Energy Conversion and Management 45 (2004) 2033–2042www.elsevier.com/locate/enconman

Form-stable paraffin/high density polyethylene compositesas solid–liquid phase change material for thermalenergy storage: preparation and thermal properties

Ahmet Sarı

Department of Chemistry, Gaziosmanpas�a University, 60240 Tokat, Turkey

Received 1 August 2003; accepted 26 October 2003

Abstract

This paper deals with the preparation of paraffin/high density polyethylene (HDPE) composites as form-

stable, solid–liquid phase change material (PCM) for thermal energy storage and with determination of

their thermal properties. In such a composite, the paraffin (P) serves as a latent heat storage material andthe HDPE acts as a supporting material, which prevents leakage of the melted paraffin because of providing

structural strength. Therefore, it is named form-stable composite PCM. In this study, two kinds of paraffins

with melting temperatures of 42–44 �C (type P1) and 56–58 �C (type P2) and latent heats of 192.8 and 212.4

J g�1 were used. The maximum weight percentage for both paraffin types in the PCM composites without

any seepage of the paraffin in the melted state were found as high as 77%. It is observed that the paraffin is

dispersed into the network of the solid HDPE by investigation of the structure of the composite PCMs

using a scanning electronic microscope (SEM). The melting temperatures and latent heats of the form-

stable P1/HDPE and P2/HDPE composite PCMs were determined as 37.8 and 55.7 �C, and 147.6 and 162.2J g�1, respectively, by the technique of differential scanning calorimetry (DSC). Furthermore, to improve

the thermal conductivity of the form-stable P/HDPE composite PCMs, expanded and exfoliated graphite

(EG) by heat treatment was added to the samples in the ratio of 3 wt.%. Thereby, the thermal conductivity

was increased about 14% for the form-stable P1/HDPE and about 24% for the P2/HDPE composite PCMs.

Based on the results, it is concluded that the prepared form-stable P/HDPE blends as composite type PCM

have great potential for thermal energy storage applications in terms of their satisfactory thermal properties

and improved thermal conductivity. Furthermore, these composite PCMs added with EG can be con-

sidered cost effective latent heat storage materials since they do not require encapsulation and extra costto enhance heat transfer in the paraffin.

� 2003 Elsevier Ltd. All rights reserved.

Keywords: Form-stable paraffin/HDPE composite; Cost effective PCM; DSC; Thermal conductivity; Thermal

properties; Expanded graphite

E-mail address: asari@gop.edu.tr (A. Sarı).

0196-8904/$ - see front matter � 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/j.enconman.2003.10.022

2034 A. Sarı / Energy Conversion and Management 45 (2004) 2033–2042

1. Introduction

Heat storage systems are utilized in energy recovery and conservation processes and for solarthermal systems. Of various methods of heat storage, latent heat storage is the most attractive onedue to its high storage density and small temperature variation from storage to retrieval. In alatent heat storage system, energy is stored during melting and recovered during freezing of aphase change material (PCM).

Several candidate inorganic and organic PCMs and their mixtures have been study recently foruse as latent heat storage materials [1–4]. Among the investigated PCMs, paraffins have been usedas thermal energy storage materials because of their high latent heat and many desired thermalcharacteristics, such as little or no supercooling, low vapor pressure, thermal and chemical sta-bility and self nucleating behavior [1,4–6]. However, the low thermal conductivity is the majordrawback of the paraffins as PCMs. Therefore, some methods have been tried to enhance heattransfer in these PCMs [7–9].

On the other hand, when the cost of encapsulation for the paraffins, as well as the addedexpenditures needed to increase their conductivity, is also considered, the development of a newcomposite PCM based on paraffin with high thermal conductivity is important. In this regard,in recent times, a new type PCM called shape-stabilized or form-stable composite PCM, whichis based on paraffin, was developed. The composite PCM can keep its shape, although thecomposite PCM is heated above the melting point of the paraffin. In such a type of PCM,paraffin, as the solid–liquid phase change material, is dispersed into the polymer network ofHDPE. The HDPE compound in the composite PCM is considered as a supporting material toprevent leakage of the melted paraffin from the composite at a temperature between the meltingtemperatures of paraffin and HDPE. Therefore, the encapsulation problem of the paraffin canbe solved.

