| Thermal energy storage is an effective means to address the energy crisis and to improve the efficiency of energy. For the advantages of stability, smaller temperature difference between storing and releasing heat, and higher storage density, phase change materials (PCMs) develop quickly. However, the low thermal conductivity of organic PCMs (～0.2W/(m·K)) is its major drawback limiting its usage. At present, adding particles with high conductivity into organic PCMs to prepare composite PCMs has attracted much more attention. Because of its high thermal conductivity (>100W/(m·K)), low density and other characterations, graphite is increasingly becoming the preferred material additives.Firstly, composite PCMs filled with three expanded graphite (EG) were made by vacuum sorption. SEM, XRD, FT-IR and other means were used to analysis its shapes, mico-structure, and chemistry stability. And the effets of the saturated sorption capacity on the thermal conductivity of composite PCMs (hexadecanol,50℃) were made. The porous structure of EG can fix PCM well; the more the saturated sorption capacity is, the lower the thermal conductivity to be. As for lauric acid, it trends similarly. Graphite nanosheets (GNS), made by ultra-sonication (300W,10s), have better effect on the thermal conductivity than EG at the same loading. As the GNS with two-dimensional structure proved to be the best on the enhancement of thermal conductivity, GNS with three different lateral sizes was made by the assistant of ultra-sonication at different minutes and dispersed into hexadecanol to prepare specimens. Results reveal that the graphite nanosheets possessing larger aspect ratios tended to achieve higher thermal conductivity enhancement, which was deemed to be the relatively large size of nanofiller that contributes to the formation of heat transfer network in PCM matrix. Nielsen model predicted the experimental value better in the form factor of A taken100to180. The thermal conductivity nearly has no relation with temperature except for the phase change temperature. In addition, effects of graphite with various shapes and types on the heat transfer properties were investigated. The lamellar graphite flake performes best, porous carbon worst. The graphene (GNP) with two-dimensional performs better than carbon nanotubes. More comprehensive comparison of various graphite materials on enhancing heat transfer was made, it found that GNS-30performs quite with GNP although in different matrix materials.Then, for the two fillers (EG, GNS) who performed best on enhancing thermaml conductivity, phase change temperature (T) and latent heat (L) of composite PCMs were measured by DSC. Theoretical analysis has been discussed to explain the possible mechanism for the decreasement of latent heat capacity and capacity heat. The size of GNS has little effect on the T and L of composite PCMs.At last, a bench was built to verify the heat transfer enhancement and observe the temperature distribution in melting and solidification by time-temperature method, including hexadecanol and composite PCMs filled with graphite with different shapes and sizes. Results show that the heating and cooling rates of PCMs were improved significantly with the increasing of graphite particles. In addition, remarkable natural convection phenomena exist in phase change heat transfer process in hexadecanol. With the addition of graphite, such fluctuations disappear as a result of the weakened intensity of natural convection due to significant increase in viscosity of the composite PCMs in liquid phase, which somewhat offsets the acceleration afforded by the increased thermal conductivity at low loading. |
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