Plume entrainment effects in solar domestic hot water systems employing variable-flow-rate control strategies

Solar domestic hot water heating systems perform more efficiently if their storage tanks are perfectly thermally stratified. In real tanks, which do not perfectly stratify, the most important mechanism destroying stratification is plume entrainment. Plume entrainment occurs when cooler water is inserted into the tank top which contains hotter water. The resultant falling plume of cool water causes mixing. This paper uses computer simulation to evaluate and compare two strategies by which plume entrainment is minimized by controlling the collector flow rate. One strategy (calleá “SCOT”) maintains a constant collector outlet temperature, and the other (called “FCTR”) strategy maintains a constant temperature rise ΔTset from inlet to outlet of the collector. The results of the study show that the SCOT strategy always produces a system that performs more poorly than the corresponding system with a fixed flow rate. The FCTR strategy, on the other hand, consistently out-performs the fixed flow strategy, but only by a few percent. When the FCTR strategy is used, the optimum ΔTset to use is 20°C for the SDHW system simulated.