Study On Preparation And Material Properties Of High Temperature Molten Salt Phase Change Heat Storage Elements

Phase change heat storage(latent heat storage)is a kind of technology using phase change material(PCM)during phase change to absorb or release heat to achieve energy storage,which has many characteristics such as large heat storage per unit mass(volume),stable working temperature,good chemical stability and safety and so on.As an effective means to solve the contradiction between time and space of energy supply,phase change heat storage technology is one of the important ways to improve energy efficiency.Therefore,it has broad application prospects in solar energy utilization,electricity peak"move",recovery of waste heat and waste heat,as well as the industrial and civil buildings and air conditioning energy conservation,and other fields and has been widely studied.At present,the main research directions of PCM include the optimization of PCM,the structure design and preparation process of PCM,the heat transfer enhancement technology of PCM,and the simulation and calculation of PCM heat transfer process.PCM is considered to be one of the most critical components in phase change thermal energy storage(LHTES)systems.It can be divided into low temperature PCM(<200?)and high temperature PCM(200-1000?)according to the working temperature(melting point).Molten salt has become one of the important candidate materials in the field of high temperature phase change due to its high melting point,high latent heat of phase change,high chemical stability,good economy and other characteristics.Among them,polychloride salt has a wide range of adjustable melting point,high latent heat value,and low liquid viscosity,so it is selected as PCM of this topic.Combined with the requirements on the working temperature and heat storage density of LHTES system of the comprehensive utilization of nuclear energy in molten salt reactor and solar thermal power generation,the ternary eutectic chloride salt Na Cl-KCl-Mg Cl₂ was selected in this paper with a mass ratio of 5:3:2.The molten salt with this ratio has a higher latent heat of phase transition and a relatively matching phase transition temperature.But at the same time,chlorine salt also has the disadvantages of low thermal conductivity,low heat transfer efficiency and easy to leak and corrode heat storage vessels.The composite material with high thermal conductivity and molten salt can enhance the heat transfer of molten salt PCM and prevent the leakage and corrosion of molten salt packaging.This requires the structural design of composite phase change materials(CPCM).The material of enhanced heat transfer and packaging has the advantages of high thermal conductivity and good stability at high temperature.In this paper,porous materials and densified graphite with high thermal conductivity were used to enhance the heat transfer and encapsulation of heat storage chloride salts.Two typical phase change heat storage element structures,porous media-PCM structure and heat storage salt core-graphite spherical shell structure,were applied respectively.Molten salt/graphite foam(GF)and molten salt/porous silicon nitride(Si₃N₄)composite phase change materials(CPCMs)were prepared by the design of porous medium-PCM structure,and the performance characterization and heat transfer simulation of the two CPCMs were studied.In order to solve the problem of large pore size and easy leakage of encapsulated graphite material,two kinds of densifying agents,MCMB and CB,were used to densify and characterize the encapsulated graphite,and the application of densified graphite in the anti-molten salt infiltration of fuel elements of solid fuel molten salt reactor was explored.The main contents of this paper are as follows:1.The chemical stability of molten salt and GF is good before and after combination,and the distribution of salt in the cavity of GF is uniform.The introduction of GF porous skeleton material can effectively improve the thermal conductivity and heat transfer rate of molten salt PCM,and the thermal conductivity can be increased by more than 4times after composition(e.g.at 200?,the thermal conductivity of salt before composition was 1.172W/m·K,and that of CPCM after composition was 4.965W/m·K).The composite CPCM showed good stability after multiple thermal cycles and had potential application value.Numerical simulation can understand the steady-state heat transfer process of the basic volume unit in CPCM,and calculate the theoretical thermal conductivity of CPCM(e.g.at 200?,the thermal conductivity of CPCM was 10.269 W/m·K).2.It was found that the molten salt and porous Si₃N₄ skeleton bonded closely after the composite and the porosity filling rate of salt in Si₃N₄ could reach to 88.14%.The changes of lattice structure and melting point of molten salt and porous Si₃N₄ before and after recombination are very small,indicating the CPCM had good chemical stability.The higher thermal conductivity of Si₃N₄ can significantly improve the thermal conductivity of PCM,and the thermal conductivity of CPCM can be more than6.5 times that of salt PCM(e.g.at 25?,the thermal conductivity of salt before composition was 3.41 W/m·K,and that of CPCM after composition was 22.23 W/m·K).Through numerical simulation,it can be seen that the temperature gradient and heat flow in CPCM are under the joint influence of transverse thermal power,Si₃N₄,molten salt and air interface.Moreover,the