Thermal properties characterization of

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Applied Energy
journal homepage: www.elsevier.com/locate/apenergy
Thermal properties characterization of chloride salts/nanoparticles
composite phase change material for high-temperature thermal energy
storage
Dongmei Hana,b
, Bachirou Guene Lougoua,b,⁎
, Yantao Xua,b
, Yong Shuaia,b,⁎
, Xing Huangc
a
Key Laboratory of Aerospace Thermophysics, Ministry of Industry and Information Technology, Harbin Institute of Technology, 92 West Dazhi Street, Harbin 150001, PR
China
b
School of Energy Science and Engineering, Harbin Institute of Technology, 92 West Dazhi Street, Harbin 150001, PR China
c
College of Metallurgy and Energy, North China University of Science and Technology, 21 Bohai Street, Tangshan 063009, PR China
H I G H L I G H T S
• Composite phase change material (CPCM) was prepared and characterized.
• The thermal diffusivity of CPCM increased with increasing temperature.
• CPCM thermal conductivity can be enhanced ≥48% by doping with Al2O3 nanoparticles.
• Al2O3 nanoparticles was the optimal additive to improve thermal properties.
• CPCMs showed excellent thermal stability over a broad working temperature range.
A R T I C L E I N F O
Keywords:
Composite phase change materials
Ternary chloride salts
Nanoparticles
Thermal property
Thermal energy storage
A B S T R A C T
Chloride salts are widely used as thermal energy storage (TES) media for high-temperature solar TES systems.
Their thermal properties are crucial for the performance of TES systems. In this study, we prepared and char-
acterized chloride salts/nanoparticles composite phase change materials (CPCMs) for high-temperature thermal
energy storage. The ternary chloride salts (MgCl2:KCl:NaCl with 51:22:27 molar ratio) were used as base salt and
Al2O3, CuO, and ZnO nanoparticles were dispersed into the base salt at 0.7 wt% to form various composite phase
change materials (CPCMs). The thermal properties of the base salt and CPCMs were measured. The results
showed that the melting temperature of the CPCMs was very close to that of the base salt. The phase change
latent heat of the CPCMs was slightly lower than that of the base salt while the addition of dopant nanoparticles
clearly enhanced the thermal diffusivity and thermal conductivity of the CPCMs. In particular, the thermal
conductivity of the CPCM doped with Al2O3 nanoparticles showed the most obvious enhancement, which in-
creased by more than 48%, compared to that of the base salt. Al2O3 nanoparticles could be considered as an
optimal additive to improve the thermal conductivity of chloride salts. Moreover, the CPCM with Al2O3 also
exhibited excellent thermal stability. These good thermal characteristics of CPCM with Al2O3 nanoparticles
endow it promising applications for high-temperature TES system.
1. Introduction
Non-renewable energy is gradually reducing as we continue to rely
on fossil fuels. Thus, problems such as energy shortage and environ-
mental pollution have emerged. Therefore, it has become important and
urgent to develop and utilize renewable energy [1,2]. With favorable
characteristics, such as environmental friendliness, availability and
abundant sources, solar energy is considered a promising renewable
energy source that has a wide range of applications in power generation
[3,4]. However, it is difficult to provide a stable and reliable energy
supply due to the intermittent nature of solar energy. Hence, research
on high efficiency and low-cost thermal energy storage (TES) systems is
necessary to ensure the continuous and stable operation of solar power
systems without being affected by weather fluctuations [5]. TES
https://doi.org/10.1016/j.apenergy.2020.114674
Received 12 December 2019; Received in revised form 10 February 2020; Accepted 13 February 2020
⁎
Corresponding authors at: Key Laboratory of Aerospace Thermophysics, Ministry of Industry and Information Technology, Harbin Institute of Technology, 92
West Dazhi Street, Harbin 150001, PR China.
E-mail addresses: bachguelou@yahoo.fr (B. Guene Lougou), shuaiyong@hit.edu.cn (Y. Shuai).
Applied Energy 264 (2020) 114674
0306-2619/ © 2020 Published by Elsevier Ltd.
T

2. systems will store excess energy when enough energy is generated and
release it when energy is insufficient to compensate for demand.
Generally, sensible heat storage (SHS) and latent heat storage (LHS)
are considered effective ways to store thermal energy [6]. Between
these two, LHS with phase change material (PCM) as TES media has
caused great interest among researchers due to its higher energy storage
density and smaller temperature variations [7,8]. It has been widely
utilized in many fields such as electronic devices [9], industrial waste
heat recovery [10], space heating and cooling of buildings [11] and
solar thermal utilization [8,12].
