journal homepage: www.elsevier.com/locate/nanoenergyAvailable online at www.sciencedirect.comREVIEWReview on nanoencapsulated phase changematerials: Preparation, characterizationand heat transfer enhancementChenzhen Liu, Zhonghao Raon, Jiateng Zhao, Yutao Huo, Yimin LiSchool of Electric Power Engineering, China University of Mining and Technology, Xuzhou 221116, ChinaReceived 18 January 2015; received in revised form 10 February 2015; accepted 12 February 2015Available online 20 February 2015KEYWORDSThermal energy sto-rage;Nanoencapsulatedphase change mate-rial;Nanoencapsulationmethod;Heat transferenhancement;Latent functionalthermal uidAbstractInrecentyears,phasechangematerials(PCM)whicharerecommendedaspotentialthermalenergy storage medium have been receiving signicant attention. The encapsulation technologyof PCM is an effective way to enhance the thermal conductivity and solve the issues of leakageand corrosion during the melting process. As a good choice of thermal energy storage materials,the nanoencapsulated phase change materials (NanoPCM) have many advantages, such as smallsize, large specic surface and high heat transfer rate. Up to now, there has been no completeliteraturereviewonthepreparation, characterizationandapplicationof NanoPCM. Inthispaper,acomprehensivesummaryhasbeenpresentedbasedontheresearchofNanoPCMinrecent years. The following four aspects have been reviewed in detail: preparation andcharacterization of NanoPCM, application of NanoPCMin latent functional thermal uid,dynamics simulationstudy of NanoPCMandheat transfer enhancement of NanoPCM. It isexpected that this review article has certain reference value for the further understanding ofNanoPCM.& 2015 Elsevier Ltd. All rights reserved.ContentsIntroduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 815The preparation methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 816Interfacial polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 817Emulsion polymerization method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 817Miniemulsion polymerization method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 818In situ polymerization method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 820http://dx.doi.org/10.1016/j.nanoen.2015.02.0162211-2855/& 2015 Elsevier Ltd. All rights reserved.nCorresponding author. Tel.: +86 516 83592000.E-mail address: raozhonghao@cumt.edu.cn (Z. Rao).Nano Energy (2015) 13, 814826Solgel method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 821Applications of NanoPCM in latent functional thermal uid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 822Heat transfer enhancement of NanoPCM. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 823Dynamics simulation study of NanoPCM. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 823Further prospective research. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 823Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 824Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 824References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 824IntroductionEnergyshortageis gettingworsewiththerapiddevelop-ment of economy and industry in the world. After theenergy crisis inthe1970s, theresearches onrenewableandsustainableenergyhavebeengainingmoreandmoreattention [1]. The technology of phase change energystoragerealizesthestorage,transportationandutilizationofthermalenergywhenthephasechangematerials(PCM)areabsorbingandreleasinglargeamounts of latent heatundergoing phase change. The phase change energy storagetechnology, while it is attracting attention gradually, solvestheproblemthat theenergysupplydoes not matchthedemand in time and space [2], increases the energy utiliza-tion [3,4] and relieves energy crisis. Therefore, phasechangeenergystoragetechnologycanbeappliedtomanyelds such as waste heat recovery [5,6], solar energystorage [710], intelligent building [1115], thermal regulat-ing fabric [1618], electronic devices thermal control[19,20], battery thermal management system [2123],and so on.The performance of phase change energy storagedepends on the properties of PCM. According to thematerial properties, thePCMcanbedividedintoorganicand inorganic [4,24]. With strong thermal stability, theorganic PCM are commonly used for energy storage in recentyears [25]. However, someproblems of organic PCMwillappear in application such as lowthermal conductivity[26,27] andleakageduringphasetransition[28,29]. Theleakagecausescertainharmtoenergystoragesystemandenvironment, whichlimits thefurther applicationof PCM[3032]. In order to solve these problems, the PCMareencapsulated in a capsule to form