Carbonate-salt-based composite materials for medium- and high-temperature thermal energy storage

Particuology15(2014)77–81

ContentslistsavailableatScienceDirect

Particuology

j ou rn a l h o m ep a g e :w w w . e l s e v i e r . c o m / l o c a t e / p a r t i c

Short

communication

Carbonate-salt-based

composite

materials

for

medium-

and

high-temperature

thermal

energy

storage

Zhiwei

Ge

_,

_Feng

_Ye

_,

_Hui

_Cao

_,

_Guanghui

_Leng

_,

_Yue

_Qin

_,

_Yulong

_Ding

a_{StateKeyLaboratoryofMultiphaseComplexSystems,InstituteofProcessEngineering,ChineseAcademyofSciences,Beijing100190,China} b_{InstituteofParticleScience&Engineering,UniversityofLeeds,LeedsLS29JT,UK}

c_{UniversityofChineseAcademyofSciences,Beijing100049,China}

d_{SchoolofEngineeringandTechnology,ChinaUniversityofGeosciences,Beijing100083,China} e_{CentreforCryogenicEnergyStorage,UniversityofBirmingham,Edgbaston,Birmingham,B152TT,UK}

a

r

t

i

c

l

e

i

n

f

o

Received12June2013

Receivedinrevisedform22August2013 Accepted3September2013

Keywords:

Thermalenergystorage Compositematerials Microstructure Thermalconductivity Phasechangematerial

a

b

s

t

r

a

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Thispaperdiscussescompositematerialsbasedoninorganicsaltsformedium-andhigh-temperature thermalenergystorageapplication.Thecompositesconsistofaphasechangematerial(PCM),aceramic material,andahighthermalconductivitymaterial.Theceramicmaterialformsamicrostructuralskeleton forencapsulationofthePCMandstructuralstabilityofthecomposites;thehighthermalconductivity materialenhancestheoverallthermalconductivityofthecomposites.Usingaeutecticsaltoflithium andsodiumcarbonatesasthePCM,magnesiumoxideastheceramicskeleton,andeithergraphiteflakes orcarbonnanotubesasthethermalconductivityenhancer,weproducedcompositeswithgoodphysical andchemicalstabilityandhighthermalconductivity.Wefoundthatthewettabilityofthemoltensalt ontheceramicandcarbonmaterialssignificantlyaffectsthemicrostructureofthecomposites.

©2013PublishedbyElsevierB.V.onbehalfofChineseSocietyofParticuologyandInstituteofProcess Engineering,ChineseAcademyofSciences.

1. Introduction

Thermal energy storage plays a vital role in the effective and efficient useof renewable energy resources and industrial wasteheat.Keystothermalstoragetechnologyincludematerials’ developmentandheatexchangeduringchargeanddischarge pro-cesses.Moltensaltsareamongthemostpromisingphasechange materials(PCMs)forthermalenergystorageatmedium-and high-temperatures.However,applicationsofmoltensaltsasPCMsare oftenhamperedbychemicalincompatibility(suchascorrosionof containers)and lowthermalconductivities(Guillotetal.,2012; Zhao&Wu,2011).Toovercometheselimitations,composites con-tainingcarbonallotropesasthermalconductivityenhancing mate-rials(TCEMs)havebeenproposed.Mixingcarbonallotropeswith moltensaltsfollowedbycompression(Acem,Lopez,&PalomoDel Barrio,2010;Lopez,Acem,&PalomoDelBarrio,2010)orinfiltration ofmoltensaltsintoaprefabricatedcarbon(suchasgraphitefoam) (Tammer,2008)havebeenshowntobeeffectiveapproachesfor significantlyincreasingthethermalconductivityofthecomposites. However,poordispersionofthecarboninliquidmoltensaltsoften

∗ Correspondingauthorat:StateKeyLaboratoryofMultiphaseComplex Sys-tems,InstituteofProcessEngineering,ChineseAcademyofSciences,Beijing100190, China.

