• Energy Research

Two tank storage systems using molten salt represent today's state of the art in energy storage for concentrating solar power (CSP) plants. This concept shows a limited potential for further cost reductions, since the capital costs are dominated by the expenses for the salt inventory. The application of solid storage materials represents a promising approach to reduce capital costs. While this approach avoids also the risk of freezing and lessens corrosion problems, the efficiency of the heat transfer between the heat transfer fluid (HTF) and the solid storage medium is crucial. This paper introduces the CellFlux concept, which uses an intermediate closed air loop to transfer energy between the HTF and the solid storage material. A modular concept is chosen to optimize the size of the air flow channels. An initial project will provide the fundamentals needed to design a CellFlux storage unit. The feasibility will be proven by a 100 kW/500 kWh pilot storage module.

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-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.

Abstract The storage system investigated in this work, namely the CellFlux system, consists of a regenerator type thermal energy storage volume which is coupled to a heat exchanger by a circulating intermediate working fluid. The numerical simulations in this work are based on experiments conducted with a large scale pilot plant having a bed length of more than 10 m. The storage volume is of a novel design with horizontal flow direction and is filled with hollow bricks as sensible heat storage material. So far, most publications focus on packed bed storage systems, often with molten salt or oil, only few consider regularly shaped channels with gaseous flow. For the investigation in this part of the publication, a one-dimensional dispersion concentric model for channel flow is implemented in MATLAB/Simulink. The analysis in part I of this publication has revealed a radial flow maldistribution, which can be predicted by a correction function correlated beforehand. The commonly used assumption of plug flow, however, will be afflicted with an error. Thus, the aim of this work is to determine the accuracy of the model both under the plug flow assumption and the application of aforementioned correction function. Three single blow experiments, where the storage is charged or discharged from a uniform initial temperature, are compared to the numerical model. The average temperature deviation between experiment and simulation reaches up to 10% without the correction function, but improves significantly through its application and then ranges between 0.4% and 4.8%. Moreover, studies based on larger experiments have either not observed or generally not addressed the effect of a flow maldistribution. This appears to be an issue for small scale experiments where typically more sophisticated models are applied. If it was possible to model the entire storage volume with the present 1D model, the flow maldistribution effects could average out. Also, computing effort would be low and allow simulations on system level. This approach was taken only by few authors so far, particularly for packed bed systems and was also not experimentally verified. In order to correctly depict the experimental set up, the model considers the different composition of the interior, such as flow distributors and inlet/outlet cones as well as the thermal heat capacity of the walls and losses to the environment. As a result the model shows good agreement with the experiments: the mean temperature difference of the exit temperature between experiment and simulation during cyclic operation remained below 5%.

AbstractStorage of electrical energy is a key technology for a future climate‐neutral energy supply with volatile photovoltaic and wind generation. Besides the well‐known technologies of pumped hydro, power‐to‐gas‐to‐power and batteries, the contribution of thermal energy storage is rather unknown. At the end of 2019 the worldwide power generation capacity from molten salt storage in concentrating solar power (CSP) plants was 21 GWhel. This article gives an overview of molten salt storage in CSP and new potential fields for decarbonization such as industrial processes, conventional power plants and electrical energy storage.

To the present date, intense research in the field of latent, thermochemical and sensible thermal energy storage has been carried out. The presentation will give an overview of possible options for the integration of these diverse technologies into CSP power plants. Specific advantages, but also their technical challenges will be considered. Using the example of molten salt thermal energy storage, current research on storage integration will be discussed.