Preparation Of Gelled/Microencapsulated Low Temperature Phase Change Materials

Latent heat energy storage is one of the most efficient ways of storing thermal energy. Latent heat storage system based on phase change materials' (PCMs) melting and freezing is particularly suitable for solar energy heating, peak-shift of electrical demand and heat recovery. Phase change materials investigation is the first step of developing a latent heat storage system and thus is always the focus in related research.Disodium hydrogen phosphate dodecahydrates possesses the largest mass heat storage capacity among the low temperature phase change materials, which are classified as inorganic, organic and then: composites, yet it experiences phase segregation and super cooling when melting and hinders its application in latent heat storage system. Patent method of avoiding the phase segregation of the hydrate by in-situ synthesizing, thickening agent, polyacrylate sodium in the melt of it was proved to be effective but poor in reproducibility, that is, heat storage property varies largely from batch to batch.A new method was investigated of in-situ synthesizing starch grafted sodium acrylate as thickening agent for disodium hydrogen phosphate dodecahydrates. The PCMs obtained at optimum polymerizing conditions exhibit steady heat storage properties during freezing-thaw cycles, which correspond to hydrate crystal sizes less than 0.2 mm in diameter at solid state. Measuremets of crystal size distribution show that the smaller the crystal, the more steady the heat storage capacity. Variations of melting points in melting and freezing cycles also relate to heat storage property of the material. Stable PCM fuses in a temperature range less than 2℃.Another method was studied for in-situ synthesizing sodium alginate grafted sodium acrylate in disodium hydrogen phosphate dodecahydrates' melt. The productfrom optimum synthesis conditions presents stable heat storage property in thermal cycling tests like starch grafted sodium acrylate material.Microencapsulation of wax through interfacial polymerization, instead of patented in-situ polymerizing method, was successfully performed using toluene-2, 4 - diisocyanate and diamines, diethylenetriamine, ethylene diamine and 1, 6-hexanediamine as soluble monomers in organic and water phases, respectively. The capsules obtained are about 0.2 urn in diameter with a spherical shape. Heat storage capacity of the microcapsules increases with the core material, wax. Capsulation efficiency of the process varies from 60% to 80% with a tendency of a high efficiency in small feed ratio of core to wall materials. Thermal analysis shows that three polyurea wall materials start to decompose in nitrogen atmosphere at about 250 ℃, which is much higher than that of wax, n-eicosane, about 160℃. All the results indicate that interfacial polymerization between isocyanate and amine to form polyurea wall material is a quick, effective way for capsulation of wax.