WEB Hydration and dehydration of thermochemical materials
In the transition from fossil fuels to renewable energy there is a mismatch in supply and demand. In summer there is more heat than needed and in winter there is less. This mismatch is the reason for increased interest in long term heat storage. A promising method for storing heat is in thermochemical materials (TCMs) using hydrates.  During the hydration process, heat is released, while dehydration (using heating) leads to energy storage. There are several performance goals for TCMs: the (un)loading should preferably take less than 10 hours, the energy density should be higher than 1 GJ/m3 and the material should be stable for many (de)hydration cycles.
A more fundamental understanding of the hydration and dehydration of TCMs would help in finding the best material for household application. The molecular crystal structures of both the hydrate and the anhydrate of the TCMs is a starting point, but the transition between the two is not well understood. In order to gain a better understanding of the (de)hydration mechanism several TCMs were compared in terms of kinetics, cyclability and energy density. In particular we examined zeolites and other channel structures that undergo little structural change during dehydration, versus materials that rearrange their crystal structure like salt hydrates. The latter store more energy, but their hydration mechanism is complicated by factors like metastable zone width and nucleation through a wetting layer.  Macroscopic changes in the crystal shape (like crack or pore formation) have a large effect on the mass transport and were studied with optical and electron microscopy.
1. Donkers, P. A. J.; Sögütoglu, L. C.; Huinink, H. P.; Fischer, H. R.; Adan, O. C. G., A review of salt hydrates for seasonal heat storage in domestic applications. Applied Energy 2017, 199, 45-68.
2. Sogutoglu, L. C.; Steiger, M.; Houben, J.; Biemans, D.; Fischer, H. R.; Donkers, P.; Huinink, H.; Adan, O. C. G., Understanding the Hydration Process of Salts: The Impact of a Nucleation Barrier. Crystal Growth & Design 2019, 19 (4), 2279-2288.