Solar Energy Materials & Solar Cells 107 (2012) 28–36

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Crystallization of supercooled PCMs inside emulsions: DSC applications

T. Kousksou a,n, T. El Rhafiki b, M. Mahdaoui a, P. Bruel c, Y. Zeraouli a

a Laboratoire des Sciences de l’Ingenieur Appliquees �a la Mecanique et au Genie Electrique (SIAME), Universite de Pau et des Pays de l’Adour, IFR, A. Jules Ferry, 64000 Pau, Franceb Ecole Nationale Superieure des Arts et Metiers, ENSAM Marjane II, BP 4024, Mekn�es Ismailia, Moroccoc Laboratoire de Mathematiques et de leurs Applications, Centre National de la Recherche Scientifique, IPRA, BP 1155, Pau 64000, France

a r t i c l e i n f o

Article history:

Received 10 January 2012

Received in revised form

12 June 2012

Accepted 18 July 2012

Keywords:

Supercooling

Nucleation probability

Crystallization

Phase change materials

Emulsion

48/$ - see front matter & 2012 Elsevier B.V. A

x.doi.org/10.1016/j.solmat.2012.07.023

esponding author.

ail address: Tarik.kousksou@univ-pau.fr (T. K

a b s t r a c t

Heat transfer characteristic during crystallization of the phase change material (PCM) dispersed inside

an emulsion is investigated theoretically and experimentally by using Differential Scanning Calorimeter

(DSC) technique. The dispersed PCMs are hexadecane, octadecane and water. Nucleation laws are used

to simulate the supercooling phenomenon. The results indicate that the crystallization of the droplets

stabilizes the emulsion temperature at a value corresponding to that at which probability of crystal-

lization J(T) increases rapidly. To describe with accuracy the thermal properties of the PCM using DSC

technique it is more appropriate to represent these properties versus the sample temperature and not

as function of the plate temperature of DSC.

& 2012 Elsevier B.V. All rights reserved.

1. Introduction

Latent heat storage is one of the favorable kinds of thermalenergy storage methods considered for effective utilization ofrenewable energy sources, such as in solar and air conditioningsystems. In recent years, research on latent functional thermal fluid(LFTF), a two-phase heat transfer fluid, has attracted more and moreattention because of its greater apparent specific heat in its phasechange temperature range compared with conventional single-phase heat transfer fluid. Phase change microcapsule slurry andphase change emulsion are two kinds of these novel two-phase heattransfer fluids composed of phase change material particles and heattransfer fluids [1–6]. These two-phase heat transfer fluids benefitfrom a number of special features including (a) a high energystorage density due to the absorption of latent heat during thephase change process, (b) relatively low variations in operatingtemperatures of systems using such fluids due to energy absorptionat approximately constant phase change temperature, (c) the pos-sibility of using the same medium for both energy transport andstorage, thereby reducing losses during the heat exchange process,(d) lower pumping power requirements due to the increased heatcapacity, (e) high heat transfer rates to the phase change materialdue to the large surface area to volume ratio, (f) the enhancedthermal conductivity of suspensions leading to increased heat

ll rights reserved.

ousksou).

transfer to the suspension, and (g) the reduction/elimination ofincongruent melting and phase separation.

By using LFTF with proper phase change temperature in solaror air-conditioning systems, the peak capacity will be available toother users and the off-peak capacity would be more fullypreserved. Paraffins are often used as thermal energy storagematerials [2] and the melting/crystallization behavior of paraffinshas been widely studied [7,8]. They are hydrocarbons or alkaneswith different numbers of C-atoms in their chain. The more thecarbon atoms in the chain, the higher the melting temperature ofthe paraffin. Compared to salt hydrates, paraffins have lowersupercooling effects, and compared to water, they allow the userto choose a melting point suitable for meeting the cooling needsby changing the number of Carbon atom in the chain butmaintaining a reasonable latent heat capacity. For example,tetradecane (C14) has a melting temperature of �6 1C, hexade-cane (C16) �18 1Cand octadecane (C18) about 28 1C. The heat ofmelting ranges from 200 to 250 kJ/kg. By mixing differentparaffins, melting points between those of pure paraffin can beobtained [9–11]. Many researchers made an endeavor to studythe heat transfer enhancement mechanism of such two phaseheat transfer fluids [12,13]. Inaba and Morita [12] prepared aphase change emulsion with tetradecane as PCM and studied theair-emulsion direct contact heat exchange. They found that theoutput temperature of hot air approached the melting point oftetradecane with an increase of the emulsion layer height. Amixture of tetradecane and hexadecane was used to prepare anemulsion by Lorsch et al. [14] and they found that there wouldnot be tube jam if the supercooling phenomenon was observed.

