Salt Hydrate Phase Change Materials Research Articles

Integrated solar collector storage system based on a salt-hydrate phase-change material

Integrated solar collector storage system based on a salt-hydrate phase-change material

Integrated solar collector storage system based on a salt-hydrate phase-change material

A new integrated collector storage (ICS) concept for low-temperature solar heating of water is described. The solar energy is stored in a salt-hydrate phase-change material (PCM) held in the collector and is discharged to cold water flowing through a surface heat exchanger located in a layer of stationary heat transfer liquid (SHTL), floating over an immiscible layer of PCM. A theoretical model for the charging process of the proposed integrated collector is presented. The model assumes one-dimensional transient heat conduction in the PCM and SHTL layers and neglects the effect of convection heat transfer in these regions. The model was solved numerically by an enthalpy-based finite differences method and validated against experimental data. The results of parametric studies on the effect of the transition temperature and of the thickness layer of the salt-hydrate PCM on the thermal performance of the charging process are also presented.

Phase change materials for energy storage nucleation to prevent supercooling

Abstract Phase change materials (PCMs) are useful for storing heat as the latent 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 formalations 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.

Organic phase change materials for thermal energy storage

Salt hydrate phase change materials used for thermal storage in space heating and cooling applications have low material costs, but high packaging costs. A more economic installed storage may be possible with medium priced, high latent heat organic materials suitable for low cost packaging, i.e. those that are insoluble in water and unreactive with air and some of the common packaging films. We have done a preliminary survey of 12 such organic materials with melting points in the range 10–43°C. Measurements of melting point, freezing point, and the latent heats of melting and fusion are presented. For distributed passive solar storages, butyl stearate, m.p. = 19°C, f.p. = 21°C, ΔH m = 120 J/g, current cost 55¢/lb, seem promising. For central storage, both vinyl strearate, m.p. = 27° C, f.p. = 29° C, †H m = 122 J/g , and mixtures of ethoxylated (C 11-C 15) linear alcohols, 30°C < m.p. < 41°C, 31°C < 40°C, 105 J/g < ΔH m < 134 J/g, current costs between 38¢/lb and 57¢/lb, for average molecular weights between 1080 and 1520, warrant further study. Isopropyl stearate, m.p. = 14°C, f.p. = 18°C, ΔH m = 142 J/g, should be considered as a “coolness” storage for desert regions. The other materials showed either supercooling or inappropriate melting points or lower latent heats than the 4 selected for further study.