materials
Article
Design and Preparation of Carbon Based Composite
Phase Change Material for Energy Piles
Haibin Yang 1 , Shazim Ali Memon 2 , Xiaohua Bao 1, *, Hongzhi Cui 1 and Dongxu Li 1
1
2
*
College of Civil Engineering, Shenzhen University, Shenzhen 518060, China;
[email protected] (H.Y.); [email protected] (H.C.); [email protected] (D.L.)
Department of Civil Engineering, School of Engineering, Nazarbayev University, Astana 010000,
Kazakhstan; [email protected]
Correspondence: [email protected]; Tel.: +86-755-8667-0364
Academic Editor: Thomas Fiedler
Received: 26 January 2017; Accepted: 5 April 2017; Published: 7 April 2017
Abstract: Energy piles—A fairly new renewable energy concept—Use a ground heat exchanger
(GHE) in the foundation piles to supply heating and cooling loads to the supported building.
Applying phase change materials (PCMs) to piles can help in maintaining a stable temperature
within the piles and can then influence the axial load acting on the piles. In this study, two kinds
of carbon-based composite PCMs (expanded graphite-based PCM and graphite nanoplatelet-based
PCM) were prepared by vacuum impregnation for potential application in energy piles. Thereafter,
a systematic study was performed and different characterization tests were carried out on two
composite PCMs. The composite PCMs retained up to 93.1% of paraffin and were chemically
compatible, thermally stable and reliable. The latent heat of the composite PCM was up to 152.8 J/g
while the compressive strength of cement paste containing 10 wt % GNP-PCM was found to be
37 MPa. Hence, the developed composite PCM has potential for thermal energy storage applications.
Keywords: composite phase change materials; expanded graphite; graphite nanoplatelet; energy
storage; energy piles
1. Introduction
The rapid increase in global energy consumption has led to serious issues such as depletion of
fossil fuels and degradation of the environment [1,2]. According to statistics, the world’s energy
consumption will grow by 48% between 2012 and 2040 [3]. Hence, energy policy makers and
researchers are paying a lot of attention to the building sector as it is responsible for around 30% of the
total global energy consumption [4,5]. Energy piles—A fairly new renewable energy technique—use
a ground heat exchanger (GHE) in the foundation piles to supply heating and cooling loads to the
supported building. Figure 1 is a schematic drawing of energy piles application in building energy
efficiency. Geothermal energy can sustainably be utilized with a ground-source heat pump, which
takes advantage of the ground as an energy storage system [6,7]. In the energy piles system, the piles
are used to absorb and transport thermal energy from the surrounding ground to buildings via fluid
circulating in pipes placed within the piles. In fact, the thermal cycle can influence the loading of
energy piles. Figure 2 displays the effect of thermal cycles of the energy piles system on pile stresses.
Many studies have been carried out to investigate the geotechnical performance of piled foundations
for ground-source heat-pump systems [8,9]. The main effects of temperature changes on pile behavior
were assessed by the geotechnical numerical analysis method [10]. Applying thermal loads could
induce a significant change in the stress–strain state of piles and the temperature change over the cross
section of the piles should be designed to avoid high stress accumulated in piles [11,12]. Phase change
materials are promising thermal energy shortage candidates that can be used to reduce the possible
Materials 2017, 10, 391; doi:10.3390/ma10040391
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mismatch between thermal energy supply and demand. Applying phase change materials (PCMs) to
the possible mismatch between thermal energy supply and demand. Applying phase change
piles
help
inmismatch
maintaining
stable
temperature
withintemperature
the
and
can
then
influence
the
axial
thecan
possible
thermal
energya supply
and piles
demand.
Applying
phase
change
materials
(PCMs)
to pilesbetween
can ahelp
in maintaining
stable
within
the piles
and can
then
load
acting the
on
the
PCMs,
paraffin
usually
preferred
it the
is generally
believed
materials
(PCMs)
toload
pilesAmong
can help
inthe
maintaining
aisstable
temperature
piles
and can
influence
axialpiles.
acting
on
piles.
Among
PCMs,
paraffinwithin
isasusually
preferred
asthen
it is to
begenerally
chemically
inert,
show
small
volume
changes
during
phase
transition,
innocuous,
influencebelieved
the
axialnon-corrosive,
load
acting
on
the
piles.
Among
PCMs,
paraffin
is
usually
preferred
as
it
is
to be chemically inert, non-corrosive, show small volume changes during phase
generally
believed
to
be
chemically
inert,
non-corrosive,
show
small
volume
changes
during
phase
inexpensive,
and
recyclable.
However,
it
has
low
thermal
conductivity,
which
in
turn,
limits
transition, innocuous, inexpensive, and recyclable. However, it has low thermal conductivity, which its
inexpensive,
and
recyclable.
However,
application
ininnocuous,
thermal
energy
storage
[13].
intransition,
turn, limits
its application
in
thermal
energy
storage
[13]. it has low thermal conductivity, which
in turn, limits its application in thermal energy storage [13].
Figure1.1.Schematic
Schematic drawing of
of energy piles
application
ininbuilding
energy
efficiency
[6].
Figure
pilesapplication
applicationin
building
energy
efficiency
Figure 1. Schematicdrawing
drawing of energy
energy piles
building
energy
efficiency
[6]. [6].
(a)
(a)
(b)
(b)
Figure
piles system
system on
on pile
pile stresses
stresses[6].
[6].(a)
(a)Ground
Groundcooling
cooling
Figure2.2.Effect
Effect of
of thermal
thermal cycles
cycles of
of the
the energy
energy piles
Figure 2. Effect of thermal cycles of the energy piles system on pile stresses [6]. (a) Ground cooling
reduces
which can
can cause
cause tensile
tensile stresses
stressesininthe
thepiles;
piles;
reduces stresses
stresses along
along the
the cross
cross section
section of the piles which
reduces stresses along the cross section of the piles which can cause tensile stresses in the piles;
(b)
section of
of the
thepiles.
piles.
(b)Heating
Heatingcan
cancause
causeincreased
increased stresses
stresses along the cross section
(b) Heating can cause increased stresses along the cross section of the piles.
Expanded
(GNPs) are
aresafe,
safe,environmentally
environmentallyfriendly,
friendly,
Expandedgraphite
graphite(EG)
(EG) and
and graphene
graphene nanoplatelets
nanoplatelets (GNPs)
Expanded
graphite
(EG) andthermal
graphene
nanoplatelets
(GNPs)
are safe,
environmentally
friendly,
have
low
and
conductivity.
These
are preferred
preferred
over
metalmacro-scaled
macro-scaled
have
lowdensity
density
and superior
superior
thermal
These
are
over
metal
have
low
density
and
superior
thermal
conductivity.
These
are
preferred
over
metal
macro-scaled
promoters,
which
have
been
used
to
increase
PCMs
thermal
conductivity
[14].
Sari
and
Karaipekli
[15]
promoters, which have been used to
thermal conductivity [14]. Sari and Karaipekli [15]
preparedwhich
paraffin/expanded
graphite
PCM
and
that
PCM
10
prepared
paraffin/expanded
graphite
composite
and found
found
that composite
composite
PCM
with
10wt
wt%%[15]
promoters,
have been used
to increase
PCMs
thermal
conductivity
[14]. Sari
andwith
Karaipekli
EGisisparaffin/expanded
suitablethermal
thermal energy
energy
storage
With
this
mass
fraction
of
composite,
the
ofofEG
aasuitable
storage
candidate.