Nomenclature

c specific heat of the emulsion, J K�1 kg�1

J probability of crystallization per unit time, s�1

h specific enthalpy, J kg�1

K Boltzmann’s universal constant, J K�1

K1,2 external exchange coefficient for the cell, W m�2

k heat conductivity of the emulsion, W m�1 K�1

LF specific latent heat of fusion, J kg�1

n(r,t) number of crystallized droplets per unit volume, m�3

nt total number of droplets per unit volume, m�3

P mass fraction of the dispersed PCMr radius, mt time, sT temperature, KVd volume of a droplet, m3

DG* energy barrier, J

Greek symbols

b cooling rate, K s�1

r mass density of the emulsion, kg m�3

j proportion of the crystallized dropletsC heat source, J m�3 s�1

Subscripts

c crystallization pointg globali initiall liquids solidm melting pointpcm phase-change material core of particle

T. Kousksou et al. / Solar Energy Materials & Solar Cells 107 (2012) 28–36 29

The main objective of the present study is to investigate theeffect of the supercooling phenomenon on the thermal behaviorof the dispersed PCM inside an emulsion during the freezingprocess by numerical simulation and DSC technique. The nuclea-tion laws are used to determine the stochastic character of thesupercooling phenomenon.

Z0=1.1mm Real cell of thecalorimeter

Cell of themodel

2R =4.25mm

ZK2

K 2

K2

Air

Z0

K1

Sample

Sample

Fig. 1. Scheme of the experimental cell.

2. Experimental

In the present work, we have investigated three PCMs namelyhexadecane, octadecane and distillated water. Because paraffin isinsoluble in water, appropriate surfactants must be used to produceparaffin in water emulsions. From previous research, it was foundthat non-ionic surfactants are better than ionic surfactants in theformulation of stable and low toxicity oil-in-water emulsion [15]. Inselecting the most appropriate surfactant for an application, the HLB(Hydrophile–Lipophile Balance) value must be taken into considera-tion. It was identified that for oil-in-water emulsions, the HLB valuesurfactants should be between 8 and 18, whilst for water-in-oilemulsions the HLB should be between 5 and 6 [16]. Orafidiya andOladimeji [17] suggested that the required HLB values for liquidparaffin are 11.8 and 12.0 for uncentrifuged and centrifuged liquidparaffin emulsions respectively. To make stable emulsion, a mixtureof two or more surfactants is better that the use of a singlesurfactant with the same HLB number [17]. Tween surfactants(polyoxyethylene-sorbitan alkylates), which have a hydrocarbon tailand a sobitan ring, have been suggested and used by otherresearchers in the preparation of emulsions [18]. To make a stableemulsion, a lipophilic co-surfactant is needed (surfactant with anaffinity to oil).

In our case, PCMs are dispersed by a high speed stirrer withinan emulsifying medium made of a mixture of water, glycerol andTween 80 surfactant for the paraffins and a mixture of paraffin oiland lanolin for water. For hexadecane due to the particular choiceof the relative concentrations of the constitutive substances of theemulsifying medium, the dispersed system is, in fact, a micro-emulsion [19]. So, it is very stable and the experiments are easierto carry out. The droplet diameters are mono-disperse about10 nm.

DSC is one of the most widely used analytical instrumentsbecause of the ease with which it can provide large amounts ofthermodynamic data. From a single DSC testthat consists inregularly cooling down and heating of >a sample, it is expectedto obtain qualitative and quantitative information on the phase

transitions of a sample, such as transition temperature, enthalpy,heat capacity, specific heat, and latent heat.

In this work, thermal analysis was carried out using a PYRISDIAMOND DSC of Perkin-Elmer. The instrument was calibrated bythe melting point of pure ice (273.15 K or 0 1C) and mercury(234.32 K or �38.82 1C). The principle of the power-compensa-tion used in dispersed droplet is detailed in Refs. [20,21]. DSCexperiments were conducted by placing approximately 6–12 mgof each emulsion in a standard aluminum DSC sample pan (seeFig. 1).