the shell-core compositematerials which can be called as encapsulated phasechange materials (EPCM) [3336].TheEPCMaretinycontainerswhichwrapPCMinthecapsule core [37,38]. The EPCM achieve the solidication ofPCM, not only enhancing the stability andheat transferefciencyofPCM[39],butalsofacilitatingitsutilization,storage and transportation [40]. The EPCM are mainlycomposed of twoparts [41]:PCM as coreand inorganics orpolymerasshell, asshowninFigure1.TheEPCMcanbemade in arbitrary shape, either irregular or regular such astubular, spherical and oval; and they have single or severalcores within the capsule and multi-walled capsules [4143],as shown in Figure 2.According toparticlesize,EPCM canbedividedintothefollowing three types [1]: Nanocapsulated phase changematerials (NanoPCM) (particlesizeranges between1and1000 nm)[44,45],microcapsulatedphasechangematerials(MicroPCM) (particlesizeranges between1and1000 m)[16,46] and macrocapsulated phase change materials(MacroPCM) [47,48] (particle size exceeds 1 mm).MicroPCMtechnologies arematureafter morethan50yearsofdevelopment.ThestabilityofthePCMcapsulesisinuencedbytheparticlesize.Intheprocessof uidowthe MicroPCM are easily broken and can increase theviscosity ofuid, which limits the application of MicroPCM.The relationship between stability and particle size ofMicroPCMwasstudiedbyYamagishi etal.[49].TheslurrywhichcontainedtheMicroPCM(sizedistributionsrangingFigure 1 Description of a capsule [41].MononuclearPolynuclearMatrixMulti-wallFigure 2 Morphology of a capsule [41].815 Preparation, characterization and heat transfer enhancementfrom5to1000 m)particles(20 vol%)inwaterwascircu-latedbyapump-circulatingsystem.Itwasobservedthatthe MicroPCM with particle size of 10001500 mwererapidlybrokenduringthepumpcirculation.Thebreakagerateof theMicroPCMwithparticlesizeof 75300 mwas40% after 500 times of pump circulation. The breakage rateof theMicroPCMwithparticlesizeof 20100 mwas lessthan10%after 4000 times of pumpcirculation. But theMicroPCMwithparticlesizeof 510 mwerealmost notbrokenduringthepumpcirculationmorethan7000times.The results demonstrated that the breakage ratesdecreasedas theMicroPCMparticlesizes decreased, andsmallersizedMicroPCMweremoredurableandwithstoodstress from the pump circulation.AddingMicroPCMtofunctionthermal uidcanincreasethe viscosity ofuid [5052], and MicroPCM easily fracture inthe process ofuid or pump circulation, which is the obstacleof long-termcirculation. For thesmall particlesize, largespecicsurfacearea,suspensionstabilityandlowbreakagerate during pumping compared with MicroPCM, NanoPCM aregaining attention gradually [53]. Sukhorukov et al. [54]observed that when the same forceisapplied thedeforma-tion of 10 nm size capsules is smaller than that of 10 m sizecapsules.Besides,theNanoPCMhavesomeuniqueproper-ties, such as volume effect, surface effect, macroscopicquantumtunnelingeffectandsoon[55],thatmakethemsteadily disperse in the thermal uid, and ensure theapplication in energy storage and thermal management[56]. Therefore, nanocapsule of PCM has good growthprospects.Currently, there are many researchers who have summar-izedandreportedresearchprogress,preparationmethodsand application status of MicroPCM [41,5759]. But compre-hensive summary for NanoPCM has not been reported yet. Inthis paper, research progress on preparation method, appli-cation in latent functionaluid, and heat transfer enhance-ment of NanoPCM in recent years will be summarized.The preparation methodsThe chemical method, physical method and physic chemicalmethodareusuallyadoptedfor microcapsulepreparation[60]. Inorganic andpolymer arecommonly usedas shell[61], and most of the particle sizes range from 5 to 400 m.TheparticlediameterofNanoPCMissmallerthanthatofMicroPCM,therefore,thetraditionalpreparationtechnolo-giesofMicroPCMarenotsuitableforNanoPCM.Currently,the main methods for preparation of nanocapsule are listedas follows.Table 1 Core and shell of NanoPCM.Core Shell Method Capsule meansizen-Tetradecane[74]Polystyrene Miniemulsion in situ polymerization 132 nmPalmitic acid[87]SiO2Solgel method 183.7 nmn-Dodecanol[80]Polymethyl methacrylate(PMMA) Miniemulsion polymerization 150 nmn-Dodecanol[73]Styrene-butyl acrylate copolymer Miniemulsion polymerization 90100 nm(range)n-Heptadecane[66]Polystyrene Emulsion polymerization 10 nm60 m(range)n-Octadecane[77]St(styrene)MMA (methylmethacrylate)copolymerMiniemulsion in situ polymerization 102 nmn-Octadecane[81]Methyl methacrylate(PMMA) Direct miniemulsion method 119 nmn-Octadecane[81]Poly(ethyl methacrylate)(PEMA) Direct miniemulsion method 140 nmn-Octadecane[83]Methyl methacrylate-co-allylmethacrylate(MMA-co-AMA)Free radical emulsion polymerization andin situ polymerization577693 nmn-Octadecane[63]Polystyrene Ultrasonic-assistantminiemulsion in-situpolymerization124 nmn-Nonadecane[67]Poly(methyl methacrylate) (PMMA) Via emulsion polymerization 100350 nm(range)n-Dotriacontane[76]Polystyrene(PS) Miniemulsion polymerizationmethod 168.2 nmn-Tetradecane[84]Urea formaldehyde In situ polymerization 100 nmParafn [65] Polyurea Interfacialpolycondensation reaction 498.2 nmParafn [82] Carboxymethyl cellulose (CMC) in situ polymerization 50 nmC. Liu et al. 816(i). Interfacial polymerization method.