Study On Storage/Release Heat Performance Of Medium-Low Temperature Composite Phase Change Materials And System

As the rapid depletion of fossil energy and the sharp deterioration of the ecological environment,renewable energy such as solar energy,wind energy and industrial waste heat recovery and utilization have gradually attracted the attention of all countries.Phase change heat storage technology is widely used in energy development and utilization because it can balance the mismatch between the supply and demand of heat energy in time and space.Latent heat storage or phase change heat storage is to use the change of phase change material(PCM)state to absorb or release latent heat for heat storage.Its heat storage density is large,and the temperature of storage/release heat process is approximately constant.In the medium-low temperature latent thermal energy storage(LTES)system,such as solar heat utilization,industrial waste or waste heat recovery,green building,electronic component cooling,textile,aerospace and other fields,it has a good application prospect.However,there are still some problems in PCM and LTES system: organic PCM generally has low thermal conductivity,easy to leak and low ignition temperature.The inorganic PCM has the characteristics of supercooling,phase separation,poor thermal conductivity,low cycle stability and strong corrosion.In addition,the LTES system still has single heat storage unit,poor heat transfer performance and lack of thorough experimental verification of the phase change heat storage process.The main contents involved in this thesis are:(1)The influencing factors of thermal conductivity of non-metallic solid materials are summarized at the microscopic phonon level.Furthermore,the micro mechanism of heat conduction is analyzed,the thermal conductivity of the composite PCM is verified by experimental and explanation of the heat conduction mechanism.Specific heat capacity,phonon group velocity and phonon mean free path are the main factors affecting the thermal conductivity of composite PCM,which can effectively regulate their thermal conductivity.(2)In order to explore high-performance composite PCMs for the most suitable porous matrix,three kinds of composite PCMs are prepared by using the method of surface modification-vacuum impregnation and melt blendingsolidification and form-stable,using palmitic acid(PA)and aluminum potassium sulfate dodecahydrate(APSD)as PCM,surface modified high thermal conductivity expanded graphite(EG)and copper foam(CF),respectively.Furthermore,we comprehensively studied the micro-morphology,chemical compatibility,storage/release heat performance,thermal conductivity and cycle stability of the composite PCMs.The results show that PA and EG are uniformly mixed and adsorbed into the porous network structure.When the sample density of 900 kg/m3,EG content of 30 wt.% PA/EG composite PCMs of the melting and solidification enthalpy are 193.01,194.21 k J/kg,its corresponding melting/ solidification temperature are 61.08oС and 60.42oС,its supercooling degree is 0.66oС.Moreover,the horizontal and vertical thermal conductivity of PA/EG composite PCMs are 38.42 and 2.68 W/(m?K),respectively.When the sample density of 900 kg/m3,modified EG content on 20 wt.% of modified EG/APSD composite PCMs melting enthalpy is 473.52 k J/kg,corresponding to the melting point of 84.3oС.The effective thermal conductivity of composite PCMs is 6.157 W/(m?K),which is pure APSD(0.55 W/m?K)about 11 times.Furthermore,pore diameter of CF is 15 PPI to prepare PA/CF composite PCMs,the phase transition enthalpy value is 174.788 k J/kg.Compared with the pure PA(0.162 W/(m?K)),the effective thermal conductivity of composite PCMs is 5.112 W/(m?K)by about 31 times.In addition,the storage/release heat rate is shortened by 26 minutes compared with the PA/CF composite PCMs with pore diameter of 35 PPI.In the aspect of adsorption of porous matrix,the number of samples is designed and predicted by theoretically deriving the relationship between porosity and sample density.In terms of thermal conductivity of samples,a thermal conductivity model is proposed to predict the thermal conductivity of carbonbased and metal foam-based composite PCMs by establishing a correlation between porosity and additive content.The predicted error is only 6.45% of the measured value,which indicates that the interactive correlation has high prediction accuracy in the theoretical calculation of thermal conductivity.(3)Based on the optimization results of composite PCMs,the heat pipe(HP)and U-tube interactive LTES system is constructed,and the PA/EG formstable composite PCMs with EG mass fraction of 20% is used as the PCM of LTES system.The temperature distribution of heat transfer modules and the storage/release heat rate of heat exchangers are studied by using different types and numbers of U-tube or HP heat exchangers.When running a single HP heat exchanger,the time required for sensible heat absorption stage is 410 mins,for solid-liquid phase change heat absorption stage is 296 mins and for cooling heat release is 282 mins.However,the temperature distribution of the transverse measuring points of the heat storage module basically decreases along the direction of high temperature circulating working medium,while increases along the direction of cooling circulating working medium.The temperature in different longitudinal positions decreases with the distance between hot end.When running the HP and U-tube,the time of sensible heat absorption is 360 mins,the time of solid-liquid phase heat absorption is 285 mins,and the time of cooling heat release is 320 mins.The temperature distribution in different longitudinal positions of HP varies greatly,while the heat storage modules of U-tube heat exchangers have a more dispersed temperature distribution in the heating stage and a more uniform temperature distribution in the latent heat storage stage.It is expected that the studied work can be used widely in future to store the cascade utilization and efficient storage of low-grade thermal energy,such as solar energy and industrial waste heat,as the technology can well address the problem to ensure both uneven temperature distribution and slow heat storage/ release rate in the heat storage system.