Renewable Energy 96 (2016) 960e965

Contents lists available at ScienceDirect

Renewable Energy
journal homepage: www.elsevier.com/locate/renene

Heat transfer enhancement of phase change composite material:

Copper foam/paraffin
Changhong Wang*, Tao Lin, Na Li, Huanpei Zheng
School of Materials and Energy, Guangdong University of Technology, Guangzhou 510000, PR China

a r t i c l e i n f o a b s t r a c t

Article history: Phase change materials (PCM) can store heat during the phase change process. Because foam metal is
Received 10 November 2015 light-weight, has a large specific surface area and conducts heat well, we chose it as a test material. The
Received in revised form phase change material paraffin was embedded in copper foam metal to form composite phase change
1 March 2016
materials. We constructed a platform to test the thermophysical properties of pure paraffin and the
Accepted 19 April 2016
composite materials, and the paraffin and composite phase change materials were processed using
numerical simulations to analyze their phase change process. For these simulations, the Fluent software
with Solidification/Melting and Porous Zone model was used. The results showed that copper foam can
Keywords:
Foam copper
effectively improve the internal heat transfer uniformity of paraffin, reduce the heat storage time of
Phase change composite material paraffin wax by 40%, and improve the relationship between the total phase transition time and the
Heat transfer characteristic heating boundary temperature in the copper/paraffin composite phase change materials.
Heat storage performance © 2016 Elsevier Ltd. All rights reserved.
Numerical simulation

1. Introduction relationship between the drop in the air flow and the heat transfer
coefficient of metal foam at different PPI. Sundarram et al. [17].
Phase change materials are widely used in electronic cooling to conducted a phase transition simulation of the melting and so-
improve energy efficiency [1,2] and renewable energy storage [3,4]. lidification of a single facing cubic metal foam structure; they used
These materials are chosen because of their ability to absorb and CFD software to research the influence of PPI and porosity on
release large amounts of heat, keeping the temperature constant phase transformation. Lachheb et al. [18,19]. researched the heat
[5,6] during the phase change process. However, the lower thermal storage and release process for phase change materials in filled
conductivity of the conventional phase change material decreases and unfilled metal foam; they proved that padding the metal foam
its energy storage efficiency and severely limits its applications in can increase the thermal conductivity and shorten the melting
practical engineering. Currently, compounds of consisting of phase and solidification times of the phase change material. In this
change materials and metals, ceramics or carbon nano-materials application, Qu et al. [20]. used metal foam composite phase
[7e9] are used to enhance the thermal conductivity of the change materials as the thermal management system of a lithium-
materials. ion battery; their results indicated that the composite material can
Copper foam is a versatile material [10] that is light-weight, has effectively reduce the surface temperature of the lithium battery
a large surface area, good thermal conductivity, and small com- and improve battery safety. Baby et al. [21]. researched the heat
plex permittivity [11e14]. However, due to the randomness of the transfer performance of filled and unfilled copper phase change
distribution of the pore structure of the metal foam itself, its heat material radiators; their results showed that copper-filled radia-
transfer process must account for the thermal conductivity, con- tors can enhance the heat transfer performance. Nazari et al. [22].
vection and radiation of the pore structure [15]. To learn more studied the convective heat transfer problems of plate heat
about the metal foam heat transfer mechanism, scholars have exchanger-filled metal foam in which the nanofluids came
conducted a great deal of research. Diani et al. [16] used X-rays to through; this study comprehensively evaluated the heat transfer
make transmission scanning images of metal foam and study the effect of metal foam padding plate heat exchange at different flow
rates.
This article studies the affects of temperature change on the
thermal storage performance of the phase change material
* Corresponding author.
E-mail address: wangchh@gdut.edu.cn (C. Wang). paraffin with and without copper foam as a filler material. It

http://dx.doi.org/10.1016/j.renene.2016.04.039
0960-1481/© 2016 Elsevier Ltd. All rights reserved.
 C. Wang et al. / Renewable Energy 96 (2016) 960e965 961

analyzes the relationship between the complete phase transition

time and the heat source temperature of paraffin and of composite
phase change materials. It simulates the phase change process of
paraffin and composite phase change material using CFD software
Fluent. Finally, this article analyzes the simulation results and
discusses the impact of copper foam at different heat source
temperatures on the heat storage performance of the phase
change material.

