Applied Energy 86 (2009) 1479–1483
Contents lists available at ScienceDirect
Applied Energy
journal homepage: www.elsevier.com/locate/apenergy
Preparation and thermal properties of polyethylene glycol/expanded
graphite blends for energy storage
Weilong Wang a,b,*, Xiaoxi Yang c, Yutang Fang a, Jing Ding d, Jinyue Yan b,e
a
The Key Laboratory of Enhanced Heat Transfer and Energy Conservation, Ministry of Education, South China University of Technology, Guangzhou 510640, People’s Republic of China
School of Sustainable Development of Society and Technology, Mälardalen University, Västerås SE 721 23, Sweden
c
Dongguan University of Technology, Dongguan 523808, People’s Republic of China
d
School of Engineering, Sun Yat-sen University, Guangzhou 510275, People’s Republic of China
e
Chemical Engineering and Technology/Energy processes, Royal Institute of Technology, Stockholm SE 100 44, Sweden
b
a r t i c l e
i n f o
Article history:
Received 22 July 2008
Received in revised form 5 November 2008
Accepted 2 December 2008
Available online 20 January 2009
Keywords:
Polyethylene glycol
Expanded graphite
Form-stable materials
Thermal conductivity
a b s t r a c t
Expanded graphite is a promising heat transfer promoter due to its high conductivity, which improves
the thermal conductivity of organic phase change materials. Moreover, it can also serve as supporting
materials to keep the shape of the blends stable during the phase transition. After various investigation,
the results showed that the maximum weight percentage of polyethylene glycol was as high as 90% in
this paper without any leakage during the melting period, with the latent heat of 161.2 J g 1 and the
melting point of 61.46 °C. It was found that the value of the latent heat was related to the polyethylene
glycol portion, increased with the increase in polyethylene glycol content. Moreover, the measured
enthalpy of the composite phase change materials was proportional to the mass ratio of the polyethylene
glycol component. The melting temperatures were almost the same with different ratios of composites.
The conductivity of blends was improved significantly with the high value of 1.324 W m 1 K 1 compared
to the pure polyethylene glycol conductivity of 0.2985 W m 1 K 1.
Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
Polyethylene glycol (PEG) serving as a type of organic phase
change materials (PCMs) has been recommended as a thermal energy storage material due to its relatively high latent heat of
187 J g 1, congruent melting behavior, better resistance to corrosion and suitable melting point [1–6]. Moreover, because PEG
was a type of materials with different molecular weights with different melting temperatures, they can be used in many applications such as industrial heat utilization, electronic device
management and protection as well as in active and passive heating or cooling of building [7–12].
Recently, a novel form-stable phase change materials draws
increasing attention in the energy storage research area [13–20].
Authors prepared form-stable energy storage materials by blending PEG and silica gel in the past work and discussed how to improve the thermal conductivities by adding b-Aluminum nitride
powder. As the value of the thermal conductivity was up to
0.5607 W m 1 K 1, improved by 87.8% compared to pure PEG
(0.2985 W m 1 K 1), the latent heat correspondingly dropped to
* Corresponding author. Address: School of Sustainable Development of Society
and Technology, Mälardalen University, Västerås SE 721 23, Sweden. Tel.: +46 21
151718; fax: +46 21 101370.
E-mail address: ce_logan@hotmail.com (W. Wang).
0306-2619/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.apenergy.2008.12.004
129.5 W m 1 K 1 [21,22]. Even thought this value was still enough
for application, the decreased heat capacity was supposed to affect
the efficiency of energy storage. Therefore, the new method of
preparation and materials were required to solve this problem.
Especially, the thermal conductivities of materials are supposed
to be as high as possible for practice uses.
Sari and Karaipekli [23] and Zhang and Fang [24] reported a
type of the paraffin/expanded graphite (EG) composite PCMs had
a high thermal conductivity without adding more additives. Thus,
the composite materials can keep the larger latent heat. Based on
the above information, a type of phase change material was prepared by blending polyethylene glycol with expanded graphite.
The phase change behavior and performance of the composite
materials were investigated and reported in the paper.
2. Experimental
2.1. Materials
Reagent grade Polyethylene glycol with molecular weight
(Mr = 1000) was purchased from Guangzhou Chemical Agent Company (Guangzhou, China). Expandable Graphite (particle size:
500 lm) was purchased from Shanghai graphite CO., LTD. All the
chemicals were analytical reagents and they don’t need further
purification.
