Also, the thermal performance during the discharging phase is evaluated under various loads based on the experimental design approach. The results show that the natural convection is the main heat transfer mechanism during the charging phase, with a higher system efficiency for the un-finned collector by about 14% due to the high average temperature of the PCM. During the discharging phase, the presence of the fin helps to overcome the poor thermal conductivity of the solidified layer of the PCM by offering another path for the energy transfer from the PCM to the water, and subsequently enhance the total effective energy discharged.