With
this
massthat
fraction
ofEG
EGin
in
composite,
the
prepared
graphite
composite
PCM
and
found
composite
PCM
with 10
wt %
thermal
conductivity
improved
bystorage
approximately
273%With
in
pure
etetal.
improved
by
approximately
in comparison
comparison
tofraction
pureparaffin.
paraffin.
Wang
al.[16]
[16]the
of thermal
EG
is a conductivity
suitable
thermal
energy
candidate.
this massto
of EGWang
in
composite,
incorporated
90 wt
wtimproved
% of
of polyethylene
polyethylene
expanded
graphite
that
the
incorporated
90
%
glycol into
expanded
graphite
and
found
thatWang
thethermal
thermal
thermal
conductivity
by approximately
273%
in comparison
toand
purefound
paraffin.
et al. [16]
−1·K−1
−1−1
−1−1
−1
conductivity
of
polyethylene
glycol
(0.2985
W·m
)
improved
to
1.324
W·m
·K
.
It
was
shown
conductivity90
of wt
polyethylene
glycol
improved
to 1.324and
W·m
·K .that
It was
shown
incorporated
% of polyethylene
glycol into expanded
graphite
found
the
thermal
thatthe
thecomposite
composite PCM
PCM is
is aa promising
promising candidate for
latent
heat
storage
applications.
Xia
etet
al.
−
1
−
1
−
1
−
1
that
latent
heat
storage
applications.
Xia
al.
[17]
conductivity of polyethylene glycol (0.2985 W·m ·K ) improved to 1.324 W·m ·K . [17]
It
was
showedthat
that by
by incorporating
incorporating 10
10 wt
wt % of EG into paraffin,
the
composite
PCM
showed
aa10-fold
showed
paraffin,
the
composite
PCM
showed
10-fold
shown that the composite PCM is a promising candidate for latent heat storage applications.
increasein
in thermalconductivity
conductivity over
over pure
pure paraffin.
paraffin. Mill
et
thermal
et al.
al. [18]
[18]improved
improvedthe
thermalconductivity
conductivity
Xiaincrease
et al. [17]thermal
showed that by incorporating
10 wt %Mill
of EG
into
paraffin,
thethe
composite
PCM showed
paraffinby
bytwo
twoorders
orders of
of magnitude
magnitude using
using porous
porous graphite
ofofparaffin
graphite matrices
matriceswith
withparaffin.
paraffin.Zeng
Zengetetal.
al.[19]
[19]
a 10-fold
increase
in thermal composite
conductivity
over
pure
paraffin.
Mill
et al. [18]toimproved
the thermal
prepared
Tetradecanol/EG
PCM
and
showed
that
in
comparison
pure
tetradeconal
prepared Tetradecanol/EG composite PCM and showed that in comparison to pure tetradeconal
conductivity
paraffin
two orders
of magnitude
using porous
graphite
matrices
with paraffin.
−1), the by
(0.433 W·m
W·mof
·K
thermal
conductivity
of
−1−1
−1
(0.433
·K
), the thermal
conductivity
of the
the sample
sample with
with 77 wt
wt %
% of
of EG
EG increased
increased toto
Zeng et al. [19] prepared Tetradecanol/EG composite PCM and showed that in comparison to pure
tetradeconal (0.433 W·m−1 ·K−1 ), the thermal conductivity of the sample with 7 wt % of EG increased
Materials 2017, 10, 391
3 of 15
to 2.76 W·m−1 ·K−1 . The optimum weight percentage of EG in Tetradecanol/EG composite PCM was
suggested as 20 wt %.
In the recent past, graphene nanoplatelets (GNPs), which have high thermal conductivity and
specific surface area, have been used and found to be suitable for PCM applications [20]. In order
to enhance the composite PCM’s thermal conductivity, Mehrali et al. [21] used GNPs with different
surface areas in palmitic acid (PA). The maximum percentage of PA absorbed by GNPs without any
sign of leakage was found as 91.94 wt %. Moreover, GNPs with a surface area of 750 m2 /g improved
composite PCM’s thermal conductivity 10-fold with respect to pure palmitic acid. GNPs were used
by Tang et al. [22] to improve the thermal conductivity of palmitic acid/high density polyethylene
composite PCM. With 4 wt % of GNPs, the thermal conductivity was nearly 2.5 times higher than
pure form-stable composite PCM. Silakhori et al. [20] showed that GNPs with 1.6 wt % improved the
thermal conductivity of palmitic acid/polypyrole with an increase of 34.3%.
In this research, two kinds of carbon-based composite PCMs (expanded graphite-based PCM and
graphite nanoplatelet-based PCM) were prepared for potential application in energy piles. Thereafter,
a systematic study was performed and different characterization tests were carried out on two
composite PCMs.
2. Materials and Methods
2.1. Materials
A technical grade paraffin, which is generally believed to be chemically inert, non-corrosive,
show small volume changes during phase transition, innocuous and inexpensive, was used for this
research [23]. As far as a carrier for PCM is concerned, expanded graphite (supplied by Qingdao
Teng Sheng Da Carbon Machinery Co., Ltd., Qingdao, China) with an expansion ratio of 300 and
technical-grade graphene nanoplatelets (supplied by Chinese Academy of Sciences Chengdu Organic
Chemical Co., Ltd., Chengdu, China) were used. The properties of EG and GNPs are enlisted in Table 1.
Table 1. Properties of expanded graphite (EG) and graphene nanoplatelets (GNPs).
Type
Particle Size
Diameter
Expanded Time
Nanosheet Number
Expanded graphite
Graphene
~0.5 mm
/
/
5–7 µm
300
/
/
<20
2.2. Preparation of CPCMs
In this research, vacuum impregnation was used to prepare two kinds of composite phase change
materials (CPCMs), i.e., expanded graphite-based PCM (EG–paraffin) and graphite nanoplatelet-based
PCM (GNP–paraffin). The procedure adopted is as follows. At first, 100 g of supporting material
(EG or GNPs) and 300 g paraffin were mixed together and put into a vacuum chamber for about 2 h to
evacuate the air from the composite PCM. Thereafter, the vacuumed composite PCM was kept on the
filter paper to remove the redundant paraffin. Finally, the percentage of PCM retained by GNPs was
determined after removing the redundant PCM from the composite and keeping the PCM in an oven
at 80 ◦ C for three days. Moreover, during this period, the high absorption cushion paper was changed
eight times on average to remove the extra PCM until the composite PCM mass became constant.
The maximum amount of paraffin retained by EG and GNPs was 92.3% and 31.5% respectively.
2.3. Characterization Tests for CPCMs
2.3.1. Micromorphology of CPCMs
The micromorphology of EG, GNPs and the CPCMs was examined using ESEM (Quanta 250 FEG,
FEI Company, Hillsboro, OR, USA). The machine was operated under low vacuum in secondaryelectron detection mode at an accelerating voltage of 15 kV. In order to obtain representative images,
Materials 2017, 10, 391
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several regions of the powdered samples were observed. The SEM micrographs (Quanta 250 FEG,
FEI Company, Hillsboro, OR, USA) were also captured for thermal energy storage cement paste while
the energy dispersive spectrometer (EDS) was used to evaluate CPCMs dispersion in the cement paste.
2.3.2. Chemical Compatibility of CPCMs
A FT-IR spectrometer was used to evaluate the chemical compatibility between the components
of the CPCMs (Nicolet 6700; Thermo Electron Scientific Instruments Corp., Waltham, MA, USA).
