Experimental investigations on structural changes of CaO/Ca(OH)2 bulks in thermochemical energy storage

Kurzfassung

Storage of thermal energy has the potential to become a major part of future energy systems due to versatile integration possibilities as well as relatively low cost. Thermochemical energy storage (TCS), in which thermal energy is stored as chemical potential, is particularly promising for certain applications such as long-term storage. However, this technology still requires considerable efforts in research and development in order to deliver market-ready solutions. This thesis investigates a widely recognised factor that forms a major obstacle to the technical implementation of TCS based on chemical reactions: As the TCS materials react during charging and discharging of the storage, they change their chemical composition and thus their chemical and physical properties. In case of gas-solid reaction systems, which represent a majority of the materials studied, the changes in solid morphology can be significant, leading to correlating changes in any related properties of the storage material, often in the order of magnitudes. Design and predictive modelling of reactors and storage systems are therefore challenging. To improve the understanding of the processes leading to changes in reaction and transport behaviour within TCS material bulks, the reaction system CaO/Ca(OH)2 is studied in depth, as challenges caused by restructuring of this system's solids are widely reported in literature. For that purpose, bulk storage materials consisting of powder, nanoparticle-modified powder, shaped and encapsulated granules are investigated over the course of several reaction cycles. Changes in gas permeability and thermal conductivity are discussed qualitatively and - where feasible - quantitatively. Additional analyses are carried out with regard to reactivity, morphology and phase composition. In summary, it is concluded that the inhomogeneities that develop within reactors are likely to be so pronounced that they lead to reaction temperature differences in the range of tens of Kelvin within a distance of centimetres. More detailed studies on the microstructural evolution of reacting storage materials are identified as promising to further TCS technologies. Based on the results regarding material tailoring, a combination of the studied approaches is proposed. Shaped materials such as granules with diameters in the range of a few millimetres are suitable to prevent agglomeration and improve handling while minimising limitations due to insufficient heat and mass transfer. Mechanical stabilisation can be achieved by a "breathing" nanocoating or/and an inert component within the particle. However, ideal material properties depend on the specific reactors and operating conditions. The proposed approach of nanocoated granules has already been demonstrated on a technical scale following the work described in this thesis.