Most of the published papers on heat/cold storage with gas-solid reactions are devoted to a single dehydration or hydration run. Experiments on cyclic hydration/dehydration experiments have shown that the hydrates may display instabilities related to changing rehydration/dehydration rates and/or changing grains sizes. The observations are mainly based on mass measurements, while no information is available on the crystal structure of the products or on water transport inside a grain.
Understanding of the behavior of a solvent during decomposition and formation reactions of salt complexes, will lead to improvements in the stability of solid-gas reactions for heat storage materials. The solid-gas reactions are studied using Nuclear Magnetic Resonance (NMR), in addition to well accepted techniques like TGA and DSC. Because NMR is selective, it can measure non-destructively the solvent molecules within a solid salt during decomposition and formation of a salt complex. We have shown that it is possible to observe the state of the solvent in the complex, e.g., in hydrated form or as solution.
In our lab, several pure hydrates are studied by cyclic loading and unloading, mostly chlorides and sulfates. These salts were selected as study materials, based on their theoretical volume variation during hydration/dehydration reaction, not because of their high potential as heat/cold storage materials. Using NMR, the water transport in these hydrates is studied in small beds of grains (±300 mg), where air is blown through. We show that the stability of some complexes is higher than others, but a clear explanation for the observed differences is not found. Salt hydrates, like CuCl2 and MgCl2, can have good cyclic behavior by applying carefully chosen hydration and dehydration conditions, based on the phase diagrams. Even with carefully chosen conditions, other salts like MgSO4 and CuSO4 display non-stable hydration/dehydration behavior. It is expected that this has to do with a combination of material properties, like volume variations, dissolution behavior and number of possible hydrates.