Numerical Simulation And Study On Thermal Energy Storage Characteristics Of Solar Energy Molten-salt-based Single-tank

Concentrated solar power(CSP)generation is an effective approach to solve the problem of sustainable development of energy resources in the future.However,the intermittent and fluctuating natures of solar energy lead to the lack of its compatibility with the existing power grid infrastructures and low grid penetration.Energy storage technology can solve this problem to a certain extent.Due to the advantages of molten salt materials(e.g.,low cost,wide operating temperature range,relatively high operation stability,good heat transfer performance,low operating pressure),they could be used as the heat transfer material and energy storage medium for CSP plants.As the molten-salt-based single-tank thermocline energy storage system has relatively lower cost,its research and development works have received widespread attention.The numerical model of a molten-salt-based thermocline tank with the solid packing bed and multi-layer wall structure is established in this study.A comprehensive parametric study on integrated thermal and mechanical performances of the tank is conducted by using the CFD approach.The influences of nine typical factors are investigated,including the inlet molten salt velocity,cold molten salt temperature,porosity of porous bed,solid filler diameter,packing area height,thermal conductivity and specific heat of filler particle,ambient temperature and tank wall structural parameter.For all factors,complete operation cycle simulations of the tank as well as mechanical calculations are conducted.The influence laws are indicated and briefly summarized.The influence analysis results of varying parameters on the thermal energy storage characteristics of the thermocline tank reveal that for the charging process,the thermocline thickness can be decreased by increasing the cold molten slat temperature,packing area height and specific heat of filler particle or by reducing the thermal conductivity of filler particle.That can improve the energy storage performance of the thermocline tank.Other parameters have very small influences on the thermocline thickness in the charging process.For the discharging process,the thermocline thickness can be decreased by increasing the inlet molten salt velocity,cold molten salt temperature,packing area height,specific heat of filler particle,firebrick thickness and ceramic thickness or by reducing the porosity of porous bed,diameter and thermal conductivity of filler particle.The effects of ambient temperature and steel wall thickness on the thermocline thickness in the discharging process are both very small.The effect analysis results of varying parameters on the mechanical performance of the thermocline tank demonstrate that five parameters(i.e.,the porosity of porous bed,solid filler diameter,steel wall thickness,specific heat and thermal conductivity of filler particle)and the mechanical stress of the steel wall are proportional.Four parameters(i.e.,the inlet molten salt velocity,cold molten salt temperature,ambient temperature and firebrick thickness)and the the mechanical stress of the steel wall are inversely related.When the tank is completely filled with solid filler particles,the mechanical stress of the steel wall is the minimum.For all the other packing areal height conditions,the mechanical stresses of the steel wall are close.The influence of ceramic thickness on the mechanical stress of the steel wall can be neglected.