Preparation Of High-performance MgCl₂-KCl/expanded Graphite Composite Phase Change Block And Its Thermal Conductivity Enhancement

Latent thermal energy storage is an important technology for increasing energy utilization efficiency and exploiting renewable energy.Inorganic molten salts have the advantages of large latent heat and low cost,making them promising phase change materials for medium and high temperature applications such as industrial waste heat recovery and solar thermal power generation.Combining molten salts with expanded graphite?EG?can not only overcome liquid leakage and corrosivity of molten salts but also make an obvious improvement on thermal conductivity.There are defects in the preparation of inorganic molten salt/EG composite phase change materials for traditional methods.In this thesis,herein a novel process was explored for preparing high-performance MgCl₂-KCl/expanded graphite composite phase change blocks.In order to improve the thermal conductivity of MgCl₂-KCl/EG composite phase change blocks,a novel route was presented,which involved adding graphite paper?GP?to prepare MgCl₂-KCl/EG/GP composite phase change blocks.This experiment explored a novel process for preparing high-performance MgCl₂-KCl/EG composite phase change blocks,which involves mixing a solid molten salt with EG thoroughly,compressing the mixture into a block with a designed shape and then heating the block to a temperature above the melting point of the molten salt followed by cooling.The optimal mass fraction of the eutectic was determined to be around 85%.The composite phase change block has a melting point of 424.14°C and a solidification point of 418.39°C,and its latent heat values are 161.37 J g^-1 for melting and 160.28 J g^-1 for solidification.Compared with the eutectic salt,the composite phase change block exhibits an enhancement in thermal conductivity by 11-fold.Furthermore,the composite phase change block has been compared with the one prepared by the conventional method?first adsorption and then compression?.It is found that the composite phase change block fabricated by the novel process exhibits better uniformity and smaller volume expansion than the one obtained from the conventional method.In order to obtain MgCl₂-KCl/EG/GP composite phase change blocks with optimal thermal characteristics,the effects of the amount of GP and the packing density of EG on the composition and thermal properties of the obtained composite phase change blocks were investigated.It is shown that when the amount of GP was set at m_GP/m_EG=0.88 and the packing density was controlled at 280 kg m^-3,the obtained composite blocks exhibited enhanced thermal conductivity at a small loss of latent heat.Based on the above investigations,an MgCl2-KCl/EG/GP composite phase change block was fabricated under the optimized conditions:m_GP/m_EG=0.84 and the packing density of EG=280 kg m^-3.And its thermal characteristics was compared with the MgCl₂-KCl/EG composite block prepared at the same packing density of EG.It is shown that the MgCl2-KCl/EG/GP composite phase change block exhibits an enthalpy of 205.35 MJ m^-3,a decrease by only 1.27%as compared with the MgCl₂-KCl/EG composite block,while its thermal conductivity is as high as 12.76 W/?m K?,2.08 times that of the MgCl₂-KCl/EG composite block.In summary,the high-performance MgCl₂-KCl/EG composite phase change block developed in this paper has the advantages of large latent heat,good uniformity,and small volume expansion.It is revealed that adding graphite paper can effectively improve its thermal conductivity.The novel proposed preparation method is easy to operate and has high efficiency,making it show great potentials for practical applications.