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W. Wang et al. / Applied Energy 86 (2009) 1479–1483
2.2. Preparation of the composite PCMs
Expandable graphite was dried in a vacuum oven at 70 °C for
20 h previously, and then expanded graphite was made in a furnace at 900 °C for 50 s. After that, polyethylene glycol was melted
at the temperature 70 °C in the water bath. Later, the expanded
graphite was put into the melted liquid polyethylene glycol. After
mixing for 3 h, the composite PCMs were obtained. Finally, the
samples were dried at the room temperature. The composites were
prepared at different mass ratios of polyethylene glycol (range
from 50 wt% to 90 wt%) to seek for the maximum ratio without
any leakage when the temperature was above the melting point
of polyethylene glycol.
2.3. Characterization of the composite PCMs
Firstly, the composites with different mass percentages of polyethylene glycol were heated above the melting point of phase
change materials to investigate the maximum fraction without
leakage using polarizing optical microscope (POM) and dynamic
mechanical analysis (DMA). In POM test, an observation of polarizing optical microscope was performed on a 12pol microscope
equipped with a digital camera. The sample was placed between
a microscope glass and a cover slip. In DMA test, the sample was
made as large as a cube with 1 cm in the length and 0.5 cm in
the thickness.
In order to investigate the physical and chemical compatibility
of the composites, the morphology of the composite PCMs was
investigated using microphotographs, which were taken of the surface made by fracturing the specimen in liquid nitrogen and then
casting it with gold (AU) powder. The spectroscopic analysis was
characterized using Fourier transformation infrared spectroscope
(FT-IR).
The melting point and the heat of fusion of the solid composite
were determined using a differential scanning calorimeter (Perkins–Elmer DSC-2C) calibrated with an indium standard in the
range from 20 °C to 120 °C. The scanning rate was at 10 °C/min.
The heat flow repeatability was 0.2 lV. A 10 mg sample was sealed
in an aluminum pan. The melting point and freezing point were obtained by drawing a line at the point of maximum slope of the DSC
peak. The latent heat was calculated by numerical integration of
the area of the DSC peak.
Thermal conductivity measurement of the sample was measured by using transient plane source (TPS) at room temperature
(Hotdisk 2500, Sweden AB). The sensor will be placed between
two samples in the same size. The samples will be tightened for
a good contact between sensor and samples.
Fig. 1. The SEM images of expanded graphite (2000).
ites decreased and therefore it could not hold melted PEG any more
[19].
3.2. Chemical properties of the form-stable composite PCMs
FT-IR measurement was carried out in this paper. The FT-IR
spectrum provides useful information about the conformation of
the molecule.
From Fig. 2a, it is obvious that there is a peak at the wave number of 1107 cm 1 caused by stretching vibration of functional
group of C–O. Additionally, peak caused by stretching vibration
of functional group of O–H is also found at 3445 cm 1. Peaks at
2921 cm 1 and 946 cm 1 represent the stretching vibration of
functional group of CH2 and crystal peak of PEG [25]. From
Fig. 2b, the FT-IR absorption spectrum of PEG/EG composite was
compared to that of pure PEG. It is found some a little shift of
the main peak. The peak of functional group of C–O shift from
1107 cm 1 to 1114 cm 1. The group of CH2 is found moving to
2889 cm 1 and 951 cm 1. The peak of functional group of O–H
shift from 3445 cm 1 to 3457 cm 1. The frequency of the composite main group shifts mentioned means that there is interaction between PEG and EG. This interaction prevents the leakage of the
melted PCM from supporting materials.
3. Results and discussion
3.1. Morphology characterization of PCMs
Fig. 1 shows SEM photographs of the microstructures of EG.
From the figure, it can be seen that the expanded graphite has a
worm-like shape. The expandable graphite has an excellent
absorbability with strong capillary force and surface tension with
multiple pores. This structure provided a reasonable mechanical
strength to the composite maintained its form in the solid state
without seepage of the melted polyethylene glycol [23,24]. The
maximum mass percentage of polyethylene glycol dispersed into
the PCM composites was determined as 90 wt%. There was no leakage of the PEG from the surface of the composite up to this mass
ratio even at 120 °C in the experiment. Namely, when the mass
percentage of expandable graphite is less than 10%, the mechanical
strength and the highest enduring temperature of these compos-
Fig. 2. FT-IR spectrum: (a) PEG and (b) PEG/EG (90/10).
W. Wang et al. / Applied Energy 86 (2009) 1479–1483
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3.3. Phase change behavior of samples
To investigate the morphology in the transition process, the micro- morphology is recorded by POM in a heating process. Fig. 3
shows POM micrographs of pure PEG and PEG/EG composite at
various temperatures during the heating process, respectively.
