Off-grid solar thermal water heating system using phase-change materials: design, integration and real environment investigation

A solar thermal water heating system using a custom-built latent heat storage tank with paraffin wax, puretemp68 and stearic acid/palmitic acid eutectic mixture based phase-change materials was designed, developed and its performance evaluated in real-time. The fully autonomous off-grid solar thermal water heating system was packaged by integrating solar thermal collector, phase change material tank, photovoltaic modules, operational controller, circulation pumps and battery, along with a data logger. A water-glycol with anti-freeze characteristics with efficient energy storage in cold weather was employed as heat transfer fluid for solar thermal energy harvesting for charging and discharging of phase change materials to heat the water tank. Solar charge-discharge and water charging results with phase change materials during May to October 2018 (Mesa, AZ 85212, USA) showed that the stearic acid/palmitic acid eutectic mixture outperformed the paraffin wax and exhibited comparable performance with puretemp68 storage system. The efficiency of water charging from stearic acid/palmitic acid (55%) is higher compared to that from paraffin wax (20%) and better than puretemp68 (45%). Even though, the paraffin wax melts quickly during charging, it exhibited super-cooling behavior during discharging, leading to incomplete discharge. The storage energy density of stearic acid/palmitic acid (similar to 0.48 MJ kg(-1)) is comparable to puretemp68 with uniform thermal energy distribution inside the storage container due to higher thermal conductivity and lower melting point. The solar thermal water heating system demonstration proved that the domestic hot water system using stearic acid/palmitic acid could be a practical solution for effectively harvesting solar thermal energy for rural areas.