(ii). Emulsion polymerization method [62,63].(iii). Miniemulsion polymerization method [64].(iv). In situ polymerization method.(v). Solgel method.Table1summarizes several NanoPCMpreparedby thefollowing methods.Interfacial polymerizationIntheprocessofpreparingphasechangecapsulesthroughinterfacial polymerization method, the core material isemulsied rstlyafter theformationof theoil/water orwater/oil emulsionby using appropriateemulsier. Thenthe polymer as capsule is formed on the surface of the corebypolymerizationofthemonomers.Finallythecapsuleisseparated from oil phase or water phase. When the methodforpreparingNanoPCMisused,thecoremustbeaddedtothesyringewithacapillarytube.Itrequiresthesettingofhighvoltageanddirectcurrent.Besides,andthedistancebetween the needle of the syringe and liquid level ofmonomer solution needs to be as near as possible.Interfacial polymerization method is suitable for theNanoPCMemployingwater solubleandoil solubleof PCMcore. Uptonow, NanoPCMhavebeenpreparedbyinter-facial polymerization method using parafn as core andpolyurea as shell.Parketal.[65]synthesizedtheNanoPCMviainterfacialpolycondensation method, whose core and shell wereparafnandpolyurea, respectively. Theresults of differ-entialscanningcalorimeter(DSC)analysisshowedthatthemelting temperature and latent heat of the NanoPCM weremeasuredtobe56.54 1Cand101.1 J/g,freezingtempera-tureandlatent heat weremeasuredtobe47.82 1Cand105.6 J/g. Figure 3 shows the scanning electron microscope(SEM) and transmission electron microscope (TEM) images ofthe NanoPCM. It is clear that the NanoPCM have a sphericalstructure. Besides, the particles size is mainly in the rangeof 400600 nm.Emulsion polymerization methodEmulsion polymerization method, one of common methods toproduceorganicNanoPCM,mainlyconsistsofthefollowingsteps. First of all, the insoluble monomer inthe solventdispersed uniformly in the reaction mediumunder thefunction of the emulsier, the surfactant and the mechanicalFigure 3 SEMand TEMimages of NanoPCM(a) with and(b) without Fe3O4 nanoparticle [65].Figure4 (a)POMimagesofPS/heptadecaneMicro/NanoPCMand (b) SEM images of PS/heptadecaneMicro/NanoPCM [66].817 Preparation, characterization and heat transfer enhancementstirring. Thenthepolymer membraneisgeneratedonthesurfaceofthecorethroughaddingtheinitiatortoinitiatepolymerization reaction. Eventually the NanoPCM comeinto being.Emulsionpolymerizationmethodisoftenusedforpoly-mer polymerization which is suitable for preparation ofNanoPCM using liquid PCM as core material. So far, NanoPCMhavebeenpreparedviaemulsionpolymerizationmethodwhich commonly uses alkane as core and polystyrene or poly(methyl methacrylate) as shell.Sari et al. [66] synthesized Micro/Nanoencapsulatedphase change materials (Micro/NanoPCM) with the n-heptadecane as core and the polystyrene (PS) as shellthroughemulsionpolymerizationmethod.TheDSCresultsdemonstrated that the melting temperature and latent heatof PS/heptadecane (1:2) Micro/NanoPCM were 21.48 1C and136.89 J/g, respectively. The freezing temperature andlatentheatwere21.37 1Cand134.67 J/g,separately.Thelatent heat of melting decreased from 136.89 J/g to128.27 J/g due to the damage of several capsules after5000 thermal cycles. The results of thermal gravimetricanalysis(TGA)showedthattheMicro/NanoPCMrepresentsgood thermal stability in the preparation process. Thepolarized optical microscopy (POM) and SEMimages arepresented in Figure 4(a) and (b), respectively. The PS/heptadecane(1:2)Micro/NanoPCMisinincompletespheri-cal which can be clearly seen frommicrographs. Theparticlesizestestedbytheparticlesizedistribution(PSD)distributed in the range of 10 nm40 m. And later Sariet al. [67] fabricated Micro/NanoPCM using n-nonadecane ascoreandpoly (methyl methacrylate) (PMMA) as shell bymeans of the emulsion polymerization. The PSD resultsexhibited that the particle sizes were in the range of10 nm40 m. The melting temperature and latent heatweremeasuredtobe31.23 1Cand139.20 J/gbyDSC.TheMicro/NanoPCM have good thermal stability after 