2.1. Crystallization kinetics inside an emulsion

Because of the smallness of the droplet size inside an emul-sion, the crystallization of dispersed PCM occurs in a range oftemperature below the melting temperature [22,23]. We can alsonote that crystallization can occur when conditions change,through cooling or supersaturation, so that the free energy of asolid phase is lower than that of liquid [24,25]. However, thephase transition is kinetically limited by the difficulty in forma-tion of small starting crystal (crystal embryo) because when thecrystal size is very small the free energy gained by the moleculesundergoing the phase transition is offset by the interfacial freeenergy of the new solid phase. High degrees of supercooling aretherefore required before the small solid phases formed by localdensity fluctuations can grow to macroscopic size (i.e. homo-geneous nucleation). In almost all bulk liquids some impurities inthe melt will act as the starting point of nucleation before thisdegree of supercooling, which can be reached and crystallizationwill occur more rapidly (i.e. heterogeneous nucleation). However,when the liquid is finely divided into emulsion droplets, the

T. Kousksou et al. / Solar Energy Materials & Solar Cells 107 (2012) 28–3630

number of droplets may significantly exceed the number ofimpurities and the bulk of the mass of the fat droplets is freefrom impurities and hence, crystallizes by an apparently homo-geneous mechanism [26–28].

The study of the kinetics of crystallization of supercooledliquids has been an area of continuing interest [29]. While therehave been numerous studies on the nucleation kinetics, onlyrelatively few have been of sufficient depth to provide the kineticsinformation needed to model the controlling process. Foremostamong these studies has been the classic work on mercury byTurnbull [30]. Succeeding investigations [31,32] on nucleationkinetics have followed essentially a similar analysis techniqueand method. The results of detailed kinetics analysis have con-firmed the essential form of classical nucleation theory. However,a number of discrepancies have arisen between the predictions oftheory and the experimental results.

Majority of the experimental studies concerning kinetics ofcrystal nucleation from the highly supercooled liquid state havefocused upon the use of a droplet dispersion sample configura-tion. Turnbull [33] developed the generation of droplets byemulsification of liquid metals in a carrier fluid. To maintain theindependence of each droplet, a surface coating was provided in acontrolled manner by dissolving a small amount of surfactants inthe carrier fluid. With this approach, internal as well as extra-neous nucleant effects could be controlled.

Most of the crystallization processes of paraffins are dominatedby the heterogeneous nucleation mechanism, and the phase-transi-tion behavior of paraffins is complicated and very sensible to smallamounts of impurities [34]. The homogeneous nucleation of bulkand emulsified n-alkanes from C15 to C60 was studied by scanningcalorimetry, using the droplet technology [35]. While the stablephase was observed for C16 and C18, in case of C20, it is the tricliniccrystalline phase [36,37] and the nucleation proceeded through atransient metastable rotator phase [35]. In contrast, two separatedpeaks that correspond to distinct liquid–rotator and rotator–crystaltransitions were observed for C19 [35]. The odd paraffins from C13 toC41 crystallize in an orthorhombic system [36]. The faster thecooling rate, the greater the degree of supercooling achieved inthe system prior to bulk nucleation [38].

The crystallization process can be seen as a two steps process[39]. The first one corresponds to the apparition within the liquidphase of a ‘‘supercritical aggregate’’ having the same crystallinestructure as the solid. It is the ‘‘germination’’ or ‘‘nucleation’’phase. The second phase corresponds to the crystalline growth,initiated by this supercritical germ and leading to the totalcrystallization of the sample. In this study, we are interested inthe crystallization of the droplets dispersed within an emulsion.As their average size is close to the micrometer and the liquid isundercooled, i.e. far from its stable state, we consider the crystal-line growth as instantaneous. Thus, the kinetics of crystallizationis assumed to be exclusively driven by the germination phase[39]. Then the complete crystallization of the sample occurs whenthe first supercritical aggregate appears.

The classical theories of nucleation [40,41] define the prob-ability of nucleation per unit volume and unit volume I(T), whichcorresponds to the probability of formation of the supercriticalaggregate initiating crystallization:

IðTÞ ¼ AðTÞexp �DGn

KT

� �ð1Þ

where K is the Boltzmann constant, T the temperature of thesupercooled liquid and DG* represents the energy barrier for acluster to overcome before it grows irreversibly as a crystal [33]

DGn¼

16

3

s3n2

L2F

Tm

Tm�T

� �2

ð2Þ

here n is the volume per particle, s is the interface solid–liquidfree energy. The factor 16/3 is related to the shape of the nucleus,which is assumed to be spherical.

