| As an inexhaustible renewable energy,solar energy can not only meet the energy needs of the society but also reduce the environmental pollution caused by the burning of fossil energy,which has great development potential.However,solar energy has the disadvantages of uneven distribution in time and space,and low energy efficiency.Phase change materials（PCMs）are used to store and release energy during the phase change process of materials.The latent heat of PCMs can solve the contradiction between the supply and demand of solar energy,and has a broad application prospect for improving the utilization efficiency of solar energy.There are a lot of magnesium salts in Qinghai salt lakes in China.The development of PCMs containing magnesium base hydrated salts can not only solve the problem of magnesium pollution in salt lakes,but also expand the application of PCMs containing magnesium base hydrated salts in solar thermal energy storage.Based on this,graphite phase carbon nitride（g-C3N4,CN）in this paper is firstly used as porous support material and composites magnesium base hydrated salts to form stable shape,low supercooling degree and excellent thermal reliability composite phase change materials（CPCMs）.On this basis,a new method of two-stage encapsulation of PCMs is proposed.Magnesium base hydrated salts are easily lost water of crystallization.The problem is solved by using porous support material CN first stage microencapsulation and commercial adhesive second stage macroencapsulation.The thermal reliability of magnesium base hydrated salts is improved.In order to improve the thermal conductivity of CPCM,graphite sheet（GS）is introduced into the magnesium base hydrated salts composite of expanded graphite（EG）.The GS filler with high thermal conductivity is innovatively introduced into magnesium base hydrated salt CPCM with low thermal conductivity,and the thousand-layer cake type CPCM with highly anisotropic thermal conductivity structure is constructed by layer upon layer interaction stacking.Based on this structure,the series and parallel model of equivalent thermal resistance is proposed.By using the method of experimental research and numerical simulation,it is revealed that this kind of thousand-layer cake CPCM has the function of enhancing the photo-thermal conversion and storage efficiency.Firstly,the porous support material CN is firstly used in this paper to solve the liquid leakage problem of Mg Cl2·6H2O-Mg（NO3）2·6H2O（MCH-MNH,mass ratio is 41:59）mixed salts PCM.The stable shape of MCH-MNH/CN CPCM is prepared.The CN has a porous structure and broad surface.Hydrophilic group such as hydroxyl exists on the surface of CN.The CN could absorb 80%mass fraction of mixed salts PCM and has not liquid leakage.The CN acts as a nucleating agent and provides nucleation sites for the mixed salts,which promotes the crystallization and nucleation of MCH-MNH mixed salts and reduces the supercooling degree of the mixed salts PCM.The CN enhances the thermal reliability of the mixed salts,and the phase change enthalpy of the CPCM decreases only 1.9%after 50heating and cooling cycles,which is much smaller than the 50.7%decrease of the phase change enthalpy of the pure mixed salts.Secondly,a new method of two-stage encapsulation is proposed to prepare Mg（NO3）2·6H2O/g-C3N4（MNH/CN）CPCM with good shape stability and thermal reliability.The CN not only acts as a support material for preventing MNH liquid leakage during the phase transition,but also has the effect of nucleating agent significantly reducing the supercooling degree of MNH.Commercial adhesives（epoxy resin structural adhesive and silicone sealant）provide a solid shell for CPCM in second-stage macro encapsulation.The second-stage encapsulation shell prevents MNH losing water of crystallization after phase transition.The phase change enthalpy of the CPCM encapsulated by the two-stage encapsulation has not change after the 100 heating and cooling cycles,while the phase change enthalpy loss of the CPCM without adhesives package reaches 22.47%.The synergistic effects of the first-order microscopic encapsulation of CN support material and the second-order macroscopic encapsulation of adhesives improve the shape stability and thermal reliability of MNH.This two-stage encapsulation method could provide guidance for solving the thermal instability of hydrated PCMs.Then,the thermal conductivity of Mg（NO3）2·6H2O/EG（expanded graphite）CPCM is strengthened by introducing GS in this paper.EG promotes the nucleation crystallization of MNH PCM and reduces the supercooling degree of MNH PCM.The compatibility of EG,GS and MNH is good.When the compacted density is 1600 kg/m3,the axial and radial thermal conductivity of MNH/EG CPCM is 12.59 W/（m·K）and 18.86 W/（m·K）,respectively.The introduction of GS further strengthens the thermal conductivity of MNH/EG CPCM.The thermal conductivity of MNH/EG-GS CPCM increases with the mass ratio increase of GS and EG.When the mass ratio of GS and EG is 0.6,the thermal conductivity of MNH/EG-GS CPCM reaches the optimal value,and the axial and radial thermal conductivity reaches 21.5W/（m·K）and 32.3 W/（m·K）,respectively.The thermal conductivity of MNH/EG-GS CPCM blocks increase firstly and then decrease with the length-width ratio of GS.When the length-width ratio of GS is 30/10,the thermal conductivity of MNH/EG-GS CPCM blocks has an optimal value,and the axial and radial thermal conductivity is 21.5 W/（m·K）and 32.3W/（m·K）,respectively.Finally,this paper innovatively imitates the thousand-layer cake structure.The GS with highly directional heat conduction structure is embedded layer by layer in MNH/CN CPCM with low thermal conductivity,which generates obviously anisotropic thermal conduction structure.Based on this structure,a serial-parallel model of equivalent thermal resistance is proposed to verify the reliability of the model.The mechanism of heat conduction channel is generated.The directional arrangement of GS with high thermal conductivity make the heat flow vary in different directions.This structure endows the PCM with rapid internal heat conduction for thermal storage,and reduces the convective heat transfer loss with the external environment.Therefore,the performance of solar energy photo-thermal conversion and storage is improved.When the mass fraction of GS in CN-GS matrix is 40%,the through-plane and in-plane thermal conductivity of MNH/CN-GS is 0.55 W/（m·K）and 15.70W/（m·K）,respectively.The anisotropy of thermal conductivity is 28.56.The average photo-thermal conversion and storage efficiency of MNH/CN-GS CPCM is 88.4%,which is 50.4%and 62.8%higher than that of MNH/EG and MNH/CN CPCMs with isotropic heat conduction,respectively.The anisotropic heat conduction structure could improve the photo-thermal conversion and storage performance of solar energy. |
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