Salt Hydrate Eutectics: Tailored Design of Equilibria, Reversibility, and Stability in Complex Phase Change Material Systems

A critical aspect of thermal energy storage systems for the built environment is the development of thermal energy storage media that will reversibly store and discharge thermal energy repeatedly within a defined temperature range. For example, to optimize efficiency, some environmental climate control and air conditioning systems require thermal energy storage between 5 to 25 °C. To address this need, eutectic salt hydrates have been identified as promising candidate systems that will allow for the tailoring of bespoke low cost thermal energy storage systems, due to the large number of potential eutectics distributed across a broad temperature range. However, these phase change materials are associated with a number of known limitations (undercooling, phase segregation, low heat transfer rate) which limit their practical use in energy storage systems. Individual strategies have been identified to overcome many of these challenges. For example, nucleation agents are utilized to decrease undercooling, while thickeners or polymers which form salogels are utilized to limit phase segregation. However, the interaction between these different strategies has not been systematically investigated.<br/><br/>Here, we present recent efforts to expand the palette of low cost high energy storage density salt hydrate eutectics developed for building thermal energy storage applications. We compare theoretical predictions against validated eutectic compositions and properties, and highlight some of the advantages and challenges associated with these materials. We specifically address efforts to overcome the known technical limitations of salt hydrates, and their relevance to salt hydrate eutectic systems, including 1) the introduction of specific nucleation agents to promote reversibility in salt hydrate systems, 2) the introduction of polymer salogels to stabilize systems from phase segregating, 3) the introduction of expanded graphite networks to improve heat transfer through these systems, 4) the use of encapsulants to encase PCM particles, and 5) the use of scaled testbeds to evaluate stability of systems under use conditions. Addressing all of these challenges results in a complex multi-phase material system which introduces additional interactions. Here, we will focus on both individual strategies and the resulting interactions, using development of zinc nitrate hexahydrate-based eutectic systems as an example model system. We will demonstrate successful implementation of tailored low-cost PCM systems, and will emphasize ongoing challenges in this area.