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In this work the first experimental results of the world’s largest molten salt thermocline storage system with filler (TCF) will be presented. The current experimental setup has a thermal capacity of 3.8 MWh and operates in a temperature range from 290 to 560°C.

For large scale thermal energy storage at temperatures above 300°C, two-tank molten salt systems mark the current state-of-the-art as they are proven technology in parabolic trough and tower solar thermal power plants. Research is focusing on the utilization of molten salts not only as storage medium but also as heat transferring fluid (HTF) in parabolic trough plants [1]. The current two-tank concept offers serveral cost reduction possibilities. Firstly, instead of storing the hot and cold phase in two separate tanks, the salt could be stored inside a single tank to avoid a large gas volume. The separation of both phases can either be achieved by a floating insulated barrier or simply by the different densities of both phases. Secondly, a high share of the total investment costs of a molten salt storage system is caused by the molten salt itself. For the two-tank system in 50 MWel power plants, this can be as high as half of the total TES costs [2]. In the thermocline with filler concept, a large fraction of the molten salt can be substituted by a cost effective solid material, offering a significant potential for further cost reductions [3]. Finally, gaining operational experience of such systems and the ability to derive optimized operation strategies, promise an additional cost reduction potential. The “test facility for thermal energy storage in molten salts” (TESIS:store) has been set up at DLR in Cologne, Germany. An outside view of the plant can be seen in Fig. 1. The facility operates at temperatures up to 560 °C and a maximum molten salt mass flow of 4 kg/s. The storage volume has a length of 5.4 m with a total tank volume of 22 m³. The plant allows the investigation of the thermocline concept with and without filler and gaining widespread operation experience. Heat tracing along the containment walls and the piping ensures adiabatic conditions.

For large scale high temperature thermal energy storage, two-tank molten salt systems mark the current state-of-the-art. New fields of application are emerging, for example, industrial furnaces can shift their energy consumption or conventional power plants could reach lower part load levels. In volatile electricity markets, like in Germany, this increased flexibility enables significant cost reduction potentials. The two-tank concept comprises potential for further cost reductions. A first step is storing the hot and cold phase inside a single tank. The separation of both phases is achieved by their different density or by a floating barrier. Gaining operational experience and deriving optimized operation strategies, promise additional cost reduction potentials. A final step is the thermocline with filler concept, where a large fraction of the salt can be substituted by a low-cost solid material. The “test facility for thermal energy storage in molten salts” (TESIS:store) has been set up at DLR in Cologne. The facility operates at temperatures up to 560°C and a mass flow up to 4kg/s. The storage volume is 22m³. The plant allows the investigation of the thermocline concept with and without filler and gaining widespread operation experience. In this work first experimental results of the thermocline concept without filler are presented. As HTF, solarsalt at maximum temperature is used for several consecutive charging and discharging cycles. 150 thermocouples measure the temperature distribution along the middle axis and additionally at four planes in two radial directions each. The experiments help to understand and improve the molten salt distribution at the inlet regions as well as the development of the thermocline zone. Interest lies in achieving a stable thermocline zone where energetically adverse mixing should be minimized. The development of the thermocline zone and its movement speed through the storage volume are compared to a 1D numerical model.

This module gives an overview of upcoming ideas and concepts for thermal energy storage. After attending the module, the listener will be able to understand which shortcomings of current thermal energy storage technologies are there and what possible improvements are currently under research. The topic is divided into three main sections according to the underlying technologies. The first technology is sensible heat storage, which is based on storing heat by changing the temperature of a substance. The technology has the highest maturity and some concepts are already commercially available. Research focus lies on the optimization of costs; hence system simulations play an important role. Experiments have left lab scale and are mostly at pilot or demonstrational scale. The second technology is latent heat thermal energy storage, where the phase change enthalpy of matter is utilized to store heat. Research is still focusing on better understanding and simulating the phase change process in real systems as well as new concepts in general. The concept has also been demonstrated on a relevant scale. Finally, the third technology is thermochemical energy storage, where the heat of reaction is used for thermal energy storage. Research is still in lab scale here, with the aim of investigating several possible reaction systems and understanding the underlying chemistry. Some concepts have been proved in a kW-scale.

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.