There are a few researches on the preparation and investigation of the thermal properties ofform-stable composite PCM with the basis of solid–liquid phase change material. Inaba and Tu[10] established the thermophysical properties of shape-stabilized paraffin as a new type latentheat storage material and reported that this material could be used as a PCM without encapsu-lation in thermal energy storage systems. Feldman et al. [11] tested a matrix type phase changethermal storage tile module with no surface covering, that was intended to transfer heat directly toand from room air at small temperature difference. In that tile module, heat is stored and releasedby melting and freezing mixtures of fatty acids with polymeric matrices. They concluded that thetiles keep their shape and dimensions with no weeping of liquid fatty acid, depending on theircomposition. Lee and Choi [12] studied the durability of HDPE (high and low)/paraffin blends asenergy storage materials by investigation of the seepage behavior of paraffin. In that study, it isexplained that the total stored energy is comparable with that of the traditional PCMs. They alsoinvestigated the effect of HDPE crystalline morphology on paraffin seepage in an injection moldedHDPE/paraffin by SEM and optical microscopy (OM) and light scattering (LS). The morpho-logical characteristics indicated that the excellent sealant property of h-HDPE/paraffin blend isdue to its molecular weight difference. Hong and Xin-shi [13] prepared a polyethylene–paraffincompound (PPC) as a form-stable PCM and analyzed its structure by SEM and measured itslatent heat by the technique of DSC. They recommended that the form-stable PCM that iscomposed of 75 wt.% paraffin is a desirable one for application in low temperature heat storage,

A. Sarı / Energy Conversion and Management 45 (2004) 2033–2042 2035

since it is cheap and easy to prepare and of a latent heat comparable with traditional PCMs.Moreover, Xiao et al. [14] prepared a shape-stabilized PCM by composing paraffin with a thermo-plastic–elastomer poly (styrene–butadiene–styrene) and established its thermal performanceduring the melting and solidification processes. They concluded that the shape-stabilized PCMexhibits the same phase transition characteristics as paraffin and up to 80% of the latent heat ofparaffin. In that study, it was reported that the thermal conductivity of shape-stabilized PCM wasincreased significantly by introducing expanded graphite (EG).

Based on the above literature survey, it is worthwhile to note that the form-stable or shape-stabilized PCM acts a direct heat storage medium. Therefore, it is cost effective for latent heatstorage applications because an outer container is unnecessary. Moreover, it can be made intogranular material with desirable dimensions.

On the other hand, corresponding with wide requirements, a proper form-stable paraffin/HDPE composite PCM can be used in heating systems by selecting a suitable kind of paraffin interms of melting temperature and latent heat. In this sense, the objective of this study is to preparethe composites of paraffin type P1 (Tm ¼ 37:8 �C)/HDPE and type P2 (Tm ¼ 56:6 �C)/HDPE asform-stable PCMs and to investigate their micro-structure by SEM and to determine theirthermal properties employing the DSC method. This study also deals with estimation of the in-crease occurring in the thermal conductivity of the form-stable composite PCMs by addi-tion of expanded and exfoliated graphite, which has function of improving the thermalconductivity.

2. Material and methods

2.1. Material and preparation of the form-stable composite PCMs

Two kinds of technical grade paraffins (type P1; Tm ¼ 42–44 �C and type P2; Tm ¼ 56–58 �C bymanufacturer), supplied from the Aldrich company, were used as phase change material. Only onetype HDPE was used as supporting material in the preparation of the paraffin/HDPE compositePCMs. The density of the selected HDPE was measured using a volume expansion meter, asdefined in Ref. [10], and found as 0.942 g cm�3. The form-stable P1/HDPE and P2/HDPE com-posite PCMs with amounts of 100 g were prepared by mixing the melted paraffin with the meltedHDPE and cooling to room temperature. Composite PCMs with different weight percentages ofparaffin (50, 60, 70, 75 and 77) were prepared to determine the maximum ratio of the paraffinunder which there is no leakage of the paraffin from the composite when it is in the melted state.

In order to improve the thermal conductivity of the prepared form-stable composite PCMs,expanded and exfoliated graphite (EG) was added to the melted composite samples, with stirring,in the ratio of 3 wt.% at about 150 �C. The EG was prepared with the heat treatment process asexpressed in Ref. [14].

The structures of the prepared form-stable composite PCMs were analyzed by SEM (JEOL6400), and the thermal properties were measured using a DSC (DuPont 2000) instrument. Thethermal conductivities of the samples were estimated by a thermal conductivity apparatus(Cussons Technology).

2036 A. Sarı / Energy Conversion and Management 45 (2004) 2033–2042

3. Results and discussion

3.1. Structure analysis of the form-stable composite PCMs

The maximum mass percentage of both paraffin types dispersed into the composites wasdetermined as 77 wt.%. There was no leakage of the paraffin up to this weight ratio even when itmelts. In other words, when the mass percentage of paraffin is over 77%, the mechanical strengthand the highest enduring temperature of these composites decreased, and thus, leakage of themelted paraffin started.