5000thermal cycles. All of the results indicated that thepreparationofPS/n-heptadecaneandPMMA/n-nonadecaneMicro/NanoPCMhadpromisingpotential indifferentther-mal energy storage applications such as solar thermalcontrolling of building envelopes, thermoregulation tex-tiles,thermal protectingofvehiclebatterysystems,ther-mal regulating application, and so on.Backet al. [68] andAlkanet al. [69] alsosuccessfullysynthesizedandcharacterizedtheNanoPCMthroughemul-sion polymerization method.Miniemulsion polymerization methodMiniemulsionpolymerizationmethodis themost commonmethodfor preparing NanoPCM at present.In this method,polymerizationreactionis carriedoutwithintinydroplets,which are stable, decentralized, size at nanometerlevel under the effect of high shear force, and containwater, monomer, emulsier andinitiator, etc. DuringtheFigure5 TEMmicrographsofSBA/n-dodecanol NanoPCMsynthesizedwithdifferentamountsofemulsierLAS:(a)2%,(b)3%,(c) 4%, and (d) 5% [73].C. Liu et al. 818miniemulsion polymerization reaction, the monomer deter-mines the chemical composition of product and the proper-ties of latex [70], and the size and morphology of thepolymeremulsionformednallyaresameasthoseoftheoriginal droplets [71,72].Comparedwithemulsionpolymerization,theminiemul-sion polymerization method in preparation of NanoPCM hastheadvantagesof lessenergy inputandhighstability.Thismethodis suitablefor thecombinationof alkaneas PCMcore and polystyrene, polyurea, styrene-butyl acrylate(SBA), styrene (St)-methylmethacrylate (MMA) copolymerand poly (methyl methacrylate) as shell.The NanoPCM containing n-dodecanol as core andstyrene-butyl acrylate (SBA) copolymer as shell weresynthesizedthrough miniemulsionpolymerization methodby Chen et al. [73]. The particle size, morphology andthermal performancesof theNanoPCMweremeasuredbyPSD, TEM and DSC, respectively. The results showed that theencapsulation efciency of NanoPCMhas reached98.4%.The spherical structure of the NanoPCM can be seen clearlyfromFigure5.Whenthemassratioofmonomers/n-dode-canol reached 1:1, the average particle size got 100 nm, thethermal decompositionwas about 195 1C, andthephasechange temperature and phase change enthalpy were18.4 1C and 109.2 J/g, respectively.Fang et al. synthesized the NanoPCM, whose shell ispolystyreneandcoreisn-tetradecane[74], n-octadecane[63,75]orn-dotriacontane[76]viaminiemulsionpolymer-izationmethod. ThesynthesizedNanoPCMweresphericalandthez-averageparticlesizewas 132 nm, 124 nmandFigure 6 SEM micrographs of NanoPCM with various amounts of polyaniline (a) 0 g, (b) 0.5 g, (c) 1.0 g, (d) 1.5 g and (e) 2.0 g [83].819 Preparation, characterization and heat transfer enhancement168.2 nm, respectively. Theresults of DSCanalysis repre-sented that the melting temperature and latent heat of then-tetradecane/polystyrenenanocapsuleswere4.04 1Cand98.71 J/g, andthefreezingtemperatureandlatent heatwere 3.43 1C and 91.27 J/g, respectively. The latent heat ofthen-octadecane/polystyrenenanocapsulesreachedupto124.4 J/g. The melting temperature and latent heat of then-dotriacontane/polystyrenenanocapsules weremeasuredtobe70.9 1Cand174.8 J/g,andthefreezingtemperatureand latent heat were measured to be 63.3 1C and 177.1 J/g,respectively.Tumirah et al. [77] fabricated the NanoPCMwith n-octadecane as core and styrene (St)-methylmethacrylate(MMA) copolymerasshell usingminiemulsionin-situpoly-merization method. The morphology, particle size andthermalpropertiesoftheNanoPCMwerecharacterizedbySEM, dynamic light scattering (DLS) and DSC. When theshell/coremassratiowas3:1,themeandiameter,meltingtemperature and freezing temperature of the sphericalNanoPCMwere102 nm, 29.5 1Cand24.6 1C, respectively.The melting and freezing latent heat reached 107.9 J/g and104.9 J/g, separately. After 360 heating/cooling cycles, thefabricatedNanoPCMstill hadgoodchemical stabilityandthermal reliability.TheNanoPCMwiththeaveragesizebelow200 nmwerealso synthesized using miniemulsion polymerization methodby Luo et al. [78], Fuensanta et al. [79], Chen et al. [80] andZhang et al. [81].In situ polymerization methodIntheprocessofpreparingphasechangecapsulesthroughinsitupolymerizationmethod,thereactionmonomerandcatalystareall locatedoutsidethecore.Themonomerissolubleinthecontinuousphase,butthepolymerisimmis-ciblewiththecontinuousphase. Therefore, thepolymer-ization reaction occurs on the surface of the core. With thedevelopment of the polymerization, the prepolymer isgeneratedgraduallyonthesurfaceof thecore, andnallythe capsule shell is formed [57].Up to now, in the in situ polymerization method, the organics,which are commonly polymers such as urea-formaldehyde