Preparation And Performances Of Novel Form-Stable Composite Phase Change Materials Based On Inorganic Salt For Cold Energy Storage

The energy consumptions of air-conditioning and refrigeration take up a large proportion in the total energy consumption of buildings.Applying cold energy storage technology with phase change material(PCM)in air conditioning system can effectively balance the power load and realize "peak shifting and valley filling",so as to improve the energy utilization efficiency.The key of cold energy storage technology is the selection of PCM.Among them,inorganic hydrated salt(hydrate)PCMs are competitive in thermal conductivity and latent heat per unit volume as well as cost-effectiveness.However,some problems exist in these PCM,such as supercooling,phase separation and liquid phase leakage,so it is necessary to find suitable nucleating agent and stabilizing carrier.In addition,the phase change temperature of the single PCM is difficult to meet the temperature requirements of cold energy storage,so the modifier should be added to PCM to adjust its phase change temperature.Therefore,based on the background of cold energy storage with PCM in air-conditioning,this paper is devoted to the development of hydrated salt or salt hydrate form-stable composite phase change material(CPCM)with high performance.Specific tasks are as follows:Firstly,the modified CaCl2·6H2O(CaCl2·6H2O-urea-ethanol)composite for cold energy storage was prepared using urea and ethanol as thermoregulation additives.The effects of different temperature regulators and their dosage on the phase change temperature and enthalpy of modified CaCl2·6H2O were investigated.Using strontium chloride hexahydrate(SrCl2·6H2O)and borax as nucleating agents,the supercooling and thermal properties of modified CaCl2·6H2O PCMs with different nucleating agents and contents were analyzed The effect of nucleating agent on the crystal morphology of modified CaCl2·6H2O was investigated.The chemical composition of modified CaCl2·6H2O were analyzed by Fourier infrared spectroscopy(FT-IR).The results of DSC manifested that CaCl2·6H2O composite containing 15 wt.%urea and 5.0 wt.%ethanol had a promising potential in air-conditioning application with a phase change temperature of 11.62? and a phase change enthalpy of 127.2 J/g.The step cooling curve showed that the effect of inhibiting supercooling of SrCl2·6H2O on modified CaCl2·6H2O was better than of that of borax with the same amounts.Meanwhile,after adding 2.0 wt.%SrCl2·6H2O,the supercooling degree of the modified CaCl2·6H2O PCM could be reduced to 0.95?,and its enhanced crystallization behavior was confirmed by optical microscope.The amount of SrCl2·6H2O added had little influence on the phase change temperature and enthalpy of modified CaCl2·6H2O PCM.FT-IR anaylsis verified the composition of modified CaCl2·6H2O PCM.Secondly,in order to solve the liquid leakage and heat conduction enhancement of modified CaCl2·6H2O,high-performance expanded graphite-based modified CaCl2·6H2O(modified CaCl2·6H2O/EG)composite phase change material(CPCM)was prepared by the melting-blending method with expanded graphite(EG)as thermal conductivity enhancer and porous carrier.The stablizing effect,thermal conductivity and phase change behavior of CPCMs with different sizes EGs(50,80,100 mesh)are discussed in detail.Results indicated that the form-stable effect and the adsorptive rate of EG-50 for modified CaCl2·6H2O PCM were superior to that of EG-80 and EG-100,which originated from its larger proportions of micropores,larger specific area and pore volume.The results of pore structure analysis and SEM observation confirmed that the pore structures of three kinds of EGs were occupied by modified CaCl2·6H2O PCM.The CPCM with EG-50 possessed a higher thermal conductivity and latent heat than that of the other two with EG-80 and EG-100 respectively,which were due to the low interfacial thermal resistance and the small confinement effects in EG-50.Besides,the addition of EG could suppress the supercooling degree of modified CaCl2·6H2O PCM.Thirdly,the hydrophilic modification of EG with TiO2 coating was conducted by sol-gel method to improve the compatibility of EG with inorganic modified CaCl2-6H2O PCM.Subsequently,MEG-based modified CaCl2-6H2O form-stable CPCM(modified CaCl2·6H2O/MEG)was prepared by a physical melt-blending method.The loading content of TiO2 on EG was optimize.The static adsorption capacity of EG and MEG on modified CaCl2·6H2O was investigated.The optimum content of MEG was determined.XRD,FT-IR,SEM-EDS and pore size analyzer were used to characterize EG,MEG and form-stable CPCM.The thermal conductivity and the cycle stability of the CPCM before and after compositing of PCM and MEG were investigated.The results of contact angle test indicated that EG loaded 15 wt.