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Optical Temperature Field Measurements With Two-Color Laser Induced Fluorescence In A Stratified Thermal Energy Storage

Clemens Naumann, Christian Cierpka

Technische Universität Ilmenau, Ilmenau, Germany


Thermal energy storage systems (TES) are an important part for storing surplus electric energy from renewable, volatile energy sources. TES using liquid storage materials operate most efficiently by stratifying the storage fluid based on its thermal density gradient into a hot and cold layer, separated by a thermocline region. Previous studies of such stratified TES have shown that heat conduction from the hot, upper layer to the cold, lower layer through the storage tank wall induces counter-directed convective flows (wall jets) along the inner walls. These wall jets carry hot fluid from the upper layer and cold fluid from the lower layer towards the thermocline region, which leads to mixing of the layers and therefore to a decreased thermal efficiency. To quantify the influence of the wall jets on the mixing process, optical temperature field measurements in the thermocline region of a 105 L stratified TES model experiment are performed with two-color planar laser induced fluorescence (2C-PLIF). The results show that the method is feasible for evaluating the thermocline temperature distribution over time in such a system with an accuracy of 1.5 °C. Further measurements in the hot wall jet region show that the temperature resolution of the measurement system limits the evaluation of the thermal boundary layers. However, the 2C-PLIF system provides sufficient temperature field information to determine the local heat fluxes in future studies in combination with simultaneous PIV measurements.

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