After mixing CPCM and KBr in a 1:30 (powder: KBr) ratio, the sample was pressed in “Manual
Hydraulic Presess” at 10 ton for 1 min. Finally, the infrared spectrum was obtained by keeping the KBr
pellets in the sample compartment. The scanning parameters were frequency ranging from 4000 to
400 cm−1 with a resolution of 4 cm−1 .
2.3.3. Thermal Capacity of the CPCMs
Thermal capacity of the CPCMs (DSC-Q200, TA Instruments Corp., Newcastle, PA, USA) was
determined by using DSC. The sample was tested under nitrogen atmosphere in a temperature range
of 0–60 ◦ C at 2 ◦ C/min heating/cooling rate at a flow rate of 40 mL/min. The results were extracted
by using TA Instruments Universal Analysis software.
2.3.4. Thermal Stability of CPCMs
The thermal stability of the CPCMs was evaluated by using TGA Q50 (TA Instruments Corp.,
Newcastle, PA, USA). The sample was tested under nitrogen atmosphere from room temperature to
600 ◦ C with a heating rate of 20 ◦ C/min and flow rate of 20 mL/min.
2.3.5. Thermal Reliability of CPCMs
The thermal reliability of the CPCMs was determined with respect to change in thermal properties
after 100 heating/cooling cycles. For this purpose, the sample was placed in the Temperature and
Humidity programmable chamber manufactured by Dongguang Bell. The sample was subjected to
a heating/cooling rate of 4 ◦ C/min from 10 ◦ C to 50 ◦ C. Moreover, the temperature was maintained
for 30 min at 50 ◦ C and 10 ◦ C, respectively. After thermal cycling, the FT-IR and DSC analyses
were performed.
2.3.6. Compressive Strength of Cement Paste Composite Containing CPCMs
The compressive strength of paste (20 × 20 × 20 mm3 ) incorporated with 0% and 10% CPCMs
(EG- and GNP-based PCM) by weight of cement was evaluated at 28 days by applying a loading rate
of 50 ± 10 N/s. The experimental matrix is given in Table 2. As far as the mixing of the ingredients
is concerned, initially, the cement and CPCM were dry mixed and then a mixture of water and
superplasticizer was added to the dry mixture.
Table 2. Mix proportion (mass ratio) of composite phase change materials (CPCM) in cement paste.
Cement Paste
Cement
Water
CPCMs
Superplasticizer (wt %)
Control specimen (OPC)
EG–paraffin-10
GNP–paraffin-10
1
1
1
0.35
0.35
0.35
0
0.1
0.1
0.15
0.3
0.3
3. Results and Discussions
3.1. Macro- and Micro-Morphology of Composite PCM
The morphologies (at macro- and micro-scale) of EG, GNPs, EG–paraffin and GNP–paraffin are
shown in Figure 3. In comparison to the light grey colour of EG powder (Figure 3a), the EG–paraffin
Materials 2017, 10, 391
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sample showed
showedaadark
darkgrey
greycolour,
colour,which
which
was
due
penetration
of PCM
in liquid
the liquid
sample
was
due
to to
thethe
penetration
of PCM
in the
state.state.
The
Materials
2017, 10, 391
5 of 15
The
micrograph
of
EG
is
shown
in
Figure
4.
It
is
known
from
literature
[24]
that
at
micro-scale,
micrograph of EG is shown in Figure 4. It is known from literature [24] that at micro-scale, EG shows
shows airregular
flattened
irregular honeycomb
network.
The of
micrograph
of was
EG–paraffin
aEG
flattened
honeycomb
network. The
micrograph
EG–paraffin
similar towas
EG similar
except
sample showed a dark grey colour, which was due to the penetration of PCM in the liquid state. The
to
EG
except
that
PCM
showed
the
honeycomb
structure
due
to
the
effect
of
capillary
and
surface
that
PCM
showed
the
honeycomb
structure
due
to
the
effect
of
capillary
and
surface
tension
forces.
micrograph of EG is shown in Figure 4. It is known from literature [24] that at micro-scale, EG shows
tension
forces.
At
level, theshowed
GNP powdercolour,
showed
blackbecame
colour,darker
which due
became
darker
due to
At
level,
themacro
GNP
powder
which
to the
penetration
amacro
flattened
irregular
honeycomb
network.black
The micrograph
of EG–paraffin
was similar
to EG
except
theparaffin
penetration
paraffin
in(Figure
the liquid
state
(Figure
3c,d).(Figure
At micro-scale
(Figure 4c),
GNPinparticles
of
theofliquid
3c,d).
Atdue
micro-scale
4c), GNP
(flaky
nature)
that PCMinshowed
the state
honeycomb
structure
to the effect
of capillary
andparticles
surface tension
forces.
(flaky
in
nature)
showed
smooth
planer
structure
with
a
large
surface
area,
which
led
to
mechanically
showed
smooth
planer
structure
with
a
large
surface
area,
which
led
to
mechanically
interconnected
At macro level, the GNP powder showed black colour, which became darker due to the penetration
interconnected
composites
[20].
When
the
PCM was
with GNP
particles
(Figure
composites
was
composed
with composed
GNP
particles
they
have 4d),
the
of paraffin[20].
in theWhen
liquid the
statePCM
(Figure
3c,d).
At micro-scale
(Figure
4c), GNP(Figure
particles4d),
(flaky
in nature)
they
have
the
tendency
to
absorb
organic
materials
on
their
surface
[20].
tendency
absorbplaner
organic
materials
their surface
surfacearea,
[20]. which led to mechanically interconnected
showedto
smooth
structure
withon
a large
composites [20]. When the PCM was composed with GNP particles (Figure 4d), they have the
tendency to absorb organic materials on their surface [20].
(a)
(b)
(a)
(b)
(c)
(c)
(d)
(d)
Figure 3. Appearance of the used materials in this study. (a) Expanded graphite (EG); (b) Graphene
Figure 3. Appearance of the used materials in this study. (a) Expanded graphite (EG); (b) Graphene
Figure 3. Appearance
ofEG–paraffin
the used materials
in(d)
thisGNPs–paraffin
study. (a) Expanded
graphite (EG); (b) Graphene
nanoplatelets
(GNPs);
(c)
CPCM;
CPCM.
nanoplatelets
(GNPs); (c)
EG–paraffin CPCM;
(d) GNPs–paraffin
CPCM.
nanoplatelets (GNPs); (c) EG–paraffin CPCM; (d) GNPs–paraffin CPCM.
(a)
(a)EG
EG
(b)EG–paraffin
EG–paraffin
(b)
Figure 4. Cont.
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6 of 15
(c) GNPs
(d) GNPs–paraffin
Figure 4. SEM micrographs of (a) Expanded graphite (EG); (b) EG–paraffin CPCM; (c) Graphene
Figure
4. SEM micrographs of (a) Expanded graphite (EG); (b) EG–paraffin CPCM; (c) Graphene
(d) GNPs–paraffin
nanoplatelets (GNPs);(c)
(d)GNPs
GNPs–paraffin CPCM.
nanoplatelets (GNPs); (d) GNPs–paraffin CPCM.
Figure 4. SEM micrographs of (a) Expanded graphite (EG); (b) EG–paraffin CPCM; (c) Graphene
3.2. Chemical
Compatibility
the CPCMs
nanoplatelets
(GNPs); of
CPCM.
