Thermal Stratification in Solar Storage Tanks: Long-Term Modelling and Efficiency Analysis

The storage tank plays a key role in solar thermal installations, as thermal stratification allows high temperatures to be maintained in the upper region while keeping the return temperature to the collectors low. This study analyses the influence of thermal stratification on short- and long-term performance of solar domestic hot water systems using a multi-node storage tank model. An algorithm was developed to compute temperature profiles along the height of a storage tank operating under time-varying temperature and flow-rate conditions. Time courses of temperatures and heat fluxes in a solar domestic hot water system were determined. In addition, the seasonal variation in the optimal locations for supplying the tank with water from the solar collector was identified. Annual simulations were performed for the climate of Kraków (Poland) and the domestic hot water demand of a single-family household. The results show that the effect of the degree of stratification on solar fraction and solar efficiency is small. It was also demonstrated that the effect of thermal stratification within the tank on stabilizing the temperature of the produced water is more significant than the effect associated with increasing the tank volume.