Energy Conversion and Management 81 (2014) 322–329
Contents lists available at ScienceDirect
Energy Conversion and Management
journal homepage: www.elsevier.com/locate/enconman
Characterization of phase change materials for thermal control
of photovoltaics using Differential Scanning Calorimetry
and Temperature History Method
A. Hasan a,⇑, S.J. McCormack b, M.J. Huang c, B. Norton d
a
Department of Architectural Engineering, College of Engineering, United Arab Emirates University, P.O. Box 15551, Al Ain, UAE
Department of Civil, Structure and Environmental Engineering, University of Dublin, Trinity College, Dublin 2, Ireland
Centre for Sustainable Technologies, University of Ulster, Newtownabbey, N. Ireland BT370QB, UK
d
Dublin Energy Lab., Focas Institute, School of Physics, Dublin Institute of Technology, Kevin St., Dublin 8, Ireland
b
c
a r t i c l e
i n f o
Article history:
Received 7 November 2013
Accepted 19 February 2014
Keywords:
Phase change materials
Thermal control
Photovoltaics
Temperature History Method
a b s t r a c t
Five solid–liquid phase change materials comprising three basic classes, paraffin waxes, salt hydrates and
mixtures of fatty acids were thermophysically characterized for thermal regulation applications in photovoltaics. The PCM were investigated using Differential Scanning Calorimetry and Temperature History
Method to find their thermophysical properties of interest. The relationship between thermophysical
properties of the PCM and their choice as temperature regulators in photovoltaics is discussed in relation
to the ambient conditions under which PV systems operate.
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
Silicon photovoltaics (PV) show a power drop of 0.3%/K up to
0.65%/K [1,2] above 25 °C panel temperature depending on type
of the PV cell and the manufacturing technology [3]. Various mathematical correlations have been developed to describe dependence
of PV operating temperature on climatic conditions and PV materials [4]. The operating temperature reached by PV panels and associated power drop largely depends on the climate of the site. In
Germany 50% of the solar radiation incident on a PV panel is above
600 W/m2 while in Sudan this value reaches 80% resulting different
operating temperatures and associated power drop [5,6]. A maximum PV operating temperature of 125 °C has been reported in
southern Libya (27.6 N and 14.2 E) resulting in a 69% reduction
in the nominal power [7]. The advisable operating temperature
limit for PV ranges from 40 to 85 °C [8] however in hot and arid
climates, PV temperature frequently rises above this temperature
Abbreviations: CL, eutectic mixture of capric–lauric acid; CP, eutectic mixture of
capric–palmitic acid; DSC, Differential Scanning Calorimetry; EG, expanded graphite; LDPE, low-density polyethylene; (P(BMA-co-MAA)), p(n-butyl methacrylate-comethacrylic acid); PCM, phase change material; PEG, polyethylene glycol; PGMA,
polyethylene glycol poly (glycidyl methacrylat); PUs, polyurethane polymers; RT20,
a commercial paraffin wax; SP22, a commercial blend of salt hydrate and paraffin
wax; THM, Temperature History Method.
⇑ Corresponding author. Tel.: +971 555454069.
E-mail address: [email protected] (A. Hasan).
http://dx.doi.org/10.1016/j.enconman.2014.02.042
0196-8904/Ó 2014 Elsevier Ltd. All rights reserved.
range [7], which results in temperature induced power failure as
well as PV cell delamination and rapid degradation [9] urging a
strong need for PV temperature regulation to maximize both panel
power output and lifetime.
In an attempt to avoid temperature dependant PV power loss,
passive cooling of PV with paraffin wax based solid–liquid phase
change materials (PCMs) was evaluated both experimentally and
numerically. The PCM was contained in a rectangular aluminium
container with internal dimension of (300 mm 132 mm 40 mm) and front surface of the container was selectively coated
to mimic a PV cell attached to the surface [9]. Temperature distributions on the front surface and inside the PCM were measured
and validated through a 2D as well as a 3D finite volume heat
transfer model [10,11]. Building on this work, Hasan et al. 2010,
replaced the selective coating meant to mimic the PV cell with
an actual PV cell encapsulated in perspex sheet and evaluated 5
PCMs under various solar radiation intensities to mimic different
weather conditions. Two PCMs, a eutectic mixture of capric acid–
palmitic acid and a salt hydrate CaCl26H2O achieved relatively
higher temperature regulation at all solar radiation intensities
[12]. A temperature regulation of 18 °C was recorded for 30 min
and 10 °C for 5 h at 1000 W/m2 insulation and 23 °C ambient temperature. Huang et al. [13] evaluated the insertion of metallic fins
in the PV-PCM systems previously investigated [10,11] and
reported an improvement in temperature regulation due to
increased natural convection. A simulation study was conducted
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A. Hasan et al. / Energy Conversion and Management 81 (2014) 322–329
Nomenclature
Symbols
Ac
Bi
cp,l
cps
cp,t
cp,w
Hm
hc
convective heat transfer area
biot number
mean specific heat capacity of liquid PCM
mean specific heat capacity of solid PCM
specific heat capacity of test tube
mean specific heat capacity of water
latent heat of fusion of PCM
convective heat loss coefficient
[14] for temperature regulation performance of BIPV containing
microencapsulated PCM with a melting point of 26 °C attached at
back of PV showed very low temperature drop from 49 to 47 °C
in summer and from 35 to 30.5 °C in winter. The low performance
can be attributed to lower thermal conductivity of encapsulation
materials and the lower mass ratio of PCM contained in microencapsulation. Biwole et al. [15] modelled heat and mass transfer of
a PCM layer at the back of PV and reported a time lag of 75 min
compared to bare PV panel to reach 40 °C which shows thermal
mass effect of adding PCM layer into PV. Author observed that
the previous research focussed on incorporating the PCM into PV
systems with least attention to characterizing PCM thermo-physical properties in relation to PV temperature regulation.
