ACMA2014
Experimental study of the thermal properties of composite stearic acid
/ coffee grounds / graphite for thermal energy storage
Dihia DJEFEL†, Said MAKHLOUF†, Souad KHEDACHE†, Nacer LAMROUS†, Gilles
LEFEBVRE*, Laurent ROYAN**
† Mouloud Mammeri University, L.M.S.E. Laboratory, Po Box 17 RP 15000, Tizi Ouzou,
Algeria.
*Paris Est Créteil University, CERTES-IUT, 61 Av. Général de Gaulle, 94010 Paris Créteil,
France.
**Matière Systeme Complexe Laboratory, University of Paris Denis Diderot , UMR 7057
CNRS 75013, Paris, France.
Abstract
Stearic acid/coffee grounds composites with graphite as thermal energy storage
materials were prepared using impregnation methods. In the composites, coffee grounds
acted as the supporting material, and stearic acid is selected as the phase change material
(PCM) due to the desirable thermal properties to the thermal storage by latent heat.
Nevertheless, it has a major disadvantage of the low thermal conductivity. In order to
improve conduction heat property of the composites, graphite was added in the
composites. Fourier transformation infrared spectroscope (FT-IR), and scanning
electronic microscope (SEM) were used to determine chemical structure, microstructure
of stearic acid/coffee grounds composites, respectively.
The thermal properties, such as phase change temperature and latent heat, were
investigated by a differential scanning calorimeter (DSC). Thermal conductivity was
determined by a Hot disk Method. The SEM results showed that stearic acid was well
dispersed in the porous network of coffee grounds. The DSC results indicated that the
composites solidify at 51.10 ℃ with a latent heat of 98.98 J/g and melt at 50.89 °C with a
latent heat of 100.14 J/g when the mass percentage of stearic acid in the composites was
50%. The thermal conductivity results showed that addition of graphite improved thermal
conductivity. The thermal conductivity of the stearic acid/coffee grounds/Graphite
composite is 53% higher than that of acid stearic/coffee grounds.
1.
Introduction
Latent heat thermal energy storage (LHTES) with phase change materials (PCMS) have
attracted more and more research interests in recent years, due to the considerable latent heat of
a phase change material PCM and characteristic of constant temperature variation between
energy storage and release[1,2].
As known, a wide range of inorganic and organic and eutectic PCMs have been studies, such as
salt hydrates, paraffin, fatty acids and their binary mixtures [3]. Among the investigated PCMs,
fatty acids considered as potential PCMs. In spite of the well-known desirable properties for
thermal energy storage that other PCMs as fatty acid stearic acid was an important PCM has
been widely preferred because of their high storage density, excellent thermal properties ( high
latent heat of fusion, good chemical stability, non-toxicity, smaller volume storage, little supercooling) [4-6]. But, the poor thermal conductivity was a major disadvantage of this acid.
Adding a material with high thermal conductivity in composites PCMs can improve this
problem. Expanded graphite (EG) has been considered as excellent thermal conductivity
promoter because of their desirable’s properties [7].
Therefore, during recent decades several PCMs with different support materials were prepared
and characterized as for different applications. From the previous review it can be concluded
that there are a lot of papers using the composites PCMs with thermal energy storage. However,
there is no publication to utilize the cellulosic waste such as support with PCM for thermal
storage in the current energetic and environmental situation to need for a more rational and
efficient use of energy.
This work is focused on the preparation, characterization and thermal properties of stearic acid/
coffee grounds composite (SA/CG) as a new kind of phase change material (PCM) for thermal
energy storage.
2.
2.1.
Experimental
Materials
Stearic acid (90% pure) with melting temperature of 59 °C and latent heat of 190 J/g was used
as thermal energy storage material. It was purchased from Prochima Sigma. Physical property
parameters of stearic acid are listed in Table 1.
Coffee Grounds which is not subjected to any chemical (expect water evaporation process) or
thermal treatment, used as adsorbent in this study.
Grounds coffee was first dried then washed with hot water several times to remove any dust and
other water-soluble impurities. The washed sample was dried in electric oven at 50 °C for 48 h.
Coffee grounds size distribution curve is shown in Fig. 2 and its size distribution parameter is
listed in Table 2.
Weight: 8.8 mg
STEARIC ACID 90%
o
melting point = 52.22 C
 190.5 kJ/kg
o
o
at 69 C
at 40 C
o
o
Cp = 2.3 kJ/kg. C
Cp = 2.58 kJ/kg. C
40
45
50
55
60
65
70
o
C
Figure 1: DSC analysis data for investigated stearic acid.
Samples
Octadecanoic
acid
Table1: Physical property parameters of stearic acid.
