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At the new TESIS research facility, DLR researchers in Cologne are investigating future high-temperature salt-based storage systems.
© Dr. Franz Meyer, BINE Informationsdienst
TESIS test facility for heat storage systems
13.07.2017

An illustration of the TESIS test facility with vertical single-tank stratified storage facility with filling materials and four storage tanks with a total salt mass of 115 tonnes. The lying tanks serve to oscillate the salt during the research operation, and are not necessary for the target application.
© DLR (CC-BY 3.0)

Salt stores heat

Molten salt can store large amounts of heat at high temperatures. In Cologne-Porz, researchers at the German Aerospace Centre (DLR) are using a new test facility to find out where storage systems with only one tank could replace the common two-tank storage technologies. This would cut costs considerably. The system was presented to the public on 5 July 2017.

Molten salt energy storage has proven itself in solar power plants. It keeps the lights on during the night. However, thermal storage tanks can also make conventional power stations more flexible when reacting to load changes. This is becoming increasingly important when it comes to balancing fluctuating feed-in coming from regenerative energy producers. Industrial processes often require high-temperature storages for large amounts of heat.
Previous systems had two large, insulated steel tanks. When energy is required, the molten salt in the hot tank is used and then pumped into the cooler one. When there is an excess of energy, the molten salt in the cold storage tank is heated and transferred into the hot tank. The advantage of this concept is the inherently simple system design. The storages operate at a constant temperature level and are therefore not subjected to temperature stress. The disadvantage, however, is the effort for two separate tanks, which increases the costs.

TESIS:STORE – Thermal stratification like in solar hot water storage tanks

The test facility on the premises of DLR in Cologne differs visually from the flat twin cylinder tanks located in solar power plants in southern climate zones. A single slender tower rises above the test facility. It houses the test system called TESIS:STORE. The scientists are using it to scientifically analyse the single storage tank concept and demonstrate its technical feasibility. The beginning of July saw the construction of the test facility (BINE reported), now the scientists were able to present the completed test facility.

The tank could take up to 135 tonnes of salt. But it is not entirely filled up with the comparatively expensive salt. Basalt stones lower the necessary amount of salt. They have about the same heat capacity as salt. This way, the scientists combine the advantage of pumpable liquid with the low price of natural stone.
Solar power systems for domestic water heating demonstrate the storage principle: in the slender, high water tanks, the lighter warm water settles in the upper area while the cold water gathers below.
Stable temperature layers also play a decisive role in salt storage. However, the challenges are far more complex. The thermal stratification of salts in the temperature range between 170 and 560 degrees Celsius has not yet been explored, and thermal stress must be mastered first. The hot salts have a corrosive effect on pumps and valves. Freezing processes can block pipelines. The scientists want to tackle these questions with detailed investigations into heat and material transport, thermomechanics and material compatibility. The size of the test storage system permits scaling, model validation and system integration of innovative liquid salt storage systems. Compared to systems available on the market, the single storage concept can significantly reduce capital costs. According to the scientists, the cost-cutting potential is in the double-digit percentage range.

Test route TESIS:COM

In addition to the storage facility TESIS:STORE, the scientists also built the test route TESIS:COM. Here, components such as valves, absorber tubes and also measurement instrumentation can be examined and qualified. Process engineering issues involving liquid salt, for example freezing processes, are also to be answered here. DLR also aims to offer this to external customers.

The facility will be officially put into operation on 15 September 2017.

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