| The energy consumption in industrial area ranks the first in total global energy consumption in China,significant amount of low-temperature heat is emited directly as waste heat. Right now,the utilization technology in our country mainly focus on high temperature waste heat, low temperature waste heat below 200? is lack of corresponding mature technology,waste heat in this temperature range is suitable for heating, meanwhile, small-scale heating systems are commonly used for distributed heat users to meet the heat demand,which is normally costly, inefficient and not environmental friendly. Recovering industrial waste heat for heating becomes the effective way to improve the energy utilization rateIn mobilized thermal energy storage system(M-TES),heat is deliverd to end users by transporting the containers carrying phase change materials(PCM),which recovers and storages waste heat from the factory.It is the effective method to solve the problem above.Thermal energy storage technology is the key of M-TES.In previous work,we choosed erythritol as phase change material considering the temperature range of waste heat and the properties of material.Erythritol(melting point 119?) is a potential candidate due to its high latent heat,good thermal stability, non-toxic and non-corrosive,but its thermal performance is effected by the low thermal conductivity.Based on this, erythritol/carbon nanotubes(CNTs) composite phase change material was prepared in which carbon nanotubes(thermal conductivity 3000 W·m-1·K-1) was used as a heat transfer promoter.The dispersibility of carbon nanotubes was observed under scanning electron microscopy and the melting point,latent heat,thermal conductivity,thermal stability was investigated experimentally. Further numerical simulation on thermal performance of the composites in the heat storage container was made.The result shows:Based on the traditional two-step method,preprocessing was used according to the structure characteristic of CNTs,carbon nantubes were first refluxed with H2SO4(purity 98%),then mixed with the liquid erythritol using electromagnetic stirring device and dispersed by ultrasonic disruptor.By this way,carbon nanotubes showed a good dispersibility in erythrtiol.Small addition of carbon nanotubes could improve the conductivity of erythritol significantly and thermal conductivity of composite PCMs increased with an increasement in CNTs content,but the increasement decreased gradually.Thermal conductivities of composite PCM were improved by 60%-89% with CNTs mass fraction from 0.25% to 0.75%.Meanwhile,CNTs have little effect on the melting point and latent heat of erythritol. The heat transfer enhancement mechanism of CNTs was simply discussed.It was considered that both the long chain strcture of CNTs which form the heat conduction net and high thermal conductivity contribute to heat transfer enhancement.Mass of composite PCM changed little during the heating process from 20? to 160?.It showed a good thermal stability in this temperature range. CNTs played a role as nucleaing agent,and could reduced the subcooling of erythritol.The subcooling of composite PCM reduced as the addition of CNTs increased.The composite PCM showed a far higher melting rate than erythritol,and the melting rate increased as then mass fraction of CNTs increased.There was little difference in solidfying rate between erythritol and composite PCM as natural convection dominated the heat transfer process.According to the result of numerical simulation,the thermal recycling time was reduced by 40% when using composite with mass fraction of 0.5% carbon nanotubes compared to pure erythritol.Composite PCM showed a good thermal performance in practical. |
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