The pre-factor term A(T) in Eq.(1) varies more slowly withtemperature than does the exponential term and can thus beconsidered as a constant in this work. For a single droplet ofvolume Vd, the probability of nucleation per unit time J(T)¼VdI(T)may be expressed as [42,43]

JðTÞ ¼A1 exp � B1

TðT�TmÞ2

� �for TrTm

0 for TZTm

8<: ð3Þ

where A1¼VdA(T) and B1 ¼ ð16=3Þððs3n2T2mÞ=ðKL2

F ÞÞ are consideredas constants for a given volume Vd and DT¼Tm�T is the degree ofsupercooling. It is important, however, to note that DT is statis-tical data since the metastability breakdown is a fundamentallystochastic process [44]. The B1 coefficient is a characteristic of thesupercooled substance (molar volume of the crystal, liquid–crystal interfacial tension) whereas A1 depends on the volumeof the crystal for the case of heterogeneous nucleation [45,46].

As we assume here that the crystalline growth is instanta-neous and begins with the formation of the first supercriticalaggregate, J(T) also represents the probability of crystallizationper unit time of a microparticle volume. The J(T) function isstrongly nonlinear. It remains very close to zero for a wide rangeof temperatures below Tm and then quickly increases when T

decreases.Due to the stochastic nature of metastability breakdown inside

the emulsion, it is necessary to use a statistical approach,simultaneously studying a large number of samples. This can bedone by considering the droplets of a near monodisperse emul-sion. Let nt be the total number of droplets per unit volume ofemulsion and n the number of crystallized droplets. Betweentimes t and tþdt, whereas the temperature is assumed to beconstant, the rate of crystallizing droplets can be expressed as theproduct of the number of liquid droplets nt�n and the crystal-lization probability J(T) [38]:

dn

dt¼ ðnt�nÞJðTÞ ð4Þ

Introducing the crystallized fraction j¼ n=nt , Eq.(4) can berewritten as

djdt¼ ð1�jÞJðTÞ ð5Þ

This expression constitutes the fundamental equation of thekinetics of the crystallization of the droplets dispersed within anemulsion.

2.2. Physical model

The experimental conditions are chosen in a way that only thedispersed PCM crystallizes during the cooling, the emulsifyingmedium remaining in the liquid state [23]. For the concernedemulsions, the dispersed droplets are small enough to considerthe emulsion as homogeneous, and all physical quantities arerelative to this homogeneous phase. Further, the studied emul-sions are supposed to be viscous enough to neglect the convectionphenomenon inside the cylindrical cell. The thermal conductivityand the specific heat capacity are supposed to be constant withregard to the temperature. However, these properties vary as thedispersed phase is solid, liquid or in an intermediate state (liquid–solid). Most of the properties of the emulsion depend on P whichis the mass fraction of the dispersed PCM (ratio of the mass of thedispersed PCM to the total mass of the emulsion). For a given P, allthese properties are supposed to be constant.

Fig. 2. Experimental curves of the crystallization probability per unit time.

T. Kousksou et al. / Solar Energy Materials & Solar Cells 107 (2012) 28–36 31

Following the assumptions given above, the energy equation isexpressed as

rc@Tðr,z,tÞ

@t¼ l

@2T

@r2þ

1

r

@T

@rþ@2T

@z2

!þCðr,z,tÞ ð6Þ

where l is the thermal conductivity of the emulsion, r the massdensity and c the specific heat capacity. The heat source C isdifferent from zero when the phase change process occurs.

The volumetric heat source C is proportional to the heattransformation of one droplet and to the number of dropletswhich crystallizes per unit time and unit emulsion volume. Wehave

C¼ r0PLFdjdt

ð7Þ

where r0 is the mass density of the dispersed phase (PCM) and LF

the latent heat of fusion (LF40).Taking into account Eqs. (5) and (7) Eq. (6) becomes

rc@Tðr,z,tÞ

@t¼ l

@2T

@r2þ

1

r

@T

@rþ@2T

@z2

!þr0PLFJðTðr,tÞÞð1�jðr,z,tÞÞ

ð8Þ

To take into account the air between the emulsion and thecover of the cell, we consider two different heat exchangecoefficients K1 and K2 (see Fig. 1). So the boundaries conditionsare