phase transition time is reduced after compound,and the heat transfer rate is significantly increased.The theoretical thermal conductivity of dense Si₃N₄ and CPCM was calculated by Chiew-Glandt model step by step(e.g.at25?,the thermal conductivity of CPCM was 19.509 W/m·K).3.Two groups of MCMB densifiers with the same particle size and different mass fraction and the same mass fraction and different particle size were used to densify and modify graphite,the packaging material of heat storage element.For the densified graphite with the same particle size and different mass fraction MCMB added,from microstructure study it is found that the internal structure of the graphite after densification was compact and the porosity was reduced,indicating that MCMB had the effect of densification.When the mass fraction is increased from 1%to 15%at 3?m,the overall densification effect is gradually enhanced,but the effect tends to be stable when the mass fraction is more than 5%.The introduction of excessive MCMB will lead to the decrease of thermal conductivity and increase of thermal expansion coefficient of graphite matrix,which has a negative effect on its thermal properties.However,the addition of less MCMB(less than 5%)has little effect on the thermal properties of graphite matrix.In general,the matrix graphite is densified by MCMB with a particle size of 3?m and a mass fraction of 5%,and the overall performance is better.4.For densified graphite with the same mass fraction and different particle sizes MCMB added,in the case of adding 15%mass fraction MCMB,MCMB with smaller particle size(2 microns)can more effectively filling the pores between natural flake graphite and artificial graphite particles,and reduce the size of the filling material after the overall average pore diameter and porosity,improve the mercury and molten salt threshold pressure impregnation,and realize effective densification.The addition of MCMB has a certain negative influence on the thermal properties of graphite matrix(thermal conductivity decreases,thermal expansion coefficient increases).When the particle size of MCMB is the smallest(2?m),the decrease degree of thermal conductivity and the increase degree of thermal expansion coefficient are the least,and the anisotropy is closer to 1,showing good thermal properties.At the same time,the addition of MCMB can also improve the mechanical properties of matrix graphite,such as the compressive strength.In general,when the matrix graphite is densified by MCMB with a mass fraction of 15%and an average particle size of 2?m,the overall performance is better.5.For nano-grade CB-densified graphite,the average pore size and apparent density of graphite decreased gradually with the increase of mass fraction(5%-20%)of added CB before purification at 1950?,while the porosity,mercury and threshold pressure of molten salt infiltration increased gradually.In addition,the final saturated impregnation volume of mercury also increases gradually.The thermal properties(thermal conductivity and thermal expansion coefficient)of ATH-1 and ATH-5 could still be maintained after a small amount of CB densification.However,excessive addition of CB can significantly reduce the overall thermal conductivity of graphite and increase the thermal expansion coefficient of graphite.After purification at 1950?,the changing rules of microstructure parameters of the graphite with the increase of the amount of CB were similar to those before purification except that the overall average pore size of the graphite after purification increased slightly(except for the abnormal ATH-1 before purification).The thermal properties of graphite after densification were similar to those before purification.However,after purification,the thermal properties(thermal conductivity,thermal expansion coefficient)of all the graphite were improved to some extent.In general,the overall performance of matrix graphite densified by 5wt%CB is relatively better.6.The densification mechanisms of graphite matrix modified by two densifiers are different,the structure of densified graphite and the law of mercury impregnation are also partly different.It can be indicated from the research that,due to the MCMB thermal sintering of autogenous shrinkage characteristics,accumulated mercury quantity into MCMB densified matrix graphite is smaller than that of A3-3,and the MCMB tend to accumulate in the pores of matrix graphite,so as the added MCMB increases,matrix graphite average pore diameter reduced,porosity lowers,apparent density increases,mercury and molten salt impregnation threshold pressure increases,And this law tends to saturate with the gradual filling of graphite pores in matrix.On the other hand,CB has no self-shrinkage of thermal sintering.After CB densification,the accumulated amount of mercury into matrix graphite is larger than that of A3-3,and the nano-grade CB whose particle size is much smaller than MCMB tends to attach to the surface of large particles of matrix graphite aggregate first.Therefore,with the increase of the addition amount of CB,the average pore size of graphite decreases,the porosity increases,the apparent density increases,and the threshold pressure of mercury and molten salt infiltration increases.In summary,the matrix graphite modified by MCMB and CB densifiers has good anti-molten salt infiltration performance in molten salt heat storage element and fuel element of solid fuel molten salt reactor.