Molecular Dynamics And Experimental Study Of The Thermophysical Properties Of Polyol Phase Change Materials For Medium-temperature Latent Heat Storage

Latent heat storage technology can alleviate the contradiction between supply and demand of heat energy,improve the efficiency of heat energy conversion,and promote the comprehensive cascade utilization of solar energy and industrial waste heat.It is of great practical significance for energy saving and emission reduction,and for striving to achieve the strategic goals of "emission peak" and "carbon neutralization" in China.High-performance phase change materials(PCMs)play a key role in the development of latent heat storage technology.In recent years,polyol PCMs,which exhibit high latent heat of fusion,have attracted much attention in the medium temperature range of about 370–520 K.Different from other common organic PCMs(e.g.,paraffin and alkanes),polyol PCMs contain multiple high-polarity hydroxyl groups in their molecular structures.Although a number of studies have measured the macroscopic thermophysical(e.g.,phase change enthalpy and thermal conductivity)of polyol PCMs,there is a lack of understanding of their “structure-property”relationships at the microscale.For example,if only relied on experimental investigations,it would be difficult to figure out the variation of phase change enthalpy of polyol phase change materials with their crystal structures,the distribution of hydroxyl groups,as well as the presence of nano-additives.Although some researchers have pointed out the effect of hydrogen bonds formed by hydroxyl groups on the thermal conductivity of monohydric alcohols,the contribution of hydrogen bonds to the thermal conductivity of polyol PCMs is insufficient and requires a quantitative analysis.Moreover,when being packaged in a heat storage tank,the thermal contact resistance(TCR)between polyol PCMs and the thin-walled metal shells will also affect the heat storage/retrieval rates of the latent heat storage system.Therefore,it is necessary to carry out measurements and microscale studies on the thermal contact resistance between polyol PCM and metal shells.In view of the above-mentioned issues,molecular dynamics simulations were performed in this dissertation to get atomistic insights into the thermophysical properties of polyol PCMs,with one of the representatives being erythritol possessing very high storage density,and the interfacial heat conduction between erythritol and metal shells.First of all,multiple force fields including OPLS,CHARMM,GAFF and GROMOS,which are possibly suitable for modeling polyhydroxy structures were tested.By comparing the numerical results with the measured density and specific heat capacity of erythritol in both solid and liquid phases,the GROMOS force field was identified as the most suitable molecular model for polyol PCMs.Using this force field,the interface/NPT method was adopted to reproduce the microscale melting progress of erythritol.It was found that the linear structure of erythritol could turn into nonlinear structure during melting,and that the variation of hydrogen bond energy accounts for 45.5% of the total melting enthalpy of erythritol.This confirms that the existence of abundant hydrogen bonds is the fundamental reason for the high phase change enthalpy of polyol PCMs.Since the subcooling degree of erythritol is as high as about 100 K,making it difficult,if not impossible,to reproduce the crystallization process in molecular dynamics simulations.So,a typical monohydric alcohol PCM,i.e.,n-hexadecanol,was studied as an alternative to explore the effects of nano-additives on the phase change enthalpy of alcoholic PCMs.The simulation results revealed that the n-hexadecanol molecules can agglomerate in the vicinity of the graphene nanosheets,thus maintaining the nonlinear liquid-like structures during solidification.These agglomerated n-hexadecanol molecules have no contribution to the solidification enthalpy,leading to the lower solidification enthalpy of graphene nanosheets/n-hexadecanol composites than that predicted by effective medium theory.The non-linear n-hexadecanol molecules absorb few latent heat during melting,which finally results in the reduction of melting enthalpy.In order to further improve the phase change enthalpy of the graphene nanosheets/n-hexadecanol composites,hydroxyl groups were added on the surface of graphene nanosheets to construct a simplified graphene oxide model.Hydrogen bonds can be generated as a result of the interaction between the hydroxyl groups added over graphene oxide and the existing hydroxyl groups in nhexadecanol,which successfully compensate part of the phase change enthalpy loss of the composites.Such simulation results were verified by preparing graphene oxide/n-hexadecanol composites and measuring their phase change enthalpy.This finding provides an idea for the development of polyol-based nano-enhanced PCMs with high storage density.In order to elucidate the effects of hydrogen bond and crystal structure on the microscopic heat conduction of polyol PCMs,erythritol and a typical solid-solid polyol,i.e.,pentaerythritol,were investigated.The variations of their thermal conductivity during phase transitions were explored.Through analyzing the intrinsic relationship among hydrogen bond,thermal conductivity and phase transition,the contribution of hydrogen bond to intermolecular heat conduction of polyol PCMs was revealed.The results suggested that in the solid phase,more hydrogen bonds and higher hydrogen bond energy will lead to higher thermal conductivity of polyol PCMs.In order to take more advantage from the hydrogen bonds,an “ideal crystal” model of monohydric alcohols was proposed and established.It was shown that the heat conduction along the chain of monohydric alcohols with the “ideal crystal” structure is ballistic and the temperature gradient along the chain is very low,and that the major temperature difference is at the interface between the molecular layers of the “ideal crystal”.Because the hydrogen bond formed at the interface can improve the interfacial heat transfer coefficient,the thermal conductivity of the “ideal crystals” was predicted to be about doubled of that of the natural monohydric alcohols.This provides a theoretical guidance for improving the inherent thermal conductivity of polyol PCMs.Last,in order to measure the TCR between the interfaces of polyol PCMs and metal shells,an improved steady-state heat flow method was proposed.An instrument was designed and built successfully based on this improved test principle.The TCR between erythritol and different metals was measured systematically using the self-developed instrument.The effects of surface roughness and contact pressure on the TCR between the interface of erythritol and metal shells were clarified.The experimental results confirmed that the lower surface roughness and higher contact pressure will lead to lower TCR.In addition,the overlap factor of phonon density of state at the interface of erythritol/metal was revealed by molecular dynamics simulations,thus distinguishing the heat conductance between the interface of erythritol and different metals.These results are useful for the selection of packaging materials and the thermal design of latent heat storage systems.In conclusion,molecular dynamics simulations and experimental studies on the some key thermodynamic and transport properties of typical polyol PCMs were carried out in this dissertation.The results will help exploit polyol-based composite PCMs with high phase change enthalpy and high thermal conductivity,and provide fundamental support for the development and promotion of medium-temperature latent heat storage technology.