Influence of Phase Segregation and Concentration Polarization on Heat Releasing Behavior of Latent Heat Storage Material Na₂HPO₄· 12H₂O

Disodium hydrogen phosphate dodecahydrate is a promising candidate for latent heat storage material due to its fast crystal growth rate and high heat storage density per unit volume. In its practical application, however, a key technical problem has been to eliminate the formation of phase segregation and concentration polarization owing to the cyclic operation of heat storage and heat release. In the present study, in order to understand the influence of the formation of such phase segregation and concentration polarization on the heat release behavior of the heat storage material, the dodecahydrate melt samples formed them were prepared artificially and their solidification behavior were observed directly by eye. Additionally, the crystal growth rates of the dodecahydrates were measured and the features of each solidification behavior were evaluated quantitatively. As a result, the crystal growth rates were dropped to about 40% in both cases of the dodecahydrate melt diluted by phase segregation and the upper layer of the melt diluted by concentration polarization. Further, in another case of the lower layer of the melt concentrated by concentration polarization, the growth rate was depressed to about 20%. It was suggested that the formation of phase segregation and concentration polarization brought about not only a latent heat reduction, but also a heat releasing speed depression.