On another front most of the PCM characterization has been
conducted on determination of the thermophysical properties of
PCM [16] with no reported characterization for PCM as temperature regulators in PV systems. Polyethylene glycol (PEG10000)/
poly (glycidyl methacrylate) (PGMA) crosslinked copolymer was
prepared as solid–solid phase change material and it was found
that the PCM reversibly stores heat from 25 to 60 °C and maintains
the solid structure as high as 100 °C [17]. Polyurethane polymers
(PUs) have been synthesized as solid–solid phase change materials
using three different kinds of diisocyanate molecules and polyethylene glycols (PEGs) through the condensation reaction of PEGs
with diisocyanates. The solid–solid PCM possessed phase change
enthalpy of 179 kJ/kg and phase transition temperature of 60 °C
[18]. Expanded graphite (EG) was used in Sodium nitrate, potassium nitrate and their mixture to enhance PCM thermal conductivity. It was reported that by addition of 10 % by weight EG increased
the thermal conductivity of PCM by about 30–40% [19]. Composite
PCM by impregnating 65% paraffin into halloysite nanotube was
prepared and tested for leakage with a melting point of 57.16 °C
and heat storage capacity of 106.54 kJ/kg. Melting and freezing
time were reported to decrease by 60.78% and 71.52% respectively
with the addition of graphite [20]. Polyethylene glycol (PEG)/silicon dioxide (SiO2) composite form-stable phase change materials
(PCMs) without co-solvent and surfactant with 80% PEG weight
percentage. The PCM possessed enthalpy value of (102.8–111.1 J/
g) and phase transition temperature of up to 56.5 °C and showed
very little change in phase change enthalpy and temperature with
50 thermal cycles [21].
Form-stable paraffin/PUPCMs composites (n-octadecane/PUPCM, n-eicosane/PUPCM and paraffin wax/PUPCM) were prepared
with phase transition temperature range (20–65 °C) and enthalpy
range of up to 141.2 J/g. An inexpensive process of preparation
via bulk polymerization was introduced and the PCM was reported
to be stable at higher temperature [22]. Low-density polyethylene
(LDPE) with paraffin waxes was prepared by melt-mixing method
with a Brabender-Plastograph and were tested for leakage. It was
found that the wax remained compact during melting cycle showing no leakage [23].
mp
mt
mw
t1
t2
To
Tr
Ts
T1,a
mass of PCM
mass of test tube
mass of water
start of solidification time
end of solidification time
cooling curve start point
reference fluid (water) temperature
start of solidification temperature
ambient temperature
Microencapsulated PCM was prepared by containing n-alkane
in p(n-butyl methacrylate-co-methacrylic acid) shell (P(BMA-coMAA)) with phase change enthalpies of melting (130.3 J/g or
123.9 J/g) and freezing (125.8 J/g or 118.4 J/g). The PCM was
subjected to thermal cycles and phase change enthalpies varied
with thermal cycles [24]. polystyrene/n-tetradecane composite
nanoencapsulated phase change material were prepared via ultrasonic-assistant miniemulsion in situ polymerization with melting
and freezing temperatures of 4.04 °C and 3.43 °C rand latent
heats of 98.71 J g1 and 91.27 J g1, respectively. Mechanical structural stability was reported through freeze–thaw cycle test [25].
The properties desired for a suitable PCM are summarized in
Table 1. A systematic PCM choice is difficult as a particular PCM
may only have some of the desired characteristics, but may not
possess the others. The current work relates PCM properties of
melting point, latent heat of fusion, specific heat capacity, thermal
conductivity, density and under-cooling in relation to PV temperature and night time ambient temperature to regenerate PCM to
achieve better temperature regulation effect.
2. Methodology
PCMs were characterized using (i) Differential Scanning Calorimetry (DSC) and (ii) Temperature History Method (THM) to
determine their thermophysical properties and compare them
Table 1
Properties of a PCM desired for photovoltaic thermal regulation.
