PCM review work

1. Research Paper
Thermal properties of beeswax/graphene phase change material as
energy storage for building applications
Muhammad Amin a
, Nandy Putra a,⇑
, Engkos A. Kosasih a
, Erwin Prawiro a
, Rizky Achmad Luanto a
,
T.M.I. Mahlia b,c
a
Applied Heat Transfer Research Group, Department of Mechanical Engineering, Universitas Indonesia, 16424 Depok, Jawa Barat, Indonesia
b
Faculty of Integrated Technologies, Universiti Brunei Darussalam, Jalan Tungku Link BE 1410, Brunei Darussalam
c
Department of Mechanical Engineering, Universiti Tenaga Nasional, 43000 Kajang, Selangor, Malaysia
a r t i c l e i n f o
Article history:
Received 22 June 2016
Revised 12 October 2016
Accepted 12 October 2016
Available online 13 October 2016
Keywords:
Thermal properties
Beeswax/graphene
Phase change material
Thermal storage
a b s t r a c t
Increased energy consumption in buildings is a worldwide issue. This research is concerned with the
implementation of a phase change material for thermal storage. This concept has gained great attention
as a solution to reduce energy consumption in buildings. Beeswax, which is a phase change material with
a high thermal capacity, is investigated in this research. This paper is intended to measure and analyze
the thermal properties of beeswax/graphene as a phase change material. The melting temperature, ther-
mal capacity and latent heat were determined using differential scanning calorimetry (DSC), and the
thermal conductivity was investigated using a thermal conductivity measurement apparatus. To discover
the change in the physical properties due to the effect of nanoparticles, the viscosity of the material was
investigated as well. Based on the result from the DSC, the latent heat of 0.3 wt% beeswax/graphene
increased by 22.5%. The thermal conductivity of 0.3 wt% beeswax/graphene was 2.8 W/m K. The existence
of graphene nanoplatelets enhanced both the latent heat and thermal conductivity of the beeswax.
Therefore, based on this result, beeswax/graphene is concluded to have the potential to reduce energy
consumption in buildings.
Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction
Buildings, as a major energy consumer, have driven the need to
address this problem. Construction of a building typically uses
common materials that consume a large quantity of energy, such
as steel, brick, and cement. The energy consumption of a building
is concentrated on the utilization of air conditioning fans and
pumps, which represent 25% of the total energy consumption in
a building [1]. Thermal storage has been developed to overcome
this building energy issue. Latent heat storage is a more promising
type of heat storage compared with common sensible heat storage
methods based on the heat capacity [2]. Phase change materials are
the most popularly used material due to their high heat capacity
and non-toxic properties, which can be used in building applica-
tions as thermal storage [3–5].
The basic properties of a phase change material are the phase
change temperature and latent heat [6]. Transfer of thermal energy
in a phase change material occurs during the phase change process
from one phase to another phase [7]. Phase changes in these
materials are separated into solid-solid, solid-liquid, and solid-
gas. The most applicable of phase change materials are solid to liq-
uid transformation materials, such as beeswax, paraffin, and other
materials. Unlike conventional (sensible) storage, this material
absorbs and releases heat in a small temperature gap and has 5–
14 times the thermal capacity [8]. However, for their application,
phase change materials have to meet some requirements, such as
physical, kinetic, and thermal properties [9,10]. Beeswax is the pri-
mary material of this research due to its high thermal capacity.
Beeswax is the result of a metabolic process of bees, and wax is
released (excreted) through the abdominal segments of bees. Bees-
wax consists of esters of fatty acids and long chain alcohols. Bees-
wax is categorized as an-organic non-paraffin PCM. Beeswax
consists of palmitate, palmitoleate, hydroxypalmitate, and oleate
esters of long chain aliphatic alcohols, and its empirical formula
is C15H31COOC30H61 [11]. Two types of beeswax are yellow bees-
wax and white beeswax. Yellow beeswax has a honey-like scent,
but is brittle in the solid phase. Conversely, white beeswax does
not have a honey-like scent; however, it is more flexible than yel-
low beeswax. Beeswax cannot be dissolved in water and can be
easily dissolved in chloroform.
http://dx.doi.org/10.1016/j.applthermaleng.2016.10.085
1359-4311/Ó 2016 Elsevier Ltd. All rights reserved.
