| TES (thermal energy storage) is a promising method in the aspect of solving the mismatching between energy supply and demand in space and time. This technique has been considered to be a potential strategy to the problem of energy shortage and environmental issues. Owing to the advantages of extraordinary large energy-storage density and approximate isothermal energy storage process,the latent heat of phase change materials(PCMs) have been broadly applied in a great number of fields in the current stage,such as building energy conservation, solar energy utilization, Waste heat recovery and textile and garment, etc. Fatty acids are becoming one of the most preferential PCMs because of the high latent heat, good chemical stability, no phase separation and no subcooling. However, the drawback of low thermal conductivity and liquid leakage of fatty acids seriously restrict the practical applications. In order to overcome those disadvantage,this paper focuses on adjusting the phase change temperature by the eutectic effect,optimizing the heat-conducting property and liquid leakage by composite technology.Moreover, the heat transfer property of fatty acids in phase change units are enhanced through means of active and passive reinforcement methods.With the guidance of the eutectic theory induced by the second law of thermodynamics and phase equilibrium theory, a series of fatty acids eutectic mixtures were prepared. The phase change temperatures of fatty acids are measured in the range of 18.61? to 68.54?,and it can be found the corresponding fatty acid PCM in every 5? section. Meanwhile,phase change latent heat, specific heat capacity, volume swelling rates and thermal conductivity of eutectic mixtuers are similar with those of pure fatty acid. Thermal cycling test showed that there was no significant change of the thermal properties fatty acids system,which reveals that fatty acids system performed well in thermal reliability.Three kinds of carbon materials with different structure: carbon nanotubes (CNTs,one-dimensional), graphene nanoplatelets (GnPs, two-dimensional) and expanded graphite(EG, three-dimensional) were added into PA-SA eutectic mixture to enhance the thermal conductivity. The influences of content, sturecture and scale of carbon materials on the thermal property of PA-SA composite PCMs were studied. The phase change temperature,latent heat, heat conduction and heat transfer performance of PA-SA are affected by the adding content and structure of the carbon materials. All the three kinds of carbon materials can effectively improve the thermal conductivity of PA-SA. The thermal conductivity of composite PCMs increases with the increasing of the content of carbon materials, and the EG is the most effective due to its three-dimensional structure. For the highest loading(8wt%), the thermal conductivity of the composite PCMs are 1.12 , 2.73 and 15.84 times higher with CNTs, GnPs and EG respectively than that of PA-SA at 25?. And further the influence mechanism is revealed by non-isothermal phase change kinetics.A series of form-stable composite phase change materials with LA-PA-SA as PCM and expanded perlite (EP), vermiculite (VMT), diatomite (DMT) as supporting materials which can prevent the leakage of melted PCMs were prepared by vacuum impregnation method.The maximum mass ratio of LA-PA-SA retained in EP, VMT and DMT was found as 55wt%,50wt% and 45wt% respectively because of the different structure and size distribution of the porous supporting materials. FT-IR and XRD results show that the LA-PA-SA was uniformly dispersed into the pores of the supporting materials by physical interaction. The phase change temperatures and latent heats of LA-PA-SA/EP, LA-PA-SA/VMT and LA-PA-SA/DMT form-stable composite PCMs were measured as 31.8?, 30.6?, 31.5?and 81.5J/g, 75.8J/g, 68.0J/g, respectively. Thermal cycling test showed that there were no significant change of the thermal property of the form-stable composite PCMs after 1000 thermal cycling, and the change was in the acceptable range in the practical application. The temperature regulation control experiment results show that the LA-PA-SA/EP form-stable composite PCM has more excellent energy storage temperature performance.PA-SA/EG form-stable composite PCM shows a high thermal conductivity, no liquid leakage and high energy storage density was prepared by using EG with the vermicular porous structure as supporting and thermal enhancement material. The optimum absorption mass ratio of PA-SA and EG is determined as 13:1, which means that the content of PA-SA in the PA-SA/EG form-stable composite PCM is as high as 92.86wt%. SEM and FT-IR results show that PA-SA was uniformly distributed in the porous network structure of EG due to the physical action. The melting and freezing temperature of the PA-SA/EG are 53.89? and 54.37?, and melting and freezing latent heat are 166.13J/g and 166.27J/g,respectively. The energy storage density has no significant reduce cause of the supporting material. The TG result shows that EG effectively inhibits the decomposition and volatilization of PA-SA. PA-SA/EG shows a good thermal stability in its operating temperature range. The thermal energy storage and release rates of PA-SA/EG are increased due to the high thermal conductivity of EG.The heat transfer performance of PA-SA in the unit were studied at first, and then the"-" and"+" structure metal grid and fin were set in the unit to enhance the heat transfer of PCM. Because of the high thermal conductivity of the metal grid and the metal fin, the heat transfer of PA-SA was effectively enhanced. And the ”+" word structure of the grid and fin enhanced heat transfer effect are better than the "-" structure. For the comparison of metal fin and grid, the enhancement of metal fin is far better than that of metal grid. Furthermore, the heat transfer of PA-SA in unit was enhanced through ultrasonic. The results show that, in the ultrasonic power range of 60-150W, the enhancement effect of ultrasonic is enhanced with the increase of power; under the certain power, the heat transfer enhancement effect is enhanced with the increase of ultrasonic time. The enhance effects are different when the ultrasonic used in the different stage of the melting process because of the ultrasonic reinforcement mechanism. In this work, the effect of ultrasonic enhancement is the best after PA-SA was heated for 10min and then use the ultrasonic for 5min. A further comprehensive study of passive and active reinforcement method found that the combination of the two reinforcement methods can have a synergy in terms of strengthening heat transfer effect. |
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