Applied Energy 88 (2011) 3125–3132

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Applied Energy

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Study on preparation, structure and thermal energy storage propertyof capric–palmitic acid/attapulgite composite phase change materials

Min Li a,b,⇑, Zhishen Wu b, Hongtao Kao c

a Jiangsu Key Laboratory for Construction Materials, Southeast University, Nanjing 211189, Chinab International Institute for Urban System Engineering, Southeast University, Southeast University, Nanjing 210096, Chinac Nanjing University of Technology, Nanjing 210009, China

a r t i c l e i n f o

Article history:Received 16 November 2010Received in revised form 16 February 2011Accepted 22 February 2011Available online 24 March 2011

Keywords:CompositePhase change materialAttapulgiteThermal energy storage

0306-2619/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.apenergy.2011.02.030

⇑ Corresponding author at: Jiangsu Key LaboratorySoutheast University, Nanjing 211189, China. Tel.:2583793232.

E-mail address: limin.li@163.com (M. Li).

a b s t r a c t

Fatty acid phase change materials (PCMs) have some advantages such as less corrosivity, no separation ofsubcooling phase and low price. In this paper, capric acid and palmitic acid are composited according to acertain mass ratio to prepare binary fatty acid. Capric–palmitic acid are absorbed into attapulgite by vac-uum method to prepare capric–palmitic acid/attapulgite composite PCMs. Analysis methods such as dif-ferential scanning analysis (DSC), scanning electron microscope (SEM), Fourier transform infrared (FT-IR)and specific surface analysis (BET method) are used to test the thermal properties, structure and compo-sition of the prepared composite PCM. The results indicate that the pore structure of the caplic–paltimicacid/attapulgite composite PCM is open-ended tubular capillary, which is beneficial to the adsorption.Capric acid and palmitic acid can be absorbed uniformly into attapulgite and the optimum absorptionratio of capric–palmitic binary fatty acid is 35%. There is no chemical reaction between the capric–palmitic acid and attapulgite. The phase change temperature of the capric–palmitic acid/attapulgite com-posite PCM is 21.71 �C and the latent heat is 48.2 J/g.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

PCM can absorb and release heat during the phase change.Therefore, it is available for thermal energy storage and tempera-ture control. Phase change heat storage technology has been themost attractive heat utilization technology and one of the mostimportant potential approaches to increase energy utilization rate[1–6]. PCMs can be used in building to improve thermal comfort ofindoor environment and conserve energy. During daytime PCM canabsorb heat through the melting process. During night PCM can re-lease heat through the solidification process. Many studies havebeen carried out in this field [7–12]. The study on PCM is veryimportant under globe energy crisis at present.

Phase change thermal energy storage materials can be inorganicor organic. Inorganic PCMs have greater heat of fusion than that oforganic PCMs. The main advantages of inorganic PCMs are thatthey have low toxicity, great thermal energy storage capacity andare cheaper than organic PCMs. However, subcooling and phaseseparation may occur during the phase change process of inorganicPCMs [13,14]. The main advantages of organic PCMs are that they

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have no corrosiveness and subcooling. In addition, they are nottoxic and have steady performance [15,16].

For the past few years, composite phase change thermal energystorage materials are studied. They can not only overcome the dis-advantages of single PCMs effectively but also expand applicationranges of PCMs. As a result, composite phase change materialshave been a hot research topic in the field of thermal energy stor-age materials. Xavier et al. [17] prepared a new supported phasechange material made of paraffin impregnated by capillary forcesin a compressed expanded natural graphite matrix. High loads ofparaffin were obtained from 65% to 95% weight depending uponthe bulk graphite matrix density. Sarı and Karaipekli [18] studiedthe preparation and thermal properties of palmitic acid (PA)/ex-panded graphite (EG) composite as the form-stable phase changematerial (PCM). The maximum mass fraction of PA retained in EGwas found as 80 wt.% without the leakage of PA in melted stateeven when it is heated over the melting point of PA. Sarı [19] alsostudied the preparation of paraffin/high density polyethylene(HDPE) composites as PCM. In this composite, the paraffin com-pound is dispersed into the network of solid HDPE.

Fang et al. [20] prepared form-stable lauric acid (LA)/silicondioxide (SiO2) composite phase change materials using sol–gelmethods. Cai [21] prepared the halogen-free flame retardantform-stable phase change materials (PCM) based on paraffin/highdensity polyethylene (HDPE) composites by using twin-screw ex-truder technique. Son and Kim [22] prepared polymer–PCM gels

3126 M. Li et al. / Applied Energy 88 (2011) 3125–3132

from polypropylene and n-octadecane for thermal energy storageand release materials, their basic properties and possible applica-tions especially in latent heat storage were also discussed.