A lot of researches on LHS systems with PCM have been conducted.
Esen et al. [13] studied the performance of a solar assisted cylindrical
energy storage tank with PCM based on enthalpy method and assessed
the effects of various thermal and geometric parameters of different
PCM and tank configurations on the performance of the storage tank.
Seddegh et al. [14] compared the thermal behavior of PCMs in a ver-
tical and horizontal shell-and-tube TES unit, using a combined con-
duction and convection heat transfer model. Wang et al. [15] conducted
experimental research on a vertical shell-and-tube LHS unit with ery-
thritol as the PCM and found that as the molten PCM region increased,
the natural convection effect increased. Natural convection had sig-
nificant impacts on the charging performance of PCM. Also presenting
the similar results, Tao et al. [16] established three-dimensional models
for a shell-and-tube LHS unit. The results showed that when PCM in
shell side and PCM in tube side, the latent heat storage rate of the
models with natural convection was increased 13.9% and 28.5%, re-
spectively. The above researches provide some feasible methods for the
design, construction, and application of LHS systems. However, the
relatively low thermal conductivities of most PCMs result in lower heat
transfer rates during melting and solidification, which restrict their
large-scale application.
In recent years, how to improve the thermal conductivity of PCM
has attracted great attention. Many efforts have been made in enhan-
cing heat transfer by adding finned configurations [17], using multi-
stage PCMs [18], impregnating a porous matrix and dispersing high
thermal conductivity materials [19,20]. Zeng et al. [21] chose Ag na-
nowires as additives to prepare 1-Tetradecanol/Ag nanowires compo-
site PCM (CPCM). The thermal conductivity of CPCM increased from
0.32 to 1.46 W/m·K when 62.73 wt% Ag nanowires were added. Xie
et al. [22] used an organic PCM (PA) mixed with expanded graphite
(EG) to obtain PA/EG CPCM with large enthalpy and high thermal
conductivity to enhance photo-thermal conversion performance. How-
ever, most of the previous studies have focused on low temperature
PCMs, which cannot meet the current industrial demand for higher
operating temperature [23]. The improvement of solar thermal cycle
efficiency requires higher operating temperatures [24]. Therefore, in-
tensive study on high-temperature PCMs for LHS systems is necessary.
High-temperature TES systems utilizing molten salts have proven to
be an effective solution for improving the solar thermal cycle efficiency
[25]. Molten salts have been widely used as heat storage media for
concentrating solar power (CSP) plants [23,26]. Villada et al. [27] re-
ported the Rankine cycle efficiency of the power block in a CPS para-
bolic trough plant could be improved from 37.6% to 40% when using
molten salts instead of thermal oil. The schematic diagram of a heat
transfer and heat storage system for CSP tower plants is shown in Fig. 1.
Due to the high melting point of molten salt, higher temperature steam
can be generated to allow for higher turbine efficiency, which can im-
prove the solar thermal cycle efficiency [26,28]. Therefore, molten salts
with higher operating temperature are being studied by research teams
worldwide for the next generation of CSP plants to improve system
efficiency and reduce power generation costs [24,28]. Fluoride salts are
less attractive due to their relatively high cost [28]. Chloride salts with
low prices, excellent thermal stability and high operating temperature
required are promising candidates for high-temperature TES systems
[26,29]. Du et al. [23] designed and prepared ternary eutectic chloride
salt (NaCl-CaCl2-MgCl2) for TES systems over 500 °C in CSP plants and
studied its thermal stability. Li et al. [30] investigated and predicted the
thermophysical properties of 16 eutectic compositions of chloride salts
to satisfy the requirement for high-temperature heat transfer fluid and
TES systems.
