WEB Crystal hydrates for thermochemical energy storage - progress and challengesWednesday (23.09.2020) 14:30 - 14:45 P: Processing and Synthesis 1 Part of:
There is an urgent demand for long term stable heat storage materials. TCM’s (Thermo-Chemical Materials) could serve a vehicle for long term compact storage. Inorganic crystal hydrates (salt hydrates) are considered as a promising class of TCMs for the built environment. Charging with thermal energy happens via dehydrating the crystal at elevated temperatures and low water pressures. Discharging occurs via rehydration, where heat is released. The major challenges are related to the fact that (dis)charging processes involve solid-solid phase transitions.
To illustrate the physics of heat storage in crystal hydrates, several aspects of the (de)hydration of crystals like MgSO4, CuSO4 and CuCl2 are discussed. Combined NMR and TGA studies on MgSO4 have revealed that (de)hydration might involve liquid-like intermediate states and that reversible cycling is not possible. At that point CuSO4 and CuCl2 behave far more stable. At present the connection between cycling stability and the nature of the lattice rearrangements is under study. Although valuable, these detailed studies did not help to select an appropriate salt hydrate for the application. Therefore a database of 563 hydration reactions has been compiled . Based on filters for the energy density and p-T characteristics a long list of about 30 salt hydrates has been compiled that seem to be the most suitable ones. A promising candidate has been identified: K2CO3 .
With the help of K2CO3 the major challenges for materials engineering of TCMs will be discussed: boosting the energy density, manipulating the phase diagram and improving the cyclic stability. The physical-chemistry of cycling (de)hydration reactions will be discussed in detail. It will be shown that a metastable zone exists in which nucleation problems reduce the kinetics of the transition severely . Presently, the nucleation behavior of the hydration reactions is investigated. Both the crystallite size and local mobility of intermediate phases are studied. It seems that a highly concentrated wetting films play an important role in the reaction process. This opens up the way towards targeted materials manipulation in order to improve the power output.
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