Numerical Comparison between Phase Change Temperatures of Latent Heat Storage Material Na₂HPO₄ Aqueous Solutions Measured by Thermal Cycling and Phase Diagram Values for the Na₂HPO₄‒H₂O System

Disodium hydrogen phosphate dodecahydrate is a latent heat storage material that is expected to be applied to in-door air conditioning and floor heating, but the heptahydrate is stable at above the melting point of the dodecahydrate and can therefore crystallize and accumulate in the melt of the dodecahydrate during a long-term thermal cycle of heat storage and heat release, thereby reducing the latent heat of storage material. To avoid such phase segregation, an excess amount of water is generally added relative to the liquidus line of the heptahydrate on the phase diagram, but this has the drawback of reducing the latent heat. In addition, it is extremely difficult to predict the transient behavior of phase transition of the heptahydrate during heat storage and heat release by means of the phase diagram in the equilibrium system. In this study, to make clear the correspondence between the phase transition behavior of the material and the phase diagram, thermal cycle tests in the temperature range of 243‒353 K were performed on aqueous solutions of 24.7‒52.9 wt% disodium hydrogen phosphate, and phase change behaviors were observed from the obtained cooling and heating curves. Three crystal phases were identified, namely, a eutectic mixture of dodecahydrate and water, do-decahydrate, and heptahydrate, and their melting points were in agreement with the phase diagram. However, the po-sitions of the liquidus lines of dodecahydrate and heptahydrate were different from the phase diagram, indicating that they were on the higher concentration side.