Requirement
Reason for requirement
High latent heat
High heat capacity
Good thermal
conductivity
Reversible phase change
Fixed melting point
Maximum heat absorption
Minimum sensible heating
Efficient heat removal
Physical
Congruent melting
Low volume expansion
High density
Minimum thermal gradient
No overdesign
Low containment requirement
Kinetic
No supercooling
Good crystallisation rate
Easy to freeze
Faster solidification
Chemical
Chemical stability
Non-corrosive
Non-flammable
Non explosive
Non-toxic
Long life
Long container life
Comply building safety codes
Environment friendly
Economic
Abundant
Cheap and cost effective
Market competitiveness
Economic viability and market
penetration
Environmental
Recyclable/reusable
Odour free
Ease to dispose of
Comfortable to apply in
dwellings environment
Properties
Thermal
Diurnal response
Consistent behaviour
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A. Hasan et al. / Energy Conversion and Management 81 (2014) 322–329
Table 2
Thermophysical characteristics determined through experiments and compared with literature of PCMs investigated for temperature regulation of photovoltaics.
PCM
Thermophysical properties of PCMs
Melting point (°C)
Experimental
Literature
Heat of fusion, (kJ/kg)
Experimental DSC
Experimental THM
Literature
Thermal conductivity (W/m/C)
Solid
Liquid
Specific heat capacity (kJ/kg/K)
Solid
Experimental (THM)
Experimental (DSC)
Literature
Liquid
Experimental THM
Experimental (DSC)
Literature
Corrosion to metallic containers
Thermal cyclic stability
Chemical classification
Density (kg/m3)
Solid
Liquid
Kinematic viscosity (m2/s) 103
Coefficient of thermal expansion (K1)
Material source
RT20
SP224A
CaCl26H2O
CL
CP
22
21 [28]
21.6
23 [28]
29.6
29.8 [31]
20.6
18.5 [35]
22.4
22.5 [35]
139
143
134 [28]
125
135
150 [28]
212
210
191 [31]
188
179
168 [35]
195
190
173 [35]
0.2 [28]
0.18 [11]
0.6 [28]
0.4 [41]
1.08 [41]
0.56 [41]
0.143 [38]
0.139 [38]
0.143 [38]
0.139 [38]
1.5
1.3
1.4 [11]
1.7
1.6
1.4 [31]
1.77
1.6
1.4 [31]
1.8
1.9
1.97 [40]
2.2
1.9
2 [40]
1.6
1.8
1.7 [11]
No [19]
Yes
Paraffin wax
1.8
1.7
1.95 [40]
Yes [28]
N.A
Mixture of paraffin wax and salt
hydrate
2.2
1.9
2.1 [40]
Yes [32]
Yes [31] No[34]
Salt hydrate
2.12
2.3
2.24 [40]
Yes [32]
Yes [36]
Eutectic mixture of
fatty acids
2.4
2.2
2.3 [40]
Yes [32]
Yes [36]
Eutectic mixture of
fatty acids
0.88 [19]
0.75 [19]
6.25 [11]
0.001 [11]
Rubith-erm
(2010)
1.49 [28]
1.44 [19]
1.23 [19]
0.0008 [28]
Rubith-erm
(2010)
1.71 [31]
1.56 [32]
1.84 [11]
0.0005 [41]
Sigma Aldrich
(2010)
0.89 [35]
0.77 [26]
0.0022 [31]
0.00067 [40]
Sigma Aldrich (2010)
0.87 [35]
0.79 [26]
0.0023 [26]
0.00078 [35]
Sigma Aldrich (2010)
with available literature research summarized in Table 2. DSC is a
standardized method to determine thermophysical properties of
small samples in the range of 3–10 mg while THM is a custom
method to accommodate larger sample size up to 40 g to represent
PCM mass at application scale. Additionally THM helps determine
under-cooling of PCM, a feature unavailable in DSC. Five PCMs are
selected for evaluation, comprising salt hydrates (SP22 and CaCl2
6H2O), paraffin (RT22) and eutectic mixtures of fatty acids
(capric–lauric acid and capric–palmitic acid). SP22, RT20 and
CaCl26H2O were sourced as ready-to-use from commercial manufacturers while fatty acid mixtures, CL (eutectic mixtures of 45%
capric acid and 55% lauric acid by weight) and CP (eutectic mixture
of 75.2% capric acid and 24.8% palmitic acid by weight) were prepared in laboratory from individual chemicals. Individual fatty
acids were mixed as solids in the aforementioned ratios to get a
net mixture of 40 g. Each mixture was heated to and kept at
80 °C for three hours to melt and form homogenous solution and
was subsequently cooled to and kept at 16 °C for five hours to let
them solidify as eutectics.
