# Inorganic Salt Hydrate for Thermal Energy Storage
## 4 Phase Separation of Salt Hydrates  
A number of salt hydrates with large latent heat which can be regarded as PCMs cannot melt congruently [11]. As a result, the amount of water released by salt hydrates is no enough to dissolve the crystalline salt formed during the process of dehydration. Residual salts will deposit on the bottom due to a larger density and then lead to phase separation. With the continuous cold-thermal cycle, the sediments will continue to increase, resulting in gradually declining of thermal storage capacity. Thus, phase separation is another major problem of salt hydrate PCMs [40]. Adding thickener agents into salt hydrates is one of common methods to minimize or avoid phase separation. Many researchers used hydroxyethyl cellulose, nanocellulose, cellulose [41; 42; 43; 44], super absorbent polymer [45], polyethylene glycol [46] and some silicone derivatives [47] as thickeners. Porous materials such as expanded graphite [29; 48], diatomic [49; 50; 51] and nano-titanium dioxide [52; 53] etc. were also reported as fine supporting materials for inorganic materials. They increase the viscosity of solution so that the solid particles can be evenly distributed. In addition to this, microencapsulation, macro encapsulation (hermetically packed in vessel) and impregnation with porous materials to prepare composite PCMs are effective methods. This cannot only solve problems of phase separation, but can also effectively prevent leakage problem attributing to the melting process of PCMs. Microencapsulation phase change materials (MEPCMs) usually consist of a polymeric material as shell and a desired PCM as core [54]. Microencapsulated salt hydrates can reduce the reactivity with the outside surroundings, which can be utilized in powder form or dispersed into a carrier fluid, and then prevent PCMs from phase separation and leakage. Physical and chemical processes are two major methods to prepare MEPCMs. Physical methods typically consist of spray drying, fluidized bed and complex coacervation. Interfacial polymerization, in situ polymerization, suspension-like polymerization, and sol-gel methods make up chemical method [55; 56]. Hitherto, vast researches have accomplished to formulate MEPCMs, whereas most of them focused on organic materials as cores because it is more troublesome to formulate water-solubility salt hydrate MEPCMs. Consequently, only a few literatures have reported hydrated salt microencapsulation. [PERSON] [57] outlined a novel microencapsulated PCM which used sodium acetate trihydrate as core materials. Its ability of dispersions into a matrix and variation of mechanical properties were investigated. The study performed two different emulsions: emulsion by using microfluidic device with acrylate as shell material and emulsions preparing by mechanically creating with polytrimethylene carbonate (PLA) as shell. Figures 1 and 2 shows SEM image and optical microscope image of the PLA microcapsules and microfluidic device as microcapsules, respectively. From the results we can clearly see more stable and less dispersive microcapsules by microfluidic device. The microcapsules showed outstanding performance without leakage and phase separation. Fully loaded microencapsulated PCM with Na\({}_{2}\)HPO\({}_{4}\cdot\)7H\({}_{2}\)O as core and methyl methacrylate as shell was prepared by [PERSON] et al. [58], which was exploited suspension copolymerization-solvent volatile method. During the procedure of polymerization reaction, the original salt hydrate Na\({}_{2}\)HPO\({}_{4}\cdot\)12H\({}_{2}\)O lost 5 crystal waters through heating process and the remainder was successfully scattered into organic solvent. It was worthy noted that the thermal conductivity did not seem to change after microencapsulated Na\({}_{2}\)HPO\({}_{4}\cdot\)7H\({}_{2}\)O formed. This excellent PMMA/Na\({}_{2}\)HPO\({}_{4}\cdot\)7H\({}_{2}\)O MEPCM can be deemed as a potential material for solar space heating.