Simulation Of Thermal Energy Storage Characteristics And Study On Phase Change Fluid State In Truncated Cone Tank

For realizing the win-win situation between economic development and environmental protection,it is an inevitable choice of the going sustainability way with low-carbon economy including adjusting energy structure and improving energy efficiency.Solar energy becomes the most potential renewable energy due of the green,economic and sustainable.In this paper,the medium-high temperature thermal energy storage?TES?system of phase change material?PCM?and heat storage device in concentrated solar thermal power?CSP?plant technology has been investigated in the light of the energy development needs of the new era countries and the technical needs of future products.The PCM of 54%KNO₃/46%NaNO₃ as the latent heat storage medium has been studied about its thermal properties and the effect of expanded graphite?EG?on its properties has been analyzed.From the point of view of the heat storage/exothermic process and phase change fluid state,the advantages of the truncated cone thermal storage tank has been designed and analyzed,and the optimal TES device is obtained.Then,the effects of the wall thickness of the heat conduction tube,the inlet condition of the heat transfer fluid?HTF?and the thermal conductivity of the PCM on the performance of the heat storage tank has been investigated by FLUENT software.Finally,in order to improve the utilization rate of heat energy in the TES system,a two-stage thermal storage device is established,and the influence of the inlet condition of HTF on the this system has been studied.It is found that the melting point and latent heat of 54%KNO3-46%NaNO3 is224.28?and 105.8 J/g,respectively.And the SEM images show that PCM and EG are uniformly mixed physically,and the effect of EG on melting point and latent heat is negligible.According to the simulation results by FLUENT software,it can be concluded that the performance of the truncated cone TES tank with the taper of 0.2072is the best.And the effect of wall thickness on the melting/solidification process can be negligible.However,increasing the inlet condition of HTF and the thermal conductivity of PCM can obviously improve the performance of this TES device.The results show that the maximum storage/exothermic rates are 120.28 J/s and 46.19 J/s,respectively,and the maximum heat storage efficiency of the truncated cone thermal storage tank is94%.For the two-stage TES device,the heat utilization efficiency of this system can be improved obviously by increasing the inlet condition of temperature and velocity of HTF,and the heat storage efficiency of the cascade TES system is optimal when the inlet temperature and velocity of the HTF are 747 K and 1.2 m/s,respectively.This study provides the significant references of the PCM option and device design in CSP thermal energy storage system.