Phase diagram prediction and experimental verification of a binary hydrated salt mixture as phase change thermal storage materials

As an important inorganic phase change materials (PCMs), hydrated salts have many advantages such as high latent heat, moderate thermal conductivity, low cost and excellent fire safety compared with organic PCMs. The phase change process of hydrated salts has a significant effect on thermal storage performance. It is meaningful to predict the phase change process by the thermodynamic models. The modified BET model is an effective method to predict the phase diagram of water–salt system. In this work, an applicability of modified BET model used for Na2SO4·10H2O and KAl(SO4)2·12H2O binary hydrated salt mixture was confirmed. According to the BET parameters and solubility constants, the solubility phase diagram of this binary hydrated salt mixture was predicted with modified BET model, and Na2SO4·7H2O was found when Na2SO4·10H2O was lower than 44 wt%. Besides, the phase diagram and the existence of Na2SO4·7H2O were validated in the DSC and XRD measurements. The function of Na2SO4·7H2O to eliminate phase separation was interpreted as that it acts as a thickener to prevent water molecules from escaping from the liquid and avoids the formation of anhydrous Na2SO4. This work could guide the design of binary hydrated salt mixtures as thermal storage materials, and provide new insights for phase separation elimination of hydrated salt mixtures.