| With the development of social economy,the problem of high resource consumption and surge of total power consumption of traditional thermal power generation technology is becoming increasingly prominent,and it has been unable to adapt to the development trend of the new era.As we all know,solar energy is a kind of clean and renewable energy,and the solar energy to generate electricity has become the mainstream direction of future development.Solar thermal power station is one of the most mature storage solar power generation technologies.However,solar salt（60wt%Na NO3+40wt%KNO3）has low thermal conductivity（0.59W/（m·K））,the phase change material used for heat storage in solar thermal power station.The low thermal conductivity of the solar salt has become the key bottleneck problem restricting the heat storage rate.At present,the heat storage rate can be improved by adding metal enhanced heat transfer materials with specific structures and forming composite phase change materials with solar salt.However,there are still major problems that metal materials are corroded by solar salt at high temperatures.Therefore,how to improve the heat storage rate of phase change materials for solar thermal power stations and prevent enhanced heat transfer materials from being corroded by high temperature is imminent.In this study,the bionic porous ceramic skeleton was used to improve the heat storage rate of phase change materials under the support of the National Key R&D Program of China-“Design and controllable preparation of high reliability bionic hierarchical porous and its composites”.Plants in nature rely on transpiration to reduce the surface temperature of their leaves and prevent them from being burned down due to excessive temperature.This is mainly because the porous structure of plant xylem can efficiently transport water and inorganic salts from the soil to the various organs of the plant.Biologically inspired by the porous structure in plant xylem that promotes water and inorganic salt transport to reduce plant body temperature.The biomimetic templates are inspired by the porous structure of the xylem of the poplar（Populus L.）,and the ceramic material with excellent thermal conductivity and anti-corrosion is used as the skeleton material.The bionic porous ceramic skeleton was prepared,and the composite phase change material was formed by the composite of porous ceramic skeleton and solar salt.The bionic design of the porous ceramic skeleton,the controllable preparation method,the heat storage and anti-corrosion of the composite phase change materials are carried out.The main experimental conclusions are summarized as follows:The porous structure of the carbonized xylem of poplar showed that the pore patterns of the related porous structures in the xylem were circular and square,and the pore diameter ranges from 3μm to 7μm,and the pore filament widths between the pore sizes were 1μm.Based on the bionic similarity principle and the fluid mechanics similarity principle,a bionic porous skeleton with circular and square pore shapes,aperture（square is the length of hole）ranging from 1.5 mm to 4.5 mm,and the ratio of aperture to the width of hole wire ranging from 3:1 to 7:1 was designed.Taking solar salt as phase change material,the bionic porous ceramic skeleton and solar salt were combined to form composite phase change material.FLUENT was used to study the heat transfer mechanism of composite phase change material.The results of numerical simulation show that the melting time of solar salt is positively correlated with the pore size of the bionic porous ceramic skeleton,and negatively correlated with the pore width.The average heat storage rate of the bionic square porous ceramic skeleton was higher than that of the bionic circular porous ceramic skeleton when the pore diameter and the pore width of the bionic square porous ceramic skeleton were equal.Therefore,this study selects porous ceramic skeleton and solar salt in the range of bionic square,aperture to aperture width ratio 3:1～7:1 to form composite phase change materials.In this study,the bionic porous skeleton green body was prepared by the low-cost photocuring 3D printing,and then the bionic porous ceramic skeleton was formed by the liquid silicon infiltration.The thermal conductivity and mechanical properties of bionic porous ceramic skeleton were optimized by adjusting the solidification printing time,solid loading of ceramic slurry and sintering temperature.Aluminum nitride（Al N）ceramic paste was printed,and the ratio of solid phase mass to photosensitive resin mass was 5:10.The curing time of each layer is 20 s,the number of layers of the bottom supporting layer is 5,and the curing time of the bottom layer is 220 s.The liquid silicon infiltration reaction sintering temperature is 1550°C.The bionic porous ceramic skeleton with good thermal conductivity and mechanical properties was obtained.In addition,there are cracks on the surface of the bionic porous ceramic skeleton after liquid silicon infiltration reaction sintering,which will seriously reduce the thermal conductivity and mechanical properties of the bionic porous ceramic skeleton.In this study,the cracks on the surface of bionic porous ceramic skeleton were repaired by the method of impregnation and secondary sintering,and the parameters of solid loading of the repaired ceramic slurry,secondary sintering temperature were optimized and analyzed.The results show that the optimal solid loading（mass ratio of ceramic phase to ceramic phase and silica sol）of ceramic slarry for crack repair is 40 wt%～50 wt%,and the cracks on the surface of bionic porous ceramic skeleton are completely repaired after three times of surface crack impregnation repair.The solar salt impregnated into bionic porous ceramic skeleton formed bionic porous ceramic skeleton/solar salt composite phase change material.To reduce the thermal resistance between the bionic porous ceramic skeleton and solar salt,the titanium nitride（Ti N）nanoparticle coating was modified on the surface of the bionic porous ceramic skeleton after crack repair.The heat transfer mechanism of the composite phase change materials with solar salt was investigated.The experimental results show that the melting time of bionic porous ceramic skeleton/solar salt composite phase change material is shortened by 33.05%,and the heat storage rate is increased by 8.63%.After surface crack repair,the heat storage rate of skeleton/solar salt composite phase change material increases by 12.16%.The heat storage rate of Ti N nanoparticles modified skeleton/solar salt composite phase change material was increased by 19.31%.The anti-corrosion of the bionic porous ceramic skeleton was studied by static corrosion testing device,and the thermal cycle stability of bionic porous ceramic skeleton/solar salt composite phase change material was studied by dynamic thermal cycle test.The experimental results showed that the bionic porous ceramic skeleton broke after 180 dynamic thermal cycles,but the bionic porous ceramic skeleton with cracks repaired and bionic porous ceramic skeleton modified with Ti N nanoparticles did not break after 180 dynamic thermal cycles.The bionic porous ceramic skeleton was fractured after being immersed in solar salt for 288 h,but the bionic porous ceramic skeleton after crack repair and the bionic porous ceramic skeleton modified with Ti N nanoparticles did not break.After dynamic thermal cycle test and static anti-corrosion test,it is found that the bionic porous ceramic skeleton,the bionic porous ceramic skeleton with cracks repaired and the bionic porous ceramic skeleton modified with Ti N nanoparticles have sharp angular structure of ceramic particles,and no signs of corrosion by solar salt.The results of energy dispersive spectroscopy（EDS）and X-ray diffraction（XRD）analysis show that no new elements and phase components are generated by this three porous ceramic skeleton,which proves that the three bionic porous ceramic skeleton have excellent anti-corrosion.In summary,the problem of low heat storage rate and corrosion of heat storage phase change materials for solar thermal power stations.Based on the idea of bionics,a bionic porous ceramic skeleton is designed and prepared which takes into account both improving the heat storage rate of solar salt and preventing corrosion by high temperature of solar salt.This provides a new idea for improving the heat storage rate of composite phase change materials and preventing corrosion.At the same time,it also provides the theoretical basis and technical support for the preparation method of the low-cost 3D printing porous ceramic skeleton. |
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