Introduction
Currently society is moving from a carbon-based society to a renewable-based society as to become less dependent on fossil fuels.  The main challenge in the transition from carbon- to renewable-based society is not the production of energy, but is to balance consumer demands with renewable energy sources. In order to balance these two independently fluctuating properties, both short (daily) and long (seasonal) term energy storage are required. In case of centralized energy storage, we can store electricity with help of techniques like flywheels, pumped hydro and compressed air. For decentralized energy storage, electricity can be stored by electrical batteries, but these are inefficient in case of long term storage [13]. As we know that the demand of thermal energy dominates thermal energy use, decentralized storage in the form of heat is one of the possible solutions. A promising class of materials are hydrates, producing heat during the hydration and storing heat during dehydration. The principle is given in figure 1.



Fig 1 Schematic diagram of  the loading and unloading by hydration and dehydration of a salc crystal in order to store heat

    Cycling
This class of materials can have energy densities potentially above 2 GJ/m3. A major disadvantage is the physical instability of these materials, i.e. the decrease in rehydration/hydration rates -thus heat absorption/release- with increasing number of cycles, accompanied with grain size variations. In order to study this we have look more closely at the performance of hydrates during cycling. In fig 2 examples are given for CuCl2 and CuSO4. As can be seem there is almost nu degeneration in performance for CuCl2, whereas there is a clear decrease in performance fo CuSO4.




    Fig 2. The loading of a sample of CuCl2 and  CuSO4 grains measured with help of NMR during different runs of dehydration/hydration.
Dehydration by a heating after a cooling period of 2 h (no active cooling), the sample was rehydrated with air over a period of 5 till 20 h.
The dashed line indicates the sample temperature.

    Dehydration
In order to get more fundamental insight we have look at the loading of various crystals during dehydration both at crystals and in a small reactor. We also looked in more detail at the dehydration in a combined NMR and TGA study in order to get a better understanding how water leaves a hydrated MgSO4·7H2O crystal  induced by heating. The TGA data showed that the dehydration process of MgSO4·7H2O depends on the heating rate. By studying the dehydration with the help of NMR, formation of pore water is observed during dehydration; i.e., an aqueous solution of MgSO4 in the crystal is formed as is schematically depicted in figure 3.


Fig 3: The loading of a MgSO4  as a function of temperature during dehydration as measured by TGA and by NMR, i.e., the pore, lattice and total water content.


     

P.A.J. Donkers,  Experimental study on thermochemical heat storage materials,  Ph.D. thesis, Eindhoven University of Technology,  the Netherlands (2015),  PhD-Donkers-2015 3.5 Mb.pdf

P.A.J. Donkers, L. Pel, O.C.G. Adan, Hydration / dehydration cycles of salt hydrates – studied with NMR–, Conference Proceeding EuroSUn 2014, Aix-les-Bains (France), 16 - 19 September 2014, http://proceedings.ises.org/paper/0154-Donkers.pdf

P.A.J. Donkers, K. Linnow, L. Pel, M. Steiger, O.C.G. Adan,  Na2SO4•10H2O dehydration in view of thermal storage, Chemical Engineering Science 134, 360–366 (2015)

Pim A.J. Donkers, Steffen Beckert, Leo Pel, Frank Stallmach, Michael Steiger, and Olaf C.G. Adan, Water Transport in MgSO4·7H2O During dehydration in view of thermal storage, J. Phys. Chem. C119 28711–28720 (2015)

P.A.J. Donkers, L. Pel, O.C.G. Adan, Experimental studies for the cyclability of salt hydrates for thermochemical heat storage, Journal of Energy Storage 5 25–32  (2016)

P.A.J.  Donkers, L. Pel, L.,M. Steiger,  O.C.G. Adan, O.C.G. Deammoniation and ammoniation processes with ammonia complexes. AIMS Energy, 4(6), 936-950 (2016).

Pim Donkers, Leo Pel, Dehydration/hydration of granular beds for thermal storage applications: a combined NMR and temperature study, Int. J. of Heat and Mass Transfer  105  826–830 (2017)