The SEM photographs indicating the microstructures of the form-stable P1/HDPE and P2/HDPE composite PCMs are shown in Figs. 1 and 2, respectively. From these figures, it can beobserved that the paraffin is dispersed into the network of solid HDPE used as the supportingmaterial. This dispersion provides a mechanical strength to the whole compound. Therefore, thecomposite material maintains its shape in the solid state without seepage of the melted paraffin. Inthe photographs, the black and white parts represent the paraffin and HDPE compounds in thecomposite PCMs, respectively, as seen in Figs. 1 and 2. Both micrographs are of similar texturessince both form-stable PCMs are composed of paraffin and HDPE in the same mass combinationand they have similar chemical structure. The obtained SEM results are in agreement with thedispersion model given in Refs. [10,14] and the SEM photographs obtained for different paraffin/HDPE composite PCMs by Refs. [12,13].

3.2. Thermal properties of the form-stable composite PCMs

Thermal properties of the prepared paraffin/HDPE composite PCMs with different mass per-centages of paraffin, such as transition temperatures, melting temperatures and latent heat, weredetermined by a DSC thermal analyzer. Indium was used as a reference for temperature cali-

Fig. 1. The SEM photographs of form-stable P1/HDPE composite PCM.

Fig. 2. The SEM photographs of form-stable P2/HDPE composite PCM.

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bration. Samples were measured in a sealed aluminum pan with a mass of 6.5 mg. The DSCthermal analyses were performed in the temperature range of 15–80 �C for the pure paraffins and15–150 �C for the composite samples with a heating rate of 5 �C/min and under a constant streamof nitrogen at atmospheric pressure. The melting temperature Tm and the transition temperaturesTt of the composite PCMs correspond to the onset temperature obtained by drawing a line at thepoint of maximum slope of the leading edge of the regarded DSC peak and extrapolating the baseline on the same side as the leading edge of the peak, as seen in Fig. 3. The latent heat DHL, wascalculated as the total area under the peaks of solid–solid and solid–liquid transitions of theparaffin in the composite by numerical integration.

Figs. 3 and 4 show the typical DSC thermograms of the pure paraffins (type P1 and P2) used inthe preparation of the form-stable composite PCMs. These thermograms present reference data to

Fig. 3. DSC thermogram of pure paraffin 1 (type P1).

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evaluate the changes in thermal properties of the composite PCMs, depending on the amounts ofparaffin. The DSC thermograms of the P1/HDPE and P2/HDPE composite PCMs including 60and 77 wt.% paraffin were also shown in Figs. 5, 6 and 7, 8, respectively. There are three peaks onthe DSC outputs, as can be seen in Figs. 5–8. The sharp or main peak represents the solid–liquidphase change of the paraffin used as PCM in the composites, and the minor peaks at the left andright sides of the main peak correspond to the solid–solid phase transition of paraffin and themelting temperature of HDPE, respectively. These figures indicate that both paraffin/HDPEcomposite PCMs exhibit similar thermal characteristics. This is because there is no chemicalreaction between the paraffin and HDPE in preparation of the form-stable composite PCMs.

In Table 1, the thermal properties of the pure paraffins and both composite PCMs, such astransition temperature (Tt), melting temperature (Tm) and the latent heat (DHL) obtained by theDSC measurements are given. As indicated in Table 1, the Tt, Tm and DHL values for bothcomposite PCMs approach those of the pure paraffins when the mass percentage of paraffin in thecomposite approaches 100. Moreover, in Table 1, a little deviation can be observed in the meltingtemperatures of the paraffins dispersed in the ratio of 77 wt.% when compared with those of thepure paraffins. This deviation is 2 �C for type P1 and 0.9 �C for type P2.

On the other hand, it can be noted that the latent heat of each form-stable composite PCM is ingood agreement with the calculated value by multiplying the latent heat of the dispersed paraffinwith its mass fractions. According to this calculation, as an example, the ratio of the latent heat ofthe form-stable composite PCMs (including 77 wt.% dispersed paraffin) to that of the regardedparaffin in the pure state are 76.5% for the P1/HDPE composite and 76.3% for the P2/HDPEcomposite PCM. The calculated ratios are a little less than the mass percentage of the dispersedparaffin (77 wt.%). In addition, the mean difference between the measured and calculated latentheat values is in the level of 0.7% for the form-stable P1/HDPE composite and 1.9% for the P2/HDPE composite PCM. However, these results are in accordance with the results given in Refs.[10,14].

