1. No category

Solar Energy Materials & Solar Cells 139 (2015) 65–70
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells
journal homepage: www.elsevier.com/locate/solmat
Energy efficient thermal storage montmorillonite with phase change
material containing exfoliated graphite nanoplatelets
Su-Gwang Jeong, Seong Jin Chang, Seunghwan We, Sumin Kim n
Building Environment & Materials Lab, School of Architecture, Soongsil University, Seoul 156-743, Republic of Korea
art ic l e i nf o
a b s t r a c t
Article history:
Received 15 September 2014
Received in revised form
7 February 2015
Accepted 11 March 2015
In this experiment, we used a vacuum impregnation method to prepare shape stabilized PCM that
contained sodium montmorillonite (Na-MMT) and Exfoliated graphite nanoplatelets (xGnP), to improve
the thermal conductivity of PCMs, and prevent leakage of the liquid state of PCMs. Na-MMT has low cost
and natural abundance, high adsorption and absorption capacities, and fire retardant heating rate. In the
used materials, xGnP, usually produced from graphite intercalated compounds, are particles consisting of
several layers of graphene sheets. As a result, we found that the FTIR adsorption spectra of paraffinic
PCMs did not change, and there was no chemical interaction between paraffinic PCMs and xGnP/NaMMT mixture. From the DSC analysis, xGnP made an impact on the thermal properties of the paraffinic
PCMs/Na-MMT composites. The oxidation rate of paraffinic PCMs based composite with xGnP was
greater than that of the composite without xGnP. FTIR, DSC, TGA and TCi were used to determine the
characteristics of the paraffinic PCMs/Na-MMT composites.
& 2015 Elsevier B.V. All rights reserved.
Keywords:
Paraffinic PCMs
Sodium montmorillonite
xGnP
Heat storage
Thermal properties
Vacuum impregnation
1. Introduction
Recently, thermal energy storage (TES) systems have been used
to reduce energy use in buildings, to contribute to more efficient
environmental energy use, and to supply thermal comport for
occupants. The main advantage of using a thermal storage system
is that it can contribute to matching supply and demand, when
they do not coincide in time [1]. The best known method of TES in
buildings involves sensible heat storage, by changing the temperature of a storage material. This can be used for the storage and
release of thermal energy in a passive way, but in comparison with
latent heat storage, by changing the phase of a storage material, a
much larger volume of material is required to store the same
amount of energy. Therefore, an effective way to reduce the energy
consumption of buildings for heating and cooling is by incorporating phase change materials (PCMs) in passive latent heat thermal energy storage (LHTES) systems of building walls, windows,
ceilings, or floors [2]. Latent heat storage technology has been
widely used in building. The storage and application of heat are
achieved through phase change materials. This has the advantages
of high heat storage density, and keeping the temperature stable
during the heat storage/release process. The application of PCM in
building can not only save energy, but also decrease the temperature fluctuation. The application of PCM in building has been
n
Corresponding author. Tel.: þ 82 2 820 0665; fax: þ 82 2 816 3354.
E-mail address: skim@ssu.ac.kr (S. Kim).
http://dx.doi.org/10.1016/j.solmat.2015.03.010
0927-0248/& 2015 Elsevier B.V. All rights reserved.
one of the hot topics in latent heat storage technology [3–6].
Unlike conventional sensible storage materials, PCMs absorb and
release heat at a nearly constant temperature. They store 5–14
times more heat per unit volume than sensible storage materials,
such as water, masonry, or rock. A large number of PCMs are
known to melt with a heat of fusion in any required range. The
PCM to be used in the design of thermal storage systems should
possess desirable thermophysical, kinetic, and chemical properties, which are as follows [7]. They also play an important role in
solving energy imbalance, by improving thermal efficiency, and
protecting the environment [8,9]. PCMs can be categorized into
two major groups: inorganic compounds and organic compounds.
