Research On Preparation And Performance Of Salt Hydrate Composite Phase Change Materials For Thermal Insulation Building Envelopes

Promoting building energy conservation technologies is an important way to realize the sustainable development of global energy and ecological environment.Improving the thermal performance of building envelope can improve thermal environment of the building,thereby reducing reliance of human being on heating,air conditioning and other equipment.Phase change materials(PCMs)can absorb and release latent heat during melting and solidofication process,which can compensate for the shortcomings of current insulation materials which have low heat capacity,thereby achieving the goals of reducing building energy consumption.Compared with organic PCMs with high price and flammability,inorganic salt hydrate PCMs deserve more attention because of the features of low cost,wide range of sources and inflammability.In this thesis,several salt hydrates whose phase transition temperature is close to comfort level of human body are selected.The aim is to develop high-performance salt hydrates for building envelopes.The main content is as follows:First,for the application of hydrated salt phase change materials in transparent envelopes such as windows,Sr Cl₂·6H₂O as nucleating agent,CTAB as dispersant and Cs_xWO₃as nanoparticles are added to Ca Cl₂·6H₂O solution for preparing modified salt hydrate PCM with NIR shielding ability.The influence of the addition of nanoparticles on morphology,phase transition characteristics and optical properties of Ca Cl₂·6H₂O is investigated.The thermal reliability and thermal storage performance of the materials are also studied.The characterization and test results show that the addition of Sr Cl₂·6H₂O and Cs_xWO₃can significantly restrain supercooling of Ca Cl₂·6H₂O.When the mass fraction of Cs_xWO₃is 0.75 wt%,the phase change temperature and enthalpy of CPCM is29.04°C and 132.8 k J·kg^-1,respectively.Besides,these thermal properties only changed a little after 100 thermal cycles,demonstrating excellent potential for energy storage.Thermal performance result shows that the double layer glass window which equip with Ca Cl₂·6H₂O/Cs_xWO₃PCM has good thermal insulation performance.Due to receiving a large amount of solar radiation,the heat energy passing through the roof can easily cause indoor temperature fluctuations,leading to an increase in the frequency of use of refrigeration equipment.Based on the needs for energy conservation of roof structure,Na₂HPO₄·12H₂O with phase change temperature of 35.5°C and latent heat of 204 k J·kg^-1 is selected as core PCM for roof structure,and expanded perlite(EP)is selected as porous material for preventing leakage problem of Na₂HPO₄·12H₂O during phase transition process.A thin layer of Ti O₂synthesized by sol-gel method is used to modify the surface of EP to increase NIR reflectance property.SEM and EDS result demonstrate that Ti O₂ has been successfully loaded on the surface of EP.Leakage test shows that the maximum Na₂HPO₄·12H₂O adsorption capacity of MEP is 55%.The result of DSC?XRD?FT-IR and VIS-UV-NIR result show that the composite PCM has suitable phase transition temperature(33.51°C)and high latent heat(103.7 k J·kg^-1),as well as a negligible supercooling degree.Besides,there was no chemical interaction between the components.Furthermore,higher NIR reflectivity of salt hydrate PCM is achieved after combining with MEP.These properties endow the fabricated composite PCM with large potential for practical application.In order to expand the application fields of salt hydrates PCMs,so they can play a role in energy-saving application of walls,Na₂HPO₄·12H₂O-Na₂CO₃·10H₂O binary salt hydrate with a phase change temperature within the range of human co mfort is fabricated,and diatomite with porous structure and hydrophilic group is u sed for stabilizing salt hydrate.In order to prevent water loss of salt hydrate PC M during long-term cycles,UV-curable polyurethane acrylic resin(PUA)was used to coat the diatomite surface to prepare shape-stable composite PCM.Subsequentl y,the composite PCM is sealed into PVC boards to make PCM walls for test roo m,and then experience thermal performance.Even though diatomite with 40%ma ss ratio can solve the leakage problem of salt hydrate by surface tension and capi llary force,the binding of crystal water by adsorption can not be maintained after long-term heating.After coating by PUA resin,the heat loss rate of composite P CM is significantly reduced.The enthalpy loss of composite PCM can be controll ed within 2%after 300 times melting-solidification cycles,which demonstrates bett er thermal reliability.The composite PCM has a low melting point(24.05°C),a high phase change enthalpy(102.6 k J·kg^-1)and a low subcooling(0.51°C)as wel l as a low thermal conductivity(0.37 W·m^-1·K^-1).Compared with the reference ro om without PCM wallboards,the maximum indoor temperature of the simulated ro om with composite phase change board on the inner wall is reduced by 17.19°C,and the minimum temperature is increased by 10.26°C,so it has Smoother temp erature fluctuations.Obvious temperature hysteresis(4.03 h)and higher thermal co mfort frequency can also be achieved.Finally,a large-size office building integrated with Ca Cl₂·6H₂O PCM window,Na₂HPO₄·12H₂O/MEP PCM roof and Na₂HPO₄·12H₂O-Na₂CO₃·10H₂O/diatomite/PUA wallboards is numerically studied.The calculation results show that the introduction of low-temperature fresh air at night can reduce energy consumption by increasing the utilization rate of PCMs,and the peak temperature of the building can be reduced by more than 3.0°C.On the other hand,when the thickness of the PCM wallboards is 15mm and when they are installed on the interior layer of the building as well as a lower thermal conductivity of 0.2 W·m^-1·K^-1,the best energy saving effect can be achieved.Compared with the reference building,the PCM building can achieve an energy-saving rate of 14.3%.