2.1. Differential Scanning Calorimetry
A heat-flux DSC was used and crucibles were purged with a
constant supply of nitrogen gas. PCM sample of 5 mg of each material was heated from 10 °C to 60 °C at a heating rate of 5 °C min1.
At the end of each heating run, the crucibles were cooled down to
10 °C with liquid nitrogen in preparation for the next run. Difference in heat input between the reference pan and the pan containing sample for a unit temperature rise was plotted and processed
to determine phase transition temperature, specific heat capacity
and latent heat of fusion of each PCM. Three runs were performed
for each PCM with the same experimental conditions to ensure
repeatability.
2.2. Temperature History Method (THM)
To characterize PCM by THM, 25 g melted sample of each of the
five PCMs was placed in 15 cm long glass test tubes with 1 cm
internal diameter and 0.8 mm wall thickness. These tube dimensions were selected to ensure that the Biot number, Bi = hR/
2K 6 0.1 where h is the convective heat transfer coefficient, R is radius of the tube and K is the thermal conductivity of the tube material. Satisfying these conditions enables heat transfer to occur
solely in one-dimension along the length of the tube being a
lumped system in front of heating or cooling source to apply
lumped capacitance method. The PCM and the reference (distilled
water) contained in the tube were heated and stabilized at 40 °C
for few minutes then subsequently exposed to cooler ambient at
5 °C to cool and solidify. Temperatures of the cooling PCM and reference were recorded and plotted against time until all the PCM
solidified.
Although the properties measured from DSC may serve as a reference, the fact that they accommodate small sample sizes in mg
range makes them irrelevant for large-scale applications in the
range of grams or kilograms. In order to determine properties for
a reasonable PCM sample size, Yinping et al. [16] introduced the
Temperature History Method (THM) based on recording the temperature of a PCM while it cools from a higher temperature liquid
state to a lower temperature to solidify. The cooling curve thus
obtained can be used to determine the (i) start and end of solidification, (ii) specific heat capacity, (iii) amount of under-cooling, (iv)
latent heat released during solidification, and (v) temperature
dependent enthalpy. The original method introduced by [16]
assumed that degree of super cooling represented the end of
melting and assumed no sensible heat due to incongruence, the
latter occurring in real melting/solidification contributes to errors
in determining thermophysical properties. The method was improved by Hong et al. [26] by (i) considering the end of melting
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A. Hasan et al. / Energy Conversion and Management 81 (2014) 322–329
to be when the derivative of the temperature–time graph on the
cooling curve becomes minimum and (ii) including sensible heating in the calculation of heat capacity and enthalpy. The THM
was further extended by Marin et al. [27] to determine temperature dependent properties such as specific heat capacity and enthalpy of the PCM and produced enthalpy temperature curves for
PCMs. The governing equations to calculate specific heat capacity,
heat of fusion and heat transfer coefficient are obtained by simple
energy balance given in Eq. (1)[16]:
ðmt cp;t þ mp cp;l ÞðT 0 T s Þ ¼ hc Ac A1
where Hm is the heat of fusion of the PCM, A2 ¼ t1 ðT s T 1;a Þdt
where T1,a is the ambient temperature and (t1 ? t2) is the time in
which the phase change occurs and:
ð3Þ
where cp,s is the mean specific heat capacity of the solid PCM,
Rt
A3 ¼ t23 ðT 0 T 1;a Þdt where Tr is the reference temperature. If a tube
containing pure water is suddenly exposed to the same ambient as
of the PCM, it also cools and releases heat following the curve
shown in Fig. 6. Considering the Bi < 0.1 similarly we have:
ðmt cp;t þ mw cp;w ÞðT 0 T s Þ ¼ hc Ac A01
ð5Þ
where mw and cp,w are the mass and mean specific heat capacity of
water respectively,
A01 ¼
Z
t 01
0
ðT T 1;a Þdt
and A02 ¼
Z
t 02
ðT T 1;a Þdt
t1
Using Eq. (5) the natural convective heat transfer coefficient (hc)
of air outside the tube is calculated which is 4.5 W/m2 K. Rearranging the equations described above, the specific heat capacities (cp,s
and cp,l) and the latent heat of fusion (Hm) of the PCM are calculated
using the Eqs. (6)–(8).
mw cp;w þ mt cp;t A3 mt
C p;t
mp
A02 mp
ð6Þ
1.