Inorganic PCMs, such as salt hydrates, salts, metals, and alloys,
generally have high volumetric latent heat storage capacity [7],
which is almost twice as much as that of organic PCMs. But their
utility is often limited by incongruent melting and supercoiling
effects. Also, organic PCMs are classified into paraffinic PCMs, and
non-paraffinic PCMs, such as fatty acids [10]. Paraffinic PCMs are
considered as one of the most promising candidates, due to their
large latent heat, low vapor pressure in the melt, good chemical
stability, self-nucleating behavior and safety [11–13]. However,
paraffinic PCMs have flowability during the phase change process,
as a solid–liquid PCM. Therefore, it is necessary to prepare formstable paraffinic PCM with a micro-capsulation methods or
incorporation methods or shape stabilized process [14–17]. Also,
paraffinic PCMs suffer from low thermal conductivity and liquid
leakage, when they undergo the solid–liquid phase change. These
drawbacks reduce the rate of heat storage and extraction during
66
S.-G. Jeong et al. / Solar Energy Materials & Solar Cells 139 (2015) 65–70
the melting and solidification cycles, and restrict their wide
application. To overcome the problem related to the low thermal
conductivity of paraffinic PCMs, a great many efforts have been
made, which include dispersing metallic or nonmetallic particles
with high thermal conductivity into PCMs [18,19], using finned
tubes with different configurations in a storage unit [20], and
impregnating PCM into high thermal conductivity material with
porous structure [21–24], such as carbon materials and metal
foams. Compared with carbon materials, metal foams, additives or
fins not only significantly increase the weight and cost of the
storage systems, but for some of them are also incompatible with
PCMs. In previous work, we studied the effect of exfoliated graphite nanoplatelets addition on the thermal properties of the
paraffin wax/xGnP composite prepared as a form-stable PCM, and
reported that the thermal conductivity of PCM increased with
increasing graphite mass fraction [25]. Therefore, we prepared
PCM/montmorillonite composite with xGnP, to improve the thermal conductivity for energy saving. This study also aims to
investigate the effect of the xGnP addition on the dispersibility,
thermal conductivity, and latent heat capacity of form stable PCM
[26,27]. Also, to solve the leakage problem of PCMs, some investigators have studied the possibility of a container that can prevent
the leakage of liquid PCMs, by using shape-stabilized PCM
(SSPCM), microencapsulated PCM (MPCM), and incorporated PCM
techniques [28–31]. Actually, paraffinic PCMs should be incorporated into porous materials, such as gypsum wallboard, plaster,
concrete, clay minerals, and others [32]. Clay minerals have been
used for years in many applications, such as nanocomposites [33],
catalysts [34], adsorbents for removal of hazardous compounds
[35], and supports for highly fluorescent probes [36]. The demand
for clays in diverse scientific and technological areas lies in their
low cost and natural abundance, high adsorption and absorption
capacities, and fire retardancy, among other properties. Among
clay minerals, the most common smectite clay mineral is montmorillonite (MMT). MMT evolves from volcanic ashes by weathering or hydrothermal effects, like other aluminum-rich minerals,
and composes the highest part of the volcanic ash clay termed
bentonite. The terminology of the word ‘bentonite’ can be summarized as follows: the rock term bentonite, which is commonly
used for the smectite group minerals (sodium montmorillonite
(Na), calcium montmorillonite, saponite (Mg), nontronite (Fe), and
hectorite (Li)), is a clay material, altered from a glassy igneous
material, usually volcanic ash. Therefore, this paper uses sodium
montmorillonite (Na-MMT) as a container of PCM [37]. We prepared thermal enhanced paraffinic PCM/Na-MMT composite by
compounding xGnP, to improve the thermal conductivity of the
PCMs. They have a very high aspect ratio, comparable to that of
carbon nanotubes. Drzal et al. successfully developed a microwave
exfoliation and ultrasonic grinding process, to prepare exfoliated
graphite nanoplatelets of different sizes and surface areas. These
particles have been incorporated into different thermoplastic and
thermoset materials, and PCMs, to improve the electrical, thermal
and mechanical properties of nanocomposites [38,39]. In this
study, we prepared thermal enhanced paraffinic PCM/Na-MMT, by
using a vacuum impregnation process with xGnP. The vacuum
impregnation method guarantees the high heat storage of paraffinic PCMs, due to capillary forces and surface tension forces
during the incorporation process. The Na-MMT is needed for
applying grouting materials of ground heat exchange system
because the Na-MMT is used to grouting materials, originally. So
we developed the ground heat energy storage composite through
mixture of Na-MMT and paraffinic PCMs. And we enhanced thermal conductivity of Na-MMT/paraffinic PCM composites by mixing
xGnP. And the thermal conductivity enhanced composite brought
preventing super-cooling phenomenon of paraffinic PCMs. In this
paper, we analyzed the microstructure, chemical bonding, heat
capacity, thermal resistance and thermal conductivity of thermal
enhanced paraffinic PCM/Na-MMT composites, from the results of
Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and TCi
thermal conductivity analyses.