3.2. Chemical
Compatibility
of(d)
theGNPs–paraffin
CPCMs
The FT-IR spectra of paraffin, expanded graphite, GNPs and composite PCMs are shown in
3.2. Chemical
Compatibility
of the CPCMs
The
FT-IR
paraffin,
expanded
GNPs
andcm
composite
PCMs
arecm
shown
−1, 2851
−1, 1459 cm
−1, 1371
−1 and in
Figure
5. The spectra
paraffinofspectrum
shows
peaksgraphite,
at 2917 cm
−1 , 2851 cm−1 , 1459 cm−1 , 1371 cm−1 and
Figure
5.
The
paraffin
spectrum
shows
peaks
at
2917
cm
−1
−1
−1
FT-IR
spectra
of paraffin,
expanded
graphite,
GNPs and
composite
PCMsvibration
are shownofinthe
720 cmThe
. The
peaks
at 2927
cm and
2851 cm
are related
to C–H
stretching
−1 . The peaks at 2927 cm−1 and 2851 cm−1 −1
−1, 2851
720methylene
cm
are
related
tocm
C–H
stretching
vibration
Figure
5.group
The paraffin
shows
2917
1459
cm−1, 1371
cm−1 and
[25–27],spectrum
while the
peakpeaks
at 720atcm
iscm
associated
to−1,the
rocking
vibration
of of
thethe
−
1
−1
−1
−1
−1) related
720 cmgroup
.group
The [25–27],
peaks
at while
2927
cm
and
2851
cmcm
related
to
C–HC–H
vibration
of of
the
methylene
the peak
at(1597
720
is associated
tostretching
thebending
rocking
vibration
of
methylene
[25,28,29].
A strong
peak
cmare
to the
vibration
thethe
−1 −
methylene
group
[25–27],[25]
while
the
720 cm
is1cm
associated
to the
the rocking
vibration
of the of
−1) corresponding
methylene
group
[25,28,29].
A and
strong
peakatpeak
(1597
cm
) related
to
C–Htobending
methylene/methyl
group
a peak
weak
(1378
the
C–Hvibration
bending
−1) related−to
1 ) the
methylene
group
[25,28,29].
A strong
peak
(1597
cm
C–H
bendingto
vibration
ofbending
the
thevibration
methylene/methyl
group
and
weak
peak
(1378
cmFinally,
corresponding
the
of the
methyl
group[25]
can
also a
be
observed
[25,28,29].
the shoulder
in
the C–H
region
near
−1) corresponding to the C–H bending
methylene/methyl
group
[25]
and
a
weak
peak
(1378
cm
−1
3430 cmof is
linked
to the
OHcan
stretching
the hydroxyl
group Finally,
[30]. the shoulder in the region near
vibration
the
methyl
group
also beof
observed
[25,28,29].
vibration
of the methyl group can also be observed [25,28,29].
Finally, the shoulder in the region near
−1 is
spectrum
of the
EG OH
shows
a wider band
3421 cm−1group
, which
is linked to the stretching vibration
3430 cmThe
linked to
stretching
of theathydroxyl
[30].
3430 cm−1 is linked to the OH stretching of the hydroxyl−1−
group
[30]. −1
1
of The
the OH
group
[31].
It
also
shows
bands
at
2923
cm
and
2858
cmlinked
(symmetric
and asymmetric
spectrum of EG shows a wider band at 3421 cm−1 , which is
to the stretching
vibration
The spectrum of EG shows a wider
band at 3421 cm , which is linked to the stretching vibration
−1 (–C=C– stretch
−1 (–C–C–
−
1
−
1
stretching
vibration
of
–CH
2
),
1653
cm
structural
vibration),
and
1466
cmasymmetric
of theofOH
group
[31].
It
also
shows
bands
at
2923
cm
and
2858
cm
(symmetric
and
−1
−1
the OH group [31]. It also shows bands at 2923 cm and 2858 cm (symmetric and asymmetric
stretch) respectively.
stretching
vibration of –CH ), 1653 cm−1−1(–C=C– stretch structural vibration), and 1466 −1cm−1 (–C–C–
2 2), 1653 cm (–C=C– stretch structural vibration), and 1466 cm (–C–C–
stretching vibration of –CH
stretch)
respectively.
stretch)
respectively.
Figure
5.5.FT-IR
EG–paraffin,GNP–paraffin.
GNP–paraffin.
Figure
FT-IRspectra
spectraof
ofparaffin,
paraffin, EG,
EG, GNPs,
GNPs, EG–paraffin,
Figure 5. FT-IR spectra of paraffin, EG, GNPs, EG–paraffin, GNP–paraffin.
Materials 2017, 10, 391
Materials 2017, 10, 391
7 of 15
7 of 15
For GNPs,
GNPs, the
the wider
wider peak
peak at
at 3417
3417 cm
cm−−11isislinked
[21]
For
linkedto
tothe
the stretching
stretching vibration
vibration of
of the
the OH
OH group
group [21]
−1
−1
−1
−
1
−
1
−
1
while the
the bands
bandsat
at2925
2925cm
cm ,, 2855
2855 cm
cm , ,and
while
and1463
1463 cm
cm are
arerelated
relatedtotothe
thesymmetric
symmetric and
and asymmetric
asymmetric
stretching
vibration
of
–CH
2 and –C–C– stretch respectively.
stretching vibration of –CH2 and –C–C– stretch respectively.
The FT-IR
FT-IR spectra
spectra of
of EG–paraffin
EG–paraffin and
and GNP–paraffin
GNP–paraffin CPCMs
CPCMs clearly
clearly depict
depict that
that interactions
interactions are
are
The
physical in
in nature.
nature. Therefore,
Therefore, the
the developed
developed CPCMs
CPCMs are
are chemically
chemically compatible.
compatible.
physical
3.3. Thermal Properties of CPCMs
DSC was used to determine the thermal properties of paraffin, EG–paraffin and GNP–paraffin
composites. The DSC curves are shown in Figure 6. The samples show two characteristic
characteristic transition
peaks, in which the minor peak represents the solid–solid phase change of paraffin while the major
represents the
the solid–liquid
solid–liquid phase
phase change
change of paraffin
paraffin [32]. The melting and freezing temperatures
peak represents
◦
for paraffin were 23.77
23.77 ◦°C
and
26.24
°C
while
these
temperatures were 22.88 ◦°C
C
C
C and 26.21 ◦°C
C for
◦
◦
GNP–paraffin respectively.
respectively. This shows that with the
EG–paraffin and 22.68 °C
C and 26.88 °C
C for GNP–paraffin
of PCM
PCM decreased.
decreased. The decrease in the melting
incorporation of EG and GNPs, the melting point of
temperature is believed to be due to the increase in heat transfer caused by the addition of EG and
GNPs, which have higher thermal conductivities. It
It can
can also
also be observed that the difference in the
peak melting and freezing temperatures is reduced by the incorporation of EG and GNPs in paraffin.
paraffin.
For example, the peak temperature difference between the melting and freezing temperature of paraffin
3.73 ◦°C,
while these
these values
values are
are 3.16
3.16 ◦°C
and 1.9
1.9◦°C
forEG–paraffin
EG–paraffinand
andGNP–paraffin
GNP–paraffin composites
composites
is 3.73
C, while
C and
C for
respectively. For
the
PCM
composites,
the
decrease
in
the
peak
temperature
difference
is
believed
to be
For the PCM composites, the decrease in the peak temperature difference is believed
to
duedue
to the
improved
thermal
conductivity
of EG
GNPs.
Moreover,
GNP–paraffin
composite
has a
be
to the
improved
thermal
conductivity
of and
EG and
GNPs.