As a phase change material, molten salts also have the problem of
low thermal conductivity. Many studies have been done to improve the
thermal conductivity of molten salts. Myers et al. [20] doped CuO na-
noparticles into nitrate salts systems to prepare salt-CuO CPCM and
found that the addition of CuO improved the thermal diffusivity and
thermal conductivity of nitrate salts. Xiao et al. [31] added expanded
graphite to sodium nitrate to prepare CPCM. Measurement results
showed that the thermal conductivity of CPCM with 5%, 10% and 20%
expanded graphite was twice, four times and seven times higher than
that of pure sodium nitrate, respectively. Tian et al. [32] performed the
experimental exploration of Mg particles additives on the thermal be-
havior of ternary carbonate salts systems as PCM. Compared with pure
ternary carbonate salt, the thermal conductivity of CPCM with 2 wt% of
Mg was enhanced by 45.11%. From the previous studies, doping high
thermal conductivity materials into molten salts is an effective way to
enhance its thermal conductivity. Most of the previous researches on
improving the heat transfer properties of molten salts have focused on
nitrate and carbonate. Tian et al. [26] prepared chloride salts (NaCl-
CaCl2-MgCl2)/expanded graphite CPCM. They found that the thermal
conductivities of the CPCMs were more than 1.3 times higher than that
of the pure salts. However, carbon materials and molten salts are in-
compatible, which makes it difficult to form stable suspensions, re-
sulting in phase separation. This will damage the stability of the heat
storage system and even worsen the heat transfer. Therefore, the studies
on the improvement of molten salts heat transfer properties for high-
temperature TES systems are not comprehensive. More studies need to
be conducted on the evaluation of heat transfer enhancement of
chloride salts for high-temperature TES systems.
In this study, ternary chloride salts (MgCl2:KCl:NaCl with
51:22:27 molar ratio) were chosen as base salt. In order to improve the
original thermal properties of chloride salts for better application in
high-temperature TES systems, different metal oxide nanoparticles
(Al2O3, ZnO, and CuO, respectively) were dispersed into the base salt to
prepare CPCMs. The microstructures of the base salt and CPCMs were
characterized through scanning electron microscope (SEM). The dif-
ferential scanning calorimeter (DSC) technique, laser flash analysis
(LFA) method and thermogravimetric analysis (TGA) were used to
measure their thermal properties. The obtained results provided some
new considerations and feasible methods for the design of CPCMs ap-
plied in future high-temperature TES devices.
2. Experimental
2.1. Materials
In this study, potassium chloride (AR, ≥99.5% purity, Shanghai
Aladdin Co. Ltd), anhydrous magnesium chloride (AR, ≥99.0% purity,
Shanghai Aladdin Co. Ltd), and sodium chloride (AR, ≥99.5% purity,
Shanghai Aladdin Co. Ltd) were selected for the preparation of ternary
chloride salts, which was set as base salt. ZnO (AR, 99.9% purity,
Shanghai Aladdin Co. Ltd), CuO (AR, 99.9% purity, Beijing DK nano
technology Co. Ltd), and Al2O3 nanoparticles (AR, 99.9% purity,
Beijing DK nano technology Co. Ltd) were selected as additives. Then
these nanoparticles were doped into the base salt to prepare the CPCMs.
The detailed thermo-physical properties of the chloride salts and metal
oxide nanoparticles are shown in Table 1.
2.2. Preparation
Different CPCMs doped with different metal oxide nanoparticles
were prepared using the solution evaporation method. The processing
diagram for the preparation of CPCMs is shown in Fig. 2. The detailed
D. Han, et al. Applied Energy 264 (2020) 114674
2

3. preparation processes are as follows:
(1) Heat MgCl2, KCl, and NaCl with a temperature setting of 120 °C for
2 h to remove moisture. Then weight MgCl2, KCl, and NaCl at a
molar ratio of 51:22:27 using a high-precision electronic balance
and mix them uniformly to prepare the base salt.
(2) Weight gum arabic (as a dispersant) and nanoparticles using the
high-precision electronic balance and add them to deionized water.
Then vibrate ultrasonically for about 2 h to form a uniformly dis-
persed suspension.
(3) Add the base salt to the suspension to form the CPCMs solutions
with 0.7 wt% nanoparticles. Then vibrate ultrasonically for about
2 h to ensure uniformity of the solutions.
(4) Evaporate the solution in a vacuum drying oven at atmospheric
pressure with a temperature setting of 110 °C for 6 h to remove the
water from the CPCMs. Then cool and pulverize the CPCMs for the
follow-up experimental measurements.
(5) Repeat the above processes to obtain 3 kinds of CPCMs doped with
Al2O3 nanoparticles, ZnO nanoparticles, and CuO nanoparticles,
respectively.
2.3. Characterization
2.3.1. Microstructure
The surface microscopic morphology of all samples (base salt, metal
oxide nanoparticles, and CPCMs with different nanoparticles) was ob-
served using a scanning electron microscope (SEM, Phenom Pro,
Phenom World, Shanghai) at room temperature. The accelerating vol-
tage and resolution of the instrument were 15 kV and 8 nm, respec-
tively.