Latent
Melting
Alternative
Molecular
heat of
point
Name
Formula
fusion
(°C)
(J/g)
Stearic acid
CH3 (CH2)16 COOH
52.20
190
Heat specific
C p (kJ / kg
°C)
Solid Liquid
2.58
2.3
Figure 2: Size distribution curve of coffee grounds.
Table 2. Size parameter of coffee ground
Sample
Coffee
grounds
d(0.1) (µm)
2,583
d(0.5) (µm)
35,058
d(0.9) (µm)
dav (µm)
57,857
32,073
S/g (m2/g)
1,48
Annotation: D50 means the corresponding particle size when the cumulative distribution percentage reaches 50%; D10 means the
corresponding particle size when the cumulative distribution percentage reaches 10%; D90 means the corresponding particle size
when the cumulative distribution percentage reaches 90%; Dav means the average particle size in the cumulative distribution of
size; S/g means specific surface on weight basis [8].
2.2.
Preparation of SA/GC and SA/GC/Gr composites
SA/GC and SA/GC/Gr composites were prepared by adding GC and Gr to melted SA. Then, the
temperature was adjusted to 70 °C using a constant temperature bath. In order to determine the
highest fraction without leakage, the composite PCMs were prepared at different mass fraction
of SA (the fraction of 50 % is retuned). In order to determine variation in thermal conductivity
improvement of SA, mass fractions of EG in the composites were selected as 7%, and 16%.
3. Characterization
3.1.
Scanning electron Microscopy environmental (ESEM)
The morphology and microstructure of CG and SA–CG composites were observed on a field
emission scanning electron Microscopy environmental (ESEM, PHILLIPS ESEM XL 30), with
20 kV of tension acceleration.
3.2.
Fourier transforms infrared spectroscopy (FT-IR)
The FT-IR spectroscopy technique was used to confirm the chemical structure and the
compatibility between the components. For the analysis, the samples were mixed with KBr and
then pressed into a pellet. The spectra of the samples were recorded at the wavenumber range
between 400 and 4000 cm-1 with a resolution of 4 cm-1 by using 8001M model FT-IR
spectrophotometer.
3.3.
Differential scanning calorimeter analysis (DSC)
Phase change properties, melting and freezing temperatures, and latent heats of the SA and
SA/CG composite were determined by differential scanning calorimeter DSC applied with
Mettler Toledo Co instrument during a thermal cycle of heating to cooling. at a heating and
freezing rate of 5 °C per min and temperature range from room temperature to 100 °C in air
atmosphere. Approximately 9 mg of each sample was used for the DSC study to avoid the
possible thermal lag. The extrapolated onset temperature (TS), peak temperature (TP) and
extrapolated end temperature (TE) of the SA, SA/CG, and SA/CG/Gr composites were obtained
from the special software of the DSC. The phase change latent heats were determined by
numerical integration of the area under the peaks. Temperatures have been registered in general
with a minor error of 0.2 °C and latent heats within 10%.
3.4.
Thermal conductivity measurements
Thermal conductivity values of different composites were measured at room temperature by
using TPS 500 hot disk thermal constants analyzer technique, which was developed by
Gustafsson [9] . The hot disk technique represents a transient plane source method for rapid
thermal conductivity and thermal diffusivity measurement. The main advantages of the hot disk
technique include: wide thermal conductivity range, from 0.005 W/(m K) to 500 W/(m K); wide
range of materials types, from liquid, gel to solid; easy sample preparation; non-destructive; and
more importantly, high accuracy [10].
4. Results and discussion
4.1.
Morphology of the grounds coffee and CG/SA composites
Fig. 3 shows the SEM images of the coffee grounds and prepared composite samples. It can be
seen from these SEM micrographs that the coffee grounds has porous structures, which allow
adsorbing the SA in liquid state. The SEM images of composite PCMs show that SA/CG
composites were well retained into the pores of grounds coffee. The porous structures of the
grounds coffee provided the mechanical strength for the whole composites and prevented the
seepage of the melted SA.
Figure 3: SEM images of coffee grounds and composite PCMs.
4.2.
FTIR analysis of the SA and stearic acid with CG
FT-IR spectroscopy can be used to reveal specific interactions in composite PCM. Fig. 4
displays the FT-IR absorption spectra of the characteristic peaks form SA, SF and SA/SF
composite. In the pure SA spectrum, there are adsorption peaks at the wave number of 3017
cm−1 and 2848 cm−1 which usually overlaps with the absorption band of aliphatic C–H vibration
caused by stretching vibration of O–H group. The peak at 1702 cm−1 is the characteristic
absorption peak for the stretching vibration of carbonyl group. The peak at 1463 cm−1 is the –
CH2 bending peak, 1389 cm−1 represents C–H and C–C bending and 729 cm−1 and 718 cm−1
corresponds to rocking vibration and bending, which are all characteristic for aliphatic chain of
SA [11, 12, 13].
