Recent studies on the flow phenomena in stratified thermal-energy-storage (TES) systems have shown that heat conduction from the hot upper fluid layer through the vertical tank sidewall into the lower cold fluid layer leads to counterdirected wall jets adjacent to the vertical sidewalls. It was shown that these phenomena destroyed half of the total exergy content in less than a tenth of the storage time constant of a $2$-${\mathrm{m}}^3$ stratified TES system. This paper investigates short-term fluctuations of the wall jets since these fluctuations can potentially mix the hot and cold zones of the thermal stratification that are separated by the thermocline region. Using particle-image velocimetry measurements in two regions of a TES model experiment (near-wall region and far-field region) and analyzing the frequency content of the velocity fields revealed characteristic oscillations for different regions. In the near-wall region, observed fluctuations agreed well with an adjusted boundary layer frequency from the literature, showing that the wall jet is transitioning from laminar to turbulent flow. In the far-field region, the oscillations are related to the Brunt-Väisälä frequency. It is shown that the fluctuations from the boundaries of the thermocline region are most dominant and propagate into deeper regions of the thermocline. A comparison to data from the large-scale test facility for thermal energy storage in molten salt at the German Aerospace Center in Cologne showed good agreement. The consensus between the two experiments proves firstly that a small-scale model experiment with water as a storage liquid can be used to analyze the physical phenomena of large-scale molten salt storage facilities and secondly that these fluctuations are relevant for exergy destruction in real-scale TES.

This study focuses on the techno-economic optimization of direct molten salt parabolic trough solar thermal power plants (STPPs) equipped with thermocline filler (TCF) thermal energy storage (TES). On one hand, this technology allows for cost reductions compared with state of the art two-tank (2T) TES. On the other hand, however, it leads to a performance decrease of the power block (PB) due to partial part load operation. To evaluate the dominating effect, annual simulations on a system level are performed for the TCF direct molten salt storage concept and, as a reference, for the two-tank direct molten salt storage concept. The levelized cost of electricity (LCOE) serves as a global measure to compare the two systems and to optimally size the TCF storage and the solar field (SF). The result of this study is that LCOE can theoretically be reduced by up to 8% by using a TCF instead of 2T storage system. The influence of temperature deviations from the nominal value at the end of charge or discharge, porosity and particle diameter of the TCF on LCOE, and system behavior is investigated in detail. This study further presents alternative operation strategies with improved system behavior and reveals determining factors for the integration of TCF storage into a system.