55
Liquid
50
-1.0
-1.0
-3.0
ð8Þ
60
Onset 29.2 [°C]
Peak: 29.7°C
Heat: 213 kJ/kg
Temperature (ºC)
0.0
Heat Flux (kJ/kg)
C p;s ¼
mw cp;w þ mt cp;t A1 mt
C p;t
mp
A01 mp
Fig. 1(a) presents the DSC thermograph of pure salt hydrate
CaCl26H2O and shows that heat absorption began at 25.9 °C and
completed at 33.9 °C with a peak at 29.7 °C. Start of heat absorption is marked where the DSC curve starts deviating from the datum (0) on vertical heat flow axis. Initially the slope of the curve
is smaller which shows sensible heat absorption below melting
point when the PCM starts heating in the crucible. Withtime the
slope increases and reaches infinite at a point which means heat
is being absorbed without temperature change in the PCM sample
which shows melting onset. A tangent is drawn at the point corresponding to infinite slope and is projected to intersect the datum
line (corresponding to 0 at vertical axis). A line is drawn from
the point of intersection between the tangent and datum line to
intersect temperature axis to determine the melting point of the
PCM. The curve continues beyond melting point and the slope of
the line reaches a point where it becomes zero which refers to a
point where maximum PCM melting happens. Slope of the line reverses from this point to reach back datum line (at 0 heat absorption) which refers to end of melting. The curve is processed in DSC
software to determine thermophysical properties of PCM. Fig. 1(a)
is a curve from the DSC which explains and labels how the temperatures and heat of fusion are determined through a built-in DSC
software with accuracy of one point after decimal. The melting
point was found to be 29.2 °C and latent heat of fusion of
213.2 kJ/kg compared to previously reported melting point of
29.8 °C and heat of fusion of 191 kJ/kg [31]. The melting point is
well in agreement with literature however the heat of fusion is
10% higher [31]. The difference in heat of fusion may be attributed
to (i) difference of PCM purity grade in experiments and literature
and (ii) difference in thermal cycles in experiments and literature.
Fig. 1(b) shows the THM cooling curve for CaCl2.6H2O when cooling from 55 to 10 °C. The curve shows PCM undergoes 11 °C of under-cooling (down to 18 °C) to crystallize and trigger solidification
ð2Þ
ð4Þ
C p;l ¼
3.1. Characterization of CaCl26H2O
R t2
ðmt cp;t þ mw cp;w ÞðT s T r Þ ¼ hc Ac A02
ð7Þ
Result for characterization of the five selected PCM using DSC
and THM are explained and discussed below.
where mt and mp are the masses of the tube and PCM respectively,
cp,l and cp,t are the mean specific heat capacities of liquid PCM and
test tube material respectively, T0 and Ts are the start of cooling
and start of solidification temperature, hc is the convective heat loss
coefficient, Ac is the convective heat transfer area of a tube and
Rt
A1 ¼ 01 ðT 0 T 1;a Þdt. Heat loss by the cooling PCM is due to natural
convection given by Eq. (2):
ðmt cp;t þ mp cp;s ÞðT s T r Þ ¼ hc Ac A3
mw cp;w þ mt cp;t A2
ðT 0 T s Þ
mp
A01
3. Results and discussion
ð1Þ
mp Hm ¼ hc Ac A2
Hm ¼
45
Solidification range
40
Solidification starts
35
Solidification ends
30
25
Under-cooling
20
-4.0
15
-5.0
0.0
10.0 20.0 30.0 40.0 50.0 60.0
Temperature (°C)
(a)
Solid
10
0
2000
4000
6000
8000
10000
Time (Sec)
(b)
Fig. 1. (a) Differential Scanning Calorimetry (DSC) and (b) Temperature History Method (THM) curves for salt hydrate CaCl26H2O [42].
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A. Hasan et al. / Energy Conversion and Management 81 (2014) 322–329
(b)
Liquid
(a)
Heat Flux kJ/kg
Solidification starts
Solidification ends
Solidification range
Solid
Fig. 2. (a) Differential Scanning Calorimetry (DSC) and (b) Temperature History Method (THM) curves for paraffins wax RT 20 [42].
(a)
(b)
Solidification starts
Solidification ends
Under-cooling
Solidification range
Fig. 3. (a) Differential Scanning Calorimetry (DSC) and (b) Temperature History Method (THM) curves for blend of salt hydrate and paraffin was, SP22 [42].
and the temperature subsequently rises to 29 °C where the PCM
actually solidifies. Solidification point obtained through THM
(29 °C) differs by 0.8 °C from melting point obtained through DSC
(29.8 °C) due to i) the PCM sample mass in THM being about
1000 times (in g) than that in DSC (in mg) and (iii) the DSC being
a standardized method while THM is custom made which may render measurement and data processing error. The heat release during solidification occurs between 29 and 27 °C which is very close
to the isothermal condition due to high thermal conductivity
(1.09 W/m K) of CaCl2.6H2O compared to RT22(0.2 W/m K). In general DSC and THM results are in agreement with the acceptable
deviations however the THM has an advantage of larger sample
size close to sample sizes in application range and helps determine
under-cooling of the PCM. The results show that CaCl26H2O is a
potential PCM for this application due to (i) shorter melting range
(ii) higher heat of fusion (iii) higher thermal conductivity (iv) higher density of 1710 kg/m3 and (v) very low volumetric expansion
4.03% between solid and liquid [21]. However the disadvantages
of the PCM are (i) higher melting point of 29.8° than the reference
PV temperature of 25 °C, (ii) higher corrosion rate in metallic
containers materials compared to the other PCMs [32,33] (iii) a
tendency to dehydrate during melting when exposed to air [34]
and (iv) under-cooling down to 18 °C may cause problems to
trigger solidification in summer nights. Using the same method,
melting point and heat of fusion were determined from DSC built
in software for the remaining PCM with higher accuracy however
the labels on DSC graphs will not be shown for Figs. 2–5 to avoid
repetition
3.2. Characterization of paraffin Waxes
Fig. 2(a) presents the DSC thermograph obtained with paraffin
wax RT20 showing the PCM started heat absorption at 18.7 °C with
a peak at 24 °C and completed heat absorption at 31.4 °C. The melting point found to be 22 °C with a latent heat of fusion of 139 kJ/kg
are in good agreement with the manufacturer corresponding data
of melting point of 21 °C and heat of fusion of 134 kJ/kg [28].
