Design and Performances of Expanded Vermiculite-based Composite Phase Change Materials for Thermal Energy Storage

Author:Deng Yong

Supervisor:li jin hong he ming yue


Degree Year:2019





The expanded vermiculite shows large storage capacity and excellent thermal stability and chemical compatibility,which is a suitable encapsulation carrier of PCMs.In this paper,aiming at the disadvantage in the applications of low heat storage capacity,poor heat transfer and large supercooling degree of PCMs,the expanded vermiculite acted as encapsulation carrier,and the polyethylene glycol(PEG),Na2HPO4·12H2O and nitrate served as energy storage media,and the properties of heat storage,heat transfer and supercooling were improved by functional modification.The main influencing factors of the phase change behavior and heat storage performance of composite PCMs were explored.The enhanced heat transfer and supercooling suppression mechanisms were clarified.The effective medium percolation theory was employed to predict the thermal conductivity of composite PCMs.The results indicated that both silver and silicon carbide nanowires showed larger thermal conductivity enhancement ability in the PAE ss-CPCMs and PSE ss-CPCMs,respectively.The three-dimensional heat transfer enhancement network or channel with silver and silicon carbide nanowires was constructed in the pore structures of expanded vermiculite and effectively reduced interfacial thermal resistance during heat transfer,which improved rapid and uniform heat transfer property,thus accelerating heat storage and release rates.To some extent,the problem of slow or uneven heat transfer of composite PCMs was solved.The surface interactions of expanded vermiculite and nanowires affected the phase change behavior and heat storage property of PEG.Non-isothermal crystallization behavior studies found that the half crystallization time of PAE ss-CPCMs containing 0 and 19.3 wt% silver nanowires was respectively prolonged by 44% and 60% when compared with PEG,and the apparent activation energy decreased to-219.7 kJ/mol and-213.4 kJ/mol,respectively.The synergistic effect of expanded vermiculite and nanowires promoted nucleation of PEG and constrained its crystalline growth,providing theoretical basis for quantitative explanation of reduction of heat storage capacity caused by the decrease in crystallinity.The heat storage capacity of composite PCMs with enhanced heat transfer with nanowires had been reduced.Hence,the carbon nanotubes were employed to enhance heat transfer.On one hand,the purpose of regulating the component of expanded vermiculite was to minimize the influence on the heat storage behavior of PEG.On the other hand,the surfaces of carbon nanotubes could adsorb a large amount of PEG to increase the encapsulation capacity of composite PCMs(13 wt%).As a result,the PCE ss-CPCMs with simultaneously enhanced latent heat and thermal conductivity were constructed.The strategy of the encapsulation of expanded vermiculite combined with the addition of inorganic alumina particles was taken to suppress the supercooling behavior of Na2HPO4·12H2O as PCM.It was found that the supercooling degree of Na2HPO4·12H2O greatly reduced to 1.4 ℃ and the suppression ratio reached 90.2% by the synergistic effect between expanded vermiculite and alumina,and the supercooling suppression mechanism on the surfaces of expanded vermiculite and alumina was revealed,which provided a new approach to reduce the supercooling behavior of hydrated salt PCMs by using nucleating agents whose lattice parameters were large different from PCMs.A series of nitrate/expanded vermiculite middle-temperature composite PCMs with high heat storage capacity(295 J/g),high thermal stability and excellent thermal reliability(latent heat reduction was less than 2% after experiencing 100 phase change cycles)were designed and prepared for the first time.The effect of expanded vermiculite on the phase change and heat storage behavior of nitrate PCMs was clarified.A new method with wide applicability for theoretical encapsulation capacity calculation had been developed.In this work,the basic research on theory and application of expanded vermiculite-based composite phase change materials was systematically carried out.The work is expected to provide theoretical and technical support for constructing low temperature/high temperature and organic/inorganic composite PCMs with excellent thermophysical properties,and present a new approach for high value-added and green utilization of rich vermiculite resources in China.