High temperature thermal energy storage in liquid molten salts is a cost effective and proven technology for process heat and power plant applications. Examples are the improved use of waste heat from industrial processes, enhancement of the flexibility of power stations and cogeneration, as well as the conversion and storage of fluctuating surplus electricity from renewable energy sources. Some of the main advantages of molten salt technology are its high maturity, low costs for the storage material, high heat transfer rates and operation at ambient pressure levels. The thermocline concept promises further potential for cost reduction by storing hot and cold molten salt inside a single tank, separated due to density stratification. By embedding a low cost solid filler material into the molten salt storage tank, further cost reductions can be achieved. A new test facility to investigate and advance this technology, namely “TESIS:store”, is currently being commissioned at DLR in Cologne. Besides new technological challenges, originating mainly from chemical stability, the sizing of such dynamic systems has taken on greater significance. In most of the studies to the present date, sizing calculations are based on constant boundary conditions, which are derived from the connected energy source (i.e. solar field, industrial process) and energy sink (i.e. power block, industrial process). In reality, these boundary conditions are not constant. For example, during charging, the mass flow of the heat transfer fluid (HTF) is varying throughout the day and the time span of solar irradiation is different over the course of the year. Towards the end of the discharging period, the exit temperature of the storage volume will drop. Consequently, an attached power block will return a decreased HTF temperature back into the storage system. If the return temperature declines too far, the cold zone of the storage volume does so as well. In the subsequent charging cycle, the HTF coming from the storage volume now has a very low temperature. As a result, if the thermal power of the energy source is limited, the charging mass flow has to be reduced. These considerations show that boundary conditions have a significant impact on the operating behavior of such thermocline systems and it is therefore advisable to take them already into account during the sizing calculations. In this work a fast numerical model for sizing calculations, implemented in Matlab®, is presented. With this model, two sizing approaches are used for an extensive parametric study, where variable geometric properties and boundary conditions are applied. The first approach is considered as the classic approach, based on regained exergy after a full storage cycle, whilst for the second approach, the storage model is coupled with a simplified solar field and power block model. In the latter case, produced electric power after a full storage cycle is used for the rating of the storage configurations. The comparison of both approaches show the significance of fluctuating boundary conditions on the cyclic behavior of the storage system and eventually, its sizing results.

Diese Arbeit behandelt die Analyse des CellFlux-Speichersystems, eines neuartigen Speicherkonzeptes für sensible Wärme. Die Analyse umfasst die Identifikation geeigneter Komponenten zur Umsetzung, bis hin zur dynamischen Abbildung des Gesamtsystems in einem Simulationsprogramm. Hierbei steht ein Einsatz des Speichersystems in Kombination mit Kraftwerksprozessen im Vordergrund. Die Funktionsfähigkeit des Konzeptes wird im Pilotmaßstab nachgewiesen und auf Basis der erarbeiteten Entwurfsgrundlagen wird abschließend eine beispielhafte Anwendung des Speichers untersucht.

Although facility scale thermal energy storage of sensible heat in the range of 200-600°C has achieved a high maturity, state-of-the-art approaches are still not very cost effective. An innovative storage concept is thus proposed here that avoids the two major cost-driving factors of the concrete storage and 2-tank molten salt systems. First, the storage volume is comprised of low-cost sensible storage material such as concrete, natural stone or clinker bricks. These materials are several times cheaper than eutectic salt mixtures used in the 2-tank-storage system. Secondly, the system uses an intermediate air cycle, allowing for direct contact with the storage material. The necessary heat exchanger for transferring the heat from the primary oil loop to the intermediate air cycle consists of significantly less steel compared to the tube register inside the concrete storage. All components are kept in close proximity inside an enclosed loop, thus avoiding long flow lengths and keeping parasitic losses within a reasonable range. Preliminary investigations have shown that the overall performance and profitability of the storage system are closely linked to the thermal efficiency and pressure drop of the heat exchanger, the effectiveness of the storage, as well as the operation strategy. To demonstrate the feasibility of the storage concept and to investigate its performance characteristic under realistic conditions, a pilot scale test facility has been set up. In parallel, dynamic models of the storage system and a SEGS-like solar thermal power plant have been implemented in a Matlab/Simulink environment to analyze its performance theoretically. Experimental data is also used for model validation. The conference presentation gives a detailed overview of potential configurations and their dynamic behavior. Furthermore, first experimental results of the demonstration facility are presented.