Abstract
Thermal energy storages (TES) are increasingly important for storing energy from renewable energy sources. TES that work with liquid storage materials are used in their most efficient way by stratifying the storage fluid by its thermal density gradient. Mixing of the stratification layers during stand-by periods decreases the thermal efficiency of the TES. Tank sidewalls, unlike the often poorly heat-conducting storage fluids, promote a heat flux from the hot to the cold layer and lead to thermal convection. In this experimental study planar particle image velocimetry (PIV) measurements and background-oriented schlieren (BOS) temperature measurements are performed in a model experiment of a TES to characterise the influence of the thermal convection on the stratification and thus the storage efficiency. The PIV results show two vertical, counter-directed wall jets that approach in the thermocline between the stratification layers. The wall jet in the hot part of the thermal stratification shows compared to the wall jet in the cold region strong fluctuations in the vertical velocity, that promote mixing of the two layers. The BOS measurements have proven that the technique is capable of measuring temperature fields in thermally stratified storage tanks. The density gradient field as an intermediate result during the evaluation of the temperature field can be used to indicate convective structures that are in good agreement to the measured velocity fields.
Highlights
Energy storage facilities as part of renewable power plants become increasingly important to achieve the aim of limiting the climate change [1] and guaranteeing a stable power grid at the same time [2,3,4]
The camera focuses on this background pattern that is placed behind the investigated region
Thermal or compressible effects which are present in the measurement region during the exposure of the measurement images lead to apparent distortion of the background pattern compared to the reference image which relies on the intermediate changes in refractive index
Summary
Energy storage facilities as part of renewable power plants become increasingly important to achieve the aim of limiting the climate change [1] and guaranteeing a stable power grid at the same time [2,3,4]. Since a major part of global primary energy consumption is consumed or wasted as heat [5], thermal energy storage systems (TES) have the potential to become one of the most widespread energy storage technologies in the future They can compensate for the disadvantages of other energy storage technologies such as electric batteries, pumped hydro storages or compressed air energy storages by the fact that they usually consist of relatively cheap construction and storage materials and do not have to meet any geological restrictions [3,4,6,7]. The first step to measure thermal or compressible effects in this region is to take a reference image of the background pattern before there are any refractive index changes due to thermal or compressible effects present in the measuring area. Thermal or compressible effects which are present in the measurement region during the exposure of the measurement images lead to apparent distortion of the background pattern compared to the reference image which relies on the intermediate changes in refractive index
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