Study On Cordierite Ceramic-PCM Composite Heat Storage Materials In Solar Thermal Power Generation

The solar energy is abundant and widely distributed in China,which has broad prospect for development and utilization.However,the solar radiation has discontinuity and instability,which seriously affected the development and application.In order to improve the solar energy utilize efficiency and realize large-scale applications,an efficient energy storage device is needed in solar thermal utilization system.Cordierite ceramic heat storage material has the advantages of easy availability,low production cost and good thermal shock resistance.Al-alloy phase change（PCM）heat storage material with high thermal conductivity has large heat storage density and good thermal cycling stability.A composite of cordierite ceramic and alloys PCM can make use of sensible heat from ceramic substrate material and latent heat from PCM to realize the storage and utilization of energy at the same time.It can not only solve the storage and transportation problem and solid-liquid separation problem of single phase change materials,but also can give full play to the stability of cordierite ceramic material and improve the thermal efficiency.Therefore,this article systematically studied the preparation and properties of cordierite ceramics-PCM composite heat storage material.Cordierite-mullite ceramics were designed to achieve an enhancement of microstructure.By adding sintering additives,the density and mechanical strength of cordierite composite ceramic materials were improved.In situ synthesized silicon carbide whisker（SiC_w）was bonded to improve the thermal conductivity of cordierite composite ceramics.The effecting factors and the mechanism of thermal shock resistance on cordierite composite ceramics were discussed in this paper.And the thermal storage properties of the composite heat storage material were studied.Through static erosion tests,the adaptability between cordierite composite ceramics and alloys PCM（Al68-Si6-Cu26）was researched,the erosion characters and erosive regularity between alloy and the ceramic materials were mainly analysized as well.The failure mechanism of the composite heat storage material was revealed.Cordierite-mullite ceramics were in situ synthesized with kaolin,talc and manufactured alumina.And a comparative analysis of mullite combined cordierite ceramic was undertook,focused on the synthesis temperature range and the influence on its structure and performance.It found that,the optimal synthesis temperature was1420 ^oC.When contrasted with mullite combined cordierite ceramic,the former showed a more uniform microstructure and more closely intergranular combination.Cordierite and mullite grains interlocked each other,which was conducive to improve thermal shock resistance.As for the in situ synthesized cordierite-mullite（8:2）ceramic sintered at 1420 ^oC,the water absorption（Wa）is 14.51%,the apparent porosity（Pa）is 27.40%,the bulk density is 1.89 g/cm³,the bending strength is 59.21MPa.Sintering aids（B₂O₃,Nd₂O₃ and Y2O3）were added to the in situ synthesized cordierite-mullite（8:2）ceramic to improve the sintering performance,the density and the bending strength.The sintering additives with low melting point can promote the formation of liquid phase before cordierite and mullite crystal phase develop.So,the mass diffusion of liquid phase played a role of promoting the sintering and the synthesis of target crystalline phases.Besides,it improved the strength of the ceramic materials.And the viscous flow of liquid filled the space between the particles promoted ceramic densification.B₂O₃ showed the most remarkable effect of sintering aids.The samples with 2.0 wt%B₂O₃ possesses a water absorption（Wa）of 0.03%and a bending strength of 116.70 MPa.In order to improve the thermal conductivity of cordierite-mullite ceramic,SiC_w was in situ synthesized from three routs（SiO₂+C,Si+C,Si+phenolic resin）,and a comparative analysis of SiC bonded cordierite-mullite ceramic was undertook.It found that,the SiC synthesized by Si+C was the most appropriate way to improve thermal conductivity.The generated SiC whisker developed to be heat conduction passages,which improved the thermal conductivity on one side,and on the other side,had an effect of the whisker enhanced toughening.As for the cordierite-mullite ceramic with 40 wt%SiC,the thermal conductivity is 6.14 W/m·K（25 ^oC）,the water absorption（Wa）is 6.83%,the apparent porosity（Pa）is 15.65%,the bulk density is2.13 g/cm³,the bending strength is 104.47 MPa.The thermal shock resistance was discussed from factors as follows:particle dispersion,composition of composite ceramic,mechanical strength and bulk density of composite ceramic,thermal conductivity of composite ceramic.It found that,there were some annular cracks distributed around mullite grains in mullite combined cordierite ceramic,which was the reason for decrease of the bending strength.A larger amount of cordierite in the composites could lead to a better thermal shock resistance.The improvement of thermal conductivity was beneficial to the uniform temperature distribution.Therefore,there were several aspects to improve thermal shock resistance of materials effectively:reduce the thermal expansion coefficient of material,improve the material strength,density and thermal conductivity.To investigate the adaptation of cordierite composite ceramics and alloy PCM,the erosive regularity and mechanism were analyzed by static erosion test.It found that,there was a certain degree of erosion effect between cordierite composite ceramic and alloy PCM,and gradually formed to be a dense transition layer.The thickness of transition layer increased with the increasing thermal cycle time and back into balance.The pores in ceramic materials were the main erosion channel,and slowly migrated under capillary force.Due to the existed silicon carbide whisker in the pores,the poor alloy wettability of PCM,and the transition layer of density was hindered the further erosion of alloy PCM.Numerical calculation was used for the heat storage material thermal stress analysis,and the temperature field and stress field in the process of heat accumulation obtained.The dangerous point in the process of thermal storage was found out through the analysis,namely the combine part of ceramic composite ceramics materials and alloy PCM.