E-mailaddresses:[email protected],[email protected](Y.Ding).

leadstotheseparationofthecarbonfromthebulkliquidphase of moltensalts.Althoughinfiltrationofliquid moltensaltsinto prefabricatedblocksofcarbonmaterialscanavoidthisseparation issue,theresultingcompositesareoftenunabletoretainsufficient PCMswithintheirstructuresatmedium-andhigh-temperatures, leadingtoleakageofmoltensaltsduringsolid–liquidphasechange, andhenceadecreaseinthermalenergystoragedensityofthe com-posites(Pincemin,Olives,Py,&Christ,2008).Relativelypoor wet-tingbetweencarbonmaterialsandliquidmoltensaltsisan impor-tantreasonfortheleakageandphaseseparation;thedifferencein densitiesofthesaltsandcarbonsisalsoacontributingfactor.

Inthispaper,weintroducemicrostructuredcomposite mate-rials(intendedformedium-andhigh-temperatureapplications), consisting of a molten salt-based PCM, a ceramic skeleton as PCMcarrier,andacarbon-basedthermalconductivityenhancer. We show that the PCM is encapsulated within the composite microstructure,and,thethermalconductivitycanbesubstantially enhanced.

2. Experimental

2.1. Rawmaterialsandcompositefabrication

ThePCMusedinthis workwasa eutecticcarbonatemolten salt (LiNaCO3) made from sodium carbonate (Na2CO3, Beijing Chemical Works) and lithium carbonate (Li2CO3, Sinopharm 1674-2001/$–seefrontmatter©2013PublishedbyElsevierB.V.onbehalfofChineseSocietyofParticuologyandInstituteofProcessEngineering,ChineseAcademyofSciences.

Fig.1.Micrographsof(a)PCM(LiNaCO3),(b)1PCM/0.01CNTs;insetisamacroscopicopticalimage.(c)1PCM/1MgO/0.015CNTs;insetisamacroscopicopticalimage,and

(d)1PCM/1MgO.Notethatintheopticalimages,thePCMappearswhiteandtheCNTsareblack.

ChemicalReagent Co.Ltd). Ceramic(MgO,SinopharmChemical ReagentCo.Ltd)andcarbonmaterials(naturalgraphiteflakesand carbon nanotubes(CNTs), BeijingDk NanoTechnology Co. Ltd) werechosen astheskeletonmaterial andthermal conductivity enhancer,respectively.Theeutecticsaltwasmadebymixing thor-oughly43%Li2CO3and57%Na2CO3(masspercentage)andheating themixturetoatemperatureabovethemeltingtemperature. Fab-ricationofeutecticbasedcompositesinvolvedthreesteps.First, theeutecticsaltwasthoroughlymixedwithappropriateamounts ofceramicandcarbonmaterials.Themixturewasthenshapedinto disk-likegreenpelletsbyuniaxialcompression.Finally,thegreen pellets weresinteredin anelectricfurnace usingthefollowing heating procedure:heatingfrom25 to400◦C at5◦C/min,then from400to550◦Cat1◦C/min,andholdingat550◦Cfor90min. Thecoolingprocedurewasthereverseoftheheatingprocess. 2.2. Samplecharacterization

Themorphologicalandmicrostructuralcharacterizationofthe compositeswascarriedoutbyscanningelectronmicroscopy(SEM, JSM-7100F,JEOL,Japan),andX-raymicrotomography(XMT, Micro-CT200,Xradia,USA). ALaserflashapparatus(LFA 427,Netzsch, Germany)and athermalanalyzer(TG-DSC,STA449F3Jupiter®, Netzsch,Germany)wereusedtoevaluatethethermophysical prop-ertiesofthesamples.

