The solid-state hydration of salts has gained particular interest within the frame of thermochemical energy storage. In this work, the water vapor pressure−temperature (p−T) phase diagram of the following thermochemical salts was constructed by combining equilibrium and nonequilibrium hydration experiments: CuCl 2 , K 2 CO 3 , MgCl 2 •4H 2 O, and LiCl. The hydration of CuCl 2 and K 2 CO 3 involves a metastable zone of ca. 10 K, and the induction times preceding hydration are well-described by classical homogeneous nucleation theory. It is further shown for K 2 CO 3 (metastable) and MgCl 2 •4H 2 O (not metastable) through solubility calculations that the phase transition is not mediated by bulk dissolution. We conclude that the hydration proceeds as a solid− solid phase transition, mobilized by a wetting layer, where the mobility of the wetting layer increases with increasing vapor pressure. In view of heat storage application, the finding of metastability in thermochemical salts reveals the impact of nucleation and growth processes on the thermochemical performance and demonstrates that practical aspects like the output temperature of a thermochemical salt are defined by its metastable zone width (MZW) rather than its equilibrium phase diagram. Manipulation of the MZW by e.g. prenucleation or heterogeneous nucleation is a potential way to raise the output temperature and power on material level in thermochemical applications.
Efficient and cheap
storage of energy from renewable resources
presents a key technology to facilitate the ongoing energy transition.
Storing heat in thermochemical materials (TCMs), such as salt hydrates,
provides a promising concept to meet this demand. TCMs can capture
heat reversibly and loss-free by relying on equilibrium hydration
reactions of the salts. Persistent bottlenecks in the full-scale application
of this technology are the low mechanical resilience of salt grains
and their tendency to coagulate or dissolve when in contact with water
vapor. To overcome this, the salt grains can be encapsulated by a
stabilizing polymer coating. Ideal coatings combine high water vapor
permeability with reversible deformability to minimize the resistance
for water transport and to accommodate the volumetric changes of the
TCM during repetitive (de)hydration, respectively. Here, a systematic
study into the applicability of commercially available polymers as
coating materials is presented. Mechanical analysis and wet-cup experiments
on freestanding polymer films revealed that cellulose-based coatings
successfully combine permeability and ductility and meet the engineering
demands for domestic TCM-based heat storage applications. The validity
of using freestanding films as model system was confirmed by encapsulating
granular TCMs in ethyl and hydroxyl propyl cellulose using fluidized
bed coating. The permeability was retained and an enhanced structural
integrity of the TCM grains during (de)hydration cycles was observed.
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