2.3.2. Melting temperature and phase change enthalpy
DSC was used to determine the melting temperature and phase
change enthalpy of the base salt and CPCMs. The DSC tests were con-
ducted using high temperature DSC analysis (DSC 404 F3 Pegasus,
NETZSCH, Germany) under a constant argon atmosphere with the flow
rate of 50 mL/min. Approximately 10 mg of the sample was placed in a
graphite pan. The graphite pan was covered with a lid with small holes
to balance the pressure inside the graphite pan to protect the instru-
ment from potential damage [37]. The heating rate was 10 °C/min from
Fig. 1. Schematic of a heat transfer and storage system.
Table 1
Detailed thermo-physical properties of chloride salts and metal oxide nano-
particles. The heat of fusion data and thermal conductivity data of chloride salts
were taken from Ref. [33]. The density data of chloride salts and nanoparticles
were taken from Ref. [30] and Ref. [34], respectively. The remaining data were
taken from FactSage [35], unless otherwise noted.
Material Tm (°C) ΔHf (J/g) cp (J/g·K)
25 °C
ρ (g/cm3
)
25 °C
λ (W/m·K)
25 °C
KCl 771 355.97 0.69 1.98 6.56
MgCl2 714 452.83 0.75 2.32 2.64
NaCl 801 482.02 0.87 2.16 6.60
Al2O3 2054 1092.59 [34] 0.78 3.97 36.30 [20]
ZnO 1975 – 0.51 5.60 30.76 [36]
CuO 1477 148.34 [34] 0.53 6.31 76.50 [20]
Fig. 2. Processing diagram for the preparation of CPCMs.
D. Han, et al. Applied Energy 264 (2020) 114674
3

4. room temperature to 600 °C. The error of temperature was within ± 3
°C and the relative error on phase change enthalpy was estimated
within 4%.
2.3.3. Thermal diffusivity
The thermal diffusivity of the base salt and CPCMs was measured
using laser flash analysis (LFA 1000, Linseis, Germany) in the helium
atmosphere. The instrument calculated the thermal diffusivity by
heating the front surface of the crucible containing the samples with a
laser pulse and measuring the temperature rise of the rear surface over
time. Given that the test time was very short, heat loss inside the sample
could be effectively avoided [38]. First, the samples were pretreated
under vacuum to eliminate gas bubbles in the samples to ensure the
uniformity of the samples, making the measurement more accurate
[39]. Then, the crucible filled with the pretreated samples was placed in
the sample holder. The thermal diffusivity was determined by heating
the bottom of the crucible by a laser pulse and measuring the change of
temperature rise as a function of time by infrared detector. The average
value was obtained by measuring each sample three times. The un-
certainty of the thermal diffusivity was estimated at 5%.
2.3.4. Thermal stability
Thermogravimetric analysis (TGA) was used to study the thermal
stability of the base salt and CPCMs. The TG tests from 50 °C up to
1000 °C of 10 mg of powder were carried out using the simultaneous
thermogravimetric-differential scanning calorimetry analysis (STA 449
F3 Jupiter, NETZSCH, Germany) in a 50 mL/min argon flow with a
heating rate of 10 °C/min. A platinum-rhodium (Pt-Rh) crucible was
used to contain the samples due to its high temperature resistance and
good thermal stability. The mass of the samples was controlled from
5 mg to 15 mg for each measurement to have enough samples to fill the
Pt-Rh crucible, without causing spillage during the measurement pro-
cess. The temperature accuracy was about ± 0.5 °C. The weighing re-
solution was about 0.1 μg and the balance precision was about 0.01%
[40].
3. Results and discussion
3.1. Microstructure analysis
The microstructure of the base salt, metal oxide nanoparticles, and
CPCMs characterized by SEM are shown in Fig. 3. From Fig. 3(a), the
SEM image under high magnification clearly shows that the base salt
surface was rough with an irregular structure. Fig. 3(c) and (g) are high-
resolution images of the raw Al2O3 and CuO nanoparticles, respectively.
The Al2O3 nanoparticles were composed of small particles, while the
raw CuO nanoparticles were composed of blocky structures. In
Fig. 3(e), the raw ZnO nanoparticles were mainly composed of thin
foliated structures. The large specific surface areas of Al2O3, ZnO, and
CuO nanoparticles allow for more heat transfer [41]. Thermal inter-
action could be available quickly and efficiently along the surface of
nanoparticles [32]. SEM images of the CPCMs doped with Al2O3, ZnO,
and CuO nanoparticles are shown in Fig. 3(b), (d), and (f), respectively.
