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Long-term storage system in lime
What potential is provided by thermochemical storage systems? Researchers at the German Aerospace Centre (DLR) have been pursuing this question. With a new test plant they are investigating the extent to which solar heat can be stored thermochemically for industrial processes and solar power plants.
Thermochemical heat storage systems absorb heat through endothermic reactions and release it again through exothermic reactions. With this type of storage the charging and discharging can be repeated as many times as required. “Up to 100 per cent of the chemically stored heat can be released again since the reaction is completely reversible,” explains Dr. Antje Wörner, who is head of the Thermal Process Technology department at the DLR.
To charge the system, hot air heated at a temperature of 600 °C flows past calcium hydroxide. This reaction creates calcium oxide, also known as burnt lime. In addition water vapour is also released. The storage system is discharged by feeding water vapour into the reactor container in a controlled manner. The burnt lime becomes calcium hydroxide again. The exothermic reaction releases the energy again in the form of heat. In contrast to traditional heat storage tanks, which despite their insulation cool down with time, the chemical storage system can store the thermal energy without loss and therefore makes a more suitable long-term storage system. Other advantages include the fact that it can be largely discharged at a consistently high temperature level and the initial material is cheaper. Such storage systems are therefore suitable, for example, for use in the ceramics industry or as storage systems for operating solar power plants at night.
Calcium oxide stores with a high energy density
The energy density of this storage system is high. In terms of the volume, calcium oxide storage systems can store five times as much thermal energy as water. With the test plant, the DLR can transfer a thermal output of 10 kW up to a temperature level of 1000 °C, whereby the scientists are researching charging and discharging concepts for specific applications.
The Ministry for Innovation, Science, Research and Technology in the German Federal State of North-Rhine Westphalia is funding the test plant. The project forms part of the joint “Chemical heat storage using reversible gases and solid reactions” (CWS) project. As part of these investigations supported by the German Federal Ministry of Economics and Technology, the scientists are determining the potential of the storage system.