Performance study of one-dimensional models for stratified thermal storage tanks

Two basic approaches are used to model the temperature distribution in thermal storage tanks for solar domestic hot water (SDHW) systems. In the multinode approach, the tank is divided into N nodes, with an energy balance written for each node. This approach results in a set of N differential equations that can be solved for the temperatures of the nodes as a function of time. In the plug flow approach, segments of liquid of different temperatures and sizes are assumed to move through the tank in a plug flow manner. The sizes of the fluid elements are determined mainly by the simulation time step and the flow rates. Whenever the incoming fluid from the heat source is colder than the fluid at the top of the tank, “plume entrainment” occurs. A model describing plume entrainment has been incorporated into both the multinode and the plug flow models in the TRNSYS program[1]. A performance study of the TRNSYS tank models has been carried out with experimental data from two different sources. Three performance numbers have been defined for quantifying the accuracy of the models compared with experimental data. Recommendations are given as to which tank model should be used under which conditions.