Seasonal storage for space heating using solar DHW surplus

Due to the seasonality of solar energy, achieving 100 % of annual solar fraction for domestic hot water (DHW) production is only possible by greatly oversizing the collector area of a solar system, thus creating a significant energy surplus in summer. This simulation study investigates the possibility of using this surplus to promote space heating during winter, in a moderate South European climate, to try achieving a total solar fraction of 100 %. Priority is given to the DHW reservoir, diverting the excess heat to an additional large-capacity seasonal thermal energy storage (STES) reservoir. The best configuration for the number of collectors and STES tank volume was assessed through a parametric study, to reach a compromise between a high solar fraction and a reasonable system efficiency. The results showed that a system with 10 m2 of solar collectors and a 30 m3 STES tank or, alternatively, 20 m2 of collectors and a 20 m3 tank achieved the desired solar fraction and efficiency for the chosen building and local climate conditions. A comparison with the literature shows that this strategy can achieve better results, requiring less collector area and storage volume.