From Fig. 3a, it can be seen that pure PEG was crystalline and its
crystal structure was orbicular at 60 °C. It indicates that polyethylene has a quite good capacity of crystalline, which contributes to
the reasonable latent heat. When the temperature reached 64 °C,
the color became faint. It is because parts of PEG started to melt
and the crystal structure was damaged. There is nothing but many
drops can be found in Fig. 3b. It in dictated that the entire PEG
melted at 68 °C totally. Namely, phase change process finished
completely. The whole process occurred quickly and PEG has a narrow melting temperature range from 60 °C to 68 °C.
The POM micrograph of the composite PCMs at 120 °C is shown
in Fig. 3d. There is no change during the entire heating process. It
indicated that expandable graphite serving as supporting material
attribute to prevent leakage of the melted polyethylene glycol. This
solid–solid phase change behavior of composite was further confirmed by visual observation. After heating the composite slowly
directly to 120 °C, it was observed that liquid leakage did not occur
during the heating process because polyethylene glycol was absorbed inside the pores of the expanded graphite well and limited
by the capillary force.
DMA measurements of PEG and PEG/EG composite are demonstrated in Fig. 4a and b, respectively. Fig. 4a shows the thickness of
PEG started to reduce when the temperature reached 69.89 °C.
After that, the higher the temperature, the thinner the PEG sample.
The highest rate of melting speed was 429.9 lm °C 1. When the
temperature reached 78.61 °C, the shape change value was equal
to the thickness of PEG sample, namely, it melted completely.
These behaviors indicated that this transition should be a solid–liquid phase change. Fig. 4b shows that composite had different
phase change behavior totally. The sample was heated at the temperature up to 120 °C (60 °C higher than the melting point of PEG),
no rapid shift occurred in the entire curve. It indicated that the
Fig. 4. DMA curves: (a) PEG and (b) PEG/EG (90/10).
composite remained in the solid state during the phase transition
and exhibited solid–solid phase change behavior [26].
Fig. 3. POM images of samples (100): (a) PEG (60 °C); (b) PEG (64 °C); (c) PEG (68 °C); and (d) PEG/EG (120 °C).
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W. Wang et al. / Applied Energy 86 (2009) 1479–1483
3.4. Thermal properties of the composite PCMs
Thermal properties including phase change temperature and
heat latent are important properties in the energy storage system.
Thermal properties of the PCMs were measured using DSC technique. Fig. 5a and b presents the thermal characteristics of PEG
and PEG/EG (90/10 wt%) composite in the heating and freezing cycle at a scanning rate of 10 °C/min, respectively.
Fig. 5a shows that the latent heat of pure PEG is 187.3 J g 1
(melting temperature Tm = 61.18 °C), which proves that PEG has a
large latent heat. This is because PEG is a linear polymer chain
made up of (CH2–CH2–O)n and has hydroxyl groups on two ends.
It can be easily crystallize and has quite large enthalpy because
of its simple structure. From Fig. 5a, it also can be seen that PEG
shows a melting temperature range. This is because PEG is a mixture of distributed molecular weight. Fig. 5b indicates that a phase
change with an enthalpy of 161.2 J g 1 happened at 61.46 °C. The
thermal characteristics of the composite are very close to those
of PEG. It shows that there is no chemical reaction between PEG
and expanded graphite [27]. In the study of PEG/acetate cellulose
(CDA) [7], this type of the PEG/CDA composite materials’ allows
the maximum of the mass percentage of PEG was 85 wt% to keep
the form-stable. Thus, the latent heat of this type of materials
was 139 J g 1. Therefore, compared to PEG/CDA composite materials, the PEG/EG composite materials had the large heat storage
capacity. Meanwhile, because the CDA was organic materials as
well, the thermal conductivities of materials were quite low. In
contrast, the PEG/EG materials have quite large thermal conductivities, which will be mentioned later in this paper.
Compared with the heating and freezing curves of PEG and PEG/
EG, it can be seen that the difference of the melting and freezing
temperatures is about 15–20 °C.The quite same difference of melting and freezing temperatures were also reported in other literatures [28,29], however, not many discussion about this
phenomena. The temperature difference indicates than PEG exists
certain supercooling, probably due to morphological constraints
and entanglements in inter-lamellar regions. Still, PEG and PEG
blends have potential due to the different temperature range with
different the molecular weight of PEG.
3.5. Relationships between the thermal properties and the mass
percentage of PEG
The effect of different mixture ratios from 50% to 100% (PEG
wt%) on the thermal characteristics of composite is listed in Table
1.
From Table 1, it can be seen that the enthalpy of composite is
decreased with the diminution of the weight percentage of PEG.
The relation between mass percentage and latent heat is linear.
