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

Abstract

Disodium hydrogen phosphate dodecahydrate (Na 2 HPO 4 •12H 2 O) 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 -1 in a four-run freeze-thaw cycling.Additives such as thickening agent or in-situ synthesized 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 Na 2 HPO 4 •12H 2 O and some water showed constant heat storage capacities. Polyacrylate sodium gelled salt was synthesized through polymerizing sodium acrylate in the melt of Na 2 HPO 4 •12H 2 O and some extra water at 50 ℃. Optimum conditions composed of sodium acrylate 3.0%-5.0% (w/w), cross-linking agent N,N-methylenebisacrylamide 0.10%-0.20% (w/w), K 2 S 2 O 8 and Na 2 SO 3 (mass ratio 1∶1) 0.06%-0.12% (w/w). As opposed to normal large crystals of pure Na 2 HPO 4 •12H 2 O 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 Na 2 HPO 4 •12H 2 O were measured by an adiabatic calorimeter.

The Potential Environmental and Social Influence of the Inorganic Salt Hydrates Used as a Phase Change Material for Thermal Energy Storage in Solar Installations

“…Disodium hydrogen phosphate dodecahydrate (DHPD) is considered an excellent hydrated salt for storing latent heat due to its high latent heat and almost no phase separation. Its greatest disadvantages are supercooling and dehydration after exposure to air [28,45]. The leakage problem can be prevented by encapsulating it in porous expanded vermiculite (EV) [28].…”

Hybrid Polymer Salogels for Reversible Entrapment of Salt-Hydrate-Based Thermal Energy Storage Materials

“… − In contrast, covalently crosslinked polymer gels, which also often require large polymer amounts (5%–60% by weight), provide shape stabilization (Figure S1). − However, the permanent nature of the crosslinks renders these materials irreversible, making filling and removal from thermal energy storage modules impossible. In contrast to these previous strategies, our group introduced temperature-responsive polymer salogels (polymer gels in inorganic salt hydrates) based on hydrogen bonding polymers such as poly­(vinyl alcohol) (PVA), for reversible shape stabilization of liquid salt hydrates. , The on-demand reversibility of gelation by heating above a gel-to-sol transition temperature ( T gel ) allows filling and removal of the salogels from thermal energy storage devices, whereas gelation below T gel provides shape stabilization above the melting temperature of the salt hydrate ( T m ).…”

Crystallohydrate Loaded Halloysite Nanocontainers for Thermal Energy Storage

“…There are several methods to solve it. One of them is to add thickening agents such as sodium polyacrylate to stop phase separation in the beginning . However, the melting enthalpy retains only for maximum 10–15 thermal cycles due to the boundary interface appeared between PCM crystals and thickening agent.…”