Flexible Heat Storage Based on Stable Supercooling of Sodium Acetate Trihydrate

Stable, efficient and flexible supply of renewable energy is important for achieving the goals on peak carbon dioxide emissions and carbon neutrality. Adding energy storage systems can improve the stability and flexibility of renewable energy systems. Typically, a short-term heat storage for example a hot water tank has a high utilization frequency, but a low energy storage density. Consequently, it lacks the ability to overcome extreme weather. While a long-term heat storage utilizing stable supercooling of sodium acetate trihydrate has a low utilization frequency and a slow response to heat demand. In this research, a flexible heat storage concept combining short-term and long-term heat storage is proposed, by decoupling the sensible heat and the latent heat of the heat storage. Flexible heat storage depends on stable supercooling of phase change material.

A phase change composite based on sodium acetate trihydrate utilizing its stable supercooling was developed in this research. Its melting point is 53~58℃, the latent heat is 200~215 kJ/kg, the solid specific heat capacity is 2.7~2.9 kJ/(kg·K), and the liquid specific heat capacity is 3.0~3.2 kJ/(kg·K). Further, a full-scale flexible heat storage was designed and manufactured based on the features of the phase change composite. Totally 137.8 kg phase change composite and 75 L water were used in the flexible heat storage.

In this research, the thermal performance of the flexible heat storage was tested in experiments. Then, the success rate of stable supercooling, the long-term stability of stable supercooling and the solidification characteristics of the composite were further investigated. After experimental investigations, the flow characteristics of the flexible heat storage were investigated by means of computational fluid dynamics (CFD) models. Targeting on the flow defects in the heat storage, i.e. unwanted mixing/flow short circuit and uneven flow distribution, three methods were investigated to optimize the flexible heat storage.

The results show that adding extra water and liquid polymer can almost completely eliminate the phase separation of sodium acetate trihydrate. The composite can be kept in stable supercooled state at room temperature, due to its low viscosity and high specific heat capacity, it was suitable for short-term heat storage. When the charging temperature was higher than 77℃, the success rate of stable supercooling was about 66%. After achieving stable supercooling, the long-term stability was satisfied. The supercooled composite was successfully used as a short-term heat storage medium in 20 thermal cycles.

The solidification of the supercooled composite can be divided into spontaneous solidification and triggered solidification. In spontaneous solidification, the composite had small supercooling degree, slow solidification rate and crystal growth rate. The growth of crystals was similar to branch growth mode and formed large-diameter needle-shaped crystals. The gaps between solid crystals were relatively larger. In triggered solidification, the composite had large supercooling degree, fast solidification rate and crystal growth rate. The growth of crystal had the characteristics of both plane growth mode and branch growth mode. The formed crystal was compact needle-shaped with small diameters.

The function of flexible heat storage was successfully realized in experiments. During charge (30~87℃), totally 21.7 kWh of heat was stored. The charging time was about 8 h. During discharge of sensible heat and discharge of latent heat, 14.0 kWh and 7.6 kWh heat was released respectively. Most sensible and latent heat was released within 1.5 h and 2 h respectively. 294 L and 334 L hot water was produced with an average temperature of 68.2℃ and 46.7℃ respectively.

The flow defects inside the heat storage can be improved or even eliminated by changing inlet position, changing inlet size and adding porous plate. After moving the inlet from the bottom to the top of the storage, the time needed to completely melt the PCM was shortened by 50%. The best storage design was identified: For charge of the storage, the top inlet should be used, while for discharge, the bottom inlet should be used. Concerning the size of inlet opening, the charging time of the heat storage was reduced from 75 min to 51 min by using 3.0 mm radius instead of a 11.2 mm inlet. The small inlet size (3.0-8.0 mm) was suggested to make a uniform temperature distribution inside the heat storage. Short circuit was completely eliminated by adding a porous plate with 10% porosity. The charging time of the heat storage was shortened by 28% after adding the porous plate. Finally, recommendations were proposed for different applications of the heat storage.