PHASE-CHANGE MATERIALS FOR ENERGY-STORAGE NUCLEATION TO PREVENT SUPERCOOLING

Phase change materials (PCMs) are useful for storing heat as the latent heat of fusion. Such storage has Potential in heating and cooling buildings, waste heat recovery, off-peak power utilization, heat pump systems, and many other applications. Among the PCMs that have proven useful in heat storage applications are calcium chloride hexahydrate, CaCl2 . 6H2O, magnesium chloride hexahydrate, MgCl2 . 6H2O, and magnesium nitrate hexahydrate, Mg(NO3)2 . 6H2O. Many salt hydrate PCMs, including those listed above, have the disadvantage that during extraction of stored heat the material supercools before freezing. This reduces the utility of the material, and if too severe can completely prevent heat recovery. Many factors determine whether a particular additive will promote nucleation, for example, crystal structure, solubility, and hydrate stability. Candidate isomorphous and isotypic nucleating additives, with crystal structures that fit well with the PCM structure, were selected from tables of crystallographic data. Epitaxial nucleators, with less obvious lattice structure features that promote nucleation, were selected mostly by intuition. Effective nucleators were discovered by both methods. Based on laboratory test results, promising materials were developed into formulations based on CaCl2 . 6H2O, MgCl2 . 6H2O, Mg(NO3)2 . 6H2O, Mg(NO3)2 . 6H2O-MgCl2 . 6H2O eutectic, and Mg(NO3)2 . 6H2O-NH4NO3 eutectic salt hydrate PCMs. Subsequently, attempts were made to correlate crystal structure and hydrate stability with nucleating efficacy, and to speculate about active nucleating structures.