resin,melamine-formaldehyde, carboxymethyl cellulose, poly (methylmethacrylate)andpoly(allylmethacrylate),havebeenmainlycoated as shell material.Hu et al. [82] synthesized the NanoPCM (parafn as coreand carboxymethyl cellulose (CMC)-modied MF as shell) viain situ polymerizationmethod.TheobtainedCMC-modiednanocapsules were spherical in shape with an averagediameterof 50 nm. Whenthemasscontentof parafninthenanocapsuleswere31.6%,49.1%and63.1%,thecorre-spondingphasechangeenthalpywere41.79 J/g,64.85 J/gand 83.46 J/g, respectively, and the corresponding crackingratio of the nanocapsules was 17.5%, 10.6% and 11.0% whenthenanocapsulessuspensionwasshearedmechanicallyat5000 rpm for 10 min.Wanget al. [83] synthesizedtheNanoPCMthroughfreeradical emulsion polymerization method and insitu polymer-ization method, using poly (methyl methacrylate-co-allylmethacrylate)asashellandn-octadecaneascore.Figure6shows the SEM micrographs of NanoPCM with various amountsofpolyaniline(PANI).Theinuencesofdifferentcontentsofpolyaniline as nucleating agent on the surface morphology, thecrystallization property and the thermal stability of NanoPCMwereinvestigated. Theresults reectedthat theshapeofcapsule is spherical, the particle size distributes between 1001000 nm and theaverage size is in the rangeof577693 nm.There is a little effect of additive amount of the polyaniline onthe morphology, the particle size and the encapsulationefciency of capsule, but increasing the amount of polyanilinewould decrease the degree of supercooling. When the addingamount of polyaniline were 0 g, 0.5 g, 1.0 g, 1.5 g, and 2.0 g,thecorrespondingsupercoolingdegreeof thecapsulewere2.3 1C, 0.8 1C, 0.4 1C, 1.1 1C and 0.5 1C, respectively.Figure7 SEMmicrographs of PA/SiO2NanoPCMpreparedatdifferent PH of the solvent:(a) 11, (b) 11.5 and (c) 12 [87].C. Liu et al. 820The NanoPCM, containing n-tetradecane as the core,werefabricatedviainsitupolymerizationmethodbyFanget al.[84]. The shellis the polymerizationproductof ureaand formaldehyde. The results of SEM analysis showed thatthe NanoPCM had mean size of about 100 nmand n-tetradecanewas well encapsulated. Themass content ofn-tetradecane exceeded 60%, and the phase changeenthalpy reached up to 134.16 J/g.Solgel methodSolgel method requires relative mild condition in thepreparation. The main processes are as follows: rstly,metal alkoxide as precursor mixes uniformly with thesolvent, catalyst andcomplexingagent, etc. Secondly, astableandtransparentcolloidal solutioncomesintobeingafterhydrolysisandcondensationchemical reaction.Thenthe gel with three-dimensional network structure wasformed after aging of the sol. Finally, MacroPCM or NanoPCMcan be prepared after drying, sintering and curing[41,85,86]. Solgel methodis suitablefor NanoPCMwithalkane, palmitic acid and indiumas core material andsilicon dioxide as shell material.Latibari et al. [87] successfully synthesized the NanoPCMwhichcontainspalmiticacid(PA)ascoreandSiO2asshellthroughsolgel method.Threesamples(S1,S2andS3)ofPA/SiO2nanocapsuleswerepreparedatthreedifferentPHvalues(11,11.5and12),respectively.ThemicrographsofPA/SiO2 NanoPCM can be seen from Figure 7, and it is clearthattheNanoPCMhasasphericalstructure.TheresultsofFourier transforminfraredspectroscope(FTIR), X-raydif-fractometer (XRD) and Energy dispersive X-ray Spectro-metry(EDS)indicatedthattheNanoPCMweresynthesizedsuccessfully, and they had compact and smooth surface. SEMand TEM tests indicated that the mean diameters of S1, S2and S3 were 183.7 nm, 466.4 nmand 722.5 nm, respec-tively. The encapsulation ration of PA for S1, S2 and S3 were82.53%, 84.28% and 88.32%, respectively. The thermalconductivity of theNanoPCMwas improvedcomparedtoFigure 8 Effect of mass concentration of slurry on temperature distribution [104].821 Preparation, characterization and heat transfer enhancementthat of pure PA. Atest of 2500 thermal cycling for S3indicated that the melting and freezing temperatures of theNanoPCMwerechangedfrom61.6 1Cto60.1 1Candfrom57.08 1C to 56.85 1C, respectively. The latent heats ofmelting and freezing were changed from180.91 J/g to177.3 J/gandfrom181.22 J/gto178.6 J/g, respectively.These results indicated that the NanoPCM have goodthermal properties and reliability and chemical stability.Hongetal. [88] synthesizedtheNanoPCMthroughsolgelmethod, whichusedsilicaasshell andindiumascore. Twotypes of silica obtained from tetraethylorthosilicate (TEOS) andsodiumsilicatehavebeenusedintheNanoPCM.Theparticlesize and the degree of super-cooling of the two kinds ofNanoPCMwereanalyzedandcompared.Theresearchresultsshowed that the core diameter and shell thickness of NanoPCMusing