2. Experimental
2.1. Materials
This study used two types of liquid paraffinic PCMs, with different melting points. In the experiment, we used n-hexadecane
and n-octadecane as PCM, which have 254.7 J/g and 247.6 J/g of
latent heat capacity, and melting points of 20.84 °C and 30.4 °C,
respectively. The n-hexadecane and n-octadecane are made of the
alkane series, and belong to the paraffinic PCMs. The paraffinic
PCMs were obtained from Celsius Korea, South Korea. xGnP is a
graphitic carbon-based material. The graphite was obtained from
Asbury Graphite Mills, Inc., NJ, USA, by applying a cost- and timeeffective exfoliation process initially proposed by Drzal's group
[10]. xGnP, which combines the layered structure and low price of
nanoclays with the superior mechanical, electrical and thermal
properties of carbon nanotubes, is very cost effective, and can
simultaneously provide a multitude of physical and chemical
property enhancements [11–13]. Table 1 shows the physical
properties of xGnP. The used Na-MMT has 8–12% free moisture,
8-14cc/2 g swelling, 0.75–0.85 g/cm3 loose bulk density and more
than 80% particle size (200 mesh pass). This Na-MMT was
obtained from Ilsung chemical Co., Ltd. South Korea.
2.2. Preparation
The paraffinic PCMs/Na-MMT with xGnP or without xGnP
composites were prepared by an impregnation method in a
vacuum, following the manufacturing process. The preparation of
paraffinic PCMs based composites with xGnP and without xGnP is
almost the same. So we only describe the preparation process of
paraffinic PCMs based composites with xGnP. The detailed preparation process is as follows: 5 wt% of xGnP according to weight
percentage of Na-MMT was mixed in Na-MMT, before the vacuum
process. The xGnP/Na-MMT mixture was placed inside a filtering
flask, which was connected to a water tromp apparatus, to evacuate
air from its porous surface. Then, the valve between the flask and
the container of 200 g of liquid paraffinic PCMs was turned open, to
allow it to flow into the flask, to cover the xGnP/Na-MMT mixture.
After the vacuum process was continued for 90 min, air was
allowed to enter the flask again, to force the liquid paraffinic PCMs
to penetrate into the pore space of the xGnP/Na-MMT mixture. To
find the maximum PCM amount in the xGnP/Na-MMT mixture, we
impregnated 200 g of paraffinic PCMs into xGnP/Na-MMT mixture.
In this case, no impregnated excess of paraffinic PCMs remained in
the flask. Therefore, we needed to remove the excess of paraffinic
PCMs in the flask through filtering. The colloid state of SSPCM was
filtered by 1 μm filter paper, until a granule type of sample
appeared on the filter paper. Then the granule type of thermal
enhanced SSPCM was dried in a vacuum drier, at 80 °C for 48 h.
Table 1
Physical properties of xGnP.
Surface area (m2/g)
Bulk density (g/m3)
Pore volume (cm3/g)
Thermal conductivity (W/m K)
Specific heat capacity (J/kg K)
20.41
0.0053–0.010
0.081
2–300
710
S.-G. Jeong et al. / Solar Energy Materials & Solar Cells 139 (2015) 65–70
67
2.3. Characterization techniques
Fourier transform infrared spectroscopy (FTIR: 300E Jasco) was
utilized to monitor the change of chemical groups upon curing.
Clear potassium bromide (KBr) disks were molded from powder,
and used as backgrounds. The samples were analyzed over the
range of 525–4000 cm 1, with a spectrum resolution of 4 cm 1.
All spectra were averaged over 32 scans. This analysis of the
composites was performed by point-to-point contact with a
pressure device. Thermal properties of thermal enhanced paraffinic PCMs/Na-MMT, such as the melting and freezing temperature
and latent heat capacity, were measured using differential scanning calorimetry (DSC: Q1000). The melting and freezing temperatures were measured by drawing a line at the point of maximum slope of the leading edge of the peak, and extrapolating to
the base line. DSC measurements were performed at a 5 °C/min
heating and cooling rate, and a temperature range of 0–80 °C and
80–0 °C. The latent heat capacity of the thermal enhanced paraffinic PCMs/Na-MMT was determined by numerical integration of
the area under the peaks that represent the solid–solid and solid–
liquid phase transitions [27]. The enthalpy value of paraffinic PCM
based composites was obtained from a universal analysis program
and cp-calculation program, which converts the heat flow of DSC
data to specific heat value. The final enthalpy value was obtained
from the sum of specific heat value up to 80 °C. Thermal durability
of the thermal enhanced paraffinic PCMs/Na-MMT was carried out
using thermogravimetric analysis (TGA: TA Instruments, TGA
Q5000) on approximately 2–4 mg samples, over the temperature
range 25–600 °C, at a heating rate of 10 °C/min, under a nitrogen
flow of 20 ml/min. TGA was measured with the composites placed
in a high quality nitrogen (99.5% nitrogen, 0.5% oxygen content)
atmosphere, to prevent unwanted oxidation [40]. The thermal
conductivity of thermal enhanced paraffinic PCMs/Na-MMT was
measured using a TCi thermal conductivity analyzer. The TCi
developed by C-Therm Technologies Ltd. is a device for conveniently measuring the thermal conductivity of a small sample,
by using the Modified Transient Plane Source (MTPS) method [41].
