The effect of solar thermal energy storage on natural gas heating systems: An experimental and techno-economic investigation

This study investigates the storage of solar thermal energy using thermal oils in Kilis, Türkiye, a region characterized by high solar potential. Both experimental and modeling studies were conducted. Mobiltherm 605 and PO Heat Transfer Oil 32 were tested as alternatives to commonly used oils. PO Heat Transfer Oil 32 was introduced to the literature for the first time. The impact of operating temperature, water flow rate, and outdoor temperature on the efficiency of stored thermal energy in the water heating system was examined. The findings revealed that system efficiency averaged 42 % at a nighttime outdoor temperature of 9 °C but decreased to 36 % at 13.5 °C. Efficiency was also 36 % at a flow rate of 0.7 m/s, decreasing to 24 % at 0.24 m/s. Mobiltherm 605 cooled down later than PO 32 in the thermal tank, indicating a higher heat transfer coefficient for PO 32. Based on experimental data, a model of the thermal storage system was developed, and its techno-economic feasibility was assessed. Using this model, a techno-economic analysis was performed and the potential for this clean energy to reduce reliance on natural gas combi boilers for heating was discussed. For a building with a maximum indoor-outdoor temperature difference of 20 °C and a heating requirement of 467 kWh over 14 h, the thermal energy cost was estimated to be $0.029/kWh. This implementation reduced carbon emissions by 130 kg/day. The total equipment cost for the thermal storage system, with a U-type Vacuum Tube Collector (U-VTC) area of approximately 380 m², was $295,697. In conclusion, both Mobiltherm 605 and PO Heat Transfer Oil 32 demonstrated comparable price/performance ratios relative to common oils.