The available research results indicate that it is feasible tocompose organic PCMs with inorganic PCMs or polymer PCMs forpreparing form-stable PCMs. It is cost effective for latent heatstorage application because containers of PCMs are unnecessary.And this preparation method is simple and easy to be used. More-over, the prepared PCMs have appropriate phase-transition tem-perature and high latent heat. The study on organic PCMs andform-stable PCMs starts from 1990s and studies on preparingform-stable PCMs with organic and inorganic materials are stillinsufficient. Therefore, the further study on the form-stable PCMsis necessary.

Attapulgite has special layer structure, chain structure and goodabsorption. Attapulgite is cheap and there is abundant resource ofit. In addition, attapulgite can endow the prepared compositematerials with the dimensional stability, thermostability and theproperty of barrier. Therefore, attapulgite is getting more andmore attention. In this study, capric acid and palmitic acid weremixed according to a certain proportion to prepare binary fattyacid with suitable phase temperature. This study aimed to preparethe form-stable PCM using the method of absorbing the capricacid–palmitic binary fatty acid into the pores of attapulgite andinvestigate its thermal properties.

2. Experimental

2.1. Raw materials

Capric acid and palmitic acid are chemically pure and are sup-plied by Chemical Reagent Corporation of Nanjing, China. Thephysical properties of capric acid and palmitic acid given by themanufacturer (Table 1). Attapulgite clay is supplied by Yuanda ClayLtd. Co. of Nanjing, China.

2.2. Characterization

Winner 2000 laser particle size analyzer is used to test the sizedistribution of attapulgite. Differential Scanning Calorimetry

Fig. 1. The sketch of the vacuum absorption equipment.

Table 1Physical parameters of fatty acid (given by the manufacturer).

Acid Molecular formula Melting point (�C) Latent heatof fusion (J/g)

Capric acid C10H20O2 31.5 155.5Palmitic acid C16H32O2 63 212.1

(DSC204, Netzsch Instrument Inc., Germany) is used to test thethermal properties of the binary fatty acid and the compositecapric–palmitic acid/attapulgite composite PCM. The heating rateswere 0.5 �C/min in a nitrogen atmosphere from 0 �C to 120 �C. Themorphology of PCMs was observed with a SEM (JSM-6700F, JEOL,Japan) at the acceleration voltage of 20 kV under low vacuum.Infrared spectra of attapulgite and the capric acid–palmitic acid/attapulgite composite PCM is obtained with a Fourier transforminfrared spectrophotometer (750FTIR, Nicolet, USA) Specific sur-face analysis (BET method, Gemini2380, Gemini, USA) is used tocharacterized the porous structure of attapulgite and the capricacid–palmitic acid/attapulgite composite PCM.

2.3. Experimental method

2.3.1. Melting blending and vacuum absorption of material7.52 g of capric acid and 2.48 g of palmitic acid were put in a

20 ml test tube, which was immersed in a constant temperaturebath at 70 �C. The mixture of capric acid and palmitic acid was stir-red at the speed of 500 rpm for 30 min and then vibrated by super-sonic wave until the mixture is mixed homogeneously. Attapulgitewas placed into vacuum absorption equipment in pressure of0.1 MPa for 30 min to prepare the capric–palmitic acid/attapulgitecomposite PCM. The sketch of the vacuum absorption equipment isshown in Fig. 1. Then the sample was dried to the constant weightin an automatic electric oven at 70 �C and grounded.

3. Results and discussion

3.1. Size distribution of attapulgite

The size distribution of attapulgite is shown in Fig. 2 and its sizeparameter is listed in Table 2.

Table 2 shows that the size distribution of attapulgite is between0.5 lm and 25 lm and the average size of attapulgite is 4.76 lm.The specific surface area of attapulgite is 11,743.28 cm2/g. There-fore, attapulgite has good absorption.

3.2. Determination of the optimum absorption ratio of capric–palmiticbinary fatty acid

The higher the mass ratio of capric–palmitic binary fatty acidadopted in attapulgite is, the greater the latent heat of thecapric–palmitic acid/attapulgite composite PCMs. However, theleakage of liquid should not happen under the melting state ofthe binary fatty acid in order to shape the capric–palmitic acid/attapulgite composite PCMs. Therefore, there is an optimumabsorption ratio of the binary fatty acid. For determining the opti-mum absorption ratio of capric–palmitic binary fatty acid, thequantity of binary fatty acid absorbed in attapulgite is increasedby 2 wt.% from 0 wt.%. The optimum absorption ratio of capric–palmitic binary fatty acid is obtained, the value is 35%.