@T

@r

� �r ¼ 0

¼ 0 ð9Þ

�l@T

@r

� �r ¼ R

¼ K2ðT�TpltÞ ð10Þ

�l@T

@z

� �z ¼ 0

¼ K2ðT�TpltÞ ð11Þ

�l@T

@z

� �z ¼ Z

¼ K1ðT�TpltÞ ð12Þ

where Tplt, the temperature of the plates, is programmed to belinear function:

Tplt ¼ btþT0 ð13Þ

At t¼0 the initial conditions are T(r,z,0)¼T0 and j(r,z,0)¼0Because the thermal conductivity of air is smaller than that of

the metal of the cell, we consider that all the energy is trans-mitted to the plate by the lower boundary of the cell. So, F is thesum of the thermal fluxes through the walls of the metallic cell.

F¼dq

dt¼�

Xi

KiðTi�TpltÞDSi ð14Þ

where Ki¼K1 or K2

Using this model, it is possible to study the effect of variousparameters on the melting kinetic inside an emulsion.

In this work, the crystallization probability per unit time J(T)has been obtained from differential scanning calorimetry (DSC)[38] (see Fig. 2). It must be noted that the above equations arevalid for both the freezing and melting cases. For the meltingprocess, the main assumptions remain the same. Since thetemperature is uniform in each control volume, the actualdifference is that all the droplets of a control volume pass throughthe phase change simultaneously at the melting temperature.

Fig. 3. Theoretical and experimental thermograms.

T. Kousksou et al. / Solar Energy Materials & Solar Cells 107 (2012) 28–3632

2.3. Numerical procedure

The finite difference equations are obtained upon integratingEq. (8) over each of the control volumes. The resulting finite

Fig. 4. Theoretical thermogram and solid fraction versus the plate temperature Tplt.

Table 1Supercooling degree.

PCM Degree of supercooling (1C)

Hexadecane 14.2

Octadecane 13.4

Water 35

T. Kousksou et al. / Solar Energy Materials & Solar Cells 107 (2012) 28–36 33

difference scheme at time tþDt has the following form:

aPTP ¼Xnb

anbTnbþrcpVP

DtTold

P þr0PLFJðTPÞVP

Dtð1�jPÞ ð15Þ

where VP is a volume associated with the Pth node point, thesubscripts P, ‘‘nb(E,W,N, S)’’ and ‘‘old’’ refer to the Pth node point,the neighboring node points and the old time value, respectively.The calculation of the coefficients aP and anb and the solution ofEq. (15) are presented in Jamil et al. [47] and it is deemed torepeat it in the present work.

Fig. 5. Variation of the emuslion temperature at the center of the sample.

3. Results and discussion

Thermal analyses were carried out using a Pyris Diamond DSCof Perkin-Elmer. The temperature scale of the instrument wascalibrated by the melting point of pure ice (273.15 K) and mercury(234.32 K). The DSC experiments were conducted by placing in theDSC cell. The sample was cooled at 2 1C min�1until crystallization ofthe dispersed droplets. The sample was then heated at variousheating rates. For the numerical calculation, we have applied thetheoretical model described in the previous section. The values ofthe physical characteristics required in different equations havebeen determined experimentally or using the literature correlations[48], except the coefficients of heat exchange (K1 and K2) that havebeen determined by simulation from exploratory experiments.

The physical validity of the proposed computational model hasbeen checked by comparison between numerical and experimen-tal results. Fig. 3 shows the time variations of specific heat flow Fversus the plate temperature Tplt for three dispersed PCMs:hexadecane, octadecane and water during cooling process. Natu-rally, no two phase region emerges during the cooling process andthe shape of the curve is similar to that for ordinary pure PCM. Asit can be seen in Fig. 3, a good agreement between numerical andexperimental results is observed. As indicated in the previoussection, the model allows us to describe the thermal transferinside the DSC cell. One of the advantages of the model is thecalculation at each instant the local and the global liquid fractioninside the sample. Using this model, we can also give a physicalmeaning to the thermogram shape and we can easily detectthe beginning and the end point of the phase transformation.As one can see in Fig. 4, Tpeak represents the value of the platetemperature when the energy exchange is maximum and doesnot indicate the end of the cooling process. The crystallizationtemperature Tc is defined as the most probable temperature,corresponding to the temperature of the beginning of the phasechange transformation where the probability J(T) increases shar-ply. The maximum total solid fraction of PCM inside the emulsionis 1 and this value is considered as the end point of the phasetransformation inside the emulsion. The melting temperatures Tm

of the dispersed hexadecane, octadecane, and water were 18 1C,28 1C and 0 1C respectively. However, during the cooling process,the phase change occurred at lower temperatures than duringheating due to the supercooling phenomenon. For example, inthe case of water emulsion, crystallization starts near �35 1C and

Fig. 6. Influence of the heating rate on the theoretical thermogram.

