Thermophysical Characterization of MgCl ₂·6H ₂O, Xylitol and Erythritol as Phase Change Materials (PCM) for Latent Heat Thermal Energy Storage (LHTES)

Abstract

The application range of existing real scale mobile thermal storage units with phase change materials (PCM) is restricted by the low phase change temperature of 58 ${}^{\circ }\mathrm{C}$ for sodium acetate trihydrate, which is a commonly used storage material. Therefore, only low temperature heat sinks like swimming pools or greenhouses can be supplied. With increasing phase change temperatures, more applications like domestic heating or industrial process heat could be operated. The aim of this study is to find alternative PCM with phase change temperatures between 90 and 150 ${}^{\circ }\mathrm{C}$ . Temperature dependent thermophysical properties like phase change temperatures and enthalpies, densities and thermal diffusivities are measured for the technical grade purity materials xylitol (C ${}_5$ H ${}_{12}$ O ${}_5$ ), erythritol (C ${}_4$ H ${}_{10}$ O ${}_4$ ) and magnesiumchloride hexahydrate (MCHH, MgCl ${}_2$ $·$ 6H ${}_2$ O). The sugar alcohols xylitol and erythritol indicate a large supercooling and different melting regimes. The salt hydrate MgCl ${}_2$ $·$ 6H ${}_2$ O seems to be a suitable candidate for practical applications. It has a melting temperature of 115.1 ± 0.1 ${}^{\circ }\mathrm{C}$ and a phase change enthalpy of 166.9 ± 1.2 $\mathrm{J}/\mathrm{g}$ with only 2.8 $\mathrm{K}$ supercooling at sample sizes of 100 $\mathrm{g}$ . The PCM is stable over 500 repeated melting and solidification cycles at differential scanning calorimeter (DSC) scale with only small changes of the melting enthalpy and temperature.