3.3. The microstructure of attapulgite and the capric–palmitic acid/attapulgite composite PCM

Fig. 3 is the SEM diagrams of attapulgite under different magni-fications. Fig. 4 shows the result of the X-ray spectrum analysis ofattapulgite.

As can be seen in Fig. 3, attapulgite is layered and laminatedstructure and there are large spaces between the layers. Thereare pore canals in the layers and the surface is uneven. Conse-quently, attapulgite has great surface area and the organic molec-ular of the capric–palmitic binary fatty acid can be absorbed inthose pores and canals.

Fig. 2. Size distribution of attapulgite.

×500 ×5000

×20000

(a) (b)

(c)

Fig. 3. The SEM diagrams of attapulgite.

Table 2Size parameter of attapulgite.

Sample D10 (lm) D50 (lm) D90 (lm) Dav (lm) S/V (cm2/cm3) S/g (cm2/g)

Attapulgite 1.30 2.43 6.69 4.76 26,422.38 11,743.28

Annotation: D50 means the corresponding particle size when the cumulative distribution percentage reaches 50%; D10 means the corresponding particle size when thecumulative distribution percentage reaches 10%; D90 means the corresponding particle size when the cumulative distribution percentage reaches 90%; Dav means theaverage particle size in the cumulative distribution of size; S/V means specific surface on volume basis, S/g means specific surface on weight basis.

M. Li et al. / Applied Energy 88 (2011) 3125–3132 3127

Element C O Mg Al Si K Ca Fe

Wt%

At%

11.45 35.16 2.18 10.18 30.31 2.52 1.32 6.88

19.39 44.69 1.82 7.67 21.94 1.31 0.67 2.51

Fig. 4. The X-ray spectrum analysis of attapulgite.

3128 M. Li et al. / Applied Energy 88 (2011) 3125–3132

The X-ray spectrum analysis shows that the main elementcontained in attapulgite is Al, Si, O, and there are small quantity ofK, Ca, Fe, Mg. Crystalline structure of pure attapulgite belongs to2:1 type clay mineral. Fig. 5 shows the crystalline structure of pureattapulgite. In every 2:1 structural layer, the angular point of tetra-hedron crystal plate reverses at a certain distance and the layeredand chained structure is formed. The channels are filled with zeo-litic water and crystal water.

Fig. 6 shows the microstructure of the capric–palmitic acid/attapulgite composite PCM. It can be observed that the quantityof pores and the distance between layers of attapulgite decreasedcompared to that in Fig. 5. That indicated that the capric–palmiticacid has been adsorbed in the layered and laminated structure of

Fig. 5. Crystal structural diagram of attapulgite clay.

the attapulgite. The attapulgite is used as the supporting materialand the capric–palmitic acid is dispersed into the layered structureof the attapulgite.

3.4. Pore structure of the capric–palmitic acid/attapulgite compositePCMs

The specific surface area and distribution of pore diameter ismeasured by automatic static volumetric method. Figs. 7 and 8are the pore diameter distribution diagram and the nitrogenadsorption–desorption isotherms of attapulgite and the caplic–paltimic acid/attapulgite composite PCM, respectively. The caplic–paltimic acid was 35% of the total mass of he caplic–paltimic acid/attapulgite composite PCM. The pore parameters of attapulgiteand the caplic–paltimic acid/attapulgite composite PCM are listedin Table 3.

Fig. 7 shows that the pore diameter distribution curve of atta-pulgite presents double peaks which is under 10 nm. The porediameters responding to the two peaks are 3.71 nm and 4.75 nm,respectively. The total pore volume was 0.1113 cm3/g and the spe-cific surface area was 100.23 m2/g. The pore diameter distributioncurve of the caplic–paltimic acid/attapulgite composite PCM is amulti-peak curve. Pore volume with diameter around 10 nm isthe maximum value. The pore diameters responding to the maxi-mum volume was 2.12 nm, the total pore volume was0.0152 cm3/g, and the specific surface area was 1.9795 m2/g.

Attapulgite have high specific surface area and good adsorptionof nitrogen due to its abundant nanometer pore canals and thethree-dimensional network structure. After the capric–paltimicacid is absorbed into attapulgite, the specific surface area of atta-pulgite decreases from 100.23 m2/g to 1.9795 m2/g, and the porevolume also decreases to one tenth. It indicated that the capric–paltimic acid had been filled into pore structure of attapulgite.

