| Floor radiant heating has been gaining popularity in new residential and commercial buildings since it can not only effectively utilize both building space and energy,but also provides a comfortable indoor thermal environment and the opportunity of utilizing low-grade energy resources.The incorporation of phase change materials?PCMs?into the floor radiant heating contributes to balancing the power loads in electric grids,realizing peak-load shifting of electricity utilization.Salt hydrates have the advantages of competitive price,high latent heat and non-flammability,making them attractive for the floor radiant heating.However,their supercooling,phase separation and leakage are non-negligible,hindering their popularization in floor radiant heating.Therefore,in this work,with the objective of promoting salt hydrates'practical application in floor radiant heating,novel high-performance form-stable composite phase change materials based on salt hydrates were prepared.The main works are as follows:Firstly,a sodium acetate trihydrate-urea?SAT-urea?non-eutectic mixture PCM for heat exchanger was developed using sodium acetate trihydrate?SAT?as PCM,urea as temperature regulator,disodium phosphate dodecahydrate?DSP?as nucleating agent and sucrose as auxiliary nucleator respectively.The mass fractions of urea,DSP and sucrose in the non-eutectic mixture were optimized.The effects of the additives on phase change properties and crystal morphology of the mixture were discussed,and its thermal stability and thermal reliability were investigated.The results showed the mixture PCM containing 8 wt%urea,1.5wt%extra DSP and 2.0 wt%extra sucrose had suitable phase change temperature?50.82??,high latent heat(245.4 kJ?kg-1)and low supercooling degree?2.51??.Optical microscope results confirmed that the addition of DSP and sucrose could promote crystallization of SAT-urea matrix.Differential scanning calorimetry?DSC?,Fourier transform infrared spectroscopy?FT-IR?and X-ray diffractometer?XRD?results indicated that the SAT-urea mixture was non-eutectic and there was physical interaction between SAT and urea.After 100cycles,the minor variations of the mixture PCM in supercooling degree,latent heat and phase change temperature could be observed.Secondly,by capillary action,expanded graphite?EG?served as both thermal conductivity enhancer and supporting material,was composited with sodium acetate trihydrate-urea non-eutectic mixture PCM to develop a sodium acetate trihydrate-urea/expanded graphite?SAT-urea/EG?form-stable composite phase change material for use in heat exchanger.The effects of EG mass fraction on phase change behaviors of the non-eutectic mixture including phase change temperature,latent heat,supercooling degree and form stability were discussed.The mass fraction of EG in the composite PCM was optimized.The morphology,pore structure,crystalline structure and chemical composition of the obtained composite PCM were characterized,and its thermal stability,thermal conductivity,thermal reliability and storage/release process were investigated systematically.The results showed that the addition of EG can significantly improve the thermal conductivity,decrease supercooling degree and prevent the molten SAT-urea mixture from leaking.The composite PCM with EG mass fraction of 12%had outstanding form stability,suitable phase change temperature?48.46??,high latent heat(216.8 kJ·kg-1),supercooling degree?1.93??as well as high thermal conductivity(4.188 W?m-1?K-1 at density of 0.98 g?cm-3).Scanning electron microscope?SEM?and pore structure analysis results displayed that the surfaces and pores?micropores and partial mesopores?of EG were nearly filled with the mixture.FT-IR and XRD results indicated that there was no chemical interaction between the SAT-urea mixture and EG.After experiencing 200 cycles,the composite PCM possessed negligible changes in phase change temperature,latent heat and crystallization property.Thirdly,a novel microencapsulated phase change material based on sodium thiosulfate pentahydrate as core and poly?ethyl-2-cyanoacrylate?as shell was successfully synthesized by interfacial polymerization.The effects of STP/H2O mass ratio,mass ratio of core-shell and mass fraction of surfactants on phase change properties of the synthesized microcapsules were discussed.The morphology,microstructure,surface elemental distribution,chemical composition and crystalline structure of the resultant microcapsules were determined by SEM,energy dispersive spectroscopy?EDS?,FT-IR and XRD.Besides,their phase change properties,thermal stability and thermal reliability were also investigated systematically.The results showed that the microcapsules synthesized under STP/H2O mass ratio of 6:4,core-shell mass ratio of 4:2 and surfactants mass fraction of 5%had the highest latent heat.SEM and TEM images showed that the microcapsules presented almost spherical profiles with a diameter of about 1.0?m and a well-defined core-shell structure,and their surfaces were compact but mildly rough.Due to the confined crystallization of STP and the heterogeneous nucleation effect of PECA shell,the microcapsules exhibited a slightly lower phase change temperature?46.44??than pure STP,and possessed latent heat of 107.0 kJ?kg-1 and and encapsulation ratio of 51.1%at the core material/monomer mass ratio of 4/2.TGA and the thermal cycling test revealed that the thermal stability of the microcapsules was improved and had a good thermal reliability.Finally,a novel phase change temperature-tuned composite phase change material for PCM floor was developed.In this composite PCM,a sodium acetate trihydrate-urea non-eutectic mixture acted as PCM and fumed silica?SiO2?served as both supporting material and temperature regulator.The effects of SiO2 mass fraction on phase change behaviors of the non-eutectic mixture including phase change temperature,latent heat,supercooling degree and form stability were analyzed detailedly,then the optimal mass fraction of SiO2 was determined.The prepared composite PCM was characterized by BET,SEM,FT-IR and XRD,and its thermal stability,thermal conductivity and reliability were evaluated.The results showed that the addition of SiO2 could adjust phase change temperature of the non-eutectic mixture in the range of 34.36-50.82?,reduce supercooling degree and prevent leakage.With SiO2 mass fraction of 30%,the obtained composite PCM had a good form stability,suitable phase change temperature?35.75??,high latent heat(151.6 kJ·kg-1)and low supercooling degree?1.14??.The results obtained from SEM and pore structure analysis showed that the non-eutectic mixture was dispersed into the pores?all micropores and part of mesopores?of SiO2.XRD and FT-IR results confirmed that SiO2 was combined with the SAT-urea mixture by physical interactions.In addition,the composite PCM possessed an excellent thermal reliability and favorable thermal conductivity. |
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