| In order to solve the problem of mismatch of energy use time and space,it is necessary to use heat storage media to store heat energy to obtain higher utilization efficiency.Among them,latent heat storage method using phase change materials as heat storage media with rich types of heat storage media and high heat density is attracting more and more researchers'attention.In the latent heat storage technology,the heat storage medium is the key,whose thermal properties directly determines the heat storage efficiency,which indirectly affects the energy utilization efficiency and their applications.Solid-liquid phase change materials are a kind of phase change materials with more practical application value.Regardless of organic or inorganic phase change materials,they have the problem of solid-liquid phase change liquid leakage,which can be effectively improved by packaging to improve the efficiency of energy use.This dissertation focuses on the development of high-performance encapsulated composite phase change heat storage materials with high heat storage density and excellent thermal properties.On the one hand,in order to prevent the liquid leakage and reduce the supercooling,as well as to improve their thermal reliability and practicality of inorganic phase change hydrated salt,a porous adsorption package method was used to synthesize polymer-coated hydrated salt/expanded graphite in low temperature composite phase change storage.For the thermal material,the inorganic salt phase change material of hydrated salt with cheap raw materials was chosen.By designing orthogonal experiments,the porous graphite expanded graphite adsorbing the hydrated salt complex was optimized,and then the UV-cured technology was used to package the optimized hydrated salt/expanded graphite.Orthogonal calculation results show that the mesh number is the most important factor affecting the target value;the morphological characterization results show that the polymer encapsulation is successful;the results of the thermal property test show that the polymer encapsulation has little effect on the phase transition temperature;the supercooling degree of polymer-coated hydrated salt/expanded graphite was significantly lower than that of pure hydrated salt;after500 cooling-heating cycles,the heat storage enthalpy of uncoated hydrated salt/expanded graphite composite decreased by approximately 28%,while the heat storage enthalpy of the polymer-coated composite remains almost unchanged,showing that the package is conducive to improve the thermal reliability of the composite;when applied it to the building gypsum board,the polymer-coated hydrated salt/expanded graphite helps to slow down the temperature rise of the gypsum board and prolong the warming time,it has a certain potential for application in building energy conservation.On the other hand,in order to improve the low thermal conductivity of organic phase change materials and prevent liquid leakage,microencapsulation technology was used to encapsulate organic paraffins through the construction of water/oil interfacial emulsification system.Through in-situ dehydration condensation of TEOS to synthesize microcapsules encapsulating organic paraffin with inorganic silica,following modifying wall materials with dye to construct novel visible light absorbing phase change microcapsules.The morphological characterization results of SEM show that the composite phase change material has a core-shell structure,and the particle size of the phase change microcapsules becomes larger after dye modification.The FTIR,XRD and XPS results show that the dye molecules can promote the hydrolysis process of TEOS.The molecule was successfully doped into the silica of the wall material.The results of the thermal properties test showed that the silica coating helps to improve the thermal stability.The modification of the dye has little effect on the phase transition temperature of the composite.After 100 heating-cooling cycles,the phase transition temperature and the phase change enthalpy did not change much from those before the cycle,indicating that the thermal reliability was good.The thermal conductivity of the modified phase change microcapsule after dye modification and the thermal conductivity of the unmodified phase change microcapsule were close to each other,however,the visible light absorption of dye-doped microcapsules is about three times that of the unmodified phase change microcapsules;the photothermal conversion efficiency of the dye-modified phase change microcapsule heat storage fluid is higher than that of the unmodified phase change capsules.The modification of organic dyes has almost no effect on the thermal conductivity,but it has a significant effect on the absorption of visible light,which can improve the photo-thermal conversion efficiency of composites.Then,in order to simultaneously improve the thermal conductivity and photothermal performance of the microcapsule composite phase change material,graphene oxide modified phase change microcapsule was synthesized.Morphological characterization results show that the composite material also has a core-shell structure,but the modified phase change microcapsules are smoother than the unmodified microcapsules;the encapsulated ratio of modified phase change microcapsules have a decrease of about 1.2%compared to unmodified microcapsules;The thermal conductivity of the GO-modified phase change material is increased by about 12.7%compared with the unmodified one,3.5 times higher than that of the pure paraffin wax;the visible light absorbance of GO-modified phase change microcapsules is obviously improved compared to the unmodified one,indicating that the graphene oxide is beneficial to increase the light absorption ability of the phase change composites;the smearing experiment results show that after the same time of light irradiation,the modified phase change microcapsule latent heat fluid has a significantly higher collection temperature than the latent heat of the unmodified phase change microcapsules.At the same temperature,the photothermal conversion efficiency of the latent heat fluid added with the modified phase change microcapsules is higher than that of the unmodified modified latent heat fluid.GO-modification helps to improve the thermal conductivity and the light absorption properties of composites.The addition of latent heat fluids with modified phase change microcapsules as cryogenic heat collectors has good potential for application in direct absorption solar collectors.Finally,in order to improve the coating efficiency and simplify the encapsulation synthesis process,a phase change microcapsule of ethylcellulose/methylcellulose-wrapped organic paraffin is synthesized via self-assembly process based on the principle of similar compatibility,following modified with GO to improve their thermal and photothermal properties.Morphological characterization results show that the synthesized phase change material composite has a core-shell structure,the particle size distribution is between several micrometers and tens of micrometers;the melting temperature of the modified phase change microcapsules is 49.7°C,and the melting enthalpy is 152.2 J·g-1,the coating rate is as high as 85.4%;the phase change temperature and latent heat of the modified phase change microcapsule before and after 100 heating-cooling cycles are almost unchanged,the packaging efficiency remains the same,showing that the modified phase change microcapsules have good heat reliability;the light-absorbance of GO-modified self-assembly phase change microcapsules is about 5 times that of unmodified ones;the photothermal temperature rise rate of modified phase change microcapsules is significantly faster than that of unmodified ones.As the solar intensity increases,the temperature increase rate increases first and then decreases;the collector temperature increases by 13.8%,and the light-to-heat conversion efficiency increases significantly.The results show that the self-assembly method successfully synthesized the phase change microcapsules.The graphene oxide modification helps to absorb and convert solar energy and has a certain saturation threshold.It does not increase with the increase of light intensity,but the photothermal conversion efficiency has improved significantly. |
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