⇑ Corresponding author.
E-mail address: nandyputra@eng.ui.ac.id (N. Putra).
Applied Thermal Engineering 112 (2017) 273–280
Contents lists available at ScienceDirect
Applied Thermal Engineering
journal homepage: www.elsevier.com/locate/apthermeng

2. A disadvantage of phase change materials (PCMs) is their ther-
mal conductivity; to overcome this disadvantage, many research-
ers have developed an encapsulation method to increase PCM
heat transfer [12]. The low thermal conductivity of this type of
material limits its application in thermal storage. The thermal con-
ductivity of paraffin is 0.22 W/m K [13]. The development of nan-
otechnology has been able to multiply the thermal conductivity
of PCMs. CuO nanoparticles can increase the thermal conductivity
of a solution by up to 212% [14]. Nanoparticles are dispersed into a
PCM to enhance its thermal conductivity, and the thermal proper-
ties of a nano-PCM can improve the melting times of a solution by
up to 28.57% for 2 wt% nanoparticles [15]. Based on previous stud-
ies, a higher concentration of nanoparticles was able to increase
nano-PCM thermal conductivity, but the latent heat was reduced.
Conversely, the size and shape of nanoparticles are important
parameters to improve the heat transfer characteristics of PCMs
[16]. The reduction of nanoparticle size increases the thermal con-
ductivity of nano-PCM [17]. There have also been some experi-
ments conducted that showed that a reduction in nanoparticle
size decreased the thermal conductivity of nano-PCM. Zi-Tao Yu
prepared paraffin with the addition of a carbon nanomaterial to
increase the thermal conductivity of paraffin [18]. Some research-
ers have investigated graphene-PCMs. Trung Dung Dao and Han
Mo Jeong reported improving the thermal conductivity of micro-
sized stearic acid/graphene with a core and shell structure using
the laser flash method, and they successfully created a stable gra-
phene inside stearic acid (SA) [19]. Luntao Liu et al. showed that
graphene oxide (GO) nanoplatelets resulted in a slight effect on
thermal storage performance [20]. Conversely, Kinga Pielichowska
et al. examined the effect of graphene on polyurethane [21]. A
remarkable thermal conversion performance was found, and the
thermal conductivity of the material was 5000 W/m K, which
was achieved using a graphene oxide-grafted microencapsulated
phase change material (SIO2/GO composite) [22]. The thermal con-
ductivity of PA/PPy PCM was increased by 34.3% when using gra-
phene nanoplatelets [23].
The development of nanotechnology is widely known to
improve the thermal properties of PCM [24]. The shape and size
of the nanoparticles have a significant effect on the thermal prop-
erties of a nanofluid. In a 0.55 vol.% Fe nanofluid, the thermal prop-
erties increased by 18% [25]. The thermal conductivity
enhancement of this nanofluid depends on the nanoparticle size,
and copper nanoparticles resulted in the best enhancement of
the thermal conductivity of the solution; however, it must be sta-
ted that agglomeration can occur inside the solution [26]. A reduc-
tion in the nanoparticle size decreases the thermal conductivity of
a nanofluid. A solution that had a nanoparticle diameter 80 nm
gave the best results with respect to thermal properties compared
with 20-nm diameter nanoparticles [27]. To analyze nanoparticles
dispersed inside a nanofluid solution, S. Harikrishnan and S. Kalai-
selvam used scanning electron microscopy and characterized the
sample using FT-IR and XRD [16].
The relationship between viscosity and the concentration of
nano-PCM was investigated by Yu et al. [28]. Viscosity increased
as the thermal conductivity and concentration of the nano phase
change material increased, and vice versa. This result showed that
a nano particle can increase the viscosity of a sample when there is
a greater percentage of the nanoparticle in the nano-PCM solution.
However, the viscosity decreases as the temperature of the nano
phase change material increases. The effect of viscosity will influ-
ence the time that is needed to complete the melting and solidifi-
cation processes of a nano-PCM [29]. Hong et al. investigated the
addition of Fe nanoparticles to ethylene glycol and showed that
this addition enhanced the thermal conductivity [25]. Lin et al.
reported the enhancement of the dynamic viscosity of an alumina
nanofluid as the concentration of the nanoparticles increased [30].