Moreover,
GNP–paraffin
composite
greater
capacity
in reducing
the temperature
gap between
the melting
freezing
has
a greater
capacity
in reducing
the temperature
gap between
the and
melting
and stage.
freezing stage.
(a)
(b)
(c)
(d)
Figure 6. Cont.
Materials 2017, 10, 391
Materials 2017, 10, 391
8 of 15
8 of 15
(e)
(f)
Figure 6.
6. DSC
and CPCMs.
CPCMs. (a)
curve of
(b) exothermal
exothermal
Figure
DSC thermograms—paraffin
thermograms—paraffin and
(a) endothermic
endothermic curve
of paraffin;
paraffin; (b)
curve
of
paraffin;
(c)
endothermic
curve
of
EG-paraffin;
(d)
exothermal
curve
of
EG-paraffin;
curve of paraffin; (c) endothermic curve of EG-paraffin; (d) exothermal curve of EG-paraffin;
(e) endothermic
endothermic curve
curve of
of GNPs-paraffin;
GNPs-paraffin; (f)
(f) exothermal
exothermal curve
curve of
ofGNPs-paraffin.
GNPs-paraffin.
(e)
In order to evaluate the better performance of GNP–paraffin composite, the morphology
In order to evaluate the better performance of GNP–paraffin composite, the morphology
characteristics of GNPs, which can obviously influence the thermal conductivity were determined.
characteristics of GNPs, which can obviously influence the thermal conductivity were determined.
Moreover, it is known that uniform thickness can help to ensure even distribution of the thermal
Moreover, it is known that uniform thickness can help to ensure even distribution of the thermal
conductivity for GNP–paraffin composite. Hence, atomic force microscopy (AFM) was employed to
conductivity for GNP–paraffin composite. Hence, atomic force microscopy (AFM) was employed to
map the topographical and structural properties of graphene by investigating how well an AFM
map the topographical and structural properties of graphene by investigating how well an AFM probe
probe tip sticks to or is repelled by a surface, or how easy it is to press the probe tip into the surface.
tip sticks to or is repelled by a surface, or how easy it is to press the probe tip into the surface. The AFM
The AFM images of GNPs are shown in Figure 7. From Figure 7a,b, it can be seen that the thickness
images of GNPs are shown in Figure 7. From Figure 7a,b, it can be seen that the thickness of most parts
of most parts of GNPs along two sections with different directions (red line and blue line) ranges
of GNPs along two sections with different directions (red line and blue line) ranges from 5 nm to 8 nm
from 5 nm to 8 nm (Figure 7b). The surface morphology of GNPs in 3D (Figure 7c) shows that GNPs
(Figure 7b). The surface morphology of GNPs in 3D (Figure 7c) shows that GNPs used in this research
used in this research have an even surface, which means that in terms of thermal conductivity, the
have an even surface, which means that in terms of thermal conductivity, the quality of GNPs is good.
quality of GNPs is good. Hence, for this reason, the efficiency of GNP–paraffin (in terms of thermal
Hence, for this reason, the efficiency of GNP–paraffin (in terms of thermal conductivity) is superior in
conductivity) is superior in cement-based materials than in EG–paraffin. Although there are some
cement-based materials than in EG–paraffin. Although there are some defects in the GNPs, proper
defects in the GNPs, proper defects in GNPs can contribute to providing good composition between
defects in GNPs can contribute to providing good composition between GNPs and PCMs.
GNPs and PCMs.
As far as the latent heats of fusion and solidification are concerned, they were found to be 163.6 J/g
As far as the latent heats of fusion and solidification are concerned, they were found to be 163.6 J/g
and 166.5 J/g for paraffin, 152.8 J/g and 155.9 J/g for EG–paraffin and 51.84 J/g and 47.22 J/g for
and 166.5 J/g for paraffin, 152.8 J/g and 155.9 J/g for EG–paraffin and 51.84 J/g and 47.22 J/g for
GNP–paraffin. The encapsulation efficiency determined by Equation 1 was found to be 93.51% for
GNP–paraffin. The encapsulation efficiency determined by Equation 1 was found to be 93.51% for
EG–paraffin and 30.02% for GNP–paraffin respectively.
EG–paraffin and 30.02% for GNP–paraffin respectively.
η(%)
= (ΔHm,EG-PCM/GNP-PCM
+ ΔH
f,EG-PCM/GNP-PCM)/(ΔH
m,PCM
+ ΔH
f,PCM
) × 100%
η(%) = (∆H
+ ∆H
)/(∆H
+ ∆H
) × 100%
m,PCM
m,EG-PCM/GNP-PCM
f,EG-PCM/GNP-PCM
f,PCM
(1)
(1)
In research conducted by Mehrali et al. [21], in which they used GNPs with surface areas of 300,
In research conducted by Mehrali et al. [21], in which they used GNPs with surface areas of 300,
2/g, the maximum percentage of palmitic acid retained by GNPs was found to be
500 and 750 m
500 and 750 m2 /g, the maximum percentage of palmitic acid retained by GNPs was found to be 77.99%,
77.99%, 83.1% and 91.94% respectively. In comparison, for the GNPs used in this research (surface
83.1% and 91.94% respectively. In comparison, for the GNPs used in this research (surface area of
area of 100 m2/g) 30 wt % retained PCM is acceptable. When the cost of GNPs with different surface
100 m2 /g) 30 wt % retained PCM is acceptable. When the cost of GNPs with different surface areas is
areas is compared, GNPs with 750 m2/g cost 150 USD/g while 100 m2/g cost only 0.2 USD/g. This
compared, GNPs with 750 m2 /g cost 150 USD/g while 100 m2 /g cost only 0.2 USD/g. This shows
shows that the GNPs used in this research with a surface area of 100 m2/g are 750 times cheaper than
that the GNPs used in this research with a surface area of 100 m2 /g are 750 times cheaper than GNPs
GNPs with a surface area of 750 m2/g. Some other reasons for the difference in the thermal energy
with a surface area of 750 m2 /g. Some other reasons for the difference in the thermal energy storage
storage capacity of GNP composites are as follows. In research conducted by Mehrali et al. [21], they
capacity of GNP composites are as follows. In research conducted by Mehrali et al. [21], they used
used palmitic acid, which has lower viscosity than paraffin. It is believed that PCM with lower
palmitic acid, which has lower viscosity than paraffin. It is believed that PCM with lower viscosity
viscosity was retained well by the GNPs. Secondly, the researchers used a hydraulic press to compact
was retained well by the GNPs. Secondly, the researchers used a hydraulic press to compact the PCM
the PCM composite, which might have allowed the extra PCM to stay with the GNPs. However, in
composite, which might have allowed the extra PCM to stay with the GNPs. However, in our case, the
our case, the percentage of PCM retained by the GNPs was determined after removing the redundant
percentage of PCM retained by the GNPs was determined after removing the redundant PCM from
PCM from the composite, by keeping the PCM in an oven at 80 °C for three days. Moreover, during
the composite, by keeping the PCM in an oven at 80 ◦ C for three days. Moreover, during this period,
this period, the high absorption cushion paper was changed eight times on average to remove the
extra PCM until the composite PCM mass became constant.
Materials 2017, 10, 391
9 of 15
the high absorption cushion paper was changed eight times on average to remove the extra PCM until
the composite
mass became constant.