3. Resultsanddiscussion

3.1. Characterizationofthecomposites

Fig.1showsSEMimagesandmacroscopicdigitalphotosofsome typicalsamples.ThePCM(LiNaCO3)particlesexhibitaroundshape andarelativelysmoothsurface(Fig.1(a)).WhenCNTsareadded, thenanoparticlescanonlybefoundatthesurfaceofthePCM par-ticles,asshowninthedigitalphotointheupperrightcornerof

Fig.1(b),andalmostnoCNTs(darkincolor)areseenwithinthebulk ofthephasechangematerial(whiteincolor).Thecross-sectional andtopviewsofFig.1(b)showthatthesampleisalmost semicircu-lar,withCNTsdistributedonlyonthesurfaceofthePCMparticles. Fig.1(c)showsanSEMimageofacompositematerialcontaining thePCM,MgO,andCNTsinamassratioof1:1:0.015.MgOcrystals, withdiametersof0.15–0.6␮m,aredistributeduniformlyonthe surfaceofthesample.ComparingtheinsetsinFigs.1(b)and1(c), wenoticethatthattheCNTsaremuchbetterdispersedinthesalt inthepresenceofMgO.Suchstructuraldifferencesbetweenthe sampleswithandwithouttheMgOskeletonmaterialarenotonly becauseoftheliquid-phase-sinteredmicrostructure(Fig.1(d)),but arealsolikelytobeassociatedwithinterfacialenergiesbetweenthe differentcomponentsofthecompositematerials.

Fig.2showsXMTimagesofthesamplesofdifferent composi-tons.Asshownin Fig.2(a),thePCM particleaftermelting and recrystallizationhasaroundshape.Althoughuniaxial compres-sionwasappliedduringthepreparationstage,manyporesremain inthePCMstructure;thesecanbeattributedtothevolumechange ofthePCMduringthemelting/recrystallizationprocess.Fig.2(b) showsthePCMcontainingCNTs.Inthiscomposite,theporesare largerthanthoseinthepurePCM(Fig.2(a)),andalltheCNTsare

Fig.2.XMTimagesof(a)PCM(LiNaCO3)atslice465,(b)1PCM/0.01CNTsatslice500,

and(c)compositematerial1PCM/1MgO/0.015CNTsatslice531intheXYplanes. Imagesareobtainedfromsegmentationofthescannedsamplesbythresholding withMimics10.0software;green,black,andpinkcolorsrepresentPCM,CNTs,and MgO,respectively.(Foractualcoloursmentionedinthetext,readersarereferredto theon-linewebversionofthisarticle.)

Z.Geetal./Particuology15(2014)77–81 79

Fig.3.Schematicsofmicrostructureformationmechanismfor(a)PCMandceramic skeletonmaterial,(b)PCMandthermalconductivityenhancer;green,black,and pinkcolorsrepresentPCM,thermalconductivityenhancer,andceramic, respec-tively.

distributedontheexteriorsurfaceofthePCMparticle. Fig.2(c) showsacompositematerialcomprisingPCM,MgO,andCNTswith a1:1:0.015massratio.ThePCMandCNTsareuniformlydistributed withinthestructureformedbytheskeletonmaterial(MgO).The XMTresultsareconsistentwiththeSEManalysesshownabove.

3.2. Microstructureformation

Fig.3(a)showsaproposedmicrostructureformation mecha-nismforacompositeconsistingofthePCMandceramicmaterial. Duringpowdermixing,porestaketheformofinterparticlevoids. Uponuniaxialcompression,theamountofvoidspaceisreduced, whilethegreendensityincreases.Uponsintering,thesolidphase PCMbeginstoturnintoliquidasthetemperatureapproachesthe meltingpoint,leadingtomultiphasecompositeswithco-existing

Fig.4. Photosofcompositeswithout(a)andwith(b)theceramicmaterial,showing thattheceramicreduceslossofmoltensaltfromthecomposite.