Murshed et al. studied a TiO2 nanofluid; the nanoparticles
enhanced the thermal conductivity of the nanofluid [31].
The present study focused on analyzing the heat transfer perfor-
mance of a beeswax/graphene nano-PCM. Graphene nanoplatelets
with different mass fractions were added to beeswax. The material
compatibility was tested using SEM, FTIR and XRD. The latent heat,
thermal capacity and thermal conductivity were the 3 main
parameters that were analyzed in this paper. The relationship
between the viscosity and concentration of the nano phase change
material was also investigated.
2. Methodology
2.1. Materials
This study used beeswax as the main phase change material. An
image of beeswax can be seen in Fig. 1(a). To increase the thermal
properties, graphene nanoplatelets were dissolved into the bees-
wax. The graphene nanoplatelets were commercial material pur-
chased from XG Sciences. The physical and chemical properties
of the graphene are shown in Table 1.
2.2. Preparation of the beeswax/graphene nano-phase change material
(nano-PCM)
Nano-PCMs with 0.05, 0.1, 0.15, 0.2, 0.25, and 0.3 wt% graphene
were synthesized using an ultrasonic method. Mixing nanoparti-
cles into PCM has an important role in improving the thermal char-
acteristic of PCM [32]. During the preparation of nano-PCM, the
steps must be performed carefully. Utilization of an ultrasonic
vibrator minimized the possibility of agglomeration within the
nano-PCM [33,34].
The sample of solid beeswax was prepared by shaving the bees-
wax into a refined form. The sample was melted at 70 °C for 30 min.
Then, graphene nanoplatelets were added to the melted sample
based on the desired nano-PCM formulation. The beeswax and gra-
pheme solution was mixed using an ultrasonic processor for one
hour until it had dispersed completely into the beeswax. The solu-
tion turned black when it was finished, as shown in Fig. 1(b). Fig. 2
shows the preparation steps of the beeswax/graphene.
2.3. Characterization and properties test method
Scanning electron microscopy (SEM), X-ray diffraction (XRD)
and Fourier transform infrared spectroscopy (FT-IR) analyses were
performed to characterize the beeswax/graphene nano-PCM sam-
ple. The size of the dispersed graphene nanoplatelets was investi-
gated using SEM. Compatibility between the graphene and base
beeswax material was analyzed using FT-IR spectrometer and a
Philips PW 1710 XRD.
A Perkin Elmer differential scanning calorimeter r(DSC) was
used to investigate the latent heat and heat capacity of the sam-
ples. To stabilize the heat flow, nitrogen was used as a cooling
medium with a 20 ml/min flow rate. The samples were held in
an aluminum DSC pan. The samples were tested using a 5 °C/min
scan rate. The melting temperature and latent heat of the samples
were calculated using numerical integration with the DSC software
[35]. The heat capacity was determined through comparison to a
reference, such as indium, based on the DSC graphic.
A thermal conductivity meter was used to determine the ther-
mal conductivity of the beeswax/graphene nano-PCM. First, the
sample was molded into cylindrical shapes with thicknesses of
2 mm and 4 mm. The cylinder mold was manufactured using steel.
The sample was removed from the mold after it had completely
solidified. Fig. 3 shows the experimental setup for this thermal
274 M. Amin et al. / Applied Thermal Engineering 112 (2017) 273–280

3. conductivity measurement. The sample holder in this measuring
instrument maintained high pressure to prevent air from leaking
into the sample. To ensure that there was no trapped air between
the interface of the sample and standard cylinder, thermal paste
was placed on the sample. Polyurethane was used as a thermal
insulation to prevent radial heat loss. Water flowed through the
bottom of the copper cylinder at a flow rate of 100 L/min to cool
the sample. To ensure the accuracy of the measurement, NI module
9111 and NI C-DaQ 9173 were used in this experiment. LabView
software was used to achieve a more significant figure regarding
the thermal conductivity measurement.
To measure the viscosity of the beeswax fluid, a characteriza-
tion was performed. The viscosity was measured using a Brookfield
DV-E viscometer. Five different spindle speeds were used to collect
five viscosity values of the beeswax to plot the characterization
graph. The beeswax was melted before being poured into an adap-
ter as the heating medium. Water at 70 °C was circulated around
the adapter to keep beeswax in the liquid phase. A schematic of
the viscosity measurement set up is shown in Fig. 4.