Fig. 7. Effect of the cooling rate on the emulsion temperature at the center of the

sample.

T. Kousksou et al. / Solar Energy Materials & Solar Cells 107 (2012) 28–3634

hence, degree of supercooling of 35 1C is observed. Table 1presents the degree of supercooling for the three investigatedemulsions. The time corresponding to the phase transformation is

the time required to release the latent heat of PCM in theemulsifying medium. The end temperature determines the totaltime of the phase change of the whole sample. Fig. 5 displays the

T. Kousksou et al. / Solar Energy Materials & Solar Cells 107 (2012) 28–36 35

variation of emulsion temperature T versus Tplt at the center of theDSC sample. We can see from Fig. 5 that for all the three dispersedPCMs, the emulsion temperature presents a typical flat shapecharacteristic of the phase change. Fig. 6 shows the thermogramsobtained by the model and DSC versus the cooling rate b. Thepeak temperatures range becomes broader and it shifts to lowertemperatures with increasing cooling rate. We can also note thatthe peak maximum temperature Tpeak decreases continuouslywhich increasing cooling rate. Fig. 7 displays the prediction ofthe emulsion temperature T versus Tplt at the center of the samplefor different cooling rates. It can be seen from this figure that thetemperature differences inside the sample becomes more impor-tant as the cooling rate increases.

Fig. 8 presents the specific enthalpy versus the plate tempera-ture for different cooling rates. We note that it is difficult todescribe with accuracy the freezing process inside the sample byincreasing b. The main reason for this difficulty is due to thetemperature gradients inside the sample. These gradients becomemore important as cooling rate increases. We can also note that,when we represent the thermal properties of PCM versus the

Fig. 8. Influence of the cooling rate on the specific enthalpy of the dispersed PCM.

Fig. 9. Specific enthalpy of the dispersed PCM versus the sample temperature.

plate temperature of the DSC, the effect of cooling rate needs to beconsidered. Unfortunately, users of DSC technique often representthermophysical properties of the sample and especially enthalpyas function of the plate temperature without taking into accountthe effect of cooling rate on the heat transfer inside the sample.This representation can cause significant errors in determiningthe thermophysical properties of the studied materials using DSCtechnique. To remedy this problem, we felt that it was moreappropriate to represent the specific enthalpy versus the sampletemperature (for example temperature at the center of thesample) as seen in Fig. 9. Using this representation, we candescribe and predict with accuracy the evolution of the enthalpyof PCM during the freezing process. Unfortunately, the sampletemperature is inaccessible from DSC technique.

4. Conclusion

In this article, heat transfer inside a phase change emulsion incooling mode for different dispersed substances, such as hexade-cane, octadecane and water, is reported. Supercooling phenom-enon during freezing process is modeled employing nucleationlaws. The results indicate the existence of important temperaturegradients and that the crystallization of the droplets locallystabilize the temperature at a value corresponding to that atwhich probability of crystallization J(T) increases rapidly. It is alsofound that to describe with accuracy the thermal properties of thePCM using DSC technique it is more appropriate to representthem versus the sample temperature and not as a function of theplate temperature of DSC.

References

[1] S. Gschwander, P. Schossing, H.-M. Henning, Micro-encapsulated paraffin inphase change slurries, Solar Energy Materials & Solar Cells 89 (2005)307–315.

[2] R. Yang, H. Xu, Y. Zhang, Preparation, physical property and thermal physicalproperty of phase change microcapsule slurry and phase change emulsion,Solar Energy Materials & Solar Cells 80 (2003) 405–416.

[3] S. Zhang, J. Niu, Experimental investigation of effects of supercooling onmicroencapsulated phase-change material (MPCM) slurry thermal storagecapacities, Solar Energy Materials & Solar Cells 94 (2010) 1038–1048.

[4] P. Charunyakurn, S. Sengupta, S.K. Roy, Forced convection heat transfer inmicroencapsulated phase change material slurries: flow in circular ducts,International Journal of Heat and Mass Transfer 34 (1991) 819–835.

