Balancing Surplus Electricity and Heat Demand in Domestic Households by Thermochemical Energy Storage Based on Calcium Hydroxide

Kurzfassung

In Germany, about 70% of the final energy demand of domestic households is used for space heating. While electricity generation by renewable sources increased considerably in the last years, heat is still predominantly supplied by combustion of fossil fuels. As a consequence, the heat sector contributes about 30% of Germany's yearly CO2 emissions. One way to increase the renewable share in the heating sector is to use electricity for space heating. Nevertheless, the heat is mainly demanded in winter whereas the supply of PV-panels prevalently occurs in summer. Thermochemical energy storages are well suited to overcome this seasonal discrepancy as the energy is mainly stored in chemical potential with a high energy density and free of thermal losses. Furthermore, the reaction system calcium hydroxide/oxide offers several advantages as the material is cheap, abundantly available and environmental friendly. The reversibility of the reaction and the storage operation in lab scale reactors has already been demonstrated. Seasonal thermal energy storage for space heating requires large capacities (for example about 8000 kWh for a single family house) but only small discharging powers (around 10 kW). Therefore, it is necessary to decouple the storage container (capacity) from the reactor (power), for example by a moving bed reactor concept. However, the movement of the material is rather difficult due to its tendency to clog and agglomerate. One approach to overcome this drawback, currently investigated at DLR, is to mechanically assist the transport of the material through the reactor. To investigate this concept a multifunctional test bench has been set into operation and a first reactor concept was investigated where a screw conveyor transports the material while simultaneously the screw is inductively heated thereby driving the charging reaction. With this concept the continuous charging of the material under incorporation of electrical energy was successfully demonstrated. However, the experiments also revealed that the heat transfer from the surface of the screw into the bulk limits the achieved charging power. To increase the power density of the reactor the heat transfer into the storage material needs to be improved. Current works therefore aim to optimize the screw geometry to enlarge the available heat transfer surface and increase the mixing of the particles. In particular, the mixing of the particles will lead to a drastically increased heat transfer rate into the moving bulk.