Study On Thermal Properties Of Nano-modified/form-stable Hydrate Salt Phase Change Material

The solid-liquid phase change materials（PCMs）,which can be used for thermal energy storage by absorbing and releasing heat energy during the phase transition process,have become a hot research topic over the past few decades.PCMs are used in a variety of applications including building thermal comfort,solar energy conversion,industrial waste heat recovery and so on.Among the large number of PCMs,inorganic hydrate salts PCMs have recently gained considerable importance in the utilization of energy because of the advantages of nontoxicity,non-flammability,low cost,etc.However,there also exists a series of problems,such as supercooling,phase separation,leakage,low thermal conductivity and specific heat,as well as the difficulty of adjusting phase transition behaviors in the inorganic hydrate salts PCMs.The main objective of this study is to overcome the above shortcomings of hydrate salts PCMs,on the basis of those traditional modification methods,taking advantage of advanced physical and chemical methods by interdiscipline,thermal performance improvement of hydrate salts PCMs are well developed.The detailed research contents are highlighted as follows:In order to eliminate phase separation of hydrate salt PCM and control its phase change temperature within the temperature scope suited for comfort of industrial and civil buildings,a novel Na₂CO₃·10H₂O-Na₂HPO₄·12H₂O eutectic hydrate salt（EHS）PCM was prepared based on the binary phase diagram and a solubility theory of inorganic salt.The composition of binary eutectic hydrate salt is 40:60 at mass ratio for Na₂CO₃·10H₂O and Na₂HPO₄·12H₂O.The phase change temperature and latent heat of the EHS are 27.3℃and 220.2J/g,respectively.And the phase separation has been entirely eliminated.In order to investigate the enhancements of specific heat and latent heat of inorganic hydrate salt based nanofluid PCM and their mechanisms,the inorganic hydrate salt based nanofluid phase change material was prepared by incorporating TiO₂ nanoparticles with different mass fractions（0.1,0.3,and 0.5 wt%）into the eutectic hydrate salt（EHS）through ultrasonic mechanical blending.The results show that the loading of 0.3wt%TiO₂ nanoparticles can increase specific heat by up to83.5%and latent heat up to 6.4%.The optical microscopy on crystal configuration confirms the increase of solid specific heat and its origin.Moreover,the chemical thermodynamics gives an explanation for latent heat improvement.These findings suggest that hydrate salt based TiO₂ nanofluid offers a promising route for the achievement of high heat energy storage targets of hydrate salt phase change materials.Two types of eutectic hydrate salt（EHS）form-stable PCMs were developed by physical blend method in terms of expanded graphite（EG）and expanded graphite oxide（EGO）as form-stable shell materials.The effects and mechanisms of chemical structures of EG and EGO on thermal properties were thoroughly discussed.The results indicate that the EHS/EGO form-stable PCM possesses higher latent heat,favorable thermal conductivity and lower supercooling degree because of the more surface oxygen-containing groups and surface defects as well as larger interlayer spacing in the EGO.Furthermore,the EHS/EGO form-stable PCM remains thermally stable after 200 thermal cycles.This work will provide insights into the performance improvement of form-stable PCM by means of tailoring the chemical structures of porous supports.A graphene oxide-modified hydrate salt/poly（acrylamide-co-acrylic acid）copolymer hydrogels（GO-EHS/PAAAM）form-stable phase change composite was presented,in which the phase transition behaviors of hydrate salt PCM could be tailored through the special interaction of GO and PAAAM with hydrate salt.The DSC results described that the peak shape during the endothermic process was converted from a single peak of a pure hydrate salt to two interconnected peaks of GO-EHS/PAAAM composite,of which the mechanism analysis was validated via SEM,FT-IR and Raman spectra.The thermal conductivity of GO-EHS/PAAAM with2wt%of GO increased by 54%when compared with EHS/PAAAM.Moreover,the GO-EHS/PAAAM composite remained chemically and thermally stable after 300 thermal cycles.The advanced phase change materials（PCMs）with tunable phase transition behaviors are significantly meaningful to the regulation and control of temperature in changeable application environments.The EHS/EGO and EHS/PAAAM form-stable hydrate salt PCMs were incorporated into cement mortar and epoxy resin by mechanical blending to prepare the thermal energy storage cement-based and resin-based composites.The present work focuses on the mechanical properties,microstructure and heat storage and release characteristics of the thermal energy storage cement-based and resin-based composites.The results show that the strengths of the thermal energy storage cement-based and resin-based composites decrease with the increasing form-stablehydratesaltPCMsamounts.However,they can still be regarded as building maintenance structure materialsto use.Besides,thermal performance test demonstrates the good endothermic and exothermic characteristics of the thermal energy storage cement-based and resin-based composites.