Thermal Stratification in Small Solar Domestic Storage Tanks caused by Draw-offs

As shown in many research studies in the past, the thermal stratification of the tank caused by draw-offs has a high impact on the performance of a Solar Domestic Hot Water (SDHW) system. Nevertheless, in most tank models for system simulations the influence of the draw-off pattern on the mixing behaviour is not taken into account sufficiently. Two typical Danish domestic water storage tanks, each with a volume of about 150 l, were investigated. In both tanks the inlet pipes are placed at the bottom and hot water is drawn from the upper part of tank. Above the inlet pipes, differently shaped plates are placed in order to reduce the mixing of the incoming cold water with the warmer storage water. To measure the thermal stratification thermocouples were placed in a vertical glass tube inside the tank. Measurements were carried out with different draw-off volumes, flow rates, and initial temperatures in the storage tank. Furthermore, for theoretical investigations of the flow patterns in the storage tank, calculations were carried out by means of the Computational Fluid Dynamics (CFD) tool Fluent. Based on the experimental and theoretical results, the sizes of fully mixed zones in the bottom part of the tanks were described for the investigated operation conditions in one-dimensional multi node tank models. In this way the variable extend of mixing due to operating conditions could be taken into account. The influence of the mixing behaviour on the annual system performance was then determined with a component oriented simulation tool for solar thermal systems.