Preparation And Properties Of Microencapsulated Phase Change Materials With High Energy Storage Density And Solar-Thermal Conversion Efficiency

The imbalance between energy supply and demand,as well as the carbon emissions and environmental pollution caused by fossil energy have attracted more and more attention.Seeking the development and utilization of clean and renewable energy such as solar energy is one of our development goals.Solar energy can be converted into other forms of energy through photothermal,photoelectric,photochemical,etc.,and converting solar energy into thermal energy through photothermal conversion is the most direct and convenient method.Thermal energy can be stored by using the characteristics that phase change materials can absorb or release heat energy during the phase change process.Solid-liquid phase change materials are the most common and most valuable phase change materials.However,they are prone to leakage and volatilization during use or react with other materials to fail.Microencapsulation of phase change materials with inert materials can solve the above problems.Phase change microcapsules can store heat in the form of heat transfer by passively using the temperature difference with the outside world,or by actively converting light energy into heat energy for heat storage.Therefore,to improve the heat storage efficiency of phase change microcapsules,microencapsulated phase change materials with high thermal conductivity and high light absorbance should be prepared.In this research,taking octadecane and disodium hydrogen phosphate dodecahydrate as examples,phase-change microcapsules of organic core materials and inorganic core materials were prepared respectively,and the reduction of graphene oxide,carbon nanotubes,naphtholpyran and oxidation Tungsten and other materials modify the light absorption and thermal conductivity of microcapsules to prepare phase change microcapsules with high light absorption and high thermal conductivity.The main research contents and results are as follows:（1）Organic phase change material octadecane was used as the core material and polyurea as the wall material,and the phase change microcapsules were prepared by interfacial polymerization method.The heat conducting fillers such as reduced graphene oxide（r GO）and carbon nanotube（CNT）were introduced into the microcapsule wall material to improve the photothermal conversion performance of octadecane@polyurea phase change microcapsules.The effects of the oxidation degree of reduced graphene oxide and the synergistic effect between reduced graphene oxide and carbon nanotubes on the morphology,thermal storage,thermal conductivity and photothermal conversion property of octadecane@polyurea phase change microcapsules were investigated.The results showed that there was little difference for the morphology and enthalpy of phase change of octadecane@polyurea phase change microcapsules after the introduction of r GO or CNT.The average phase change enthalpy of the microcapsules containing r GO-CNT was 162.9 J/g,and the encapsulation efficiency reached66.7%.After the addition of r GO or CNT,the thermal conductivities and light absorbance of the phase change microcapsules all increased at a certain degree.When the amount of r GO-CNT composite filler was 0.6 wt%,the thermal conductivity of octadecane@polyurea phase change microcapsules increased to 0.38 W/（m·K）,and the light absorbance increased to 0.95～1.（2）Naphthol pyran was selected as photochromic additive modifier,and isophorone diisocyanate（IPDI）and diethylenetriamine（DETA）as wall monomers to encapsulate octadecane-naphthol pyran via interfacial polymerization,and the phase change microcapsules with photochromic property were obtained.The effects of emulsifier dosage,emulsification rate,monomer ratio and core-wall ratio on the phase change enthalpy and encapsulation efficiency of the microcapsules were investigated,and the photochromic property and solar-thermal conversion efficiency were also evaluated.The results showed that the phase change enthalpy of the microcapsule core material changed with the addition of 8 wt%naphthol pyran solution in octadecane,and the phase change enthalpy and encapsulation efficiency of the prepared microcapsules were 143.9 J/g and 65.3%,respectively.Compared with octadecane@polyurea phase change microcapsules,the heating rate of the octadecane-naphthol pyran@polyurea microcapsules was faster,and the solar-thermal conversion efficiency increased by 25%.（3）Disodium hydrogen phosphate dodecahydrate（Na₂HPO₄·12H₂O,DHPD）was used as inorganic core material and encapsulated by the polyurea formed through the polymerization between IPDI and water or DETA at the interface of water-in-oil inverse emulsion,and the Na₂HPO₄·12H₂O@polyurea microcapsules were prepared.The technological conditions including water addition,reaction temperature,reaction time,IPDI/DETA mass ratio and the total addition of IPDI and DETA were optimized,and the phase change enthalpy and encapsulation efficiency of the obtained phase change microcapsules prepared by IPDI and DETA were 163.7 J/g and 64.6%respectively,and the corresponding thermal conductivity was0.39 W/（m·K）.SEM,TEM,FT-IR and DSC confirmed that the Na₂HPO₄·12H₂O@polyurea microcapsules appeared spherical shape,and Na₂HPO₄·12H₂O was in the interior of the microcapsules,and the chemical composition and crystallinity were unchanged.（4）To improve the solar-thermal conversion efficiency of microcapsules with disodium hydrogen phosphate dodecahydrate as core material,tungsten oxide（WO₃）as photochromic material was ultrasonically dispersed into the melt Na₂HPO₄·12H₂O and used as core material,and then was emulsified into liquid paraffin to form water-in-oil inverse emulsion.Thermal conductivity of DHPD increased to 3.35 W/（m·K）when the addition amount of WO₃ was 3wt%.After the polymerization between IPDI and DETA and encapsulation for Na₂HPO₄·12H₂O-WO₃,the thermal conductivity of the microcapsules with core/shell ratio of 3:1 increased to0.87 W/（m·K）with phase change enthalpy of 155.3 J/g.SEM and TEM results showed that the microcapsules exhibited spherical shape with obvious core-shell structure,and the particle size was 1-2μm.FT-IR results indicated that no reaction occurred between the core material and wall material.The microcapsules modified by WO₃had better ability to absorb light.After irradiation for 3000 s by the imitate solar light,the temperature of the heat-storage liquid with10 wt%phase change microcapsules containing 5 wt%of WO₃ was 8.2 ^oC higher than that without WO₃.