3. Results and discussion
3.1. FTIR analysis and chemical stability test of n-octadecane based
composite PCMs
Fig. 1 shows the FTIR absorption spectra of the n-hexadecane/
Na-MMT and n-octadecane/Na-MMT composites, and n-hexadecane/Na-MMT and n-octadecane/Na-MMT composites with
xGnP. The molecular formulae of n-hexadecane and n-octadecane
are C16H34 and C18H38, respectively. Therefore, these PCMs contained –CH2 and –CH3 bonding. Because all the PCMs have the
same group bonding, the FTIR absorption spectra shown are
similar. These FTIR absorption spectra were almost the same, with
absorption peaks of 2918, 2850, 1468, and 720 cm 1, caused by
stretching vibration of functional groups of –CH2 and –CH3
bonding. These FTIR peaks are shown in Table 2. Na-MMT is
composed of more than 60% silica, of which the molecular formula
is SiO2. So, the silica peak showed in the FTIR graph. The most
intense band at 1085 cm 1 is due to asymmetric stretching of the
Si–O–Si bonding. This band usually appears between 1200 and
1000 cm 1. These stretching vibration bands have appeared after
the compositing process with PCMs. This means that the chemical
properties of silica fume were not changed. Of course, other peaks
also appeared in the FTIR graphs, but these peaks did not indicate
the characteristics of the n-hexadecane and n-octadecane. In this
experiment, we impregnated the paraffinic PCMs into the 5 wt% of
xGnP/Na-MMT mixture for shape stabilization and thermal
Fig. 1. FTIR spectra of paraffinic PCM based composites.
Table 2
FTIR spectra of paraffinic PCM based composites.
Vibration
Wave number range (cm 1)
C–H3 Asymmetric stretch
C–H3 Symmetric stretch
C–H2 Asymmetric stretch
C–H2 Symmetric stretch
C–H3 Umbrella bending mode
C–H2 Rocking vibration
Si–O–Si Asymmetric stretch
2962 7 10
28727 10
2926 7 10
2855 7 10
13777 10
720 7 10
1200–1000
enhancement of the paraffinic PCMs. In previous works, we confirmed from FTIR analysis that the xGnP has no strong peak [28].
Therefore, we found that the FTIR peaks of thermal enhanced
paraffinic PCMs/Na-MMT composites have almost the same peaks.
However, in the case of the paraffinic PCMs/Na-MMT composites
that contained the xGnP, these composites showed low value of
the FTIR peaks, as loaded with xGnP. We confirmed that the FTIR
peaks that indicated the characteristics of paraffinic PCMs did not
shift in the FTIR graphs. This means the characteristics of paraffinic
PCMs did not change. Therefore, this indicates that the heat storage properties of PCM remained after the compositing process. As
a result, in this experiment, we found that the FTIR adsorption
spectra of paraffinic PCMs did not change, and there was no chemical interaction between paraffinic PCMs and xGnP/Na-MMT
mixture. In other words, the paraffinic PCMs molecules were
retained easily in the pores of xGnP and Na-MMT by these physical
interactions, such as capillary and surface tension forces, and
leakage of the melted PCMs from the composite was prevented by
these forces. Consequently, we determined that the heat storage
characteristics of paraffinic PCMs could integrate into the structure
of xGnP and Na-MMT due to their physical bonding, without
changing their chemical properties.