Materials
2017, 10, PCM
391
9 of 15
(a)
(b)
(c)
Figure 7. Tapping mode image of atomic force microscopy (AFM). (a) AFM Image of graphene
Figure 7. Tapping mode image of atomic force microscopy (AFM). (a) AFM Image of graphene
(scan
(scan size:
size: 55 ××55μm);
µm);(b)
(b)Cross
Crosssections
sectionsofoftwo
twodifferent
differentdirections;
directions;(c)
(c)Surface
Surfacemorphology
morphology of
of graphene
graphene
by
AFM
3D
image.
by AFM 3D image.
The values of latent heat thermal energy storage obtained from this research were compared
The values of latent heat thermal energy storage obtained from this research were compared with
with those available in literature (Table 3). The results depicted in Table 3 are promising and therefore
those available in literature (Table 3). The results depicted in Table 3 are promising and therefore
the developed composite PCMs are potential thermal energy storage candidates for energy piles
the developed composite PCMs are potential thermal energy storage candidates for energy piles
and buildings.
and buildings.
Materials 2017, 10, 391
10 of 15
Table 3. Thermal properties comparison of composite PCMs with phase change temperature in the
10 of 15
human comfort zone.
Materials 2017, 10, 391
Melting Point
Latent Heat
Composite
Reference
Table 3. Thermal
propertiesPCMs
comparison of composite PCMs(°C)
with phase change
temperature
in the
(J/g)
human comfort
zone.
Dodecanol
(25–30 wt %)/gypsum
20
17
[33]
Capric-myristic acid (20 wt %)/Vermicuilite
19.8
◦
PCMs
Melting Point
Capric-lauric acidComposite
+ fire retardant(25–30
wt %)/gypsum
17( C)
Butyl
stearate
(25–30
wt %)/gypsum
Dodecanol
(25–30
wt %)/gypsum
20 18
Capric-myristic
acid (20 wt %)/Vermicuilite
19.816.9
Emersest2326/gypsum
Capric-lauric acid + fire retardant (25–30 wt %)/gypsum
17
Capric-lauric acid (26 wt %)/gypsum
19
Butyl stearate (25–30 wt %)/gypsum
18
Capric-myristic
acid (25 wt %)/gypsum
Emersest2326/gypsum
16.921.1
Erythritol
tetrapalmitate
ester
(18 wt %)/ cement
Capric-lauric
acid (26 wt
%)/gypsum
19 21.9
Capric-myristic
wtwt
%)/gypsum
21.118.49
Capric-lauricacid
acid(25(26
%)/gypsum
Erythritol
tetrapalmitate
ester (18wt
wt%)/gypsum
%)/ cement
21.9 19
Propyl
palmitate (25–30
Capric-lauric acid (26 wt %)/gypsum
18.49
Capric-palmitic acid (25 wt %)/gypsum
22.9
Propyl palmitate (25–30 wt %)/gypsum
19
Capric-palmiticParaffin/GNPs
acid (25 wt %)/gypsum
22.922.68
Paraffin/GNPsacid/Diatomite
22.6816.7
Decanoic/Dodecanoic
Decanoic/Dodecanoic
acid/Diatomite
16.7 23
RT20 (58%)/Montmorillonite
RT20 (58%)/Montmorillonite
23
PCM-clay composite (PMMT1-4)
16–17
PCM-clay composite (PMMT1-4)
16–17
Paraffin/Diatomite/CNTs
27.12
Paraffin/Diatomite/CNTs
27.12
Capricacid
acid
(55%)/Expanded
perlite
Capric
(55%)/Expanded
perlite
31.831.8
Octadecane
(70%)/Expanded
graphite
29.629.6
Octadecane
(70%)/Expanded
graphite
Paraffin/Expanded
graphite
22.82
Paraffin/Expanded
graphite
22.82
27
[34]
Latent Heat
28 (J/g)
Reference
[35]
30
17
27
35
28
35.2
30
36.2
35
37.2
35.2
36.2
39.13
37.2
40
39.13
42.5
40
47.22
42.5
47.22
66.8
66.8
79.3
79.3
82–128
82–128
89.40
89.40
98.1
98.1
138.8
138.8
152.8
152.8
[35]
[33]
[34]
[36]
[35]
[37]
[35]
[38]
[36]
[39]
[37]
[38]
[40]
[39]
[33]
[40]
[41]
[33]
This[41]
study
This
study
[42]
[42]
[43]
[43]
[44]
[44]
[45]
[45]
[46]
[46]
[47]
[47]
Thisstudy
study
This
3.4. Thermal Stability of the CPCMs
The thermal stability of composite PCM was determined
determined to ensure
ensure that
that it is stable in the working
temperature
range.
The
TGA
thermograms
of
pure
paraffin
and
composite
PCMs
areinshown
temperature range. The TGA thermograms of pure paraffin and composite PCMs are
shown
Figurein
8.
Figure
8. It
canthat
be seen
that the
initial decomposition
temperature
of composite
PCMs shifted
to a
It can be
seen
the initial
decomposition
temperature
of composite
PCMs shifted
to a higher
higher
temperature
when compared
to paraffin,
pure paraffin,
indicating
an increase
in the
thermal
stability
temperature
when compared
to pure
indicating
an increase
in the
thermal
stability
of
of
composite
PCM.
results
also
indicate
GPN
were
advantageous
in slowing
down
composite
PCM.
TheThe
results
also
indicate
thatthat
EGEG
andand
GPN
were
advantageous
in slowing
down
the
the
degradation
process.
is believed
that
thermal
energyisisinitially
initiallyabsorbed
absorbedby
byEG
EG and
and GPN;
degradation
process.
It isItbelieved
that
thethe
thermal
energy
hence, enough energy to initiate paraffin decomposition only becomes available at a slightly higher
temperature [48,49]. It is also suggested that the surfaces of EG and GPN might have adsorbed the
volatile decomposition products
products which in turn retarded
retarded its diffusion
diffusion out of the sample and hence the
mass loss
was
only
observed
at
a
slightly
higher
temperature
[48,49].
Finally,
the the
observed
weight
loss
loss was only observed at a slightly higher temperature
[48,49].
Finally,
observed
weight
of
paraffin
in composites
is in lineiswith
impregnation
results, indicating
homogeneous
loss
of paraffin
in composites
in the
linevacuum
with the
vacuum impregnation
results,
indicating
preparation
of
composite
PCM.
Conclusively,
composite
PCMs
are
thermally
stable
in
the
homogeneous preparation of composite PCM. Conclusively, composite PCMs are thermallyworking
stable in
temperature
the working range.
temperature range.
Figure
8. TGA
the paraffin
paraffin and
and CPCMs.
CPCMs.
Figure 8.
TGA thermograms
thermograms of
of the
Materials 2017, 10, 391
11 of 15
Materials 2017, 10, 391
11 of 15
3.5. Thermal Reliability of CPCMs
The
developed
DSC
Materials
2017, 10, 391composite PCMs were subjected to 100 thermal cycles, and FT-IR and 11
of 15were
3.5. Thermal Reliability of CPCMs
used to determine the changes in chemical structure and thermal properties. The FT-IR spectra of the
3.5. Thermal
Reliability
of CPCMs
developed
composite
PCM
are PCMs
shownwere
in Figure
9. Thetofinger
print ofcycles,
the composite
PCM
and
The
developed
composite
subjected
100 thermal
and FT-IR
and(before
DSC were
after thermal
cycling)
noinobvious
difference,
suggesting
that
thermal
cycling
did
affect
used
to determine
the shows
changes
chemical
structure
and
thermal
properties.