TEOS-derived silica as shell were 200 nm and 100 nm andthose of the other NanoPCM were 200 mm and 50 nm, respec-tively.Whenthechangerateofthetemperaturewere1 1C/min, 5 1C/min, 10 1C/min, 20 1C/min and 40 1C/min, the super-cooling were 32 1C, 33 1C, 34 1C, 36 1C and 41 1C, respectively.And the corresponding super-cooling of the other NanoPCM are3.9 1C, 6.1 1C, 8.3 1C, 10.2 1C and 14 1C at the same tempera-ture change rate, respectively.Amongthemethodsintroducedabove, interfacial poly-merizationmethodiswidelyusedinencapsuleofdyeandpesticides, which has the advantages of simple process andwide commercial application. However, less researchesfocusontheinterfacial polymerizationandthechoiceofshell material is relatively fewer for the synthesis ofNanoPCM.ThepreparationofNanoPCMusinginsitupoly-merization has good capsule morphology and thermal prop-erties. Furtherresearchshouldbecommittedtonotonlysimplifytheprocess,butalsoreducethecostofindustrialscale production. The miniemulsion polymerization methodisagreatwaytopreparecore/shell polymers[89].Therehave been many studies that using this method successfullysynthesized NanoPCM which have good thermal perfor-mance and stability. In the preparation process of NanoPCMusing miniemulsion polymerization method, the demands ofhigher stability of system and polymerization rate moderatecan be easily met [90]. Besides, the size of the capsule canbe adjusted by controlling the stabilizer dosage. Theadvantages of miniemulsion polymerization indicate thattheprocessofpreparationNanoPCMiseasytocontrolandconducive to the implementation of industrial production.Applications of NanoPCM in latent functionalthermal uidThelatentfunctional thermal uid, composedof thermaluid as continuous phase and PCMparticle additives asdispersion, is a kind of solidliquid two phase liquid [91,92].Due to the fact that the latent functional thermal uid hashigherheatstoragecapacityandheattransfercoefcientthantraditional singlephase uid, researchers havepaidmoreandmoreattentiontoitinrecentyears.TheEPCMslurryisakindoflatentfunctional thermal uidwhichiscomposed of encapsulated PCM and single phase heattransfer uid[93]. EPCMslurry, as efcient heat transfermediumintheheat storage, not onlyimproves theheatstoragecapacityandheattransfer rate, butalsoreducestheheatexchangersize, uidconveyingpipelinesizeandtransportenergyconsumption. EPCMslurrycanbewidelyused in various energy storage systems to achieve thestorage and transportation of energy.CurrentlyMicroPCMslurry, as latent functional thermaluid,hasbeenstudiedbymanyresearchers[9498].HeattransfercoefcientsofMicroPCMslurryweremeasuredbyWangetal.[99].TheMicroPCMslurryconsistedofmicro-encapsulated 1-bromohexadecane and water, with the massfraction of MicroPCM varying from 5% to 27.6%. The resultsshowed that the thermal storage capacity and heat transfercoefcients were both improved after adding MicroPCM. ThesameresultscanalsobefoundintheworkofDelgado[100]andRao[101].AlthoughEPCMslurryhasmoreadvantagesinheat storage capacity and heat transfer performance comparedwith traditional heat transferuid, it has some disadvantages,such as easily fracturing in the process ofow during pumping,increasingtheuid'sviscosity,makingthepipeeasytowearand jam [102,103] and so on. NanoPCM has the advantages ofsmall size and larger specic surface area. It was more stableFigure 9 The typical structures of NanoPCM [107].C. Liu et al. 822thanMicroPCMinstructure.Thefracturerate,theeffectofincreasing theuid's viscosity and wearing the pipeline are allsmaller thanthoseof MicroPCM[39,54]. NanoPCMslurryaslatent functional thermaluid has broad application prospectsintheeldofintelligentbuilding, thermal regulatingfabricandelectronicdevices thermal control, etc. Therefore, thedevelopment of NanoPCM slurry is inevitable.Fang et al. [74] synthesized polystyrene/n-tetradecaneNanoPCMusingultrasonicassistantminiemulsioninsitupoly-merization, and added the NanoPCM to water as latentfunctional thermal uidwhichwas utilizedincoldthermalenergy storage. The research results showed that the thermalconductivityofthewaterwasimprovedfrom0.6226 W/(m K)to0.63806 W/(m K) andfrom0.7296 W/(m K) to0.84676 W/(m K) at the temperature of 5 1C and 25 1C after addingNanoPCMwiththemass fractionof 15%, respectively. Whenthetemperatureis 5 1Candthemass concentrationof theNanoPCMare15%, 7.5%, 3.75%and0%, theviscosities are16.16 mPa s,12.18 mPa s,8.56 mPa s,and4.68 mPa s,respec-tively. It is visible that the NanoPCM slurry in the low viscositiesrange is suitable for using as latent functional thermal uid.Seyf et al. [104] studiedthethermal characteristicof amicrotube (NanoPCM slurry