Abstract
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.
Highlights
Space heating and domestic hot water consumes the majority energy demanded by residential buildings, e.g. about 82% in Europe [1]
This drawback can be treated as a valuable merit for a different purpose – the long-term thermal energy storage; because of the large supercooling degree, the melted salt hydrate can remain at liquid state at relatively low temperature without losing any stored latent heat, only sensible heat is lost throughout the storage period, the system is not necessarily thermal insulated but can still discharge the majority of the stored thermal energy
The present paper modelled and simulated the solidification process of the supercooled sodium acetate (SA) aqueous solution to evaluate the thermal discharge performance of the storage system
Summary
Space heating and domestic hot water consumes the majority energy demanded by residential buildings, e.g. about 82% in Europe [1]. Sodium acetate trihydrate (SAT) is one of the typical salt hydrate PCMs, which has the melting temperature of 58.0 °C and latent heat of 264.0 kJ/kg [4]. The test of SAT with 9% extra water (199.5 kg) in flat rectangular container showed the reduction of discharged thermal energy from 194 kJ/kg to 179 kJ/kg after 20 test cycles; while SAT with 1% CMC (220 kg) can maintain the discharge thermal energy at 205 kJ/kg over six test cycles Both additions led to 45 °C discharge temperature when the heat transfer fluid flow rate at 2 L/min, and the discharge power reduced from 4.5 to 5.0 kW at the beginning to about 1.0 kW within about 4 h [8]. Thermo-physical properties of liquid SA aqueous solution and solid SAT were both summarized, and the experimentally determined SAT crystal growth rate and the enthalpy change of the solidifying SA aqueous solution were introduced to the model in this paper for elaborating modelling
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