The morphological structures of the CPCMs were quite different from
the base salt. Due to the change of the interaction force between the
charged ions of the base salt and the surface atoms of nanoparticles, the
surface of the CPCMs formed nanostructures with larger specific surface
area and surface energy.
The surface energy is defined as the excess energy at the surface of a
material compared to the bulk, or it is the energy required to create unit
area of a surface. Compared with bulk material, the nanoparticles have
smaller particle size and larger specific surface areas. The number of
surface atoms is larger than that of internal atoms. Due to the lack of
atomic coordination, the activity of the atoms on the surface of the
nanoparticles is high and the surface energy is high [42]. When the
nanoparticles were doped into the base salt, the surface atoms of the
nanoparticles combined with the charged ions of the base salt, espe-
cially the positive ions. This caused a change in the local molar com-
position of the base salt in the vicinity of the nanoparticles [42]. Then,
the salt recrystallized, and the nanoparticles would act as nucleation
sites, eventually forming interconnected nanostructures with special
shapes. This provided some convenient channels for heat transfer,
which may be the reason for the significant improvement in thermal
performance of CPCMs.
3.2. Thermal properties analysis
3.2.1. Melting temperature analysis
The melting temperature of the base salt and CPCMs with different
nanoparticles was determined by DSC. The DSC heat flow curves during
the melting process are presented in Fig. 4. The sharp single en-
dothermic peaks represented the change from solid to liquid state [32].
In DSC curves, the intersection of the extrapolated baseline and the
tangent line at the inflection point on the lower temperature side of the
endothermic peak, is used to represent the melting temperature of
samples [43]. As shown in Fig. 4, the melting temperature of the base
salt was 399.7 °C which is close to that of ternary chloride salts system
of 396 °C reported by Janz [44]. The agreement obtained with the lit-
erature indicates the accuracy of our measurement result. The melting
temperature of the CPCM with Al2O3, ZnO, and CuO nanoparticles were
398.8 °C, 397.6 °C, and 401.3 °C, respectively. It is noticeable that the
melting temperature of CPCMs did not change significantly compared
with that of the base salt. By considering the conclusion presented in
Ref. [45] that the melting temperature is positively correlated with the
specific surface area of the nanomaterial at the same mass fraction, the
highest melting temperature observed in the case of CPCM with ZnO
nanoparticles can be attributed to its largest specific surface area.
3.2.2. Thermal energy storage capacity analysis
TES capacity is a key parameter for TES systems, including LHS
capacity and SHS capacity [44]. Fig. 5 shows the phase change latent
heat of the base salt and CPCMs obtained by integrating the fusion peak
using the extrapolating baseline in the DSC curves [20]. The TES ca-
pacity of latent heat of the base salt, the CPCM with Al2O3 nano-
particles, the CPCM with ZnO nanoparticles, and the CPCM with CuO
nanoparticles was 283.3 kJ/kg, 276.5 kJ/kg, 265.2 kJ/kg, and
261.8 kJ/kg, respectively. The TES capacity of latent heat of the CPCMs
was slightly lower than that of the base salt, which may be caused by
the decrease of the proportion of ternary chloride salts due to the ad-
dition of metal oxide nanoparticles. The TES capacity of latent heat of
the CPCMs decreased by around 2.4–7.6%. These results implied that
the metal oxide nanoparticles did not contribute to the phase change
latent heat while the base salt underwent solid-liquid phase transition
[32].
To compare the TES capacity of sensible heat of the base salt and
different CPCMs, 480–600 °C was selected as the working temperature
range. The TES capacity of sensible heat was calculated by considering
Ref. [37].
∫
=
°
°
q T dT
mdT dt
Q
( )
/
S
C
C
480
600
(1)
where QS, q, T, m and dT dt
/ are the TES capacity of sensible heat, heat
flow, working temperature, mass of the samples, and the heating rate,
respectively.
The TES capacity of sensible heat is shown in Fig. 6. The results
show that the TES capacity of sensible heat of the base salt and CPCMs
has a nearly linear growth trend with the increasing temperature, which
is in coherence with that of nanocomposite salts from Ref. [37]. The
TES capacity of sensible heat of base salt, the CPCM with Al2O3 nano-
particles, the CPCM with ZnO nanoparticles, and the CPCM with CuO
D. Han, et al. Applied Energy 264 (2020) 114674
4

5. nanoparticles were 140.2 kJ/kg, 159.4 kJ/kg, 145.7 kJ/kg, and
152.4 kJ/kg, respectively. TES capacity of sensible heat of the CPCMs is
slightly higher than that of the base salt in the working temperature
range. The TES capacity of sensible heat of the CPCMs increased by
around 3.9–13.7%. In particular, the CPCM with Al2O3 nanoparticles
showed the maximum increasement.
