(506e) Salt Hydrate Composites with Improved Cyclability for Thermochemical Energy Storage in Buildings

Fossil fuels are widely used for heating, especially in the buildings and industrial sectors. In the United States, 60% of the energy consumption in the residential buildings sector is attributed to thermal loads such as space heating and hot water. Heat decarbonization is thus critical, motivating the development of a thermal battery that can be charged with low-cost electricity or solar energy and can discharge zero-carbon heat to meet thermal loads on demand. Thermochemical reactions can be leveraged for heat storage and release given their high volumetric energy densities and negligible self-discharge for long duration applications. Thermochemical materials (TCMs), such as salt hydrates, undergo reversible solid-gas chemical reactions at temperatures below 100°C that make them suitable for integration with buildings. The charging reaction is an endothermic reaction that dehydrates the salt, whereas the discharging reaction is an exothermic reaction where the salt hydrates. For a practical application, TCMs must be able to withstand hundreds of charge-discharge cycles, however literature has demonstrated that significant structural changes limit the reversibility of the reaction. For example, CaCl₂ experiences a 60% volume reduction as it dehydrates from its hexahydrate to anhydrous state. This cycling-induced stress concentration leads to material pulverization, decreasing energy storage capacity after only a few cycles. In addition to structural changes, the hydration reaction can cause deliquescence, which then leads to agglomeration when the salts are dehydrated. This hygrothermal instability increases water vapor diffusion resistance, which reduces the kinetics and energy density of these materials.

To address these challenges, we develop composite architectures that reduce thermo-mechanical degradation of salt hydrates and subsequently enhance thermal battery performance. Two approaches are demonstrated to improve material cyclability: (i) salt encapsulation within a porous polymer matrix, and (ii) salt impregnation within mesoporous silica. In the first approach, cyclability is improved by synthesizing different composites that improve the mechanical properties of salt hydrates, such as CaCl₂, by incorporating it within a hydrogel matrix with elasticity, such as alginate and polyacrylamide. To address agglomeration, in the second approach CaCl₂ is nanoconfined within porous silica that shifts the hydration behavior by leveraging capillarity. The energy density of the different composites is characterized and the hygrothermal stability over multiple hydration and dehydration cycles is investigated. The hydration kinetics of these salt composites are evaluated through measuring the water uptake using a TGA-DSC for various relative humidity and temperature settings. It is then possible to compare various salt composite hydration extents through obtaining experimental kinetic constants of these salts and computationally modeling the reaction advancement using solid-gas reaction models. Overall, this study compares a variety of composite salt hydrates, and assesses the coupled relationship between their mechanical, kinetic, and thermophysical properties to achieve materials that can significantly enhance long-term energy storage performance for building applications.