2. Experimental platforms to test material properties

The experimental test system is shown in Fig. 1a, b; it includes a

phase change energy storage device, a temperature logger, and a
constant temperature heating platform. The phase change energy
storage device is shown in Fig. 1(b); it has a volume of 100 mm

k vT vT
vn ¼ q0 90 mm
k vn ¼ q0 50 mm and is surrounded by 5 mm
thick PVC insulation. The top and bottom are made of 8 mm thick
aluminum and the sides and top are 10 mm of cotton insulation. Fig. 2. The DSC curve of paraffin.
During the test, six thermocouples were used, of which five (points
1e5) are distributed inside the storage material evenly spaced
10 mm apart; the sixth set of thermocouples was used to test the
ambient temperature, as shown in Fig. 1.
In this experiment, the phase change material is 46 # industrial
wax. The foam metal selected is bubble copper with a pore size of
approximately 2e3 mm and porosity of approximately 97.3%. The
paraffin wax heating into a liquid and foam copper was embedded
into Liquid paraffin, the composite material of solidification is the
composite phase change material that we want by a constant
temperature heating way. The thermal analysis of the pure paraffin
was made using DSC Q20 differential scanning calorimetry in a
temperature range of 20e95  C; the rate of the temperature in-
crease was 1  C/min, with nitrogen protection. The DSC test curve is
shown in Fig. 2, which shows that the phase transition point
temperature is 42.24  C, and the latent heat is 170.4 J/g. Fig. 3 shows
the temperature change curve of the solidification process of the
paraffin measured by the experimental test system. A thermal
diffusivity-NETZSCH LFA analysis of the pure paraffin and com-
posite materials was made using the flash method thermal analyzer Fig. 3. The temperature curve of the pure paraffin during solidification.
LFA 447. The thermal parameters are shown in Tables 1 and 2.

Table 1
The thermal physical property parameter table of paraffin.

T/( C) A(m2/s) L(W$m
1$K
1) Cp (J$g
1$K
1)

25  C 0.097 0.487 5.012

38  C 0.082 0.169 2.065
48  C 0.042 0.065 1.548

Table 2
Thermophysical property parameters of the composite phase change material.

T/( C) a)m2/s) L(W$m
1$K
1) Cp (J$g
1$K
1)


25 C 0.856 2.879 3.351
38  C 0.770 2.217 2.879
48  C 0.249 3.112 10.547

In the tables, a is thermal diffusivity, l is Coefficient of thermal conductivity Cp is

Specific heat capacity.

3. Simulation

3.1. Geometric modeling of the boundary conditions

The numerical simulation model structure is shown in Fig. 1 (b).

Fig. 1. (a and b) Schematic diagram of experiment testing system. The boundary conditions of the sides and the top were set to
962 C. Wang et al. / Renewable Energy 96 (2016) 960e965

adiabatic boundaries. According to the experimental heating tem- The calculation formula of the effective thermal conductivity of
perature, the temperature of the bottom of the aluminum plate was metal foam composite phase change material [26,27] is:
set to 70  C, 80  C and 90  C, respectively. The parameters of the " ! #
paraffin and the composite phase change materials were set ac-
qffiffiffiffiffiffiffi
 

ks þ p 1
d
1
d kf
ks ks þ 1
d k
k
3p 3p 3 f s
cording to the parameters in Table 1. For this paper, the phase
change material is paraffin; it is important to note that although the keff ¼ " #
qffiffiffiffiffiffiffi qffiffiffiffiffiffiffi