Compared with the base salt, although the addition of nanoparticles
enhanced the TES capacity of the sensible heat of CPCMs, the decrease
in the TES capacity of latent heat was observed. Combined with the
above results, the addition of nanoparticles had little effect on the total
TES capacity. Therefore, for the improvement of the thermal perfor-
mance of chloride salts, more emphasis should be placed on the im-
provement of the thermal conductivity [45].
3.2.3. Thermal diffusivity analysis
The thermal diffusivity of the base salt and CPCMs determined by
LFA is shown in Fig. 7. The thermal diffusivity of the base salt decreased
with increasing temperature. Moreover, the same trend was observed in
Fig. 3. SEM images: (a) base salt; (b) CPCM with Al2O3; (c) Al2O3 nanoparticles; (d) CPCM with ZnO; (e) ZnO nanoparticles; (f) CPCM with CuO; (g) CuO nano-
particles.
Fig. 4. DSC curves of base salt and CPCMs.
Fig. 5. Phase change latent heat of base salt and CPCMs.
D. Han, et al. Applied Energy 264 (2020) 114674
5

6. CPCMs. Compared with the base salt, the thermal diffusivity of the
CPCMs was significantly improved. This may be due to the formation of
nanostructures with special shapes that enhance the energy transfer
process. The effect of different metal oxide nanoparticles on the thermal
diffusivity is shown in Fig. 8. While the thermal diffusivity of the CPCM
with Al2O3 nanoparticles exhibited the most obvious improvement, the
least change in the thermal diffusivity of the CPCM with ZnO nano-
particles is obtained.
As shown in Fig. 8, the enhancement degree of the thermal diffu-
sivity of CPCM with Al2O3 nanoparticles increases with increasing
temperature. For the CPCM with CuO nanoparticles, its enhancement
degree of the thermal diffusivity initially decreases with temperature,
before increasing gradually after 400 °C. However, for the CPCM with
ZuO nanoparticles, its enhancement degree increased with temperature
before 400 °C. From Fig. 8, the average thermal diffusivities of the
CPCMs with Al2O3, CuO, and ZnO nanoparticles as additives were en-
hanced by about 61.79%, 21.25%, and 6.25%, respectively. The Al2O3
nanoparticle provides the greatest improvement ability to thermal dif-
fusivity, followed by the CuO nanoparticle and ZnO nanoparticle. Based
on the above results, the Al2O3 nanoparticle may be an optimal additive
to improve the thermal diffusivity of chloride salts.
3.2.4. Thermal conductivity analysis
A critical factor for the molten salt applied as TES media is the
thermal conductivity that can be calculated by the following equation.
=
k α ρ c
· · p (2)
where k is the thermal conductivity, α is the thermal diffusivity, ρ is the
density, and cp is the specific heat.
The CPCM density can be obtained by considering Ref. [46].
= + −
ρ φ ρ φ ρ
· (1 )·
CPCM np salt
np np (3)
where φnp is the concentration of nanoparticle, ρCPCM, ρnp and ρsalt are
the density of CPCM, nanoparticle, and base salt, respectively.
The empirical correlations of density with temperature for single-
component chloride salt are shown in Table 2. It can be assumed that
the density of metal oxide nanoparticles is a constant value (see
Table 1) because its change with temperature is negligible. The density
changes of the base salt and CPCMs as a function of temperature are
plotted in Fig. 9.
For simplicity, the specific heat of single-component chloride salt
and the metal oxide nanoparticles was obtained from FactSage [35].
Given that the melting temperatures for the base salt and CPCMs were
around 400 °C, the specific heat value of the chloride salts was selected
from the solid region when the temperature was below 400 °C. Other-
wise, the specific heat value of the chloride salts was selected from the
liquid region. The specific heat value of the nanoparticles was selected
from the solid region, because the nanoparticles did not participate in
the phase transition and remained solid state all the time during the
heating process [26]. The specific heat of CPCM can be calculated by
considering Ref. [20].
=
+ −
c
φ ρ c φ ρ c
ρ
· · (1 )· ·
p CPCM
np p np salt p salt
CPCM
,
np , np ,
(4)
where cp CPCM
, , cp np
, , and cp salt
, are the specific heat of CPCM, nano-
particle, and base salt, respectively. Detailed values of the specific heat
of single-component chloride salt, metal oxide nanoparticles, base salt
and different CPCMs at different temperatures are shown in Table 3.