%TiO2 had better wettability to modified CaCl2·6H2O PCM.The results of XRD and FT-IR analysis indicated that TiO2 was successfully supported on EG.The static adsorption curves showed the significantly improved adsorption capacity and adsorption rate on modified CaCl2·6H2O PCM while comparing to that of EG.The leakage test indicated that 21 wt.%MEG could prevent modified CaCl2·6H2O PCM from leaking.The results of SEM-EDS and pore analysis revealed that TiO2 particles accumulated in layers on the surface of EG,and MEG still retained its inherent mesoporous structure.After compositing with modified CaCl2·6H2O,the pore structure of MEG was occupied basically.The obtained MEG-based modified CaCl2·6H2O CPCM melted at 10.67? with a melting enthalpy of 88.39 J/g,negligible supercooling degree(0.18?)as well as an enhanced thermal conductivity(8.831 W·m-1·K-1).Besides,the composite PCM exhibited an excellent thermal stability after 100 thermal cycles.Fourth,tetra-n-butyl ammonium bromide(TBAB)hydrate as cold energy medium was composited with silicon dioxide(SiO2)porous carrier to prepare SiO2-based TBAB hydrate form-stable CPCM(TBAB hydrate/SiO2).The contents of TBAB in hydrate,sodium phosphate dibasic dodecahydrate(Na2HPO4·12H2O)as nucleating agent and SiO2 as supporting material were optimized,respectively.The phase change properties,pore structure,morphology and the chemical composition of SiO2-based TBAB hydrate form-stable CPCM were characterized.And its thermal stability,thermal reliability and thermal conductivity also investigated.The results indicated that 40 wt.%TBAB in hydrate had an appropriate phase change temperature(11.81?)and high latent heat(211.9 J/g).Additionally,Na2HPO4·12H2O as nucleating agent could minimize its supercooling degree.The enhanced crystallization behavior was confirmed by optical microscope.The form-stable CPCM containing 30 wt.%SiO2 possessed large latent heat(134.0 J/g),low supercooling degree(2.01?)and excellent form-stable performance without any liquid leakage above melting temperature(8.33?).Also,the changes of its thermal properties were negligible after 100 thermal cycles.The results of pore structure analysis and SEM observation indicated that the pores of SiO2were basically filled by TBAB hydrate.The results of FT-IR analysis showed that TBAB hydrate was composited with SiO2 by physical interaction.The results of thermogravimetry(TG)analysis and thermal conductivity test showed that the SiO2-based TBAB hydrate form-stable CPCM had an excellent thermal reliabilityFinally,the porous support material of CNT@MIL-101(Cr)with the enhanced thermal conductivity was synthesized using mixed solvent method,and then the MOF-based TBAB hydrate form-stable CPCM(TBAB hydrate/CNT@MIL-101(Cr))was prepared.The crystal structure,chemical composition,pore structure and the morphology of CNT@MIL-101(Cr)and MOF-based TBAB hydrate form-stable CPCM were characterized by XRD,FT-IR,BET and SEM.The phase change properties and thermal conductivity of form-stable CPCM were analyzed.XRD crystal phase analysis showed that the amount of CNT below 11 wt.%did not affect the crystal formation of MIL-101(Cr).When the amount of CNT exceeded 11 wt.%,MIL-53(Cr)was obviously generated in the product.The leakage test showed that 30 wt.%of CNT@MIL-101(Cr)could effectively prevent the leakage of molten TBAB hydrate,and further showed that the maximum amount of CNT for CNT@MIL-101(Cr)was 11 wt.%.The results of pore structure analysis and SEM observation showed that TBAB hydrate could be well dispersed in the pores of CNT@MIL-101(Cr).FT-IR analysis further showed that the addition of 11 wt.%CNT did not affect the crystal formation of MIL-101(Cr),and TBAB hydrate/CNT@MIL-101(Cr)were formed by physical interaction.Meanwhile,the MOF-based TBAB hydrate form-stable CPCM had a phase change temperature of 7.99?with a phase change latent heat of 126.3 J/g,and subcooling degree of 1.64? as well as an enhanced thermal conductivity.