Numerical Simulation Of The Dynamic Charact-Eristics Of The Phase Change Thermal Energy Storage Of Molten Salt In Confined Space

People have launched a revolution of energy innovation to deal with the serious problems of the resource constraint and ecological damage.Today some new technologies that are widely used in the solar engineering and in the industrial waste heat recovery have played an important role in the energy supply and the sustainable development.The molten salt has been attracted more attention on some features of strong heat transfer capability,wide temperature range,low vapor pressure,and wide source with the development of energy storage technology.However,most of the current studies are based on the high temperature sensible thermal energy storage of the molten salt,the convection of heat transfer in the tube and so on.There are few papers about the melting process of the molten salt considering the phase change,not to mention the effect of the heat transfer of the natural convection.In this paper,an unsteady dynamic heat transfer of the molten salt was studied during the melting process of a phase change material by the FLUENT software.The effect of the natural convection of liquid part of the salt was considered in the numerical simulation.(1)A numerical model of the enclosed square cavity filled with the molten salt with an inner heating bar and an ideally adiabatic surrounding was established.The influence of the heating power and the size of the heating bar on the dynamic characteristic of the melting were addressed.(2)A shell-and-tube model of the latent thermal energy storage unit for the molten salt and the supercritical air was established.The heat transfer and the thermal energy storage process of the molten salt was analyzed to solve the problem of the waste heat of the compressed air and to improve the efficiency of the supercritical compressed air energy storage system.The influence of the velocity inlet of the supercritical air on the thermal energy storage process and the melting fraction of molten salt were discussed.The numerical result has revealed that the melting process of the salt is generally divided into three stages,i.e.the conduction-dominating stage at the beginning,the phase-changing stage in the middle,and the convection-dominating stage in the last.In different stage the temperature distribution of the salt is different.Either increasing the heating power or enlarge the heating bar size are all helpful to the salt melting process.In the shell-and-tube thermal storage unit,the heat transfer along the axial direction is obvious because of the intrinsic feature of the heat transfer of the natural convection during the melting process.And the isotherm of the molten salt after melting is almost perpendicular to the wall of the unit.The influence of the velocity inlet of the supercritical air during the thermal energy storage is taken into account,which is more effective at the phase change stage than the conduction-dominating stage of the solid at the beginning.At last the melting fraction of the molten salt has a nonlinear relationship with time.