solid,liquid,andgasphases.AccordingtothefollowingYoung’s equation:

sv=sl+lv×cos, (1)

thewettabilityoftheliquidphaseonthesolidphaseisrelatedto thecontactangle(),whichisafunctionoftheinterfacialenergies (_,wherethesubscriptss,l,andvcorrespondtothesolid,liquid, andgasphases,respectively)(German,Suri,&Park,2009;German, 2010).Asceramicmaterialsoftenhavearelativelyhigh interfa-cialenergy,asmallcontactangleisexpected.Asaconsequence, theliquidPCMphaseislikelytobeabletowettheskeleton mate-rial,andspreadonitssurfacetodisplacethesolid–gasinterface, andinsteadofformingliquid–solidandliquid–gasinterfaces.At thesametime,thewettingliquidprovidesacapillaryforcetopull theceramicparticlestogetherandrearrangethemtoformadense composite.Thiswillincreasethelocalrigidityofthestructure.After sintering,thefinalstructureconsistsofgrainsofskeletonmaterial, boundbysolidifiedliquidsalt.Suchamicrostructurecouldprevent PCMleakageduringsolid–liquidphasechange.

CarbonmaterialssuchasCNTsandgraphitearelesslikelyto bewettedbytheliquidmoltensalts,eventhoughtheyhave suffi-cientsurfacetensiontoallowwettingbywater(∼72mN/m)and mostorganicsolvents(<72mN/m)(Dujardin,Ebbesen,Hiura,& Tanigaki,1994;Pinceminetal.,2008).Thispoorwettabilitycould lead toswelling, as illustrated inFig. 3(b), and thus forminga relativelyloose,porousstructure.Thisexplainstheseparationof theCNTsfromthebulkPCMparticleswhenthecomposites con-tained onlyPCMandcarbon, asdiscussedearlierandshown in Figs.1(b)and2(b).

ThisisalsoconsistentwiththeobservationthatlittlePCMwas foundincompositesconsistingonlyofcarbonmaterialandPCM,as showninFig.4(a).However,forthecompositescontainingthePCM, theceramicskeleton,andthecarbonmaterial,therearrangement anddensificationoftheskeletonmaterialwiththeliquidmolten salt cansignificantlyrestrict theswellingcaused bythecarbon material,allowingtheformationofauniformcomposite,asshown inFigs.1(c)and2(c),andeffectiveencapsulationofthePCMinthe composite,asshowninFig.4(b).

To furtherelucidate theinfluence ofcarbon materialonthe microstructureofthecomposite,theeffectsofgraphiteandCNT loadingswithinthecompositeswerestudied.Fig.5showsthebulk densityofthesamplesasafunctionofloadingofthecarbon mate-rial.Thebulkdensitiesofboththegraphitecomposites(Fig.5(a)) andtheCNTcomposites(Fig.5(b))showaturningpoint.The den-sityofthesinteredcompositesishigherthanthegreenpelletsand thistendencydecreaseswithincreasingloadingofcarbon,untila criticalloadingisreached(attheturningpoint).Furtherincrease inthecarbonloadingleadstoasignificantdecreaseinthe den-sityofthesinteredcompositesandalowerdensitythanthegreen composites.Suchbehaviorsuggeststheexistenceofacompeting

Fig.5.Bulkdensityasafunctionofcarboncontent:(a)graphiteand(b)CNTs.

processduetodifferentwettabilitiesoftheceramicmaterialand thecarbonmaterialwiththeliquidPCMphase.Highwettabilityof theceramicdensifiesthefinalcomposite,whilepoorwettabilityof thecarbonmaterialswellsit.InspectionofFig.5(aandb)shows thatthedensityattheturningpointforthegraphitecomposites ismuchhigherthanthatfortheCNTcomposites.Thisisprobably becausethelayered(two-dimensional)structureofthegraphite flakesmakesiteasiertoorientthegraphiteperpendicularlytothe compressiondirection(Acemetal.,2010;Yuanetal.,2012),leading toahighercompositedensityforgraphitethanthatof compos-itescontainingCNTs,whichhaveaone-dimensionalhollowtubular structure.

