Session: 03-01 Low Temperature Thermal Storage

Paper Number: 141710

141710 - Thermochemical Energy Storage for Buildings: Improved Hydration Kinetics and Cycling Stability Using Salt Mixtures 

Abstract: 

Approximately 60% of primary energy consumption in buildings is attributed to thermal end-uses, underscoring the importance of heat decarbonization within this sector. Solutions such as solar heating and renewable electrification show promise, but thermal energy storage (TES) is needed to align demand and supply given that these are intermittent sources. Among various TES categories, thermochemical materials (TCMs) stand out with average volumetric energy densities that are 5-10 times higher than sensible and phase change materials; this compactness is crucial considering the limited space in buildings. Inorganic salt hydrates represent a class of TCMs that have garnered recent attention for building applications as a thermal battery that can store and release heat via reversible solid-gas chemical reactions. In addition to their high volumetric energy storage capacity (>500 kWh m⁻³), salt hydrates demonstrate several advantages such as reaction temperatures suitable for low-grade heat storage, negligible heat loss (i.e., no self-discharge), low cost, and minimal health and safety risks.  The battery charging step comprises an endothermic reaction in which the salt is dehydrated using external heat, and water vapor is released. In the discharging step, water vapor is introduced to hydrate the salt, and the resulting exothermic reaction releases heat. Despite these advantages, material-level challenges due to structural changes and hygrothermal instabilities during charge-discharge cycling have limited their practical application in buildings. A notable limitation involves the formation of a liquid state through melting during dehydration or deliquescence during hydration. In both cases, agglomeration reduces the surface area-to-volume ratio, which creates a kinetic resistance for water vapor transport. Another limitation arises from nucleation barriers (metastable states) during hydration, which lead to slow reactions. This means that a higher driving force, such as a higher vapor pressure or lower temperature, is required to induce the phase transition of the salt from the dehydrated to hydrated state.

To address these inherent limitations of individual salts, this presentation will discuss the development of a binary mixture of SrCl₂ and MgCl₂. To enable a direct comparison between the individual salts and salt mixture, a standardized fabrication process is used of ball milling to particle sizes <50 µm and preconditioning to yield a structurally stable material with minimum water vapor transport resistance. Outcomes from this study demonstrate that the mixing of salts with a common ion (chloride in this case) produces no side reactions and formation of new compounds. Instead, the introduction of a highly hygroscopic and deliquescent salt, such as MgCl₂, into the mixture enhances reaction kinetics by overcoming the nucleation barrier of the other salt (SrCl₂) due to formation of a wetting layer. The binary salt mixture is thus capable of operating at a wider range of relative humidities (25-60% at 25°C) compared to its constituent salts. Lastly, a 50/50 by mass salt mixture demonstrates improved storage performance with a specific energy density of 1100 J g^-1, peak thermal power output of 1.4 W g^-1, and cycling stability needed for building applications.

Presenting Author: Erik Barbosa Georgia Institute of Technology

Presenting Author Biography: Currently, Erik is finishing his 3rd year as PhD student in Mechanical Engineering at Georgia Tech, where his goal is to become an expert in thermal science and engineering to contribute to climate change-focused research by using innovative technologies that decarbonize energy. He is an inaugural member of the Water Energy Research Lab (WERL) under Dr. Akanksha Menon, where his research is focused on developing thermochemical energy storage to decarbonize heat for building application. Particularly, he focuses on characterization and improvements of inorganic salt hydrate materials, as well as their packed bed reactor modeling and experimentation. He is also passionate about contributing to DEI initiatives in STEM and is a National Graduate Committee Lead of the Society of Hispanic Profession Engineers (SHPE).

Authors:

Erik Barbosa Georgia Institute of Technology
Akanksha Menon Georgia Institute of Technology