3. Results and discussion
3.1. Characterization of nano-phase change material (nano-PCM)
Beeswax was tested to characterize its properties. The melting
temperature, latent heat and density are the three main properties
for heat transfer applications. Table 2 shows a comparison of the
properties of beeswax between the samples and references. A high
latent heat of the sample is an advantage for thermal storage
applications.
The morphology of the beeswax/graphene nano-PCM was
investigated using SEM. Fig. 5 shows the SEM image of the
beeswax/graphene nano-PCM. The distribution of graphene nano-
platelets in beeswax was even, and there was no sign of agglomer-
ation in the beeswax/graphene nano-PCM solution based on the
SEM images.
To ensure chemical compatibility between graphene and bees-
wax, FT-IR and XRD analyses was performed. Fig. 6 shows the
results of the FT-IR analysis of the beeswax and beeswax/graphene
nano-PCM. In the spectrum of beeswax, the peaks at 2916.64 cm 1
and 2848.93 cm 1
belong to the elongating vibration of ACH2 and
ACH3. Conversely, the greatest peak at 1734.93 cm 1
corresponds
to the in-plane bending vibration of ACH2 and ACH3, and the peak
at 1170.78 cm 1
matches the in-plane vibration of A(CH2)nA. In
the spectrum of beeswax/graphene nano-PCM, peaks that were
assigned to beeswax at 2916.59 cm 1
, 2848.91 cm 1
,
1735.14 cm 1
and 1170.93 cm 1
and that were assigned to the gra-
phene nanoplatelets at 1397.96 cm 1
still exist, and there were no
new significant peaks in this solution. Therefore, no unwanted
chemical reactions were found between beeswax and graphene
nanoplatelets, and there was only a physical interaction.
Fig. 7 shows the XRD patterns of beeswax and beeswax/
graphene nano-PCM. The broad peak at 2h = 26.29° indicates ran-
dom pucking of the graphene sheets. This peak is due to the plane
of graphite with interlayer spacing, which indicates the removal of
oxygen atoms during the intercalation process. The weak and
broad peaks from the XRD pattern of beeswax/graphene at
2h = 19.75° and 24.02° belong to beeswax. The XRD pattern of
beeswax/graphene indicated that there was only a physical inter-
action between the constituents of the composite. This was shown
in the XRD pattern in which the beeswax and graphene nanoplate-
lets patterns were observed in the XRD pattern of nano-PCM. The
intensity of beeswax/graphene is compared to the intensity of gra-
phene. The crystallinity of beeswax/graphene is better than the
intensity of graphene. The peak of beeswax/graphene had the
greatest intensity. One peak of beeswax/graphene demonstrates
crystallinity.
Fig. 1. Sample of (a) beeswax and (b) beeswax/graphene.
Fig. 2. Procedure of sample preparation.
Table 1
Properties of graphene nanoplatelets (GNPs).
Property Value
Appearance Black granules/powder
Bulk density (g/cc) 0.2–0.4
Relative gravity (g/cc) 2.0–2.25
Specific surface area (m2
/g) 300
Width (lm) Less than 2
Thickness (nm) Less than 2
Thermal conductivity (W/m K) Parallel to surface: 3000
Perpendicular to surface 6
Density (g/cm3
) 2.2
Carbon content >99%
M. Amin et al. / Applied Thermal Engineering 112 (2017) 273–280 275

4. Based on the results of SEM, FT-IR and XRD analyses of beeswax
and graphene nanoplatelets, the shape stability of the composite
was obtained. Graphene nanoplatelets were completely dispersed
within beeswax, and the compatibility between the constituents
shows that there was a good combination within the solution.
3.2. Heat storage properties of nano-phase change material (nano-
PCM)
The heat storage properties consisted of three main parameters:
the phase change temperature, latent heat and heat capacity. The
latent heat and phase change temperature of beeswax and
beeswax/graphene nano-PCM were measured using differential
scanning calorimetry (DSC). Fig. 8 shows the DSC curves of bees-
wax and beeswax/graphene nano-PCM. The results of the phase
change temperatures and latent heat are presented in Table 3. As
shown in Fig. 8, each DSC curve has one peak each in the heating
and cooling processes. This results shows that each material has
a phase change temperature for the melting and solidification
process.
