Heat Storage Performance of Disodium Hydrogen Phosphate Dodecahydrate: Prevention of Phase Separation by Thickening and Gelling Methods

Abstract Disodium hydrogen phosphate dodecahydrate (Na2HPO4·12H2O) is an attractive candidate for phase change materials. The main problem for its practical use comes from incongruent melting character during thermal cycling. Experimentally, heat of fusion of the pure salt decreased from 200 to 25 J·g−1in a four‐run freeze‐thaw cycling. Additives such as thickening agent orin‐situsynthesized polyacrylate sodium in the molten salt can prevent its phase separation to some extent. In the test, sodium alginate 3.0%–5.0% (w/w) thickened mixture containing Na2HPO4·12H2O and some water showed constant heat storage capacities. Polyacrylate sodium gelled salt was synthesized through polymerizing sodium acrylate in the melt of Na2HPO4·12H2O and some extra water at 50 °C. Optimum conditions composed of sodium acrylate 3.0%–5.0% (w/w), cross‐linking agentN,N‐methylenebisacrylamide 0.10%–0.20% (w/w), K2S2O8and Na2SO3(mass ratio 1:1) 0.06%–0.12% (w/w). As opposed to normal large crystals of pure Na2HPO4·12H2O in solid state, the gelled salt existed in a large number of tiny particles dispersed in the gel network at room temperature, commonly less than 2 mm. But only those sample particles with sizes less than 0.2 mm may have relatively stable thermal storage property. A problem encountered was the poor reproducibility of the synthesis method:heat storage capacity of the product was often very different even though the synthesis was carried out in the same conditions. An alternative gelling method by sodium alginate grafted sodium acrylate was tried and it showed a fairly good effect. Heat capacities and heat of fusion of Na2HPO4·12H2O were measured by an adiabatic calorimeter.