Role of Ba-containing Compounds in the Nucleation of Solid Calcium Chloride Hexahydrate

Currently, heating and cooling of commercial and residential buildings is one of the leading consumers of energy in the country, accounting for 40% of building energy demand and about 75% of all electricity use. One method used to increase HVAC system efficiency and to allow for more efficient load displacement, is to incorporate phase change materials (PCMs) into thermal energy storage systems, as PCMs are able to passively store and release heat, allowing for a pathway to displace peak energy load for buildings to better align with available excess energy generation. However, robust and economical phase change materials that are tailorable to a specific desired energy storage temperature are needed to accomplish this task. Salt hydrates represent one class of PCMs of interest due to their high volumetric energy densities, low cost, and the ability to tailor the energy storage temperature through the design of custom salt hydrate eutectics. However, they can also experience undercooling, a nucleation-limited phenomena which results in the existence of a metastable liquid below the melting point. This phenomena can lead to undesired issues, including incongruent melting and phase segregation. To induce nucleation, and thereby improve the reversibility of the thermal energy storage process, nucleation agents can be added to a PCM system to induce nucleation, and thereby suppress the formation of metastable phases. In many cases, the underlying mechanisms governing which nucleation agents introduce potent nucleation sites and which do not, are not well understood.<br/>In this study, we investigated the dependence of undercooling in calcium chloride hexahydrate (CaCl₂.6H₂O), a low-cost salt hydrate that melts at 29°C, on the crystal structure, chemistry, and the solubility of a family of Ba-based compounds. We utilized calorimetric techniques to measure undercooling in small quantities of calcium chloride hexahydrate with different nucleation agents, and differing weight percentages of the nucleation agents. We observed in several cases that the addition of barium-based nucleation agents resulted in at least a 10°C reduction in the undercooling, enabling practical utilization of CaCl₂.6H₂O. However, some variability existed between different compounds, as nucleation agents such as barium carbonate and barium hydroxide were observed to be effective nucleators and resulted in a larger reduction of the undercooling, while nucleation agents such as barium iodide or barium chloride dihydrate had minimal effects on the reduction of the undercooling. Additionally, we observed that the presence of insoluble particles of the nucleation agents aided in reducing the undercooling of the system and also resulted in more consistent melting behavior. The structure of the nucleation agents was also noted to have little effect on the undercooling, however it was observed that a cation substitution reaction can occur while the Ba-based nucleation agent is in solution. Therefore, a resulting phase from the reaction of the Ba-based nucleation agent with calcium chloride hexahydrate could be responsible for the reduction in undercooling, which is an area that will be further investigated. If we can pinpoint the attributes and mechanisms that make Ba-based compounds effective nucleation agents, then it will allow us to design more efficient phase change materials that are better suited for thermal energy storage applications.<b> </b>