3.2. Thermal properties analysis
Fig. 2 presents the heating and freezing curves from DSC
measurements of the paraffinic PCMs/Na-MMT composites and
the composites with xGnP. In the graph, the phase transition of the
n-hexadecane/Na-MMT composite occurred between 15 °C and
23 °C during heating, and the corresponding heat capacity was
61.36 J/g. The solidification temperature range of this composite
was lowered to between 10 °C and 15 °C, compared to the melting
temperature. In the case of the n-octadecane based composite, this
68
S.-G. Jeong et al. / Solar Energy Materials & Solar Cells 139 (2015) 65–70
composite showed melting temperature range from 20 °C to 35 °C,
and freezing temperature range from 17 °C to 25 °C, and the corresponding heat capacity was 56.58 J/g. The value for latent heat
capacity of each of the paraffinic PCMs based composite PCMs was
nearly 24% that of the pure paraffinic PCMs. This incorporated rate
is smaller than the general incorporated rate of other PCMs, which
have nearly 50% of the incorporated rate. Also we analyzed thermal properties of the paraffinic PCMs/Na-MMT with xGnP composites. Table 3 shows the phase transition temperature and latent
heat capacity of paraffinic PCMs/Na-MMT composites and thermal
enhanced composite PCMs that contained the xGnP. From the DSC
analysis, we confirmed that the paraffinic PCMs/Na-MMT composites that contained the xGnP have similar phase change
temperature range, compared to the composite PCMs without the
xGnP. The n-hexadecane/Na-MMT composite with xGnP and
n-octadecane/Na-MMT composite with xGnP have 68.95 J/g and
80.16 J/g of latent heat capacity, respectively. However, from the
latent heat capacity analysis, the paraffinic PCMs/Na-MMT composites with xGnP showed high latent heat capacity, in comparison with the prepared composite without xGnP. This means that
the xGnP is more porous than Na-MMT powder, so more PCM
could incorporate into the porous structure of xGnP. From this
result, we confirmed that the xGnP makes an impact on the
thermal properties of the paraffinic PCMs/Na-MMT composites. As
a result, we confirmed that thermal enhanced composites with
xGnP have high latent heat capacity and high thermal efficiency.
Therefore, we expect that this high latent heat capacity of thermal
enhanced paraffinic PCMs based composites will have important
potential for heating and cooling applications in various fields.
3.3. Enthalpy analysis
We analyzed the enthalpy of the paraffinic PCMs/Na-MMT composites. Fig. 3 shows its graph, which confirms that the enthalpy
slope is steep at the phase change range of each paraffinic PCMs
based composite, because of the effect of the latent heat capacity of
paraffinic PCMs in the composites. As a result, the enthalpy value of
paraffinic PCMs based composites with xGnP or without xGnP at
60 °C shows the range from 849.44 J/g to 891.01 J/g. In this experiment, we confirmed that the n-hexadecane/Na-MMT composite and
its composite with xGnP showed 849.44 and 878.82 J/g of enthalpy
value, respectively. And n-octadecane/Na-MMT composite and its
composite with xGnP showed 863.38 and 891.01 J/g of enthalpy
value, respectively. This result shows that all the samples are almost
the same, regardless of the xGnP. However we confirmed that the
paraffinic based composite with xGnP shows higher enthalpy value
compared with paraffinic based composite without xGnP. Also, we
confirmed that paraffinic PCMs based composite with xGnP shows a
steep slope at each phase change range, compared to the others. This
means that paraffinic PCMs based composites with xGnP have high
thermal conductivity. Therefore, we determined that the thermal
efficiency of paraffinic PCMs based composites with xGnP is higher
than that of others, because of the high latent heat capacity in the
phase change range of paraffinic PCMs.
3.4. Thermogravimetric analysis
Fig. 2. DSC graph of (a) paraffinic PCM based composite PCM and (b) paraffinic
PCM based composites with xGnP.
Fig. 4 and Table 4 show thermal gravimetric analysis of the paraffinic PCMs based composites and the composite with xGnP. In
previous research, we found that the n-hexadecane and n-octadecane
have one curve of thermal oxidation degradation. As shown in the
derivative weight curve of paraffinic PCMs, we found that the thermal
oxidation degradation peak of the n-hexadecane and n-octadecane
based composites occurred at 178.78 °C and 183.87 °C, respectively.
The n-hexadecane based composite with xGnP and the n-octadecane
based composite with xGnP show 171.03 °C and 188.71 °C of peak
of derivative weight, respectively. Also, we measured the thermal
Table 3
Heat storage properties of paraffinic PCM based composites.
PCM samples
n-Hexadecane þ Na-MMT composite
n-Hexadecane þ Na-MMTþ xGnP composite
n-Octadecane þ Na-MMT composite
n-Octadecane þ Na-MMTþxGnP composite
Melting point (°C)
18.65
20.96
31.47
30.31
Freezing point (°C)
14.56
12.13
22.13
22.80
Latent heat (J/g)
Incorporation rate (%)
Solid–liquid melting
Liquid–solid freezing
61.36
68.95
56.58
80.16
59.33
65.85
54.10
78.78
23.29
26.60
21.24
31.82
S.-G. Jeong et al. / Solar Energy Materials & Solar Cells 139 (2015) 65–70
69
Table 4
Thermogravimetric analysis of paraffinic PCM based composites.
