Study on solidification process of sodium acetate trihydrate for seasonal solar thermal energy storage

The phase change of sodium acetate (SA) aqueous solution to sodium acetate trihydrate (SAT) requires large supercooling degree, then the aqueous solution can be at liquid state at fairly low temperature without releasing the stored latent heat. Such a feature makes SAT a promising material for seasonal solar thermal energy storage. The present study firstly summarized the thermo-physical properties of the solid SAT and liquid SA aqueous solution at different temperatures and concentrations, including equilibrium temperatures, densities, specific heats and thermal conductivities. The calculation methods of these properties have been established. Secondly, with the aid of the above properties, a mathematic model of the thermal discharge process of the storage system, i.e. the solidification process of supercooled SA aqueous solution, was built based on the heat transfer between the phase changing material within a single storage tube and the external flowing heat transfer fluid (HTF). The experimentally obtained SAT crystal growth rate and the enthalpy change of solidifying supercooled SA aqueous solution were employed to aid the modelling. The discharge temperature and thermal power of the storage system were numerically obtained and analysed. The influence of the ambient temperature, the mass flow rate as well as the heat transfer coefficient of the HTF on the thermal discharge performance were discussed. Finally, the seasonal thermal storage density of SAT was given and compared to that of water and some sorption materials.