Specifically, the onset and melting temperatures were both
measured because a complete phase transformation did not occur
Fig. 4. Experimental setup of viscometer.
Fig. 3. Schematic thermal conductivity measurement apparatus.
276 M. Amin et al. / Applied Thermal Engineering 112 (2017) 273–280

5. at the onset temperatures; however, phase transformation was
achieved at temperatures greater than the melting temperature.
The addition of graphene nanoplatelets affected the phase change
temperatures of beeswax. The melting temperatures had a ten-
dency to decrease as the mass fraction of the graphene nanoplate-
lets increased. Conversely, the solidifying temperatures had a
tendency to increase. The melting temperature of beeswax/
graphene nano-PCM decreased to 61.57 °C from 62.28 °C. This
demonstrates that the phase change at a lower temperature for
melting and a higher temperature for solidification process with
respect to beeswax due to the heat transfer rate of the sample.
As for latent heat, the effect of graphene nanoplatelets
increased the latent heat of the samples. The melting latent heat
increased by 22.32% compared to pure beeswax. The latent heat
increase can be explained by Brownian motion and particle clus-
tering mechanisms [36]. The random motion of the graphene
nanoplatelets increased the probability of agglomeration within
the beeswax base fluid. Additionally, the Van der Walls forces
between the graphene nanoplatelets attracted each other and
formed particle clusters [36]. However, a lower concentration of
nanoplatelets allowed the thermal storage to be more operational
per unit volume. The same results were shown for the solidifying
process of the samples. The latent heat increased as the mass frac-
tion of the nanoplatelets increased.
The specific heat capacity was measured by comparing the cal-
culated value of Cp with indium as the reference. In this work, the
observation of the heat capacity enhancement was not correlated
to the existence of a micro structure. The Brownian motion of
the nanoplatelets should cause an improvement in the thermal
capacity because thermal capacity is not a conveyance property.
The heat capacity of the 0.3 wt% sample increased by 12%. The for-
mation of a solid nano layer on the surface of the nano platelets
should be considered to be the cause of the heat capacity increase.
This layer has high thermal properties compared with the base liq-
uid. Table 4 shows the results of the Cp calculation based on the
DSC curve at the peak melting temperature.
3.3. Thermal conductivity analysis
The thermal conductivity dominates the thermal transfer rate of
the material with respect to the thermal power capacity [36]. The
thermal conductivity of beeswax is only 0.25 W/m K [37], which is
Fig. 5. SEM image of beeswax/graphene nano-PCM.
Fig. 6. FT-IR spectrum.
Fig. 7. XRD pattern of beeswax and beeswax/graphene.
Heating
Cooling
Fig. 8. DSC curves of beeswax and beeswax/graphene.
Table 2
Properties of beeswax and references.
Properties Beeswax References
Melting temperature [°C] 62.28 61.8 [39,40],
64.4 [41],
62–65; 60–67 [42],
61–67 [43]
Latent heat [kJ/kg] 141.49 122 [41],
(Melting) 177 [40]
145.62 –
(Solidify)
Density [kg m 3
]
Melting 789.47 –
Solidification 819.75 –
M. Amin et al. / Applied Thermal Engineering 112 (2017) 273–280 277

6. inadequate for a thermal storage application to meet the power
capacity. Fortunately, the addition of nanoplatelets dramatically
increased the thermal conductivity. The nano-PCM composite
had an at least 2-fold greater thermal conductivity than pure
beeswax.
Fig. 9 shows the effect of graphene nanoplatelets on thermal
conductivity enhancement. The thermal conductivity measure-
ment was performed at 40 °C and with a water cooling medium
temperature of 27 °C. The addition of graphene nanoplatelets
increased the thermal conductivity of the sample to 2.89 W/m K.
This demonstrates that graphene nanoplatelets clustering would
not only affect the latent heat but also the heat transfer rate of
samples. As the mass fraction increased, the thermal conductivity
of the nano-PCM also increased. This result indicates that the ther-
mal conductivity enhancement has a linear relationship with the
mass fraction of nano-PCM. However, if the addition of nanoparti-
cle increased further, the thermal conductivity improvement
would not be estimated to be linear due to agglomeration of the
nanoparticles.