as coolant) heat sink with tangen-tial impingement through three dimensional numerical simula-tion. The NanoPCM slurry consists of polyalphaolen (PAO) asbase uid and octadecane as nanoparticle. As shown inFigure 8, when the Reynolds number (Re) is 400, thetemperature boundary layer growth for NanoPCM slurry cool-ant is slower than that of pure PAO, and increasing the massconcentration of slurry can reduce the wall temperature anduniformthe temperature distribution in solid and liquidphases. Andlater Seyf et al. [105] analyzedtheeffects ofmassconcentrationandReynoldsnumberofNanoPCMslurryon convection heat transfer of steady laminarowing past anisothermal squarecylinder bymeans of numerical solution.The NanoPCM slurry consists of water and n-octadecaneNanoPCMwith an mean diameter of 100 nm. The resultsshowed that the NanoPCM has signicant effect on enhancingthe heat transfer ability and the increases in volume fractionandReynoldsnumberleadtoenhancementofheattransferand shear stress over the cylinder.Wu et al. [103] synthesized the polymer/parafn NanoPCMusing miniemulsion polymerization method. The NanoPCMwereaddedinwater toformNanoPCMslurrywhichcouldenhance the heat transfer coefcient of jet impingement andspray cooling. The results showed that the volume fraction ofthe NanoPCM has a great effect on heat transfer coefcient.Compared to water, slurry with 28% volume fraction ofNanoPCM enhances heat transfer coefcient by 50% and 70%,for jet impingement and spray cooling, respectively.Heat transfer enhancement of NanoPCMAlthoughNanoPCMslurryaslatentfunctionalthermal uidhashigherheatstoragecapacityandheattransfercoef-cientthantraditional singlephaseuid,theheattransfercoefcient is still low. And PCMhas a larger degree ofsupercoolinginphasechange,soitsapplicationislimited[106]. Therefore, theheat transfer of NanoPCMmust beenhanced in order to reduce the degree of supercooling andimprove the heat transfer efciency.Park et al. [65] synthesized the magnetic Fe3O4 nanopar-ticles(NPs)-embeddedPCMnanocapsules(Mag-PCM)basedonaparafncoreandpolyureashell viainterfacial poly-condensationmethod. Threeweightpercentagesof Fe3O4were added to PCM nanocapsules, respectively. The weightpercentagesofFe3O4nanoparticlesinMag-PCMweremea-sured to be 3.1%, 5.7% and 6.6% by TGA. The Fe3O4nanoparticleswereembeddedinthepolyureashell.Whenthe amount of Fe3O4 NPs is added, the thermal conductivityof the nanocapsules increased and the supercooling degreeof parafn decreased.Dynamics simulation study of NanoPCMWiththeunceasingdevelopmentof computertechnology,the computer simulation has been widely used in studies ofPCM. Some researchers studied thermal properties, selfdiffusion, mesoscopic morphology and evolution mechanismof the NanoPCM by dynamics simulation.Rao et al. [107] studied the self diffusion of the NanoPCM bymolecular dynamic simulation. In this study two NanoPCMmodels were fabricated by using n-octadecane as core mate-rial and SiO2 as shell materials: one with constrained shell andanother with free shell. Figure 9 shows the typical structuresof NanoPCM. The molecular dynamic simulation resultsshowedthat thickness andhardness of NanoPCMshell hasimpact on the self diffusion properties of NanoPCM. TheNanoPCMwithconstrainedshellwillrestrainthetorsionandstretch strength of molecular chain outside the core materials,and the diffusion coefcient of NanoPCM will decreased. TheNanoPCMwithfreeshell will increasethe uidity of corematerial, reduce the thermal contact resistance betweencapsules and enhance the heat transfer of the whole capsule.Later, Rao et al. [108] studied the evolution mechanismsand mesoscopic morphologies of the NanoPCM by dissipativeparticle dynamics simulation method. The simulation resultsshowedthat thetwo types ofNanoPCM can besynthesizedusing n-docosaneas a corematerials andethyl acrylate(EA),styrene(St)orallyloxynonyl-phenoxypropanolpoly-oxyethylene ether ammoniumsulfate (DNS-86) as shellmaterials, respectively. A typical coreshell structure ofthe NanoPCM can be synthesized when the proportion of thecomponentsissuitable.TheNanoPCMfailedtobesynthe-sized when the surfactant and shell materials is excess. Andthe optimal encapsulation rate of n-docosane is 54.51%analyzedbydissipativeparticledynamics simulation. Theresults of particledynamics simulationresearchcouldbeuseful in the design and experiment of the NanoPCM.Further prospective researchMicroPCM has been studied for many years, but theresearches on NanoPCM start in recent years. Manyresearchers studiedthepreparationandcharacterizationof the NanoPCM, but there are still many aspects worth ourfurtherinvestigation.Suggestionsforthefutureworkscanbe summarized into the following several aspects:(1) Raisetheefciencyof preparationof NanoPCM: sincetheproductionefciencyof NanoPCMis quite low, it isdifcult tomeet theneeds of industrial applications.823 Preparation, characterization and heat transfer enhancementTherefore,increasingtheproductivityoftheNanoPCMis inevitable.