Preparation And Properties Of High Temperature Shape Stable Phase Change Materials Based On Carbonate

Thermal energy storage contributes to energy conservation and efficient utilization,addresses supply and demand imbalances,which plays a critical role in improving the performance and reliability of existing energy systems.This paper focused on heat storage materials and devices,aiming to establish the relationship between the performance of heat storage devices and the performance of materials,which is a multi-scale problem.The preparation,characterization and application of carbonate base phase change heat storage materials in high temperature were studied.K2CO3 and Na2CO3-K2CO3 were used as PCMs.Moreover,modified coal fly ash was used as matrix material,and expanded graphite was used as heat conduction enhancement agent.The numerical simulation of the cascade plate heat reservoir was also carried out.The following was a brief summary of the work.Firstly,a novel shape-stabilized phase change material containing K2CO3 and coal fly ash was prepared by two-step method(coal fly ash modification and cold-pressing sintering molding),which solved the problem that carbonate salt and fly ash could not coexist at high temperature.The latent heat and thermal conductivity of the shape-stabilized phase change materials increased with the increase of salt content.After sintering,the shape-stabilized phase change materials with K2CO3 content of 50 wt.%and 60 wt.%could maintain good shape without cracks.In addition,they showed good cyclic stability in mass,latent heat and chemical compatibility during 150 thermal cycling tests.Secondly,nine binary mixed carbonates(Na2CO3-K2CO3)were prepared according to different mixing ratios.The latent heat and phase change temperature of nine kinds of mixed salts were analyzed by differential scanning calorimetry(DSC).A kind of mixed carbonate(Na2CO3-K2CO3,1:1,mass ratio)was selected as phase change material,and modified coal fly ash was used as matrix material.By mixing sintering method,five kinds of shape-stabilized phase change heat storage materials with different salt content were prepared.The thermal conductivity and heat storage density of PCM increased with the increase of PCM content.When the mass content of binary carbonate was 50%,60%and 70%,the shape-stabilized phase change material had good appearance without crack after sintering,and had good heat storage ability and chemical compatibility.Thirdly,in order to further improve the thermal conductivity of binary carbonate-coal fly ash shape-stabilized PCM,thermal conductivity enhancement agent expanded graphite was added.The microstructure,thermal conductivity,chemical compatibility,specific heat capacity and latent heat of shape-stabilized PCM with expanded graphite were studied.The results showed that the overall morphology of the heat storage material was changed by the addition of expanded graphite,and the pore size,pore size and specific surface area increased.The heat capacity and latent heat of the shape-stabilized PCM decreased slightly,however,the thermal conductivity increased with the increase of the mass content of expanded graphite.Moreover,the effective thermal conductivity calculated by the quadratic parallel model was in good agreement with the experimental results.Fourthly,the addition of expanded graphite changed the wettability of PCM on the matrix material,and the wettability of the material had a great influence on the stability of PCM.In this paper,the wetting behavior of Na2CO3-K2CO3 on coal fly ash ceramic matrix with different expanded graphite content was studied by contact angle measurement at 707?.In the contact angle measurement step,the experiment took the influence of the microstructure of the substrate surface,the adhesion of molten salt to the substrate and the penetration of molten salt into consideration.The results showed that the eutectic carbonate had good wettability on the coal fly ash ceramic matrix,and the wettability was affected by the surface microstructure of the ceramic matrix and the adhesion between molten salt and matrix.In order to eliminate the effect of salt infiltration,it was necessary to modify the actual contact angle.With the increase of the content of expanded graphite in the matrix,the surface roughness of the matrix and the salt infiltration decreased,while the contact angle increased obviously.Furthermore,the matrix prepared by high forming pressure could effectively decrease the surface crack,reduce the surface roughness,and improve the modified contact angle.Finally,a one-dimensional numerical model was established and verified to analyze the thermal performance of a high temperature plate cascade heat reservoir containing the prepared PCM,aiming to establish the relationship between the performance of the heat storage device and the material performance.The results showed that the larger the cut-off threshold was,the longer the charging time was,and the higher the charging efficiency was.Moreover,when the cut-off threshold Rch exceeded 0.4,the phase change behavior of the third stage PCM would not occur,and the heat storage mode was sensible heat storage.The mass flow of heat transfer fluid,the thickness of the plate and the length of the plate of each stage had no effect on the heat storage efficiency.The higher the mass flow of heat transfer fluid was,the faster the heat storage rate of PCM was.The smaller the plate thickness was,the faster the heat storage rate of PCM was.What's more,the heat storage increased with the increase of the proportion of the first stage PCM length to the total length.