paraffin chosen for the experiment is 46 #, which represents the ks þ 4 1
d ð1
dÞ þ p 1
d
ð1
dÞ kf
ks
3 3p 3p
melting temperature of approximately 46  C, paraffin has no spe-
cific melting point, because it is non-crystalline. This means that it (2)
has an amorphous structure, and that its space lattice is not dis-
rupted by the absorption of heat in the melting process [23]. In the formula:
Crystalline structures are disrupted by heat absorption, because it
causes molecules or atoms to become arranged irregularly. After keffdThe effective thermal conductivity of the composite ma-
comparing the paraffin DSC curve and the solidification process terial, W/(m$K)
temperature curve (Fig. 3), the melting temperature in the simu- ksdThe thermal conductivity of the matrix in the porous media
lation process was set to a range of 42  Ce48  C, and the volume matrix, W/(m$K)
expansion caused by melting was ignored. kfdThe thermal conductivity of the metal foam, W/(m$K)
ddThe metal foam porosity
3.2. The model assumes
Although metal foam can effectively improve the phase change
materials’ thermal conductivity, in Formula (2), the effect of the
1) Liquid paraffin can be regarded as an incompressible Newtonian
thermal conductivity is only an ideal maximum. Studies [28] have
fluid due to its poor mobility, according to the Boussinesq
given detailed derivations of the effective heat transfer coefficient
approximation;
of the composite materials. Taking into account the contact resis-
2) The change in the parameters of the thermal performance of the
tance between the foam metal and the surface of the foam copper,
paraffin’s phase transition process from a glassy, highly elastic
and noting that the copper can be oxidized easily, we see that the
state to a viscous state can be ignored, as can the effects of
thermal conductivity of pure copper has a correction factor of 0.1.
radiation;
3) When the composite phase change material is solid or liquid, its
thermal properties are constant and isotropic. These thermal 4. Data analysis
properties vary linearly with temperature when in a molten
state. Because paraffin is an incompressible Newtonian fluid, its This article studied the change in the heat storage efficiency of
viscous dissipation effects are ignored, following the Boussinesq paraffin, with and without copper foam, and under conditions of
assumption [24]. changing temperature, by conducting an experiment in which foam
metallic copper was added to the phase change material paraffin. The
experiments were also simulated using the Fluent software with the
Solidification/Melting Model and the Porous Zone model. Experi-
3.3. Melting/freezing model mental and numerical simulation results are shown in Fig. 4(aec).
After analyzing the temperature variation curves of pure
In this study, the composite phase change process is simulated paraffin and paraffin wax/foam copper composite phase change
through Fluent software using the Solidification/Melting model and materials under different heating conditions, our conclusions are as
the Porous Zone model. According to Fluent software, the Solidifi- follows:
cation/Melting model uses the enthalpy e porous formula. The
Porous Zone model in essence uses a momentum equation with an (1) There is a certain discrepancy between the temperature
additional momentum source term; specific circumstances are change curves of the simulation and the experiment; this is
expressed in the literature [25]. The liquid-solid mixing zone is due to various factors, such as the environment, the device
seen as an area in which the porous zone is equal to porous medium used, and the means of measuring used during the experi-
of the area of the liquid phase region, and the phase transition of ment. The simulation itself may contain some errors, but the
mushy zone is the area in which the fluid fraction is between 0 and change trends are similar. Therefore, we are able to conclude
1. This pasty zone of porous and solidified materials assumes that that the using Fluent’s Solidification/Melting model and
the porous material decreases from 1 to 0 during the simulation. Porous Zone model is effective for analyzing the phase
When these materials are completely solidified into a unit, porosity change energy storage container and the foam metal filling in
becomes 0, so the speed is also reduced to 0. the paraffin to optimize the design.
Solidification/Melting model energy conservation equation: (2) As can be seen from the Fig. 4aec, the melting/solidification

 and porous media model of fluent can effectively optimize
vðrHÞ=vt þ V$ r!
n H ¼ V$ðkVTÞ þ S (1) the design of what the paraffin filled in the phase change
energy storage container or foam metal.It can be found from
RT
Where H ¼ h þ DH, h ¼ href þ Tref cp dT, r is density, ! n is fluid ve- the Fig. 4 that the temperature change of the simulation
locity, S is source term, and H is the total enthalpy. Including the process is similar to the experimental temperature in the
sensible heat enthalpy h and the latent heat enthalpy DH, href is the second part of the phase change process, but in the initial
reference enthalpy, Tref is the reference temperature, and cpis con- stage of the transformation process, the difference between
stant pressure specific heat capacity. the simulation temperature and experimental temperature is
Initial conditions: T(x,y,z,0) ¼ Tinit greatly, and with the increase of heat source temperature,
Boundary conditions:
k vT vT
vn ¼ q0 ;
k vn ¼ hsur ðT
To Þ the difference is more obvious. Perhaps the main reason,
q0 is the heat flux, hsur is the air surface heat transfer coefficient, however, the correction factor for 0.1 of thermal conductivity
and T0 is the ambient temperature. was introduced because of the copper foam oxidation, and
 C. Wang et al. / Renewable Energy 96 (2016) 960e965 963