That means the value of latent heat of the composite PCMs can
be calculated by multiplying the latent heat of pure PEG and mass
percentage of PEG in the composites. However, melting temperature and crystallizing temperature did not change much. There
are several reasons why the addition of expandable graphite causes
the decrease in the enthalpy. The addition of expanded graphite
would decrease the mass percentage of PEG, thus the enthalpy declines. In addition, from the further analysis, it can be found that
expanded graphite is quite stable in the phase transition of PEG.
Therefore, expanded graphite can be regarded as the impurity for
PEG and affects the perfection of the crystallization process.
3.6. Thermal conductivity of the composite PCMs
Thermal conductivities of the PCMs with different mass percentages of EG was measured by using a thermal conductivity
apparatus (Hotdisk 2500, Sweden), the thermal conductivity of
pure polyethylene glycol in the solid state was estimated as
0.2985 W m 1 K 1. PEG belongs to the organic phase change material and has an unacceptable low thermal conductivity. Because expanded graphite has a large conductivity (4–90 W m 1 K 1), the
conductivity value of the PEG/EG composite PCM was increased
much. When the mass percentage of EG was 10%, the composite
conductivity was 1.324 W m 1 K 1. At the same time, the composite has a reasonable heat latent of 161.2 J g 1. The enhancement of
the thermal conductivity was most likely because of the thermal
conductive network formed by the pore structure of the expanded
graphite. Therefore, PEG (90 wt%)/EG is recommended as the
promising composite PCM in this paper.
3.7. Thermal cycling test of the composite PCMs
In this experiment, the composite materials with PEG mass percentage of 90 wt% was chosen to test its stability of thermal perfor-
Table 1
Thermal activities of composite PEG/EG.
Fig. 5. The melting and freezing curves: (a) PEG and (b) PEG/EG (90/10).
PEG (%)
DHm (J g 1)
Tm (°C)
Tc (°C)
DHc (J g 1)
50
60
70
80
100
87.6
103.5
128.6
141.7
187.3
60.41
62.32
58.59
60.93
61.18
45.13
43.96
45.04
44.81
42.39
70.7
87.1
103.5
121.1
161
Tm = melting temperature; Tc = crystallization temperature.
DHm = heat latent of fusion; DHc = heat latent of crystallization.
W. Wang et al. / Applied Energy 86 (2009) 1479–1483
Table 2
Thermal properties of the composite PEG/EG (90/10) under thermal cycling.
Cycling number
1
50
100
Thermal properties of the PEG/EG materials
Tm (°C)
DHm (J/g)
Tc (°C)
DHc (J/g)
61.46
60.55
61.13
161.2
165.3
164.7
46.91
45.39
46.77
146.9
148.0
144.5
Tm = melting temperature; Tc = crystallization temperature.
DHm = heat latent of fusion; DHc = heat latent of crystallization.
mance after 1, 50, and 100 times thermal cycling. Table 2 shows
thermal properties after thermal cycling, respectively. After repeated 1, 50, 100 times of thermal cycling, the melting temperatures of composite PCM changed by 0.91 and 0.33 °C, and the
crystallization temperature changed by 1.52 and 0.14 °C, respectively. The results showed the temperature had a little change,
which were not significant for LES-based applications. It also could
be seen that the form-stable composite PCM had good thermal reliability in terms of the latent heat value. The latent heat of melting
changed by 2.5% and 2.2%, while the latent heat of crystallization
changed by 0.7% and 1.6%, respectively. These changes were negligible for LES-based applications.
4. Conclusions
The composite made by polyethylene glycol blended with
expandable graphite was a novel form-stable phase change material. PEG and EG were chosen as phase change material and supporting material, respectively. Because of the effects of capillary
force and surface tension, there was no leakage of liquid PEG from
the porous of EG network. The solid–solid transition properties of
these composites were sensitive to the proportion of components.
The maximum mass percentage for PEG dispersed in the PCM composites without any leakage of the melted PEG was found as high
as 90 wt%. At this percentage, PEG (90 wt%)/EG composite had a
large enthalpy of 161.2 J g 1 and suitable melting temperature(Tm = 61.46 °C). Meanwhile, the composite conductivity was
1.324 W m 1 K 1. The value of thermal conductivity was increased
much because of the thermal conductive network formed by the
pore structure of EG.
As above-mentioned, PEG/EG composite PCMs are promising
candidates with large latent heat of fusion and suitable melting
point as well as high thermal conductivity compared with most organic-based PCMs.
Acknowledgements
The research was supported by National Science Foundation of
China (No. 90610023). I am also greatly thankful to China scholarship council for supporting my doctoral study in Sweden.
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