×5000 ×10000 (a) (b)

Fig. 6. SEM diagrams of capric acid–palmitic acid/attapulgite composite PCM.

Fig. 7. Pore diameter distribution of attapulgite and the caplic–paltimic acid/attapulgite composite.

Relative pressure,

Pore

vol

ume/

Adsorption of Attapulgite

Desorption of Attapulgite

Adsorption of Capric/ Paltimic acid/ Attapulgite

Desorption of Capric/ Paltimic acid/ Attapulgite

Fig. 8. Nitrogen adsorption–desorption isotherms of attapulgite and the caplic–paltimic acid/attapulgite composite PCM at 77.3 K.

M. Li et al. / Applied Energy 88 (2011) 3125–3132 3129

Table 3Pore parameters of attapulgite (A) and the caplic–paltimic acid/attapulgite compositePCM (C–P/A).

Pore parameters SBET (m2/g) Vm (cm3/g) R (nm)

SamplesA 100.23 23.028 4.256C–P/A 1.9795 0.4548 30.079

Annotation: SBET is the specific surface area; Vm is saturated extent of adsorption ofmonomolecular layer; R is the average diameter of pore.

200 4010

20

30

40

50

Temperature ( )

Hea

t Flo

w E

ndo

Up

(mw

)

Fig. 10. DSC thermogram of capric–palmitic acid.

20

30

40

50

Hea

t Flo

w E

ndo

Up

(mw

)

3130 M. Li et al. / Applied Energy 88 (2011) 3125–3132

The nitrogen adsorption–desorption isotherms of attapulgite andthe caplic–paltimic acid/attapulgite composite PCM are shown inFig. 8. As can be seen in Fig. 8, the nitrogen adsorption–desorptionisotherms of attapulgite is IV type isotherms. The curve is upwardin the low pressure zone, which indicated that strong interactionforce exists between attapulgite and nitrogen. And in the high pres-sure zone, the isotherms rise rapidly due to the capillary condensa-tion. There exists obvious ‘‘desorption hysteresis’’ phenomenon inthe nitrogen adsorption–desorption isotherms which result frommicrospore action in attapulgite.

The nitrogen adsorption–desorption isotherms of the caplic–paltimic acid/attapulgite composite PCM is typical BET polymolec-ular layer adsorption curve. The curve is downward in the entirepressure range and attributes to III type isotherms. This meansthe pore structure of the caplic–paltimic acid/attapulgite compos-ite PCM is open-ended tubular capillary, which is beneficial to theadsorption. In the low pressure zone, small adsorption capacity ofnitrogen illustrated that the acting force between nitrogen and thecaplic–paltimic acid/attapulgite composite PCM is weak. Thisshows that the surface properties of attapulgite have changedand the binary fatty acid have covered on the surface layer. Inthe high pressure zone, adsorption curve rise. Nitrogen condensedin the pore with little desorption hysteresis. The reason is that thepores were largely filled with the capric–palitimic acid and theporosity decreased sharply.

0 10 20 30 40 5010

Temperature ( )

Fig. 11. DSC thermaogram of capric–palmitic acid/attapulgite.

3.5. Infrared spectroscopic analysis of attapulgite and the capric–palmitic acid/attapulgite composite PCM

FTIR spectral peaks obtained for attapulgite and the capric–palmitic acid/attapulgite composite PCM is shown in Fig. 9. As

Fig. 9. FTIR spectral peaks obtained for attapulgite (A) and the capric–palmitic acid/attapulgite composite PCM (C–P/A).

Table 4Thermal properties of some composite PCMs in literature.

Composite PCMs Phase change temperature (�C) Latent heat of phase change (J/g) References

Capric–myristic acid (20 wt.%)/Vermiculite 19.8 27.0 Karaipekli and Sarı [23]Lauric acid (60 wt.%)/expanded perlite 44.1 93.4 Sarı et al. [24]RT20/Montmorillonite 20.8 53.6 Fang et al. [25][21] Capric–myristic acid/VMT/2 wt.%EG 19.7 26.9 Karaipekli and Sarı [23]Capric–palmitic acid/attapulgite 21.7 48.2 Present study

M. Li et al. / Applied Energy 88 (2011) 3125–3132 3131

can be seen in Fig. 8, The self association peaks (AOH stretching)are at 3428 cm�1 and 3619 cm�1 for A and at 3425 cm�1 and3616 cm�1 for C–P/A. The range of the peak at 3428 cm�1 for C–P/A broadens evidently due to the formation of hydroxyl groupor hydrogen band formed by hydrogen of carbon chain of fatty acidand attapulgite. The characteristic absorption peak of SiAOASi andSiAO, as well as the characteristic absorption peak of SiAOAA1 andAlAO for C–P/A is observed at 1032 cm�1 and 793 cm�1 for CAP/A.These peaks are seen at 1035 cm�1 and 795 cm�1 for A. Thechanges in the peaks of AOH stretching are due to the interactionsof carbonyl and hydroxyl groups between A and C–P/A.

