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The test plant TESIS in Cologne.

© DLR, CC-BY 3.0

In connection with solar-thermal power plants, salt storage tanks have already established themselves as a way of maintaining the energy supply overnight. Due to the expansion of renewable energies, they are also becoming more important in terms of making conventional power plants more flexible. Furthermore, salt storage tanks can also provide industrial processes with large amounts of heat. Until now, a system had to be used, whereby heated molten salt was taken from an insulated steel tank, as a heat storage device, and pumped into a second, cold steel tank. In the event of surplus energy, the cold tank was heated up, causing the molten salt to flow back into the hot tank. Heat carriers absorb the heat in a controlled manner and release it as required.

Due to the complexity of this process, researchers from the German Aerospace Centre (DLR) are endeavouring to simplify the concept, within the scope of the MS-Store research project, to just one tank. In a vertically-embedded tank, the up to 560 °C hot molten salt is collected at the top end of the tank and stored in layers, with each layer decreasing in temperature. The researchers estimate that up to 40 percent of cost savings can be achieved by removing one of the tanks.

Tests will be carried out on an industrial scale in a salt storage test plant, which was built in Cologne-Porz within the space of 14 months. Here, researchers will, among other things, be examining measures for preventing frozen pipes or plant corrosion caused by the hot salt. A solution will also be sought for keeping the temperature layers stable within the tank. Another aim is to optimise the components and to demonstrate the technical feasibility of the storage solution.

The 20-cubic-metre test storage facility is to be filled with 85 tons of salt. Even cheaper storage media, such as ceramics or stone, could be used to replace at least some of the salt. The researchers are therefore testing basalt stones that have the same volumetric heat capacity as molten salt.