Synthesis And Characteristics Of Composite Phase Change Materials With High Thermal Conductivity For Thermal Energy Storage

Phase change energy storage with the characteristics of high energy-storing density and stable operating temperature is an efficient way for thermal energy storage.Thermal energy storage is able to increase energy utilization and solve the problem of energy maldistribution depended on time and space.Different operating time periods require correspondent rate of energy storage and release.As far as phase change energy storage is concerned,the working medium needs different heat response rates.This paper mainly presents the thermal conductivity of the composite phase change materials by adding little powder with high thermal conductivity for the demand of short working periods.In the first chapter,the general situation of phase change energy storage technologies is introduced,including development background,principle,characteristic and range of study.The thermal properties of eutectic phase change materials added with multiwalled carbon nanotubes is investigated in the second chapter.The third chapter presents the synthesis and thermal properties of form-stable phase change materials based on high density polyethylene,which is modified by nano-Al2O3,nano-graphite,graphene nanoplatelets and expended graphite.1.Phase change materials with high thermal conductivityMyristic-stearic acids(MA-SA)binary eutectics with appropriate phase change temperature were prepared based on theoretical calculation.The melting temperature of the MA-SA eutectics and the mass fraction of the myristic acid(MA)in the eutectic mixtures are 42.70℉ and 54%,which was measured by Different Scanning Calorimeter(DSC).In order to enhance the thermal conductivity of the MA-SA eutectics,the carbon nanotubes(CNTs)were added into the MA-SA eutectics,and the effects of the CNTs on thermal conductivity of the MA-SA eutectics were investigated experimentally.The minimum mass fraction of the CNTs in the composite PCMs(CPCMs)was determined to be 9%without stratification.The thermal properties and microstructure of the CPCMs were investigated by the DSC and scanning electron microscopy(SEM),respectively.The DSC results showed that the CPCMs exhibited the similar solid-liquid phase change properties as the MA-SA eutectics,and the supercooling degrees of the CPCMs decrease obviously with increase of the CNTs content in the CPCMs.The thermal conductivity Meter(TCM)was used to measure the thermal conductivities of the CPCMs,and the test results indicated that the thermal conductivities of the CPCMs increase significantly from 0.173 W/(m·K)to 0.213 W/(m·K),0.258 W/(m·K)and 0.283 W/(m·K)as the mass fractions of the CNTs in the CPCMs were 9 wt.%,12 wt.%and 15 wt.%.The CPCMs with enhanced thermodynamic property will contribute to the energy management and storage in building systems.2.Form-stable phase change materials with high thermal conductivityFor the purpose of improving the thermal conductivity of the form-stable phase change materials(FSPCM),two types of nano-powders with high thermal conductivity were added into the samples.In the modified FSPCM,myristic acid was used as a solid-liquid phase change material(PCM),high density polyethylene(HDPE)acted as supporting material to prevent the leakage of the melted MA.Nano-Al2O3(NAO)and nano-graphite(NG)were the additives for thermal conductivity enhancement.Scanning electronic microscope,Fourier transformation infrared spectroscope(FT-IR)and X-ray diffractometer(XRD)were used to analyze the microstructure,chemical structure and crystalline phase of the samples,respectively.Furthermore,the specific latent heat and phase transition temperature,thermal conductivity and thermal reliability were investigated using differential scanning calorimeter,thermal conductivity meter and thermo-gravimetric analyzer(TGA).The results showed that the MA was uniformly absorbed in the HDPE matrices and there was no leakage during the melting process when the mass fraction of the MA in the MA/HDPE composite was less than 70%.The DSC results revealed that the modified FSPCM have a constant phase change temperature and high specific latent heat.The thermal conductivity of the FSPCM was measured in the solid(30℃)and liquid(60℃)states of the MA.When the mass fraction of nano-powder additives is 12%,the thermal conductivities of the FSPCM increase by 95%(NAO)and 121%(NG)at 30℃.It is anticipated that the FSPCM possess a potential application for thermal energy storage.Form-stable phase change materials consisting of palmitic acid(PA)and high density polyethylene were modified by graphene nanoplatelets(GNP).In the FSPCM,PA was used as a solid-liquid phase change material for thermal energy storage,HDPE was a supporting material to prevent the leakage of the melted PA,and GNP were added for improving thermal conductivity and shape stabilized of the FSPCM.Thermal properties and shape stability of the composites vary with their different mixture ratios.According to the results of Fourier transformation infrared spectroscope and X-ray diffractometer,the composites have advantages like stabilized chemical structure and crystalline phase.The differential scanning calorimeter(DSC)results show that the FSPCM has a constant melting temperature of around 62℃and a high latent heat of at least 155.8 J/g.With the help of scanning electronic microscope,a layer structure and uniform dispersion of the PA is observed in the modified FSPCM.The thermos-gravimetric analyzer and thermal cycling test results indicate that the modified FSPCM has a good thermal reliability,and the leakage of the PA drops significantly with the assistance of the GNP.The thermal conductivity of the FSPCM was measured by thermal conductivity meter and it increased to 0.8219 W·(m· K)-1 which is nearly 2.5 times as high as that of the pure FSPCM,when the mass fraction of the GNP is 4%.It is anticipated that the modified FSPCM possess a potential application in solar energy and building heating systems.Form-stable phase change materials(FSPCM)with high thermal energy storage capacity and thermal conductivity were manufactured by adding expanded graphite(EG)into stearyl alcohol(SAL)and high density polyethylenemixtures.In the composites,HDPE was used to prevent SAL leakage,and EG was not only a supporting material just like HDPE but also a thermal conductivity promoter.The influences of EG on the thermal properties of the SAL/HDPE phase change materials were investigated with a series of measuring means.Several lines of evidence showed that SAL and HDPE were mixed uniformly and EG was evenly dispersed in the SAL/HDPE composites.Approximately constant melting temperatures of FSPCM at around 57℃ and high latent heat of at least 200 J/g were presented by the differential scanning calorimete.It was found that the leakage rate of the SAL phase change materials decreased obviously when EG was added into the FSPCM.Furthermore,the thermal conductivity of FSPCM with 3%EG could increase up to 0.6698 W·(m· K)-1 while thermal conductivity of FSPCM without EG was only 0.1966 W·(m· K)-1.Finally,the FSPCM with EG could provide considerable thermal energy storage capacity and high thermal conductivity for latent heat storage system.