Fig. 2(b) presents temperature–time curve obtained for RT20 using
THM. The curve shows a larger gradient from 40 to 21 °C which
points to the sensible cooling while at 21 °C, the slope of the graphs
decreases sharply which shows start of melting and keeps decreasing till 8 °C where the slop starts increasing pointing out to sensible heat release in the solid PCM below 8 °C which shows end of
solidification. The results show that RT20 is a potential candidate
for PV temperature regulation due to its higher heat of fusion
and appropriate melting point well below reference PV temperature of 25 °C however it has drawbacks of (i) low end of solidification down to 8 °C which means in summer night the material will
not go back to solid completely as summer nights in hot climate
have ambient temperature much higher than 8 °C (ii) low density
(880 kg/m3 solid and 750 kg/m3 liquid) needs larger containment
size and higher difference in solid–liquid leads to large volume
expansion of 14% from solid to liquid phase change which needs
huge containment over-sizing [28] and (iii) low thermal conductivity of 0.2 W/m K reduces the rate of heat transfer. The problem of
low thermal conductivity however can be solved by inserting
thermally conductive metallic fins in PCM containment to effect
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A. Hasan et al. / Energy Conversion and Management 81 (2014) 322–329
(b)
Liquid
(a)
Solidification starts
Solidification ends
Solidification range
Solid
Fig. 4. (a) Differential Scanning Calorimetry (DSC) and (b) Temperature History Method (THM) curves for capric–lauric acid [42].
(a)
(b)
Solidification starts
Solidification ends
Under cooling
Solidification
range
Solid
Fig. 5. (a) Differential Scanning Calorimetry (DSC) and (b) Temperature History Method (THM) curves for capric–palmitic acid [42].
melting in narrow temperature range [10–12]. Combustibility of
RT20 with flash point of 154 °C [28] can also be a potential problem
prohibiting its use in building integrated photovoltaics (BIPV) due
to fire safety issue.
3.3. Characterization of SP22
Fig. 3(a) presents DSC thermographs of a mixture of paraffin
wax and salt hydrate, SP22 showing that the PCM has a wide melting range from 14 to 26 °C with melting point of 21.6 °C and the latent heat of fusion of 125 kJ/kg. The manufacturer’s catalogue data
shows a melting range of 13–28 °C with melting point of 23 °C and
latent heat fusion of 150 kJ/kg [29]. The larger difference in latent
heat can be attributed to difference of melting ranges in experiment (14–26 °C) and catalogue (13–28 °C). Fig. 3(b) represents
THM result of SP22 which shows PCM needs under-cooling down
to 9 °C to trigger solidification however bulk solidification occurs
nearly isothermally between 15 and 16 °C due to high thermal conductivity (0.6 W/m K) of SP22 compared to RT20 (0.2 W/m K).
Though under-cooling is expected in salt hydrates [20] the solidification at a lower temperature (16 °C) is due to the salt hydrate
being blended with paraffin wax. PCM SP22 shows promise for this
application due to (i) reasonable heat of fusion (ii) higher thermal
conductivity (iii) higher density of 1490 kg/m3 and (iv) lower
volume expansion of 4.03% from solid to liquid however undercooling down to 9 °C makes it unsuitable in hot climates where
higher night time ambient temperatures hamper PCM solidification [30].
3.4. Characterization of capric–lauric acid (CL)
Fig. 4(a) showing DSC results for CL indicates that heat absorption commenced at 18 °C and completed at 27 °C with the peak at
24.6 °C. The melting point and latent heat of fusion were found to
be 20.6 °C and 188 kJ/kg which are in good agreement with previously reported values of 19.6 °C and 168 kJ/kg respectively [35].
Fig. 4(b) shows the THM results for CL while cooling from melted
state at 40 °C down to 5 °C. The graph shows that solidification
commenced at 24 °C and completes at 19 °C without any undercooling which is desired for PCM solidification in summer nights.
