While considerable progress has been made in renewable electricity generation in recent years, the share of heating demand covered by renewable technologies remains very low. However, about 83% of the final energy demand in an average German household accounts for space heating and hot water and is still mainly supplied by the combustion of fossil fuels. The heating sector alone is therefore responsible for approximately 30 % of Germany's total yearly energy-related CO2 emissions. Technologies that can provide an efficient, decentralized and CO2 neutral heat supply are not yet sufficiently available. One reason for this is that the demand for space heating occurs mainly in the winter months, whereas a local surplus of renewable electricity, e.g. from photovoltaic systems, occurs mainly in the summer months. The Group of Thermochemical Systems is addressing this challenge by developing a cost-effective, efficient long-term storage system based on limestone as a storage material.
Thermochemical storage systems are particularly suitable for the long-term storage of energy due to the loss-free storage principle. When energy is supplied, calcium hydroxide (slaked lime), can be decomposed to calcium oxide and water vapor (charging phase). At the same time the water vapor is separated from the solid by condensation. The calcium oxide (slaked lime) can then be stored in powder form for any length of time without losses of chemical energy potential (storage phase). Only when the calcium oxide is brought back into contact with water vapor or liquid water the exothermal back reaction takes place and heat is released (discharge phase).
The theoretical potential of using lime for energy storage is enormous. The material is available worldwide at low costs and in industrial quantities. The reaction has a high energy density and can be repeated for a large number of cycles (reversibillty). In addition, all substances involved in the reaction (lime and water) are ecologically completely harmless. Even after the material is used in energy storage, it can be easily disposed and it even reabsorbs the CO2 emitted during production from the atmosphere.
The basic idea behind the concept developed at DLR is to charge the storage material with electricity during the summer months, e.g. from an in-house PV system or at times of favorable electricity prices. After the charging process, the material can be stored at room temperature in a storage container, e.g. in a cellar room, for several weeks or even months. If thermal energy is required in the winter months, part of the material is taken from the container and reacts with liquid water (tap water ), under the release ofthermal energy.. If electrical energy is available again, the material can be regenerated again. This process can be repeated as often as required. In cooperation with DLR Technology Marketing, a laboratory prototype in the performance class of a single-family house is currently being operated at the DLR Cologne site.
Further literature: Publications Dr. Matthias Schmidt