[5] T. El Rhafiki, T. Kousksou, Y. Zeraouli, Thermal performance of microencap-sulated phase change material slurry: laminar flow in circular tube, Journal ofThermophysics and Heat Transfer 24 (2010) 480–489.

[6] T. Kousksou, A. Jamil, S. Gibout, Y. Zeraouli, Thermal analysis of phase changeemulsion, Journal of Thermal Analysis and Calorimetry 96 (2009) 841–852.

[7] Y. Zhong, Q. Guo, S. Li, J. Shi, L. Liu, Heat transfer enhancement of paraffin waxusing graphite foam for thermal energy storage, Solar Energy Materials &Solar Cells 93 (2009) 136–142.

[8] T. kousksou, F. Strub, J. Castaing-Lasvignottes, A. Jamil, J.P. Bedecarrats,Second law analysis of latent thermal storage for solar system, Solar EnergyMaterials & Solar Cells 91 (2007) 1275–1281.

[9] B. He, E.M. Gustafsson, F. Setterwall, Tetradecane and hexadecane binarymixture as phase change materials (PCMs) for cool storage in district coolingsystems, Energy 24 (1999) 1015–1028.

[10] E. Choi, Y.I. Cho, H.G. Lorsch, Thermal analysis of the mixture of laboratoryand commercial grades hexadecane and tetradecane, International Commu-nications in Heat and Mass Transfer 19 (1992) 1–15.

[11] T. Kousksou, A. Jamil, T. El Rhafiki, Y. Zeraouli, Paraffin wax mixtures as phasechange materials, Solar Energy Materials & Solar Cells 94 (2010) 2158–2165.

[12] H. Inaba, S. Morita, Flow and cold heat storage characteristics of phasechange emulsion in a coiled double tube heat exchanger, Journal of HeatTransfer 117 (2) (1995) 440–446.

[13] H. Inaba, S. Morita, Cold heat-release characteristics of phase changeemulsion by air-emulsion direct contact heat exchange method, Interna-tional Journal of Heat and Mass Transfer 39 (9) (1996) 1797–1803.

[14] H.G. Lorsh, K. Murali, Y.I. Cho, Improving thermal and flow properties ofchilled water—part I: material selection and instrument calibration, ASHRAETransactions 103 (1) (1997) 188–197.

[15] P. Schalbart, M. Kawaji, K. Fumoto, Formation of tetradecane nanoemulsionby low-energy emulsification methods. In: Proceedings of the 8th IIR

T. Kousksou et al. / Solar Energy Materials & Solar Cells 107 (2012) 28–3636

Conference on Phase Change Materials and Slurries for Refrigeration and AirConditioning, Karlsruhe, 2009.

[16] ICI Americas, The HLBB System—A Time Saving Guide to Emulsifier Selection,Chemmunique ICI Americas, Wilmington, 1976, pp. 345–352.

[17] L.O. Orafidiya, A.F. Oladimeji, Determination of the required HLB values of someessential oils, International Journal of Pharmaceutics 237 (2002) 241–249.

[18] Y. Zeraouli, Etude thermique des transformations des emulsions concentrees.Application �a la calorimetrie �a balayage, Thesis, Pau, France, 1991.

[19] A. Jamil, T. Kousksou, Y. Zeraouli, J.P. Dumas, E. Schall, Isothermal and non-isothermal melting of the binary solution inside an emulsion, ThermochimicaActa 460 (2007) 22–27.

[20] T. Kousksou, A. Jamil, Y. Zeraouli, J.P. Dumas, Equilibrium liquidus tempera-tures of binary mixtures from differential scanning calorimetry, ChemicalEngineering Science 62 (2007) 6516–6523.

[21] T. Kousksou, A. Jamil, Y. Zeraouli, J.P. Dumas, Liquidus temperatures deter-mination of the dispersed binary system, Thermochimica Acta 471 (2008)1–6.

[22] D. Kashchiev, N. Kaneko, K. Sato, Kinetics of crystallization in polydisperseemulsions, Journal of Colloid and Interface Science 208 (1998) 167–177.

[23] T. El Rhafiki, T. Kousksou, A. Jamil, S. Jegadheeswaran, S.D. Pohekar, Y. Zeraouli,Crystallization of PCMs inside an emulsion: supercooling phenomenon, SolarEnergy Materials & Solar Cells 95 (2011) 2588–2597.