material paraffin does not have a significant theoretical

phase transition in the phase change process. This means
that the temperature remained constant at the melting point
of the phase change phase, and that in different heating
conditions, the inflection point temperatures are not the
same. This occurred because the phase change process is a
complex process of heat and mass transfer, and paraffin itself
is an amorphous structure (i.e., it is not a crystalline structure
in which a phase transition that damages the lattice would
require a high energy transfer). Therefore, for paraffin, there
is no constant melting point, meaning that the latent heat
absorption and release process of paraffin has no significant
thermal effects and temperature may be kept relatively sta-
ble throughout the phase transition process. At the same
time, due to the improvement of the heating operation, the
temperature near the thermal boundary is higher, leading to
the phenomenon that the inflection point of the phase
change material after the phase change improves as heating
conditions improve.

Fig. 5 shows the copper foam/wax melting process cloud when

the temperature is 70  C. From the melting cloud, we can see there
is not a linear relationship between time and the liquid growth
process in the melting process of paraffin. At the beginning time of
500 s, the paraffin wax melted faster, and with the passage of time,
the melting rate gradually slowed. One reason for this is that in the
melting process, paraffin wax further from the heat boundary must
pass through the melted liquid paraffin to be absorbed. The liquid
paraffin thermal conductivity was 0.2e0.3 W/(m$K), resulting in
significant resistance and decreasing the paraffin melting speed;
therefore, the thickness of the phase change material is an impor-
tant factor affecting paraffin phase change time.

(4) Comparing the storage time under different temperature

conditions, the complete phase transition time and heating
boundary temperature relationship is shown in Fig. 6. Add-
ing foam copper can effectively improve the heat transfer
performance of phase change materials. When the heat is
70  C, the phase change material’s complete phase transition
time decreased from 10000 s to 6000 s; when the temper-
ature was 80  C, the complete transformation time decreased
from 6000 s to 3500 s; when the temperature was 90  C, the
complete phase transition time decreased from 4000 s to
2500 s. Thus, we can see that adding foam metal to the phase
change material can shorten the storage time by nearly 50%,
which greatly improves the storage efficiency of solar energy.

The variation process of the curve substantially fitted a power

function curve, wherein the fitting curve for pure paraffin is
t1 ¼ 5  1010 T
3.65, and the fitting curve for composite PCM is
t2 ¼ 2  1010 T
3.496. The power function curve’s characteristics
Fig. 4. aThe temperature change curve of the paraffin and composite phase change show that when the temperature is close to the phase transition
material under heat source temperatures is 70  C. b. The temperature change curve of temperature, the complete phase transition time is longer. How-
the paraffin and composite phase change material under heat source temperatures is
ever, as the temperature gradually increases, the time required for
80  C. The temperature change curve of the paraffin and composite phase change
material under heat source temperatures is 90  C.
phase changing will first descend rapidly, followed by the trans-
formation time decreasing until it approaches a constant. This is
generally consistent with the heat transfer law and is caused by the
the change of thermal performance parameters of paraffin temperature increasing, which increases the temperature differ-
from the glass state, high elastic state to the viscous state as ence, which in turn causes an increase in the heat flux and a
well as the influence of paraffin flow and heat radiation. But decrease in the heat transfer time.
the parameters are set to a certain value in the process of
simulation, this is the primary error factor and its heat 5. Conclusions
transfer mechanism will be further studied in the following.
(3) Through the analysis of the temperature change data in each To detect the heat storage properties of the composite phase
group, it can be seen that the low temperature phase change change material made by filling paraffin with foam copper, this
964 C. Wang et al. / Renewable Energy 96 (2016) 960e965

Fig. 5. The melting process of the bubble copper/paraffin at a temperature of 70  C.

Science Foundation of China (Grant No. 51306040) and the Science

and Technology project of Guangzhou-International Cooperation
Project (Grant No. 2016201604030056).

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