Untersuchungen zur chemischen Speicherung thermischer Energie im Temperaturbereich bis 400 °C mittels Magnesiumhydroxid
Due to the decomposition of magnesium hydroxide in the temperature range up to a maximum of 400 °C, reactive magnesium oxide and water vapour is formed, whereby thermal energy is stored in a chemical form by means of material conversion. To release the stored energy, the magnesium oxide is hydrated with water vapour to form magnesium hydroxide. In the theoretical part of the thesis, the reaction system MgO/Mg(OH)2 from a thermodynamic, reaction kinetic and materials science point of view is analyzed based on a literature study. In order to extend the understanding of the operation of a thermochemical energy storage based on MgO/Mg(OH)2, thermodynamic analyzes and a theoretical characterization of heat and mass transfer in a packed bed are carried out. In the practical part of the thesis, reactive MgO-powder is granulated by means of wet hydration into coarse-grained Mg(OH)2 in order to make it available for use as a thermochemical storage material. For the experimental characterization of heat and mass transfer, a laboratory test bench with laboratory reactor was built. This is a directly gas flowed packed bed of MgO/Mg(OH)2 with nitrogen as the heat transfer fluid and steam as reaction fluid. The granulated Mg(OH)2 has a high porosity inside the particles, whereby water vapour can be transported uninhibited with low pressure drop over the fixed bed. Parameter studies on the laboratory reactor show that the water vapour partial pressure and the heat capacity flow during gas phase hydration have the greatest influence on the heat of reaction in the packed bed. The maximum chemical reaction conversion of the gas phase hydration is determined to be 0.66, thus significantly reducing the theoretical energy storage density. A 10-fold de- and rehydration of a 200 g sample in the laboratory reactor shows a further decrease in the reaction con-version by about 50 %.Since the granulated Mg(OH)2 in the laboratory reactor already partially decomposes after ten cycles, the storage material must be mechanically stabilized for further experiments in a demonstrator.
Erlangen-Nürnberg, Univ., Diss., 2020