Fig. 4. DSC thermogram of pure paraffin 2 (type P2).

Fig. 6. DSC thermogram of form-stable P1 (77 wt.%)/HDPE composite PCM.

Fig. 5. DSC thermogram of form-stable P1 (60 wt.%)/HDPE composite PCM.

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3.3. Thermal conductivity of the form-stable composite PCMs added with EG

As mentioned before, the maximum mass percentage of dispersed paraffin (type P1 or type P2)in both form-stable composite PCMs could go as high as 77% and no leakage is observed of themelted paraffin until this value although the composite PCM is heated above its melting tem-perature. So, the studies of the improvement of thermal conductivity were performed for only the

Fig. 7. DSC thermogram of form-stable P2 (60 wt.%)/HDPE composite PCM.

Fig. 8. DSC thermogram of form-stable P2 (77 wt.%)/HDPE composite PCM.

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form-stable composite PCMs constituted of 77% paraffin. For this aim, the expanded and exfo-liated graphite (EG) was added to these composite samples in the ratio of 3 wt.%. Thus, the newcombination percentages for both composite PCMs is about P(74.7 wt.%)/HDPE(22.3 wt.%)/EG(3 wt.%).

Table 1

Transition temperature (Tt), melting temperature (Tm), and latent heat (DHL) of form-stable composite PCMs with

different mass percentage of the paraffins

Tt (�C) Tm (�C) DHL (J/g)

Paraffin 1: HDPE (wt.%)

50:50 16.3 34.9 95.7

60:40 16.8 35.8 114.8

70:30 17.0 36.6 134.6

75:25 17.4 37.4 143.9

77:23 17.6 37.8 147.6

100:0 23.1 39.8 192.8

Paraffin 2: HDPE (wt.%)

50:50 42.9 53.8 103.8

60:40 43.6 54.4 125.1

70:30 44.2 54.9 146.1

75:25 44.9 55.4 158.5

77:23 45.2 55.7 162.2

100:0 45.8 56.6 212.4

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Using the thermal conductivity apparatus, the thermal conductivity in the solid state wasestimated as 0.24 Wm�1 K�1 for the form-stable P1/HDPE/EG and 0.26 Wm�1 K�1 for thecomposite of P2/HDPE/EG composite PCM. When these values are compared with those of thepure paraffin (0.21 Wm�1 K�1) given in Ref. [1], it can be said that the thermal conductivity forthe form-stable P1/HDPE and P2/HDPE composite PCMs is increased about 14% and 24%,respectively. The increase in the thermal conductivity is most likely because of the thermal con-ductive network formed thanks to the pore structure of the EG. This idea was verified by com-paring the melting time and solidification time of the pure paraffin with that of the compositePCM as reported in Ref. [14].

4. Conclusion

It is possible to prepare composite compounds of two types of paraffins (Tm ¼ 42–44 �C and56–58 �C) with HDPE as form-stable, solid–liquid phase change materials. In each composite, theparaffin compound is dispersed into the network of solid HDPE, and it serves as latent heatstorage material while the HDPE acts as a supporting material. Therefore, the solid HDPEprevents leakage of the melted paraffin thanks to its structural strength.

The maximum weight percentage for both paraffins dispersed in the PCM composites withoutany seepage of the paraffin when it is in the melted state were found as high as 77%. The meltingtemperatures and latent heats of the form-stable P1/HDPE and P2/HDPE composite PCMs weredetermined as 37.8 and 55.7 �C, and 147.6 and 162.2 J g�1, respectively, by the DSC analysis.Moreover, the thermal conductivity for the form-stable P1/HDPE and P2/HDPE compositePCMs is increased approximately by 14% and 24%, respectively, by the addition of EG as little as3 wt.%. This is probably due to the thermal conductive network occurring in the composite withthe pore structure of the EG.

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From the point of view of satisfactory thermal characteristics and improved thermal conduc-tivity, it can be concluded that the prepared form-stable paraffin/HDPE blends as composite typePCMs have great potential for thermal energy storage applications. Furthermore, these compositePCMs added with EG can be considered cost effective latent heat storage materials since they donot require encapsulation and the extra cost to enhancing heat transfer in the paraffin.

Acknowledgements

The author thanks Prof. Dr. Teoman Tinc�er and Sevim Ulupunar in Middle East TechnicalUniversity because of DSC measurements and also thanks Prof. Dr. Dursun Pehlivan and hisassistant, Dr. Melek Yılgın in Fırat University due to the thermal conductivity measurements.

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