Thermal destratification in small standard solar tanks due to mixing during tapping

Most small solar domestic hot water systems, SDHW systems, are not equipped with circulation piping. In many systems the pipes, in which the hot water is transported from the solar tank to the draw-off locations, are relatively long. Hence, the waiting time for hot water during draw-off is relatively long. In order to reduce this waiting time to an acceptable level, the flow rate during draw-off is often very large - typically about 20 l/min - at least at the start of the draw-off. As long as the flow rate during draw-off is small, the mixing rate inside the tank is small. However, if the flow rate is large, as mentioned above, the mixing rate can be relatively large if the cold water inlet design is poor. Mixing results in destratification in the solar tank and with that reduced thermal performance of the SDHW system. Investigations indicate that the decrease of the yearly thermal performance caused by mixing during draw-offs can be as high as 23 % if a marketed cold water inlet design is used. Other tested inlet designs result in a decrease of 2-3 % of the yearly thermal performance caused by mixing. Based on the investigations recommendations on the design of the cold water inlet and on a test method for solar tanks concerning mixing during draw-offs are given.