| Solar energy has become a very promising alternative energy owing to its advantages of non-pollution and large reserves.Light-thermal conversion in solar energy utilization is one of the most important approaches to cope with problems like energy crisis,environmental pollution,etc.Heat storage is an important part of solar thermal utilization.Stearic acid,as one kind of organic phase change thermal storage materials,has aroused extensive research due to lots of merits,such as high phase change enthalpy,good stability,no supercooling,adjustable phase change temperature,constant temperature in phase change process and so on.Like other organic phase change materials,low thermal conductivity and fluidity in molten state limit its use in thermal energy storage and increase the cost of thermal energy storage system.Carbon material has high thermal conductivity and a variety of scales and forms.It has received widespread attention to increase the thermal conductivity and heat transfer capacity of the substrate via adding high conductive carbon materials into streaic acid.In this study,thermal conductivity of stearic acid was improved by adding multi-scale and multi-form carbon materials.Furthermore,the relationship among composition,structure and property of composite phase change materials was established.In this study,composite phase change materials with different addition amounts of EG,CNTs and GnPs were prepared by melt blending/ vacuum adsorption method,melt blending/ ultrasonic dispersion method,and surface modification/ melt blending/ultrasonic dispersion method,respectively.The phases,microstructures,phase change temperatures,phase change enthalpy,thermal stability,thermal diffusivity and thermal conductivity of pure stearic acid and modified composite phase change materials were studied by characterization methods like XRD,SEM,DSC,TG and LFDA.Package effect of expanded graphite on stearic acid was studied by leakage test,and heat storage and release performance of composite phase change materials were studied by heat storage and release test.After adding modified carbon materials and surfactant PVP,there is no chemical reaction but physical combination among all the raw materials.Microstructure testresults show that expanded graphite has a worm-like and micron-sized porous structure,carbon nanotubes has a nanoscale tubular structure,and graphene nanoplatelet has a nanoscale flaky structure.Porous structure of expanded graphite can package stearic acid,and the leakage is less than 5% by adding 7 wt.% expanded graphite.The added carbon materials are covered by streaic acid in composite phase change materials while they are still interconnected to each other.The addition of modified materials has little effect on the phase change temperature of stearic acid,and there is no dependence on the addition amounts of carbon materials.In general,the phase change enthalpy decreases with the increase of carbon materials,but the phase change enthalpy of CNTs-added and GnPs-added composite phase change materials is lower than the theoretical phase change enthalpy because of the nano-effect and defects.The thermal diffusivity and thermal conductivity increase with the increase of carbon materials.Expanded graphite and GnPs can improve the thermal diffusivity and thermal conductivity better than CNTs because of the formation of effective thermal conductive network.The package effect of expanded graphite increases the stability of stearic acid,followed by GnPs,while CNTs can hardly increase the stability of stearic acid.The time for heat storage and release is the most intuitive performance of the thermal conductivity of composite phase change materials,which greatly decreases the time for heat storage and release by adding EG and GnPs.CNTs can only slightly increase the solid-state thermal conductivity,which can hardly decrease the time for heat storage.However,the free CNTs in liquid composite phase change materials are repelled to grain boundaries by the crystallization of stearic acid,and then recombine to form a more effective and ordered connection than before,as a result,the time of heat release process is reduced more than that of heat storage process. |
