Encapsulation of Salt Hydrates by Polymer Coatings for Low-Temperature Heat Storage Applications

Abstract

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.

Enhanced Thermal Conductivity of Phase Change Microcapsules Based on Boron Nitride/Graphene Oxide Composite Sheets

“…12−14 In fact, the encapsulation of inorganic PCMs like ionic liquids and salt hydrates via a variety of polymer materials has also been adopted to improve the cycling phase change capability. 11,15,16 Generally, high-performance phase change microcapsules (PCMCs) shall have the attributes including high thermal conductivity (TC), high thermal-energy storage capacity, and good thermal stability. 9,17,18 Limited by the intrinsically low TC values of common polymers and organic PCMs, the thermal conduction ability of polymeric PCMCs is relatively poor.…”

“…However, organic PCMs, like paraffin, are often hampered by leakage problems when they are undergoing solid to liquid phase transition. Encapsulation of PCMs through polymeric, inorganic, or hybrid shells (SiO 2 , polystyrene, poly­(methyl methacrylate), polyurethane, polyurea, urea–formaldehyde resins, melamine–formaldehyde (MF) resins, and biodegradable polymers) to form nanocapsules or microcapsules has been widely used to solve the leakage problem of organic PCMs. − In fact, the encapsulation of inorganic PCMs like ionic liquids and salt hydrates via a variety of polymer materials has also been adopted to improve the cycling phase change capability. ,, …”

Reactive Grand-Canonical Monte Carlo Simulations for Modeling Hydration of MgCl2

“…Recently, much research focuses on enhancing pure salts to overcome the previously mentioned drawbacks, for example, a mixture of multiple salts, 14 double salts, 15,16 encapsulation, 17 composites, 18 impregnation of salts in porous materials, 19 or doping of pure salts. 16,20,21 For most common salts, the equilibrium conditions can be estimated from thermodynamic tables.…”

“…Recently, much research focuses on enhancing pure salts to overcome the previously mentioned drawbacks, for example, a mixture of multiple salts, double salts, , encapsulation, composites, impregnation of salts in porous materials, or doping of pure salts. ,, For most common salts, the equilibrium conditions can be estimated from thermodynamic tables. , However, due to the increased complexity of interacting elements that influence the equilibrium conditions or missing thermodynamic tables for new enhanced salts, the reaction equilibrium description becomes increasingly complex compared to pure salts. As a result, the equilibrium cannot always be described by simple thermodynamic rules without assumptions or educated guesses for unknown model parameters.…”

Macro‐porous permeability aspects of MgSO4 salt hydrate foams for energy storage applications

“…This could lead to long-term stability issues, due to the triggering of internal stresses in the host matrix due to the expansion of the saline solution during the hydration step that limit the material stability. 25,26 Furthermore, the matrices have usually an open porosity, which (especially for composites with large amount of salt) is not able to retain the hydrated solution in case of supersaturation, causing a release of the salt with consequent loss of efficiency as well as degradation and corrosion risks for the reactor. 19 Instead, polymeric foams, characterized by a flexible and ductile structure, are able to suffer the volume expansion induced by the salt hydration.…”