| Thermal energy is widely distributed in nature,and has a wide application in social life.The storage and utilization of thermal energy is one of the effective ways to alleviate the energy crisis and solve environmental problems,meanwhile,the problem of imbalance between energy supply and demand can be solved.In practical applications,thermal energy storage systems play an important role because most of the heat sources are uncontrollable and unpredictable.Phase change materials have been increasingly favored in the field of thermal energy storage due to their large latent heat storage capacity,constant operating temperature and abundant types.In this paper,fatty acids are selected as phase change materials,and fatty acids were combined with other backing materials by different methods to overcome the defects of leakage in molten state and low conductivity.In the first chapter,the general situation of phase change energy storage technology,including the development background,principle,characteristics and research scope is introduced.The second chapter presents the synthesis and thermal properties of fatty acid form-stable phase change material based on polyvinyl butyral and nano-SiO2.Besides,the form-stable phase change materials were modified by expanded graphite and graphene nanoplatelets.The third chapter introduces the preparation process and thermal properties of two kinds of microencapsulated phase change materials,whose shell materials are ethyl cellulose and silica,respectively.1.Synthesis and properties of form-stable phase change materials for thermal energy storageA palmitic acid/polyvinyl butyral form-stable phase change material was prepared by solution blending,in which palmitic acid was used as phase change material to store and release heat,and polyvinyl butyral was used as supporting material to prevent leakage.In addition,the expanded graphite with high thermal conductivity and porous structure was used as an additive,which can not only improve the thermal conductivity of the form-stable phase change material,but also enhance the adsorption of the form-stable phase change material to palmitic acid.The Fourier transformation infrared spectroscope(FT-IR)and X-ray diffractometer(XRD)results showed that the composite has a stable chemical structure and crystal structure.Scanning electronic microscope(SEM)showed that the expanded graphite was successfully mixed with the palmitic acid/polyvinyl butyral composite.Thermogravimetric analyzer(TGA)indicated that the form-stable phase change material had satisfactory thermal stability and there was almost no loss of weight within its operating temperature range.Differential scanning calorimeter(DSC)was employed to analyze the thermal properties,indicating that the form-stable phase change material with 70 wt%palmitic acid possesses melting latent heat of 128.08 J/g and melting point of 59.5?.The results of the thermal conductivity meter(TCM)revealed that the expanded graphite is a good thermal conductivity promoter,and the higher the content of the expanded graphite,the better the thermal conductivity enhancement effect.The modified form-stable phase change material has great application potential in low temperature solar systems.Palmitic acid as phase change material and nano-silica as supporting material were mixed to prepare a form-stable phase change material.A series of form-stable phase change materials containing different palmitic acid content were synthesized.The leakage tests indicates that the maximum content of PA in the form-stable phase change material is 70 wt%.Different amounts of graphene nanoplatelets were added into the form-stable phase change materials to investigate the effect of graphene nanosheets on the thermal conductivity.The chemical structures,the crystal phases and the morphologies of the form-stable phase change material were measured by Fourier transformation infrared spectroscope(FT-IR),X-ray diffractometer(XRD)and scanning electronic microscope(SEM),which showed that the raw materials are well mixed by physical action.The differential scanning calorimeter(DSC)results indicates that the form-stable PCM possess phase change temperature at about 60?and high latent heat at least 128.42 J/g.Thermogravimetric analyzer(TGA)and thermal cycling tests determine that the form-stable PCM have outstanding thermal stability and reliability.Thermal conductivity meter(TCM)results reveal that the thermal conductivity of the form-stable phase change material with 5 wt%graphene nanoplatelets is 1.65 times that of pure form-stable phase change material.2.Synthesis and properties of microencapsulated phase change materials for thermal energy storageThe novel microencapsulated phase change materials(MPCMs)with myristic acid and ethyl cellulose were fabricated by emulsification-solvent evaporation method,where myristic acid is core and ethyl cellulose is shell.The role of myristic acid is to store and release heat,and the role of ethyl cellulose is to prevent leakage.The morphology and particle size of the MPCMs were measured by scanning electronic microscope(SEM),and similar and complete spheres were observed.Chemical structure and crystal phase of MPCMs were analyzed by Fourier transformation infrared spectroscope(FT-IR)and X-ray diffractometer(XRD).It can be found that EC and MA are well combined by physical action without chemical reaction.The results of Differential scanning calorimeter(DSC)show that ethyl cellulose has a certain hindrance to the crystallization of myristic acid,and the inhibition of crystallization is enhanced as the content of ethyl cellulose is increased.According to the DSC results,when the mass ratio of myristic acid to ethyl cellulose is 2:1,the effect of ethyl cellulose on crystallization is negligible and the MPCM has a relatively stable phase change temperature and latent heat.Thermogravimetric analyzer(TGA)and thermal cycling test results reveal that MPCMs possess excellent thermal stability and thermal reliability at working temperature.Since ethyl cellulose is non-toxic and harmless,it is foreseeable that the MPCM has application potential in food and medical fields.For improving performances of phase change material,stearic acid was encapsulated in silica shell by sol-gel method to form microencapsulated phase change materials(MPCM),where methyl triethoxysilane(MTES)was used as precursor of silica.In order to obtain the maximum latent heat under the premise of preventing leakage,the optimum ratio of MTES to stearic acid is 1:1.Besides,graphene oxide(GO)was attached to surface of silica by a self-assembly process to form GO@MPCM,so as to further improve performances of MPCM.The chemical structure and crystal phase of MPCM were measured by Fourier transformation infrared spectroscope(FT-IR)and X-ray diffractometer(XRD).Raman spectrometer was used to further verify that GO was attached to the MPCM.The morphology of MPCM was observed through a scanning electronic microscope(SEM).The results show that MPCM are complete spheres with tight outer shells.The results of Differential scanning calorimeter(DSC)reveal that the MPCM have stable phase transition temperature and high latent heat,the encapsulation efficiency of the MPCM without GO reaches 83.22%.What is more,the addition of graphene oxide promotes thermal conductivity. |
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