Thermal energy storage systems are a central component of solar thermal systems for domestic hot water heating to bridge the time between energy production and use. In modern stratified storage tanks, a stable stratification of warm fluid over cold fluid is created by utilizing the thermal density gradient of water. In principle, the low thermal conductivity of water allows stratification to be maintained for long periods, contributing to high storage efficiency.
However, the storage walls of thermal stratified storage tanks are mostly made of steel, which has a thermal conductivity about two orders of magnitude higher than water. As a result, the steel wall forms a thermal bridge between the upper hot and lower cold water layers. The resulting thermal equalization process causes heated water to rise from below and cooled water to fall from above. These near-wall secondary flows are to be characterized in the context of the work, in order to be able to quantify later their negative influence on the stratification and to compile suitable solution concepts. For this purpose, high-spatial-resolution flow measurements of the natural convection in the wall boundary layer are carried out using PIV and LDA methods. They provide an in-depth understanding of the drive of the wall flow and additionally serve to validate flow simulations carried out in parallel with the flow solver ANSYS Fluent.
Editor:
Current publications:
H. Otto, C. Resagk, C. Cierpka: Convective near-wall flow in thermally stratified hot water storage tanks, 13th International Symposium on Particle Image Velocimetry, 22.-24.7.2019, Munich, Germany
H. Otto, S. Resagk, C. Cierpka: Kombinierte Geschwindigkeits- und Temperaturfeldmessungen in thermischen Schichtenspeichern, Symposium "Lasermethoden in der Strömungsmesstechnik", 03.-05.09.2019, Erlangen, Germany
H. Otto, C. Resagk, C. Cierpka (2020) Optical Measurements on Thermal Convection Processes Inside Thermal Energy Storages during Stand-By Periods, Optics 1, 155-172, DOI: 10.3390/opt1010011, open access
