Flexible Phase Change Thermal Storage Study Based On Stable Supercooling Of Sodium Acetate Trihydrate

Stable,efficient and flexible supply of renewable energy supply is important for achieving peak carbon dioxide emissions and carbon neutrality.Adding energy storage system can improve the stability and flexibility of renewable energy system.Until now,short-term heat storage has a high recycling utilization rate,but a low energy storage density,and generally it lacks the ability to overcome extreme weather.The long-term heat storage has a large volume,a low recycling utilization rate and slow response to heat demand.In this research,a flexible heat storage concept combining short-term and long-term heat storage is proposed,releasing the sensible heat and latent heat separately after decoupling the sensible heat and latent heat of the heat storage.Flexible heat storage depends on the 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 commercial 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 was added 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 stbale supercooling and the solidification characteristics of the composite was further investigated.After experimental investigations,the flow characteristics of the flexible heat storage were numerically investigated.According to the flow defects,three methods were used 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 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 crystal was similar to branch growth mode,and formed large diameter needle-shaped crystal.The gaps between solid crystals was relative 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 crystal with small diameter.The flexible heat successfully realized the flexible heat storage function 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 domestic hot water was produced with the average temperature of 68.2℃ and 46.7℃.The flow defects inside the heat storage can be improved even eliminated by changing inlet position,changing inlet size and adding porous plate.According to the simulation results,when high-temperature heat transfer fluid entered through upper inlet during charge and low-temperature heat transfer fluid entered through the lower inlet during discharge,the flexible heat storage had the shortest charging and discharging time.However in this way,the natural convection inside liquid composite was weak,the long-term stability may be influenced.Adding porous plate can eliminate the flow defects.The flexible heat storage with porous plate successfully achieved the original design concept,that is,high-temperature heat transfer fluid firstly fully mixed in the lower part,and then uniformly charged the whole flexible heat storage.Since the internal space of the flexible heat storage was enough,it is suggested to add porous plate to enhance the charging performance of the flexible heat storage.