The
FT-IR
spectra
the
The developed
composite
PCMs
were
subjectedclearly
to 100
thermal
cycles,
and
FT-IR
and
DSC
wereof
the
chemical
structure
of
the
developed
composite.
used
to
determine
the
changes
in
chemical
structure
and
thermal
properties.
The
FT-IR
spectra
of
the
developed composite PCM are shown in Figure 9. The finger print of the composite PCM (before and
developed
composite
PCMno
areobvious
shown in
Figure
The
fingersuggesting
print
of the
composite
PCM
(beforedid
andaffect
thermal
properties
of
composite
PCM9.(EG–paraffin
and
GNP–paraffin)
before
and
after
after The
thermal
cycling)
shows
difference,
clearly
that
thermal
cycling
after
thermal
cycling)
shows
no
obvious
difference,
clearly
suggesting
that
thermal
cycling
did
affect
thermal
cycling
are shown
indeveloped
Figure 10. After
thermal cycling, the melting and freezing temperatures
the
chemical
structure
of the
composite.
the chemical structure of
developed
for EG–paraffin
bythe
−0.44
°C andcomposite.
0.03
°C(EG–paraffin
respectively, and
while
the latent heatbefore
of melting
and
The thermalchanged
properties
of composite
PCM
GNP–paraffin)
and after
The thermal properties of composite PCM (EG–paraffin and GNP–paraffin) before and after
freezingcycling
changed
1.2 J/g
1.110.
J/gAfter
respectively.
For GNP–paraffin,
the freezing
melting temperatures
and freezing
thermal
areby
shown
in and
Figure
thermal cycling,
the melting and
thermal cycling are shown in Figure
10. After thermal
cycling, the melting and freezing temperatures
temperatures
changed
byby0.41
°C ◦and
0.02
°C◦ C
respectively,
while
the
latent
heat
of
melting
for
EG–paraffin
changed
−
0.44
C
and
0.03
respectively,
while
the
latent
heat
of
for EG–paraffin changed by −0.44 °C and 0.03 °C respectively, while the latent heat of melting and and
changed
by
J/g J/g
and
1.87
shows
that thethe
changes
observed
in phase
freezing
changed
by0.86
1.2
J/g
and
1.11.1J/g
J/g
respectively.
ForGNP–paraffin,
GNP–paraffin,
the
melting
freezing
freezing
changed
by 1.2
and
J/grespectively.
respectively.This
For
melting
andand
freezing
◦ C and
◦ C respectively,
change
temperature
and
latent
heat
storage
capacity
are
smaller
and
therefore
the
developed
temperatures
changed
by
0.41
0.02
while
the
latent
heat
of
melting
and
freezing
temperatures changed by 0.41 °C and 0.02 °C respectively, while the latent heat of melting and
composite
PCMs
are
thermally
reliable
and
suitable
for
thermal
energy
storage
applications.
changed
by 0.86
J/g and
1.87J/g
J/g
respectively.
This shows
the
changes
observed
in phase
change
freezing
changed
by 0.86
and
1.87 J/g respectively.
This that
shows
that
the changes
observed
in phase
change and
temperature
andstorage
latent capacity
heat storage
capacityand
are therefore
smaller and
therefore the
developed
temperature
latent heat
are smaller
the developed
composite
PCMs
compositereliable
PCMs are
thermally
and suitable
thermalapplications.
energy storage applications.
are thermally
and
suitablereliable
for thermal
energyfor
storage
Figure 9. FT-IR of composite PCMs before and after thermal cycling.
Figure
cycling.
9. FT-IR of composite PCMs before and after thermal cycling.
0.0
0.0
EG-Paraffin-Befor
EXO
0.8
Tpeak=27.53℃
Tmelt=22.70℃
Tpeak
=27.53
℃
△H
=151.6J/g
-0.6
Tpeak=27.03℃
Tmelt=22.26℃
△H =152.8J/g
=27.03
℃
-0.8
Tmelt=22.70℃
△H =151.6J/g
Tpeak
Tmelt=22.26℃
△H =152.8J/g
-1.0
0
10
Heat Flow
(W/g)(W/g)
Heat Flow
Heat Flow (W/g)
Heat Flow (W/g)
-0.4
-0.4
-1.0
Tpeak=23.56℃
Tfree=26.18℃
T =23.56℃
△H peak
=154.8J/g
0.4
Tfree=26.21℃
△H =155.9J/g
0.2
0.2
0.0
0.0
20
30
40
-0.2
0
10
Temperature ( C)
10
Tpeak=23.87℃
Tfree=26.21℃
Tpeak=23.87℃
△H =155.9J/g
Tfree=26.18℃
△H =154.8J/g
0.4
o
0
EG-Paraffin-After
EXO
0.6
-0.2
-0.8
EXO
0.6
-0.2
-0.6
EG-Paraffin-Befor
EG-Paraffin-After
EG-Paraffin-Befor
0.8
EG-Paraffin-After
EG-Paraffin-Befor
EG-Paraffin-After
EXO
20
(a) ( C)
Temperature
20
30
40
o
30
40
Temperature ( C)
-0.2
o
0
10
(b)
20
30
o
Temperature ( C)
(a)
(b)
Figure 10. Cont.
40
Materials 2017, 10, 391
Materials 2017,
2017, 10,
10, 391
391
Materials
0.0
0.0
12 of 15
12 of
of 15
15
12
0.3
0.3
GNPs-Paraffin-Befor
GNPs-Paraffin-Befor
GNPs-Paraffin-After
GNPs-Paraffin-After
EXO
EXO
-0.2
-0.2
=26.90℃
peak=26.90℃
TTpeak
=22.68℃
melt =22.68℃
TTmelt
△H
H =47.22J/g
=47.22J/g
△
-0.3
-0.3
-0.4
-0.4
=25.00℃
peak=25.00℃
TTpeak
=26.38℃
free=26.38℃
TTfree
△H=51.84J/g
H=51.84J/g
△
0.2
0.2
Heat
HeatFlow
Flow(W/g)
(W/g)
Heat
HeatFlow
Flow(W/g)
(W/g)
-0.1
-0.1
GNPs-Paraffin-Befor
GNPs-Paraffin-Befor
GNPs-Paraffin-After
GNPs-Paraffin-After
EXO
EXO
0
0
10
10
=26.38℃
peak=26.38℃
TTpeak
=22.27℃
melt =22.27℃
TTmelt
△H
H =46.36J/g
=46.36J/g
△
20
20
o
o
30
30
40
40
=24.86℃
peak=24.86℃
TTpeak
=26.36℃
free=26.36℃
TTfree
△
H
=49.97J/g
△H =49.97J/g
0.1
0.1
0.0
0.0
-0.1
-0.1
0
0
10
10
20
20
30
30
40
40
o
Temperature (( C)
C)
Temperature
Temperature ((oC)
C)
Temperature
(c)
(c)
(d)
(d)
Figure
10.
DSC
thermograms
of
composite
PCM
before
and
after
100
thermal
cycles.
(a)
endothermic
Figure
Figure 10.
10. DSC
DSCthermograms
thermograms of
of composite
composite PCM
PCM before
before and
and after
after 100
100 thermal
thermal cycles.
cycles. (a)
(a) endothermic
endothermic
curve
of
EG-paraffin;
(b)
exothermal
curve
of
EG-paraffin;
(c)
endothermic
curve
of
GNPs-paraffin;
curve
curve of
of EG-paraffin;
EG-paraffin; (b)
(b) exothermal
exothermal curve
curve of
of EG-paraffin;
EG-paraffin; (c)
(c) endothermic
endothermiccurve
curveof
ofGNPs-paraffin;
GNPs-paraffin;
(d)
exothermal
curve
of
GNPs-paraffin.
