Influence Of Mesopore Size, Morphology And Interface Effect On The Solidification Characteristics Of Solar Sal

In the face of the growing energy demand and in response to the“double carbon”goal,research on clean and sustainable energy development technology and energy storage has become a hot spot.Solar energy has been widely valued and applied due to its advantages of cleanliness,safety,and ease of access.Thermal energy storage is used for centralized solar energy（CSP）system to improve the seasonal and regional imbalance of solar energy.Phase change materials have become an important component of CSP due to their large latent heat capacity.In particular,molten salt phase change materials are widely used in CSP thermal storage systems due to their high operating temperature,good stability,and low cost.However,it has the disadvantages of low thermal conductivity and leakage during melting.Currently,an effective solution is to prepare composite phase change materials by impregnating them into porous carriers.With the trend of small-scale industrial preparation of porous carriers,the phase change behavior of mesoporous composite phase change materials is affected by constraint effects,and their thermophysical parameters（such as phase change point and latent heat）are significantly different from those of macroscopic volume.However,the current research on mesoporous skeleton/molten salt phase change materials,which are widely used in the field of medium and high temperature research,mostly focuses on the changes in physical properties in macroscopic experiments,while the research on solidification mechanism is insufficient.This paper aims to study the apparent morphology,freezing point,supercooling,and solidification enthalpy of mesoporous composite phase change materials.Through simulation and experiment,the effects of scale effect,morphology effect,and interface effect on the solidification characteristics of mesoporous skeleton solar salt were comprehensively obtained.At the same time,the effects of cooling rate on the three effects were further considered in the simulation and experiment,providing a theoretical basis for proposing a method for controlling the phase change performance parameters of mesoporous composite phase change materials.The research content is as follows:Firstly,Material Studio software was used to establish solar salt models at six different scales with molecular numbers of 460,920,1380,1840,2300,and 2760,as well as nanowire solar salt models with molecular numbers of 460 and 960.The freezing point,supercooling,and phase change latent heat were calculated through Lammps simulation;Solar salt composite phase change materials based on MCM-48,MCM-41,SBA-15,SBA-3,and SBA-16 were prepared by static melting method.The apparent morphology,freezing point,supercooling,and enthalpy of solidification of the composite phase change materials were studied using transmission electron microscopy and differential scanning calorimetry.The comparison of simulation and experimental results shows that with the increase of mesopore size,the freezing point of solar salt first increases and then decreases,and the supercooling first increases and then decreases,indicating that there is an optimal pore size of the skeleton;the latent heat of phase transformation gradually increases and eventually approaches the solidification enthalpy in the macro state;the mode of solidification nucleation gradually changes from bulk nucleation to competition between bulk nucleation and interface nucleation.Composites with nanowire morphology exhibit a similar pattern to nanoparticles as the size increases.At the same scale,nanowires have a higher freezing point,lower supercooling,and greater latent heat value than nanoparticles,and their phase transformation latent heat increases by 30%to 37%,indicating that nanowires have better phase transformation parameters than nanoparticles.Secondly,Material Studio software was used to establish a solar salt with a molecular number of 2760 and an Al₂O₃ content of 25 wt.%,33wt.%,40wt.%,50wt.%,67wt.%,75wt.%.A series of models of solar salt/porous aluminosilicate ceramic composite phase change materials were developed using Lammps to dynamically simulate the phase change process and calculate the solidification point,supercooling,and latent heat of phase change;porous aluminosilicate ceramic/solar salt composite phase change materials with different ratios of alumina to silica were prepared by freeze drying and melt impregnation methods.The apparent morphology,freezing point,supercooling,and enthalpy of solidification of the composite phase change materials were studied using specific interface area testing,scanning electron microscopy,and differential scanning calorimetry.The comparison and verification of simulation and experimental results show that Al₂O₃ and Si O₂ in the skeleton interface provide a substrate for the nucleation of molten solar salts,reduce the degree of supercooling,promote heterogeneous nucleation of solar salts,and have a more significant impact on the aggregation characteristics of Na NO₃;Al₂O₃ has a greater impact on heterogeneous nucleation.Finally,by increasing the solidification cooling rates of composite phase change materials at different scales,morphologies,and interfaces in the above simulation and experiments,the effect of cooling rate on the solidification characteristics of composite phase change materials was studied.The results show that increasing cooling rate has no significant impact on the change rules of phase change parameters at different scales and morphologies;However,it will increase the supercooling of solar salt,reduce the freezing point and latent heat of phase transformation,leading to undesirable changes in phase transformation parameters;it has a significant promoting effect on interface nucleation and heterogeneous nucleation processes,and has a greater impact on three-dimensional morphology than on two-dimensional morphology.