[24] J.W. Mullin, Crystallization, 4th ed., Butterworth-Heinemann, Oxford, 2001,pp. 214–223.

[25] R. Hartel, Crystallization in Foods, Aspen Publishers, Gaithersburg, 2001,pp. 411–431.

[26] W. Skoda, M. Van den Tempel, Crystallization of emulsified triglyerides,Journal of Colloid Science 18 (1963) 568–584.

[27] P. Walstra, E.C.H. Van Beresteyn, Crystallization of milk fat in the emulsifiedstate, Netherlands Milk and Dairy Journal 29 (1975) 35–65.

[28] J.N. Coupland, Crystallization in emulsions, Current Opinion in Colloid andInterface Science 7 (2002) 445–450.

[29] J.H. Perepezko, Nucleation reactions in undercooled liquids, Materials Scienceand Engineering A 178 (1994) 105–108.

[30] D. Turnbull, Kinetics of solidification of supercooled liquid mercury droplets,Journal of Chemical Physics 20 (1952) 411–415.

[31] Y. Miyazawa, G.M. Pound, Supercoolings observed for droplets of tin, alloys oftin and certain other pure metals, Journal of Crystal Growth 23 (1974) 45–49.

[32] D. Turnbul, R.L. Cormia, Kinetics of crystal nucleation in some normal alkaneliquids, Journal of Chemical Physics 34 (1961) 820–824.

[33] D. Turnbull, Phases Change in Solid State, vol. 2, Academic Press, New York,1956 (pp. 226–306).

[34] M.J. Olivier, P.D. Calvert, Homogeneous nucleation of n-alkanes measured bydifferential scanning calorimetry, Journal of Crystal Growth 30 (1975)343–349.

[35] H. Kraack, E.B. Sirota, M. Deutsch, Measurements of homogeneous nucleationin normal-alkanes, Journal of Chemical Physics 112 (2000) 6873–6877.

[36] A. Muller, K. Lonsdale, The low-temperature form of C18H38, Acta Crystal-lographica 1 (1948) 129–131.

[37] E.B. Sirota, A.B. Herhold, Transient phase-induced nucleation, Science 283(1999) 529–533.

[38] S. Gibout, A. Jamil, T. Kousksou, Y. Zeraouli, J. Castaing-Lasvignottes, Experi-mental determination of the nucleation probability in emulsions, Thermo-chimica Acta 454 (1) (2007) 57–63.

[39] B. Wunderlich, One hundred years on supercooling and superheating,Thermochimica Acta 461 (2007) 4–13.

[40] E. Ruckenstein, Y. Djikaev, Recent developments in the kinetic theory ofnucleation, Advances in Colloid and Interface Science 118 (2005) 51–72.

[41] D.H. Rasmussen, R. Appleby, Mary, G.L. Leedom, S.V. Babu, R.J. Naumann,Homogeneous nucleation kinetics, Journal of Crystal Growth 64 (1983)229–238.

[42] T. Kousksou, T. El Rhafiki, K. El Omari, Y. Zeraouli, Y. Le Guer, Forcedconvection heat transfer in supercooled phase-change material suspensionswith stochastic crystallization, International Journal of Refrigeration 33(2010) 1569–1582.

[43] T. Kousksou, J.-P. Bedecarrats, J.-P. Dumas, A. Mimet, Dynamic modelling ofthe storage of an encapsulated ice tank, Applied Thermal Engineering 25(2005) 1534–1548.

[44] J.H. Perepezko, P.G. Hockel, J.S. Paik, Initial crystallization kinetics in under-cooled droplets, Thermochimica Acta 388 (2002) 129–141.

[45] V.P. Skripov, Metastable Liquids, John Wiley and Sons, New York, 1974.[46] P.W. Wilson, A.F. Heneghan, A.D.J. Haymet, Ice nucleation in nature: super-

cooling point (SCP) measurements and the role of heterogeneous nucleation,Cryobiology 46 (2003) 88–98.

[47] A. Jamil, T. Kousksou, Y. Zeraouli, S. Gibout, J.-P. Dumas, Simulation of thethermal transfer during an eutectic melting of a binary solution, Thermo-chimica Acta 441 (2006) 30–34.

[48] David R. Lide (Ed.), CRC Handbook of Chemistry and Physics, 87th edition,2006.