Peak at 1640 cm�1 for A belongs to the flexural vibration bandof coordinated water and absorbed water molecular. While thepeak for C–P/A at 1634 cm�1 fades obviously. It indicated thatthe coordinated water and the absorbed water in attapulgitedecrease.

The C@O stretching peak for C–P/A is observed at 1706 cm�1,however, there is no C@O stretching peak for C–P/A. The peak in2952 cm�1, 2917 cm�1 and 2848 cm�1 for C–P/A is symmetricand asymmetric stretching peak of CAH, and the peak in1462 cm�1 and 1381 cm�1 is the flexural vibration peak. This indi-cated that there exists fatty acid in attapulgite.

Fig. 9 shows that fatty acids have been absorbed into attapulgiteand no bond except hydrogen bond exists between A and C–P/A.Therefore, there is no chemical reaction between the capric–pal-mitic acid and attapulgite.

3.6. Property of thermal energy storage of capric–palmitic acid/attapulgite composite PCM

Figs. 10 and 11 show the DSC thermograms of capric–palmiticacid (C–P) and the capric–palmitic acid/attapulgite (C–P/A) compos-ite PCM. The main peaks on the DSC thermograms correspond to thesolid–liquid phase change of C–P and C–P/A, respectively. Figs. 10and 11 indicates that C–P and C–P/A exhibit similar DSC curve. Theyhave similar melting temperature of 21.80 �C for C–P and 21.71 �Cfor C–P/A. The latent heat of C–P and C–P/A are obtained as the areaunder the peaks of the solid–liquid transitions of them. Figs. 10 and11 show that the latent heat of C–P and C–P/A is 138.2 J/g and 48.2 J/g, respectively. The latent heat of C–P is 34.9% of that of C–P/A. Thisvalue is in agreement with the mass percentage of the capric–palmitic acid in C–P/A. The capric–palmitic acid is confined in thelayered structure of attapulgite and the molecular heat movementof the capric–palmitic acid is confined consequently. Therefore,the mass percentage of the capric–palmitic acid affects the valueof the latent heat of the capric–palmitic acid/attapulgite compositePCM. The thermal properties of the composite PCMs available inliterature are listed in Table 4 [23–25]. It can be seen that the latentheat of the capric–lauric acid/diatomite composite PCM prepared inthe present study is higher than some composite PCMs available inliterature.

4. Conclusions

The preparation, structure and thermal energy storage propertyof capric–palmitic acid/attapulgite form-stable PCM are presented

in this study. The following conclusions are obtained based on theresults.

(1) The capric–palmitic acid/attapulgite composite PCM can beprepared by the vacuum absorption method. The optimumabsorption ratio of capric–palmitic binary fatty acid is 35%.

(2) Attapulgite is layered and laminated structure and there arelarge spaces between the layers. Attapulgite is used as thesupporting material and the capric–palmitic binary fattyacid can be absorbed in those pores and canals.

(3) The pore structure of the caplic–paltimic acid/attapulgitecomposite PCM is open-ended tubular capillary, which isbeneficial to the adsorption. The pores of attapulgite werelargely filled with the capric–palitimic acid and the porositydecreased sharply.

(4) FTIR results show that there is no chemical reaction betweenthe capric–palmitic acid and attapulgite.

(5) DSC results indicated that the melting temperature of thecapric–palmitic acid is similar to that of the capric–palmiticacid/attapulgite composite PCM. The mass percentage of thecapric–palmitic acid has little influence to the melting tem-perature of the capric–palmitic acid/attapulgite compositePCM. However, it affects the value of the latent heat of thecapric–palmitic acid/attapulgite composite PCM.

Acknowledgements

The authors gratefully acknowledge the financial support forthis research from the National Natural Science Foundation of Chi-na (50808042) for youths, Six Projects Sponsoring Talent Summitsof Jiangsu Province (1112000053), the Project-sponsored by SRF forROCS, SEM. Outstanding young teacher funding schemes of south-east university (4012001017).

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