Preparation And Properties Of Expanded Graphite / Hydrated Salt Composite Shaped Phase Change Materials

The inorganic hydrated salt phase change energy storage material occupied an important position in the applications of solar energy medium-low temperature phase change energy storage material, which had the advantages of high latent heat value, high energy density, higher thermal conductivity than the organic, low price and easy to get. However, there were two fatal flaws that were supercooling and phase separation, which had become the problems of inorganic hydrated salts phase change material to be solved. Three inorganic hydrated salt phase change materials with high latent heat value and suiting for solar heating had been selected to be modified in this paper, the "solid- solid" composites phase change material was prepared by conducting the vacuum absorption between the material after adding nucleating agent and expanded graphite. And the supercooling degree and phase separation of this material were analyzed and experimental test,the results showed that the composite phase change material with the characteristics of smaller cooling degree, no phase separation, no leakage, higher thermal conductivity and stable performance had been prepared by the methods in this paper.Expanded graphite had good nucleation properties for inorganic hydrated salt, when nucleating agent was not added, the supercooling degree of sodium sulfate decahydrate decreased from 11.4℃ to within 0.5℃ by adding expanded graphite of 5%. The supercooling degree of sodium acetate trihydrate reduced from above 38℃ to about 14℃ by adding expanded graphite of 10%. The supercooling degree of octahydrate decreased from 11.4℃ to about 4℃ by adding expanded graphite of 1% ~ 3%. The mechanism of which was that the solid expanded graphite provided the binding sites for crystal, which reduced the driving force required for crystallization, thus resulted in the decrease of supercooling degree.The optimum composition of sodium sulfate decahydrate/expanded graphite composite phase change material was 2% Borax + 8% expended graphite + 90% Sodium sulfate decahydrate, the supercooling degree, latent heat and energy density of composites phase-change material were within 0.6℃, 225.77 k J/kg and 218.09 MJ/m3, respectively, in this case, the thermal conductivity was also greatly improved. Compared with the sodium sulfate decahydrate of only adding nucleating agent borax, the time of heat storage could be shortened by 52.6%, and the time of heat release could be shortened by 55.1%. There was no attenuation phenomenon in the performance of sodium sulfate decahydrate/expended graphite after 500 times rapid heating and cooling cycles, and the volume change was small in phase change process, no liquid produced, encapsulation was convenient.The optimum performance ratio of expanded graphite/sodium acetate trihydrate composite phase change material was 91% CH3COONa·3H2O + 6%EG+ 1% Na2HPO4·12H2O, the latent heat was 233.5 k J/kg, no supercooling and phaseseparation in this case, when compared with the sodium dihydrogen phosphate ofonly adding nucleating agent, the time of heat storage could be shortened by75.3%.The optimum composition ratio of expanded graphite/octahydrate composite phase change material was 93% Ba(OH)2·8H2O + 6% EG+1% KH2PO4, meanwhile there was no liquid exudation in the phase transition process of composite phase change material. The latent heat was 248.3 k J/kg, supercooling degree was within 0.5℃, when compared with the pure substance, the heat storage time could be shortened by 45.1% and the time of heat release could be shortened by 54.5%.