Thermochemical energy storage is an emerging technology for compact and long term heat storage, which is needed to synchronise renewable energy supply and demand and make industrial waste heat transportable and tradable. Salt hydrates are suitable materials for this purpose, because they can undergo reactions with high energy densities in the required temperature range and are inexpensive. However, operation related to the intrinsic salt hydrate properties often requires more favourable conditions (temperature and pressures) and the salt stability during cycling (physical and chemical) is limited.
Microencapsulation is employed to overcome the above-mentioned limitations and in addition decrease the corrosive nature of salt hydrates. The working principle of microencapsulation is the confinement of the salt hydrate in an inherently stable matrix. Moreover, structuring of the salt/matrix particle allows increasing the available surface for thermochemical reaction and thereby favourably manipulating the reaction kinetics. First, a suitable polymeric material (cellulose-derivative) was selected based on water vapour permeability, low water uptake, mechanical properties and stability during cycling conditions. Second, a procedure was developed for encapsulation based on spray-drying. This yields a particle that shows both characteristics of a matrix-type structure as well as of a core-shell structure. The salt and polymer are present throughout the particle (matrix-type), but the polymer is enriched on the outer surface and the salt is predominantly present in the core (core/shell).
An elaborate investigation with respect to the corrosion and physical stability of the prepared material, as well as its thermal and mass-transport properties is presented here. A comparison is made between encapsulated and bulk CaCl2 hydrate. Morphological characterization is performed by scanning- and transmission electron microscopy as well as X-ray diffraction. Based on these results an explanation is provided for the particle formation. The composite’s thermal properties are characterized by thermogravimetric analysis and differential scanning calorimetry. This yields information on the energy storage density and shows significantly enhanced dehydration kinetics with respect to bulk calcium chloride. The extent of corrosion on cupper foil and physical stability at elevated temperatures is measured gravimetrically and by visual inspection. The measurements did not show an indication of corrosion for the encapsulated materials and the physical stability is significantly improved: the materials did not liquefy upon water uptake and at elevated temperatures.
In conclusion, microencapsulation greatly improved the stability and reaction kinetics of calcium chloride for the use in thermochemical energy storage. The improved performance can be attributed to the stable nature of the stabilizing matrix (cellulose-derivative) and the particular morphology of the encapsulated CaCl2 particles.
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