PCM samples
Peak of derivative
weight (°C)
Oxidation rate
(%)
n-Hexadecane þ Na-MMT composite
n-Octadecane þ Na-MMT composite
n-Hexadecane þ Na-MMTþxGnP
composite
n-Octadecane þ Na-MMTþxGnP
composite
178.78
183.87
171.03
68.51
67.64
62.94
188.71
59.33
Fig. 3. Enthalpy analysis of paraffinic PCM based composites.
Fig. 5. Thermal conductivity of paraffinic PCM based composites.
were well incorporated into the nanostructure of xGnP. For this reason, the paraffinic PCM/Na-MMT composites with xGnP show a little
low thermal durability properties at 600 °C compared to the paraffinic PCM/Na-MMT composites without xGnP. However, its small
difference of oxidation rate does not affect the total thermal durability
of paraffinic PCM based composites. Therefore we determined that
the prepared composite with xGnP which has non-flammability
properties has high thermal durability.
3.5. Thermal conductivity analysis
Fig. 4. Thermogravimetric analysis of (a) n-hexadecane based composites
and (b) n-octadecane based composites.
oxidation degradation of paraffinic PCMs based composites and the
composites with xGnP. From the thermal degradation analysis, we
confirmed that the oxidation rates of n-hexadecane/Na-MMT and
n-hexadecane/Na-MMT with xGnP show 68.51% and 62.94%, respectively. n-Octadecane/Na-MMT and n-octadecane/Na-MMT with xGnP
show 67.64% and 59.33%, respectively. As a result, the oxidation rate of
paraffinic PCMs based composite with xGnP is greater than that of the
composite without xGnP. This means that xGnP has a more porous
structure, compared to the Na-MMT. Therefore more paraffinic PCMs
Fig. 5 shows the thermal conductivity analysis of the n-hexadecane and n-octadecane based composite PCMs. The analysis
indicated the thermal conductivity of n-hexadecane and n-octadecane based composite PCMs to be 0.484 and 0.833 W/m K,
respectively. n-Hexadecane and n-octadecane show 0.154 and
0.557 W/m K, respectively. Also, we measured the thermal analysis
of the thermal enhanced composite PCMs with xGnP. As a result,
the n-hexadecane based thermal enhanced composite PCM shows
1.113 W/m K thermal conductivity value. It has a 230% increase of
thermal conductivity, compared to that of the n-hexadecane based
composite PCM. The n-octadecane based thermal enhanced composite PCM shows 2.119 W/m K of thermal conductivity value. It
has a 254% increase of thermal conductivity, compared to that of
the n-octadecane based composite PCM. This means xGnP led to
an enhancement of the thermal conductivity of composite PCMs.
Through the thermal conductivity analysis, we confirmed that
thermal enhanced paraffinic PCMs are more useful than composite
PCMs without xGnP, for application in various fields.
4. Conclusion
In this experiment, we used a vacuum impregnation method to
prepare shape stabilized PCM that contained sodium montmorillonite
70
S.-G. Jeong et al. / Solar Energy Materials & Solar Cells 139 (2015) 65–70
and xGnP, to improve the thermal conductivity of PCMs, and prevent
leakage of the liquid state of PCMs. As a result, we found that the FTIR
adsorption spectra of paraffinic PCMs did not change, and there was
no chemical interaction between paraffinic PCMs and xGnP/Na-MMT
mixture. From the DSC analysis, we confirmed that the values for
latent heat capacity of each paraffinic PCMs based composite PCMs
were nearly 24% of the pure paraffinic PCMs. The n-hexadecane/NaMMT composite with xGnP and n-octadecane/Na-MMT composite
with xGnP have 68.95 J/g and 80.16 J/g of latent heat capacity,
respectively. Also, we determined that thermal enhanced composites
with xGnP have high latent heat capacity and high thermal efficiency.
In the enthalpy analysis, we determined that the thermal efficiency of
paraffinic PCMs based composites with xGnP is higher than that of
others, because of the high latent heat capacity in the phase change
range of paraffinic PCMs. The oxidation rate and thermal conductivity
of paraffinic PCMs based composite with xGnP are higher than those
of the composite without xGnP. Also, we determined that the thermal
efficiency of paraffinic PCMs based composites with xGnP is higher
than that of the others, because of the high latent heat capacity in the
phase change range of paraffinic PCMs. Consequently, we expect the
paraffinic PCMs based composites with xGnP to be useful in applications in various fields, due to their high thermal properties.
Acknowledgment
This work was supported by the National Research Foundation
of Korea (NRF) grant funded by the Korea government (MSIP) (No.
NRF-2014R1A2A1A11053829).