CL shows 5 °C deviation from the isothermal heat release during
solidification which is more than salt hydrates (SP22 and CaCl2.6H2O) which can be attributed to the low thermal conductivity
(0.14 W/m K) of the PCM compared to salt hydrates.
This PCM is suitable for PV temperature regulation due to its (i)
suitable solidification at 24 °C and melting at 20.6 °C which is close
to the desired control temperature of 25 °C (ii) relatively higher heat
of fusion of 188 kJ/kg and (iii) good thermal cyclic stability [35–37].
This PCM however has disadvantages of (i) a wide heat absorption
temperature range (18–27 °C) due to its very low thermal conductivity (ii) relatively low density of 890 kg/m3 in solid phase [38]
which shows volume expansion of 8–10% from solid to liquid.
3.5. Characterization of capric-palmitic acid
Fig. 5 (a) represents DSC thermographs for CP showing that heat
absorption commenced at 19 °C and completed at 31.4 °C with a
328
A. Hasan et al. / Energy Conversion and Management 81 (2014) 322–329
60
60
55
T°
50
Ambient
Reference
(a)
45
T°
(b)
50
Ambient
C aCl2
45
T(C)
40
T (C)
55
35
30
40
35
30
25
25
20
20
15
15
10
10
0
20
40
60
80
100
120
0
20
40
Time (min)
60
Time
80
100
120
t (min)
Fig. 6. Temperature History Method (THM) curves for the distilled water taken as reference (left) and the sample PCM, CaCl2.6H2O (right) when cooling from a higher
temperature in the same ambient [42].
peak at 26.4 °C. The melting point and latent heat of fusion were
found to be 22.4 °C and 195 kJ/kg respectively compared to literature values of 22.5 °C and 173.6 kJ/kg respectively [37,39]. Fig. 5 (b)
presenting THM results shows that solidification starts at 25.5 °C
and completes at 18 °C with negligible under-cooling of 1 °C. Heat
release during solidification occurs in temperature range (7.5 °C)
slightly higher than that of CL (5 °C) although both PCM have same
thermal conductivity (0.14 W/m K).
CP shows promise for PV cooling application due to (i) higher
heat of fusion of 195 kJ/kg, (ii) higher solidification point of
25.5 °C suitable for summer nights and (iii) lower melting point
of 22.4 °C close to PV reference temperature of 25 °C. CP has disadvantages of (i) an undesired wide range of melting (19–31.4 °C)
and solidification (25.5–18 °C) caused by its very low thermal conductivity (ii) relatively low density of 870 kg/m3 in solid phase [38]
which causes a moderate volume expansion of 8–10% from solid to
liquid.
Specific heat capacity and heat of fusion of each of the PCMs in
solid and liquid phases is calculated using Eqs. (1)–(8) and are
summarized in Table 2. The calculated values from THM are within
maximum 5% deviation from DSC results and 10% from literature
values. The deviation can be attributed to the fact that (i) THM relies on overly simplified custom made set up which are not precisely calibrated like conventional DSC (ii) DSC results also show
variation based on sample size and (iii) literature may use different
purity grades of the PCM. Considering these factors, the values obtained from THM are reasonably reliable.
4. Conclusion
Main contribution of the paper is to establish a qualitative
relationship between thermophysical properties of PCM and PV
temperature regulation. The research mainly investigates into
thermophysical properties of three different classes of PCM in
one study, discuss their merits and helps selection of PCM for temperature control applications in PV. Some novel results related to
under-cooling of PCM are also determined. CaCl26H2O and SP22
shows higher under-cooling unachievable in summer nights in
hot climates while paraffin wax RT20 melts and solidifies over a
wide temperature range along with higher volumetric expansion
which render these PCM unsuitable for the temperature control
of PV. Although eutectic mixtures of fatty acids have very low thermal conductivities, their higher heat of fusions, reasonable range of
melting and solidification between 19and 25 °C, no under-cooling
and reasonable volumetric expansion renders them suitable for
PV temperature control at 25 °C. Amongst fatty acid, CP has a
melting point closer to PV control temperature and heat of fusion
higher than CL makes it most suitable candidate for PV temperature regulation.
Acknowledgements
The authors would like to acknowledge the Higher Education
Authority through Strand 3 funding, Science Foundation Ireland
through their Research Frontiers Program and the Research Support Unit at Dublin Institute of Technology. They would also like
to acknowledge COST Action TU0802: Next generation cost effective
phase change materials for increased energy efficiency in renewable
energy systems in buildings for providing an invaluable platform to
discuss and develop this work.
References
[1] Radziemska E. The effect of temperature on the power drop in crystalline
silicon solar cell. Renew Energy 2003;28(1):1–12.
[2] Radziemska E, Klugmann E. Thermally affected parameters of the currentvoltage characteristics of silicon photocell. Energy Convers Manage
2002;43(14):1889–900.