As stated previously, the addition of graphene nanoplatelets
influenced the thermal conductivity of the samples. However, tem-
perature is as important to the increase in thermal conductivity.
The thermal conductivity increased with the addition of graphene
nanoplatelets for temperatures of 40 °C, 50 °C and 60 °C, but as the
temperature increased, the thermal conductivity was reduced.
Fig. 10 shows the relationship between thermal conductivity and
temperature with the mass fraction of samples. This phenomenon
is due to the melting point of beeswax, which is 62.28 °C. As the
temperature approaches the melting point, the structure of bees-
wax is influenced and begins change from solid to liquid.
Fig. 9. Relation of thermal conductivity and latent heat enhancement.
Fig. 10. Thermal conductivity of beeswax/graphene.
Fig. 11. Viscosity reading of beeswax/graphene.
Table 3
Thermal storage properties of samples.
Samples Beeswax 0.05 wt% 0.1 wt% 0.15 wt% 0.2 wt% 0.25 wt% 0.3 wt%
Beeswax (g) – 30 30 30 30 30 30
Graphene (g) – 0.015 0.03 0.045 0.06 0.075 0.09
TMo (°C)a
47.61 52.99 47.31 47.31 51.37 47.71 47.37
TMp (°C)b
62.28 62.05 62.59 62.59 61.57 62.12 62.42
TSo (°C)c
57.45 58.25 58.42 58.26 58.44 58.29 58.35
TSp (°C)d
52.02 52.51 52.54 52.26 52.90 52.51 52.05
HMelting (kJ/kg) 141.49 152.66 154.25 163.23 163.40 180.07 186.74
HSolidification (kJ/kg) 145.62 161.30 164.13 167.17 167.57 184.10 190.77
a
Onset temperature for melting.
b
Peak temperature for melting.
c
Onset temperature for solidification.
d
Peak temperature for solidification.
Table 4
Heat capacity of samples.
Samples Heat capacity (kJ/kg K)
Beeswax 0.508
Beeswax/graphene 0.05 wt% 0.527
Beeswax/graphene 0.1 wt% 0.530
Beeswax/graphene 0.15 wt% 0.534
Beeswax/graphene 0.2 wt% 0.535
Beeswax/graphene 0.25 wt% 0.556
Beeswax/graphene 0.3 wt% 0.561
278 M. Amin et al. / Applied Thermal Engineering 112 (2017) 273–280

7. 3.4. Physical properties analysis
Fig. 11 shows the results of the viscosity measurements of sam-
ples using the ratio method. The viscosity measurements showed a
tendency to increase with an increase in the weight concentration
of graphene nanoplatelets due to the attractive forces among the
nanoplatelets. He et al. [38] obtained the same results, showing a
similar tendency with respect to an increase in viscosity.
4. Conclusions
Beeswax/graphene with various mass fractions of nanoparticle
was prepared with the objective of investigating the thermal prop-
erties of this nano-PCM to reduce energy consumption in build-
ings. Samples of the beeswax/graphene were successfully
synthesized using an ultrasonic method. SEM analysis showed that
beeswax/graphene was completely dispersed. No chemical reac-
tion was found between the beeswax and graphene nanoplatelets,
as shown by FT-IR and XRD analyses. The addition of graphene
nanoplatelets increased the thermal conductivity, latent heat, heat
capacity and viscosity of nano-PCM. The latent heat and heat
capacity were tested using DSC analysis, and the thermal conduc-
tivity was measured using a thermal conductivity measurement
apparatus. The addition of graphene nanoplatelets increased the
thermal conductivity of the sample to 2.89 W/m K, and the heat
capacity of the 0.3 wt% sample increased by 12%. Additionally,
the viscosity was determined using a Brookfield DV-E viscometer.
The enhancement of the latent heat and heat capacity could be
explained by Brownian motion and particle clustering. Conversely,
the increase in viscosity of nano-PCM was caused by an attractive
force between graphene nanoplatelets. Based on the results,
beeswax/graphene has potential for building applications.
Acknowledgements
The authors would like to express gratitude to DRPM Universi-
tas Indonesia for funding this research through the ‘‘Hibah PUPT”
scheme.
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