References
[1] H. Mehling, L.F. Cabeza, Heat and Cold Storage with PCM: An Up to Date
Introduction into Basics and Applications, Springer, 2008.
[2] N. Soares, J.J. Costa, A.R. Gaspar, P. Santos, Review of passive PCM latent heat
thermal energy storage system towards buildings' energy efficiency,
Energy Build. 59 (2013) 82–103.
[3] F. Kuznik, D. David, K. Johannes, J.J. Roux, A review on phase change materials
integrated in building walls, Renew. Sustain. Energy Rev. 15 (2011) 379–391.
[4] V.V. Tyagi, S.C. Kaushik, S.K. Tyagi, T. Akiyama, Development of phase change
materials based microencapsulated technology for buildings: a review, Renew.
Sustain. Energy Rev. 15 (2011) 1373–1391.
[5] D. Zhou, C.Y. Zhao, Y. Tian, Review on thermal energy storage with phase
change materials (PCMs) in building applications, Appl. Energy 92 (2009)
593–605.
[6] M. Li, Z. Wu, J. Tan, Heat storage properties of the cement mortar incorporated
with composite phase change material, Appl. Energy 103 (2013) 393–399.
[7] A. Sharma, V.V. Tyagi, C.R. Chen, D. Buddhi, Review on thermal energy storage
with phase change materials and applications, Renew. Sustain. Energy Rev. 13
(2009) 318–345.
[8] X. Wang, Y.P. Zhang, W. Xiao, R.L. Zeng, Q.L. Zhang, H.F. Di, Review on thermal
performance of phase change energy storage building envelope, Chin. Sci. Bull.
54 (2009) 920–928.
[9] W. Cheng, R. Zhang, K. Xie, N. Liu, J. Wang, Heat conduction enhanced shapestabilized paraffin/HDPE composite PCMs by graphite addition: preparation
and thermal properties, Sol. Energy Mater. Sol. Cells 94 (2010) 1636–1642.
[10] A. Karaipekli, A. Sarı, K. Kaygusuz, Thermal conductivity improvement of
stearic acid using expanded graphite and carbon fiber for energy storage
applications, Renew. Energy 32 (2007) 2201–2210.
[11] Z. Zhang, X. Fang, Study on paraffin/expanded graphite composite phase
change thermal energy storage material, Energy Convers. Manag. 47 (2006)
303–310.
[12] J.N. Shi, M.D. Ger, Y.M. Liu, Y.C. Fan, N.T. Wen, C.K. Lin, N.W. Pu, Improving the
thermal conductivity and shape-stabilization of phase change materials using
nanographite additives, Carbon 51 (2013) 365–372.
[13] S.G. Jeong, O. Chung, S. Yu, S. Kim, S. Kim, Improvement of the thermal
properties of bio-based PCM using exfoliated graphite nanoplatelets, Sol.
Energy Mater. Sol. Cells 117 (2013) 87–92.
[14] C.C. Chang, Y.L. Tsai, J.J. Chiu, H. Chen, Preparation of phase change materials
microcapsules by using PMMA networks-silica hybrid shell via sol–gel process, J. Appl. Polym. Sci. 112 (2009) 1850–1857.
[15] G. Fang, H. Li, S. Wu, Experimental investigation of performances of microcapsule phase change material for thermal energy storage, Chem. Eng. Technol. 33 (2010) 227–230.
[16] A. Sarı, A. Karaipekli, Thermal conductivity and latent heat thermal energy
storage characteristics of paraffin/expanded graphite composite as phase
change material, Appl. Therm. Eng. 27 (2007) 1271–1277.
[17] M. Li, Z. Wu, H. Kao, J. Tan, Experimental investigation of preparation and
thermal performances of paraffin/bentonite composite phase change material,
Energy Convers. Manag. 52 (2011) 3275–3281.
[18] J.F. Wang, H.Q. Xie, Z. Xin, Y. Li, L.F. Chen, Enhancing thermal conductivity of
palmitic acid based phase change materials with carbon nanotubes as fillers,
Sol. Energy 84 (2010) 339–344.
[19] J.F. Wang, H.Q. Xie, Z. Xin, Thermal properties of paraffin based composites
containing multi-walled carbon nanotubes, Thermochim. Acta 488 (2009)
39–42.
[20] F. Agyenim, P. Eames, M. Smyth, A comparison of heat transfer enhancement
in a medium temperature thermal energy storage heat exchanger using fins,
Sol. Energy 83 (2009) 1509–1520.