When two or more substances are mixed, physical blends versus chemical interactions are
reflected by changes in characteristic spectra peaks [14].
But the absorption peaks of the SA also appear in the composite PCM spectra. This result
indicates that there is no chemical interaction between the SA and CG.
Stearic acid
Comp SA/GC
60
Transmittance (%)
50
40
30
20
10
0
4000
3500
3000
2500
2000
1500
1000
500
0
-1
Wavenumbers (cm )
Figure 4: FTIR spectra of (−) stearic acid and (−) CG/SA composite.
4.3.
DSC analysis
In this study, the peak temperature and both the extrapolated onset and extrapolated end
temperatures of the DSC curve are used as characteristic temperatures. The experiments were
repeated for three times on each sample and the average values were reported recorded in table
.3. Fig. 5 shows the melting–freezing DSC curves of Comp1, Comp2 and Comp3. The DSC
curves (Fig. 1) for the pure SA present the extrapolated onset, peak and extrapolated end
temperatures during the melting process are respectively 52.85, 59.14 and 56.30 °C. The
extrapolated onset, peak and extrapolated end temperatures during the freezing process are
respectively 52.23, 46.90 and 50.42 °C. The DSC curve of SA was utilized as reference. Fig. 5
shows the DSC analysis of the different composites PCMs (Comp1, Comp2, and Comp3)
during the melting /freezing process, and the corresponding characteristic temperatures are
shown in Table 3. For COMP1 prepared with SA/CG mass ratio of 50% PCM, it can be seen
that the extrapolated onset temperature (Tmo) during melting process decrease by 1.44 °C
compared with the pure SA. This is also noticed for composite MCPs with various additions of
graphite. However, the peak temperature (Tmp) of stearic acid during the melting process varies
between 55.10 and 58.17 °C. On the other hand, the extrapolated end temperatures (Tme) of
different composites PCMs increased in comparison with the SA. And the values referred in the
table.3 of the melting temperature range, it can be seen that the melting temperatures range
increase in the different composites.
During the freezing process, the extrapolated onset temperatures (Tfo) of the composite MCPs
are closer to that of pure stearic acid. The extrapolated onset temperature same as that stearic
acid alone (52.23 °C) occurs when the mass fraction of graphite is equal to 7 wt%. Note that the
different temperatures extrapolated end and peaks of the various composites decrease compared
to those of pure stearic acid.
15
COMP1
COMP2
COMP3
Exo
DSC (mw)
10
5
0
-5
-10
-15
20
30
40
50
60
70
o
Temperature ( C)
Figure 5: DSC curves of the PCMs composites (Comp1: 50 wt% PCM, Comp2: 7wt% Gr,
Comp3: 16 wt% Gr).
Table 3. characteristic temperatures and enthalpies during melting /freezing process.
Freezing process characteristic
temperature (°C)
Samples
ΔH pc
(kJ/kg)
Melting process characteristic
temperature (°C)
Range
Onset End
Peak
Tmem
m
m
T o
T e
T p
Tmo
ΔH pcm
(kJ/k)
f
Onset
T fo
End
T fe
Peak
T fp
Range
T fe - T fo
Stearic
acid
52.23
46.90
50.42
5.32
190.47
52.85
59.14
56.30
6.28
189.36
Com1
51.12
42.76
48.11
8.36
99.93
51.41
60.45
56.84
9.04
100.44
Com2
52.23
42.64
50.74
9.59
98.05
51.70
59.22
55.10
7.50
97.95
Comp3
52.16
41.10
46.93
11.06
77.27
50.06
61.30
58.17
11.24
78.64
On the other hand, the latent heats of melting and freezing were found to be 189.36 J g−1 and
190.47 J g−1 for SA. The melting and freezing latent heats of Com1 by using 50 wt% PCM were
slightly lower those of pure stearic acid. Based on the data in this table, it can remarkably be
noted that the values of the melting and freezing latent heats of Comp2 and Comp3 decreased
with increasing amount of graphite. But, these latent heat values obtained from of the different
composites PCMs were suitable for latent heat storage in various applications.
4.4.
Hot disk conductivities
Thermal conductivity is an important property [15] in energy storage applications. In this study,
the thermal conductivities of differents composites were measured by the hot-disk method at
room temperature (~ 23 °C). Table 4 summarized the experimental results of conductivity for
SA/CG composite and SA/CG/Gr composites at two weight fractions (7% and 16%). Thermal
conductivity of COMP1 at the room temperature was measured as 0,3235 Wm−1 K−1. In order to
improve thermal conductivity of the stearic acid, the graphite was added to the composites
PCMs in mass fraction of 7% and 16%. The thermal conductivity was measured to be 0,472
Wm−1 K−1 for the COMP2 and for the COMP3 0, 596 Wm−1 K-1. It can be seen that the thermal
conductivity of Stearic acid was ameliorated.