(2) Molten salt and hydrated salt encapsulation: molten saltand hydratedsalthavehighlatentwhenthey occurredin phasetransition,sotheyareverysuitableasenergystoragematerials. But at present theresearchof thecoreof NanoPCMmainlyfocuses onorganicmaterial.Therefore, molten salt and hydrated salt as core ofNanoPCM need further research.(3) Applications of NanoPCM: uptonow, therehas beenlittleresearchworkofNanoPCMappliedinwasteheatrecovery, solar energy storage, intelligent building,thermal regulating fabric, electronic devices thermalcontrol andbatterythermal managementsystem,etc.Thereforefurther studies of NanoPCMapplicationareneeded.(4) Further researches on preparation of NanoPCM with higherencapsulationefciency,betterstability, betterthermalperformanceandmoreuniformparticlesizedistributionneed to be conducted. The diversity in choice of core andshell material, theoptimal process conditions andthereduction of production cost can not be neglected.ConclusionsThis paper mainly reviewed the research progress ofNanoPCM in recent years. This review consists of four parts:preparationandcharacterizationof NanoPCM, applicationof NanoPCM in latent functional thermaluid, heat transferenhancement of NanoPCM and dynamics simulation study ofNanoPCM. Fivekindsof availablepreparationmethodsonNanoPCMhavebeenintroduced, that is, interfacial poly-merization method, emulsion polymerization method, mini-emulsion polymerization method, in situ polymerizationmethod and solgel method. The characterizations ofNanoPCM preparation using the above methods weredescribed. Applications of NanoPCMin latent functionalthermal uid were summarized. Not only the thermalstorage capacity of the thermal transfer uid can beincreased, but also the performance of thermal conductivityoftheuidcanbeenhancedbyaddingtheNanoPCM.Andthen, this paper introducedthe self diffusion, evolutionmechanismsandmesoscopicmorphologiesoftheNanoPCMbydynamicssimulation.Finally,thelimitationsofcurrentresearch of NanoPCM were explained, and the furtherprospective research of NanoPCM were discussed.AcknowledgmentsThis workwas supportedbytheNational Natural ScienceFoundation of China (Nos. 51406223 and U1407125) and theNational Natural Science Foundation of Jiangsu Province(No. BK20140190).References[1] P.B. Salunkhe,P.S.Shembekarl,Renew.Sustain. EnergyRev.16 (2012) 56035616.[2] M.M. Farid, A.M. Khudhair, S.A.K. Razack, S. Al-Hallajl,Energy Convers. Manag. 45 (2004) 15971615.[3] T.C. Ling, C.S. Poonl, Constr. Build. Mater. 46 (2013) 5562.[4] B. Zalba, J.M. Marin, L.F. Cabeza, H. Mehlingl, Appl. Therm.Eng. 23 (2003) 251283.[5] J. Shon, H. Kim, K. Leel, Appl. 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Heis currently pursuing hisPh.D.underthesupervisionofProf.Zhon-ghaoRaoandProf. YiminLi atChinaUni-versity of Mining and Technology. Hisresearch mainly focuses on nanouid.Zhonghao Rao received his Bachelor degreein Thermal and Power Engineering andMaster degree in Thermal Engineering fromGuangdong University of Technology in 2008and2010, andPh.D. in Chemical ProcessEquipment fromSouthChinaUniversityofTechnologyin2013.Heiscurrentlyapro-fessor working at China University of Miningand Technology. His research mainly focusesonthermal energyconversionandstorageespecially by using phase change materials.JiatengZhaoreceivedhisBachelordegreeinThermal EnergyandPower EngineeringfromChinaUniversityof MiningandTech-nology in 2013. He iscurrently pursuing hisMaster degree under the supervision of Prof.ZhonghaoRaoandProf. YiminLi at ChinaUniversity of Mining and Technology. Hisresearch mainly focuses on thermal physicalproperties of nanouid.YutaoHuoreceivedhisBachelordegreeinThermal Energy and Power Engineering fromChina University of Mining and Technology in2014. Heis currently pursuing his Masterdegree under the supervision of Prof. Zhon-ghaoRaoatChinaUniversityofMiningandTechnology.Hisresearchmainlyfocusesonheat and mass transfer and nanouid.YiminLi receivedhisPh.D. inEngineeringMechanicsfromChinaUniversityof Miningand Technology in 1998. He worked atSchool of Electric Power Engineering, ChinaUniversity of Mining andTechnology. Cur-rently he is Professor and Dean of School ofElectric Power Engineering, ChinaUniver-sityofMiningandTechnology.Hisresearchmainly focuses on the theory ofuid.C. Liu et al. 826