3.3. Thermophysicalpropertiesofthecomposites

TheTG–DSCcurvesfortheeutectic carbonate(LiNaCO3)and thecompositecontaininggraphite,LiNaCO3,andMgOaregiven inFig.6.TheTGcurvesofFig.6(a)showthatboththePCMand the composite have good thermal stability. The DSC curves in Fig.6(a) yieldenthalpyof melting valuesof about348.5J/gfor theeutecticcarbonateandabout178.3J/gforthecomposite.This differenceissimplybecausethereislessmoltensaltinthe com-positethaninthepuremoltensalt,foranygivensamplemass. Toevaluatethechemicalandphysicalstabilityofthecomposites, thermalcyclingexperimentswereconductedandthe correspond-ingTG–DSCcurvesareplotted inFig.6(b);theTGcurvesshow

Fig.6.TG-DSCcurvesfor(a)thePCMandthecomposite,and(b)thecomposite over28thermalcycles.

negligiblemasschangeoverthe28thermalcycles,indicatingthat thecompositematerialhasagoodthermalstability.Theimagein theinsetofFig.6(b)showsthatthecompositehasmaintainedits originalshapeafter28thermalcycles.Thechemicalstabilityofthe compositecanbeevaluatedfromtheDSCcurvesinFig.6(b).No significantchangestothemeltingpointortheheatoffusionare seenoverthe28thermalcycles.

ThethermalenergythatcanbestoredinacompositePCM mate-rialincludesthelatentheatofthePCMandthesensibleheatof boththesolidandliquidphasesinthecomposite;themajorityof thestoredenergyisthroughthelatentheat.Ontheotherhand, thethermal conductivityofthecompositedependsonthe con-centration,size,shape,orientationandspatialdistributionofthe thermalconductivityenhancer;thehighertheconcentrationofthe enhancer,thehigherthethermalconductivity.However,ahigher concentrationofthermal conductivity enhancermeansa lower energystoragedensity.Asaconsequence,therightbalancemust bestruckbetweenthermalconductivityandenergystorage den-sityrequirementsforaspecificapplication.Fig.7plotsthethermal conductivityandthermalenergystoragedensityofthecomposites asafunctionofgraphiteloading.Onecanseethatadditionof car-bonmaterialcangreatlyenhancethethermalconductivity,with theenhancementincreasingwithincreasingcarbonloadinginthe composite.Thermalconductivityover5W/(mK)canbeachieved withacarbonloadingof20%.A10%carbonloadinggivesa com-positewithathermalconductivityover4.3W/(mK)andanenergy storagedensityover530kJ/kg.

Z.Geetal./Particuology15(2014)77–81 81

Fig.7.Thermalconductivityandthermalenergystoragedensityofcompositesas afunctionofgraphiteloading.

4. Conclusions

Weinvestigatedinorganic-salt-basedcompositematerialsfor medium- and high-temperature thermal energy storage. Using a eutectic salt of lithium and sodium carbonates as the PCM, magnesiumoxideastheceramicskeleton,andcarbonallotropes asthethermalconductivityenhancer, weproducedcomposites with good physical and chemical stability and high thermal conductivity.Wefoundthatgood wettabilityof thesaltonthe ceramicmaterialdensifiesthecompositestructure,whereaspoor wettabilityofthesalt onthecarbon materialsswellsthe com-positestructure.Thebalancebetweensuchcompetingprocesses playsakeyroleinthepropertiesandbehaviourofthecomposite materials.Our resultsshow that a 10% carbon loading gives a

composite withathermal conductivityover4.3W/(mK)andan energystoragedensityover530kJ/kg.

Acknowledgements

PartsofthisworkweresupportedbytheFocusedDeployment ProjectoftheChineseAcademyofSciences(KGZD-EW-302-1),Key TechnologiesR&DProgramofChina(No.2012BAA03B03)and Nat-uralScienceFoundationofChina(GrantNo.21106151),andtheUK EngineeringandPhysicalSciencesResearchCouncil(EPSRC)under grantEP/K002252/1.

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