Table 4. Experimental thermal conductivities of different composites PCMs.
Sample
Weight fraction of graphite (%)
Conductivity (W / mK)
COMP1
0
0,3235
COMP2
7
0,472
COMP3
16
0,596
Conclusion
A novel composite PCM was prepared within the coffee grounds (CG) as the novel support for
PCM. The optimized percentage SA confined in the CG was found as 50 wt%. The SA/CG
composite PCM was characterized by SEM and FT-IR spectroscopy techniques. The
microstructure analysis proved that the stearic acid was dispersed in the porous network of CG.
FTIR analysis improved the no chemical interaction between stearic acid and CG. It was
concluded that the melting and freezing temperatures and latent heats of composites PCM were
acceptable determined using DSC analysis. Thermal conductivity of the composite PCM was
also increased by addition for Gr. Based on all results, it was concluded that the research
concerning the coffee grounds as support material for the PCM has demonstrated the ability to
tailor PCMs with acceptable thermal properties.
References
[1] A. Abhat, Sol. Energy. 30 (1983) 313.
[2] I. Dincer and S. Dost, A Perspective on Thermal Energy Storage Systems for Solar
Energy Applications, INTERNATIONAL JOURNAL OF ENERGY RESEARCH. 20
(1996) 547-557.
[3] P.E. Arndt, J.G. Dunn and R.L.S. Willix, Organic Compounds as Candidate Phase
Change Materials in Thermal Energy Storage, Thermo. Acta. 79 (1984) 55-6, Elsevier
Science Publishers B.V., Amsterdam - Printed in The Netherlands
[4] D. Feldman, M.M. Shapiro, D. Banu and C.J. Fuks. Fatty acids and Their Mixtures
as Phase-Change Materials for Thermal Energy Storage. Sol. Energy. Materials 18
(1989) 201-216
[5] A. Hasan and A. A. Sayig, Some Fatty Acids as Phase-Change Thermal Energy
Storage Materials, Renewable. Energ. 4 (1994) 69-76. Elsevier Science Ltd
[6] A. Sari and C. Alkan, Preparation and Thermal Energy Storage Properties of
Poly(n-butyl methacrylate)/Fatty Acids Composites as Form-Stable Phase Change
Materials, Wiley Online Library POLYMER COMPOSITES—-2012
[7] Y.Yuan, N. Zhang, W. Tao, X. Cao and Y. He, Fatty Acids as Phase Change
Materials: Areview, Renewable and Sustainable Energy Reviews 29 (2014) 482–498.
[8] M. Li, Z. Wub and H. Kao, Study on Preparation, Structure and Thermal Energy
Storage Property of Capric–Palmitic Acid/Attapulgite Composite Phase Change
Materials, Applied Energy 88 (2011) 3125–3132.
[9] S.E. Gustafsson, Transient Plane Source Techniques for Thermal Conductivity and
Thermal Diffusivity Measurements of Solid Materials, Rev. Sci. Instrum. (USA) 62 (3)
(1991) 797–804.
[10] Y. He, Rapid Thermal Conductivity Measurement with a Hot Disk Sensor Part 1.
Theoretical Considerations, Thermochimica Acta 436 (2005) 122–129.
[11] Y. Wang, T. D. Xia, H. Zheng and H. X. Feng, Stearic Acid/Silica Fume
Composite as Form-Stable Phase Change Material for Thermal Energy Storage. Energy
and Buildings 43 (2011) 2365–2370].
[12] T. Zhang , Y. Wang, H. Shi and W. Yang, Fabrication and performances of new
kind microencapsulated phase change material based on stearic acid core and
polycarbonate shell, Energy Conversion and Management 64 (2012) 1–7.
[13] Z. Chen, L. Cao, F. Shan and G. Fang, Preparation and Characteristics of
Microencapsulated Stearic Acid as Composite Thermal Energy Storage Material in
Buildings, Energy and Buildings 62 (2013) 469–474.
[14] Z. Wang, J. Zhou, X. Wang, N. Zhang, X. Sun and Z. Ma, The Effects of
Ultrasonic/Microwave Assisted Treatment on The water Vapor Barrier Properties of
Soybean Protein Isolate-Based Oleic Acid/Stearic Acid Blend Edible Flms, Food
Hydrocolloids 35 (2014) 51-58.
[15] H. Nagai, F. Rossignol, Y. Nakata, T. Tsurue, M. Suzuki and T. Okutani, Thermal
Conductivity Measurement of Liquid Materials by a Hot-Disk Method in ShortDuration Microgravity Environments, Materials Science and Engineering A 276 (2000)
117–12.