Based on the obtained density, the specific heat and thermal diffusivity,
the thermal conductivity of the base salt and CPCMs estimated using
Eq. (2) is illustrated in Fig. 10. The thermal conductivity of the base salt
and CPCMs decreases with increasing temperature in the solid region.
The same trend is also observed in the liquid region. However, there is a
relatively significant change in the thermal conductivities of the base
salt and CPCMs during phase transition, which was due to the dramatic
change in the specific heat during the phase transitions. The thermal
conductivities of CPCMs were improved, mainly due to the high
Fig. 6. TES capacities of base salt and CPCMs.
Fig. 7. Thermal diffusivity of base salt and CPCMs.
Fig. 8. Effect of different metal oxide nanoparticles on thermal diffusivity of
chloride salts.
Table 2
Densities of single-component chloride salt [33]. Note, the
unit of temperature is K.
Salt Density equation (g/cm3
)
KCl = − × −
ρ T
2.1359 5.831 10 4
MgCl2 = − × −
ρ T
1.976 3.02 10 4
NaCl = − × −
ρ T
2.1393 5.43 10 4
D. Han, et al. Applied Energy 264 (2020) 114674
6

7. thermal conductivity of nanoparticle additives (see Table 1), which
makes the CPCMs form effective heat conduction links [7]. Moreover,
the nanoparticles impelled the CPCMs to form nanostructures that en-
hance the energy transfer process. However, Tao et al. [7] pointed out
that the aggregation of nanomaterials and the contact thermal re-
sistance between different materials will reduce the thermal con-
ductivity. Therefore, it is necessary to comprehensively consider the
above factors when analyzing the effect of different nanoparticles on
the thermal conductivity of the base salt.
Fig. 10 shows that the thermal conductivity of the three CPCMs is
higher than that of the base salt. Thus, it could be considered that for
the three CPCMs, the effect of forming the heat conduction links to
improve the thermal conductivity was greater than the effect of the
contact thermal resistance and the aggregation of nanomaterials. The
effect of different metal oxide nanoparticles on the thermal con-
ductivity of chloride salts is shown in Fig. 11. The average thermal
conductivities of the CPCMs with Al2O3, CuO, and ZnO nanoparticles as
additives were enhanced by about 62.59%, 21.58%, and 6.47%, re-
spectively. In particular, the thermal conductivity of CPCM with Al2O3
nanoparticles shows the greatest improvement, which is reflected in the
enhancement of thermal conductivity at all points in the measurement
temperature range by more than 48%. This can be understood by the
fact that Al2O3 nanoparticles were more efficient in forming heat con-
duction links. In contrast to CPCM with Al2O3, the thermal conductivity
of CPCM with ZnO nanoparticles exhibited the least improvement.
Considering Fig. 3 as described above, the least improvement in the
thermal conductivity of CPCM with ZnO could be attributed to the less
contribution of the thin foliated structure of ZnO nanoparticles in the
formation of heat conduction links. Moreover, the aggregation of ZnO
nanoparticles was significant due to its large specific surface area. The
above results indicate that Al2O3 nanoparticles were an effective ad-
ditive for improving the thermal conductivity of chloride salts. The
relatively high thermal conductivity of CPCM with Al2O3 nanoparticles
can increase thermal charging and discharging rates, further improving
the performance of TES systems [7]. The CPCM with Al2O3 nano-
particles may have good application prospects as heat storage and heat
Fig. 9. Density of base salt and CPCMs as a function of temperature.
Table 3
Specific heat values of single-component chloride salt, metal oxide nanoparticles, base salt, and different CPCMs.
Temperature (°C) Specific heat cp (J/g·K)
NaCl KCl MgCl2 Al2O3 ZnO CuO Base salt CPCM + Al2O3 CPCM + ZnO CPCM + CuO
350 0.960 0.764 0.842 1.113 0.594 0.645 0.897 0.899 0.895 0.895
370 0.966 0.770 0.845 1.121 0.597 0.648 0.902 0.904 0.900 0.900
400 0.974 0.779 0.849 1.133 0.601 0.653 0.910 0.912 0.908 0.908
430 1.240 0.987 0.967 1.145 0.605 0.657 1.135 1.135 1.132 1.132
450 1.237 0.987 0.967 1.153 0.608 0.660 1.134 1.134 1.130 1.130
Fig. 10. Thermal conductivity of base salt and CPCMs as a function of tem-
perature.
