Performance Analysis of a New Thermal Stratification Device for Hot Water Storage Tank Heated at the Bottom

Thermal stratification in heat storage tank has a major influence on the thermal performance of the heating system. For heat storage tank heated from the bottom, the whole tank is heated in order to provide hot water for the consumer. A new stratification device has been developed to place the heated water at the top of the tank, therefore allowing the energy usage to be reduced since the whole tank does not need to be heated to provide hot water. The aim of the paper is to investigate the performance of the newly developed thermal stratification device by means of experiments and computational fluid dynamics (CFD) simulations. Experiments have been carried out to measure water temperatures at different levels of the tank during charging by a vertical heating element integrated in the stratification device. A simplified CFD model of the cylindrical heat storage tank with the stratification device and a heating element at the bottom of the tank is created in Ansys Fluent 15.0. Transient fluid flow and heat transfer in the tank during charging are calculated. The CFD calculated water temperatures are compared to the measured temperatures. A parametric study has been carried out to investigate how thermal stratification in the tank is influenced by the design of the stratifier such as pipe thickness, material type, diameter of the pipe, by dimensions of the tank and by power of the heating element, etc. The investigations elucidate how well a hot water tank heated at the bottom with a vertical heating element can be stratified by using the new stratifier design. Based on the CFD calculations, design of the stratification device is optimized.