[21] Y.J. Zhong, Q.G. Guo, S.Z. Li, J.L. Shi, L. Liu, Heat transfer enhancement of paraffin wax using graphite foam for thermal energy storage, Sol. Energy Mater.
Sol. Cells 94 (2010) 1011–1014.
[22] K. Lafdi, O. Mesalhy, A. Elgafy, Graphite foams infiltrated with phase change
materials as alternative materials for space and terrestrial thermal energy
storage applications, Carbon 46 (2008) 159–168.
[23] K. Lafdi, O. Mesalhy, S. Shaikh, Experimental study on the influence of foam
porosity and pore size on the melting of phase change materials, J. Appl. Phys.
102 (2007) 083549.
[24] Z. Zhang, N. Zhang, J. Peng, X. Fang, X. Gao, Y. Fang, Preparation and thermal
energy storage properties of paraffin/expanded graphite composite phase
change material, Appl. Energy 91 (2012) 426–431.
[25] S. Kim, L.T. Drzal, High latent heat storage and high thermal conductive phase
change materials using exfoliated graphite nanoplatelets, Sol. Energy Mater.
Sol. Cells 93 (2009) 136–142.
[26] J. Jeon, S.G. Jeong, J.H. Lee, J. Seo, S. Kim, High thermal performance composite
PCMs loading xGnP for application to building using radiant floor heating
system, Sol. Energy Mater. Sol. Cells 101 (2012) 51–56.
[27] S.G. Jeong, J. Jeon, O. Chung, S. Kim, S. Kim, Evaluation of PCM/diatomite
composites using exfoliated graphite nanoplatelets (xGnP) to improve thermal
properties, J. Therm. Anal. Calorim. 114 (2013) 689–698.
[28] X. Wang, J. Niu, A.H.C. van Paassen, Raising evaporative cooling potentials
using combined cooled ceiling and MPCM slurry storage, Energy Build. 40
(2008) 1691–1698.
[29] B.M. Diaconu, Transient thermal response of a PCS heat storage system,
Energy Build. 41 (2009) 212–219.
[30] Y. Wang, T.D. Xia, H. Zheng, H.X. Feng, Stearic acid/silica fume composite as
form-stable phase change material for thermal energy storage, Energy Build.
43 (2011) 2365–2370.
[31] A. Karaipekli, A. Sarı, Capric–myristic acid/expanded perlite composite as
form-stable phase change material for latent heat thermal energy storage,
Renew. Energy 33 (2008) 2599–2605.
[32] S.S. Ray, M. Okamoto, Polymer/layered silicate nanocomposites: a review from
preparation to processing, Prog. Polym. Sci. 28 (2003) 1539–1641.
[33] G. Nagendrappa, Organic synthesis using clay and clay-supported catalysts,
Appl. Clay Sci. 53 (2011) 106–138.
[34] A. Dąbrowski, Adsorption – from theory to practice, Adv. Colloid Interface Sci.
93 (2001) 135–224.
[35] M. Danko, M. Mičušik, M. Omastová, J. Bujdák, D. Chorvát, Spectral characterisation of new organic fluorescent dyes with an alkoxysilane moiety and
their utilisation for the labelling of layered silicates, Chem. Pap. 67 (2013)
18–28.
[36] C. Bilgiç, D.T. Yazıcı, N. Karakehya, H. Çetinkaya, A. Singh, M.M. Chehimi,
Surface and interface physicochemical aspects of intercalated organo-bentonite, Int. J. Adhes. Adhes. 50 (2014) 204–210.
[37] K. Kalaitzidou, H. Fukushima, L.T. Drzal, Multifunctional polypropylene composites produced by incorporation of exfoliated graphite nanoplatelets, Carbon 45 (2007) 1446–1452.
[38] J. Xiang, L.T. Drzal, Investigation of exfoliated graphite nanoplatelets (xGnP) in
improving thermal conductivity of paraffin wax-based phase change material,
Sol. Energy Mater. Sol. Cells 95 (2011) 1811–1818.
[39] S.G. Jeong, J. Jeon, J. Seo, J.H. Lee, S. Kim, Performance evaluation of the
Microencapsulated PCM for wood-based flooring application, Energy Convers.
Manag. 64 (2012) 516–521.
[40] B. Lee, H. Kim, H. Yang, Polymerization of aniline on bacterial cellulose and
characterization of bacterial cellulose/polyaniline nanocomposite films, Curr.
Appl. Phys. 12 (2012) 75–80.
[41] J. Cha, J. Seo, S. Kim, Building materials thermal conductivity measurement
and correlation with heat flow meter, laser flash analysis and TCi, J. Therm.
Anal. Calorim. 109 (2012) 295–300.