Fig. 11. Effect of different metal oxide nanoparticles on thermal conductivity of
chloride salts.
Fig. 12. Weight loss curves of base salt and CPCMs during heating process.
D. Han, et al. Applied Energy 264 (2020) 114674
7

8. transfer media.
3.2.5. Thermal stability analysis
The thermal stabilities of the base salt and CPCMs were evaluated
by TGA. The weight loss curves upon heating are shown in Fig. 12. The
first weight loss of base salt and CPCMs observed around 200 °C was
about 2% and 3%, respectively, due to the removal of the adsorbed
water of salts (mainly dehydration of hydrated magnesium chloride
[47]). Due to the loss of salt hydrolysis (mainly hydrolysis of hydrated
magnesium chloride [47]), the second weight loss of base salt, CPCM
with Al2O3 nanoparticles, CPCM with ZnO nanoparticles, and CPCM
with CuO nanoparticles observed around 250 °C was about 2%, 1.3%,
1.8%, and 2%, respectively. The third weight loss of base salt and
CPCMs observed around 400 °C was about 1%, which was caused by the
solid-liquid phase transition of salts. The weight loss curves changed
sharply after 700 °C. These experimental results indicate that all the
three CPCMs doped with Al2O3, ZnO, and CuO nanoparticles, could be
stably operated at high temperature in an argon atmosphere.
According to the experimentally measured TG curves in Fig. 12, all
the three CPCMs doped with Al2O3, ZnO, and CuO nanoparticles have a
wide operating temperature range, indicating that they have excellent
thermal stability. Moreover, combined with the definition of the upper
limit of thermal stable working temperature reported by Tian et al.
[32], the base salt and the three CPCMs doped with Al2O3, ZnO, and
CuO nanoparticles measured in argon atmosphere, had upper limits of
800 °C, 820 °C, 805 °C, and 810 °C, respectively. The upper temperature
limit of the thermal stability of three CPCMs is slightly higher than that
of the base salt. In particular, the CPCM with Al2O3 nanoparticles ex-
hibited the best upper temperature limit of thermal stability, which
could make it suitable for high-temperature thermal energy storage.
4. Conclusion
In this study, the thermal properties of ternary chloride salts and
CPCMs with different metal oxide nanoparticles (Al2O3, ZnO, and CuO)
were investigated. The melting temperature, phase change enthalpy,
TES capacity, thermal diffusivity, and upper limit of thermal stability
were measured and analyzed. The following conclusions have been
drawn:
(1) The phase change latent heat of the CPCMs decreased by around
2.4–7.6% and the sensible heat of CPCMs increased by around
3.9–13.7%. Compared with the base salt, the effects of nano-
particles additives on the total TES capacity were not significant.
The melting temperature of CPCMs had negligible change.
(2) The thermal diffusivity and the estimated thermal conductivity of
the CPCMs were significantly improved, compared to the base salt.
In particular, the CPCM with Al2O3 showed the most obvious en-
hancement, which was reflected in its average thermal diffusivity
increased by 61.97%. Moreover, the highest thermal conductivity
improvement of more than 48% was obtained with the CPCM using
Al2O3 dopant.
(3) CPCM with Al2O3, ZnO, and CuO metal oxide nanoparticles have a
wide operating temperature range with excellent thermal stability.
The upper temperature limit of the thermal stability of all samples
in an argon atmosphere is above 800 °C. In particular, CPCM doped
with Al2O3 showed the best upper limit of thermal stability tem-
perature of 820 °C, which implied that it had good application
prospects in high-temperature TES systems.
(4) CPCM with Al2O3 nanoparticles exhibited the best overall TES
performance, indicating that Al2O3 nanoparticles were the optimal
additive to improve the thermal properties of chloride salts.
CRediT authorship contribution statement
Dongmei Han: Writing - original draft, Writing - review & editing.
Bachirou Guene Lougou: Writing - original draft, Writing - review &
editing, Supervision, Funding acquisition. Yantao Xu: Investigation.
Yong Shuai: Resources, Supervision, Funding acquisition. Xing
Huang: Investigation, Resources.
Acknowledgments
Funding: This work was supported by the National Natural Science
Foundation of China [Nos. 51876049; 51950410590], the Chang Jiang
Young Scholars Program of China [Q2016186], and Fundamental
Research Funds for the Central Universities of China
[HIT.NSRIF.2020054]. We are grateful for Dr. Yanwei Hu’s support for
the preparation of samples, and researcher Zhongfeng Tang's support in
the measurement of thermophysical properties of the samples.
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