(d)
(d) exothermal
exothermal curve
curve of
of GNPs-paraffin.
GNPs-paraffin.
3.6. Compressive
Compressive Strength of
of Cement Paste
Paste Containing CPCMs
CPCMs
3.6.
3.6. Compressive Strength
Strength of Cement
Cement Paste Containing
Containing CPCMs
The compressive
compressive strength results
results of cement
cement paste
paste incorporated
incorporated with
with 00 and
and 10%
10% CPCMs
CPCMs (EG(EG- and
and
The
The compressivestrength
strength resultsofof
cement paste
incorporated with
0 and 10%
CPCMs (EGGNP-based
PCM)
by
weight
of
cement
are
presented
in
Figure
11.
The
compressive
strength
of
GNP-based
PCM)
by weight
of cement
areare
presented
in in
Figure
and GNP-based
PCM)
by weight
of cement
presented
Figure11.
11.The
Thecompressive
compressivestrength
strength of
of
cement paste
paste containing 10
10 wt %
% EG-PCM and
and GNP-PCM was
was found to
to be 14.6
14.6 MPa and
and 37 MPa
MPa
cement
cement paste containing
containing 10 wt
wt % EG-PCM
EG-PCM and GNP-PCM
GNP-PCM was found
found to be
be 14.6 MPa
MPa and 37
37 MPa
respectively. The
The percentage reduction
reduction in compressive
compressive strength for
for these mixes
mixes was 77.9%
77.9% and 44%
44%
respectively.
respectively. The percentage
percentage reduction in
in compressivestrength
strength for these
these mixeswas
was 77.9%and
and 44%
respectively.
In
research
conducted
by
Zhang
et
al.
[24],
the
decrease
in
percentage
of
cement
mortar
respectively.
respectively. In
In research
research conducted
conducted by
by Zhang
Zhang et
et al.
al. [24],
[24], the
the decrease
decrease in
in percentage
percentage of
of cement
cement mortar
mortar
with 2.5%
2.5% n-octadecane/EG
n-octadecane/EG composite
composite PCM
PCM was
was found
found to
to be
be 55%.
55%. We
We would
would like
like to
to mention
mention here
with
with 2.5% n-octadecane/EG composite PCM was found to be 55%. We would like to mention here
here
that the
the compressive strength
strength of GNP-PCM
GNP-PCM cement paste
paste was 37
37 MPa and
and is acceptable
acceptable for many
many
that
that the compressive
compressive strength of
of GNP-PCM cement
cement paste was
was 37 MPa
MPa and is
is acceptable for
for many
applications as
as mentioned in
in literature [24,50–52]
[24,50–52] and Chinese
Chinese National standard
standard (GB 50574-2010)
50574-2010) for
applications
applications as mentioned
mentioned in literature
literature [24,50–52] and
and ChineseNational
National standard (GB
(GB 50574-2010) for
for
building materials.
materials.
building
building materials.
Compressive
CompressiveStrength
Strength(MPa)
(MPa)
70
70
60
60
50
50
40
40
30
30
20
20
10
10
00
Control specimen
specimen (OPC)
(OPC)
Control
EG-Paraffin-10
EG-Paraffin-10
Cement
paste composites
composites
Cement paste
GNP-Paraffin-10
GNP-Paraffin-10
Figure 11.
11. C
Compressive
strength of
of cement
cement paste
paste incorporated
incorporated with
with 000 and
and 10
10 wt
wt %
% CPCMs
CPCMs (EG
(EG and
and
Figure
C
ompressive strength
incorporated
with
and
10
wt
CPCMs
(EG
and
Figure
ompressive
GNP-based
PCM).
GNP-based PCM).
PCM).
GNP-based
4. Conclusions
Conclusions
4.
Conclusions
4.
In this
this research,
In
this
research, two
two kinds
kinds of
of composite
composite PCM
PCM i.e.,
i.e., expanded
expanded graphite-based
graphite-based PCM
PCM and
and graphite
graphite
In
nanoplatelet-based
PCM,
were
prepared
The
conclusions
drawn
nanoplatelet-based
PCM,
were
prepared
using
vacuum
impregnation.
The
conclusions
drawn
are
nanoplatelet-based PCM, were prepared using vacuum impregnation. The conclusions drawn are
as follows.
as
follows.
as
(1)
The
maximum
percentage
of
paraffin
retained
by
EG
and
GNPs
through
vacuum
impregnation
(1)
(1) The
Themaximum
maximumpercentage
percentageof
ofparaffin
paraffinretained
retainedby
byEG
EGand
andGNPs
GNPsthrough
through vacuum
vacuum impregnation
impregnation
was
92.3%
and
31.5%
respectively.
ThisThis
datadata
is similar
similar
to the
thetodata
data
obtained
from TGA
TGA
results
was
respectively.
This
data
is
to
from
was92.3%
92.3%and
and31.5%
31.5%
respectively.
is similar
theobtained
data obtained
fromresults
TGA
(93.1%
and
30.9%),
thus
testifying
the
homogenous
preparation
of
composite
PCM.
From
micro(93.1% and 30.9%), thus testifying the homogenous preparation of composite PCM. From micromorphology, it
it was
was found
found that
that PCM
PCM possessed
possessed aa honeycomb
honeycomb structure
structure of
of EG
EG due
due to
to capillary
capillary
morphology,
Materials 2017, 10, 391
(2)
(3)
(4)
(5)
(6)
13 of 15
results (93.1% and 30.9%), thus testifying the homogenous preparation of composite PCM.
From micro-morphology, it was found that PCM possessed a honeycomb structure of EG due to
capillary and surface tension forces while for GNPs, the larger surface area provided favourable
conditions to absorb PCM.
From the FT-IR analysis, the interaction between components of composite PCM was physical
in nature and therefore the components of prepared carbon-based composites are chemically
compatible with each other.
The DSC analysis showed that the developed carbon-based PCMs possess considerable latent
heat and can therefore be a potential candidate for energy piles.
From thermal stability results, it was found that the incorporation of carbon-based materials
in PCM shifted the initial decomposition to a higher temperature, indicating an increase in the
thermal stability of composite PCM. Furthermore, the developed composite PCM did not show
any sign of degradation below 100 ◦ C. Hence, it is thermally stable and can be utilized for thermal
energy storage applications.
The chemical structure and thermal properties of developed carbon-based composite PCMs were
not affected by thermal cycling. Therefore, the composite PCM is thermally reliable and can be
used for latent heat storage applications.
The compressive strength of cement paste containing 10 wt % GNP-PCM was found to be 37 MPa
and it therefore has potential application for structural purposes.
Acknowledgments: The work described in this paper was fully supported by grants from Natural Science
Foundation of China (51678367), the Natural Science Foundation of China (51678369) and the Shenzhen
Fundamental Research Funding (JCYJ20160422092836654).
Author Contributions: Haibin Yang did the experiments and data analysis. Shazim Ali Memon did the data
analysis and wrote part of this paper. Xiaohua Bao provided the original idea and did the experiments.
Hongzhi Cui wrote part of this paper. Dongxu Li wrote part of this paper.
Conflicts of Interest: The authors declare no conflict of interest.
Abbreviations
PCM
CPCM
EG
GNP
EG-PCM
GNP-PCM
Phase change materials
Composite phase change material
Expanded graphite
Graphene nanoplatelet
Expanded graphite-based PCM
Graphene nanoplatelets-based PCM
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