[3] Makrides G, Zinsser BG, George E, Schubert M, Werner JH. Temperature
behavior of different photovoltaic systems installed in Cyprus and Germany.
Sol Energy Mater Sol Cells 2009;93(6–7):1095–9.
[4] Skoplaki E, Palyvos JA. Operating temperature of photovoltaic modules: a
survey of pertinent correlations. Renew Energy 2009;34(1):23–9.
[5] Bücher K. Site dependence of the energy collection of PV modules. Sol Energy
Mater Sol Cells 1999;47(1–4):85–94.
[6] Breteque AE. Thermal aspects of c-Si photovoltaic module energy rating. Sol
Energy 2009;83(9):1425–33.
[7] Nassar YF, Salem AA. The reliability of the photovoltaic utilization in southern
cities of Libya. Desalination 2007;209(1–3):86–90.
[8] Suntechnics. Product Manual; 2008 Suntechnics STP065-12/Sb PV Panel.
[9] Saly V, Ruzinsky M, Redi P. Indoor study and ageing tests of solar cells and
encapsulations of experimental modules. Electronics Technology 2001. In:
24th international spring seminar on concurrent engineering in electronic
packaging.
[10] Huang MJ, Eames PC, Norton B. Thermal regulation of building-integrated
photovoltaics using phase change materials. Int J Heat Mass Transf
2004;47(12–13):2715–33.
[11] Huang MJ, Eames PC, Norton B. Phase change materials for limiting
temperature rise in building integrated photovoltaics. Sol Energy
2006;80(9):1121–30.
[12] Huang MJ, Eames PC, Norton B. Comparison of a small-scale 3D PCM thermal
control model with a validated 2D PCM thermal control model. Sol Energy
Mater Sol Cells 2006;90(13):1961–72.
[13] Hasan A, McCormack SJ, Huang MJ, Norton B. Evaluation of phase change
materials for thermal regulation enhancement of building integrated
photovoltaics. Sol Energy 2010;84(515):1601–12.
[14] Ho CJ, Tanuwijava AO, Lai CM. Thermal and electrical performance of a BIPV
integrated with a microencapsulated phase change material layer. Energy
Build 2012;50:331–8.
[15] Biwole PH, Eclache P, Kuznik F. Phase-change materials to improve solar
panel’s performance. Energy Build 2013;62:59–67.
[16] Yinping Z, Yi J. A simple method, the T-history method, of determining the
heat of fusion, specific heat and thermal conductivity of phase-change
materials. Meas Sci Technol 1999;10(3).
A. Hasan et al. / Energy Conversion and Management 81 (2014) 322–329
[17] Chen Changzhong, Liu Wenmin, Yang Hao, Zhao Yiyang, Liu Shanshan.
Synthesis of solid–solid phase change material for thermal energy storage by
crosslinking of polyethylene glycol with poly (glycidyl methacrylate). Sol
Energy 2011;85:2679–85.
[18] Alkan Cemil, Günther Eva, Hiebler Stefan, Ensari Ömer F, Kahraman Derya.
Polyurethanes as solid–solid phase change materials for thermal energy
storage. Sol Energy 2012;86:1761–9.
[19] Xiao X, Zhang P, Li M. Thermal characterization of nitrates and nitrates/
expanded graphite mixture phase change materials for solar energy storage.
Energy Convers Manage 2013;73:86–94.
[20] Zhang Jiangshan, Zhang Xiang, Wan Yazhen, Zhang Dandan Meiand Bing.
Preparation and thermal energy properties of paraffin/halloysite nanotube
composite as form-stable phase change material. Sol Energy 2012;86:1142–8.
[21] Tang Bingtao, Cui Junshuo, Wang Yunming, Jia Chao, Zhang Shufen. Facile
synthesis and performances of PEG/SiO2 composite form-stable phase change
materials. Sol Energy 2013;97:484–92.
[22] Chen Keping, Yu Xuejiang, Tian Chunrong, Wang Jianhua. Preparation and
characterization of form-stable paraffin/polyurethane composites as phase
change materials for thermal energy storage. Energy Convers Manage
2014;77:13–21.
[23] Karkri Abdelwaheb Trigui Mustapha, Krupa Igor. Thermal conductivity and
latent heat thermal energy storage properties of LDPE/wax as a shapestabilized composite phase change material. Energy Convers Manage
2014;77:586–96.
[24] Qiu Xiaolin, Song Guolin, Chu Xiaodong, Li Xuezhu, Tang Guoyi.
Microencapsulated n-alkane with p(n-butyl methacrylate-co-methacrylic
acid) shell as phase change materials for thermal energy storage. Sol Energy
2013;91:212–20.
[25] Fang Yutang, Yu Huimin, Wan Weijun, Gao Xuenong, Zhang Zhengguo.
Preparation and thermal performance of polystyrene/n-tetradecane
composite nanoencapsulated cold energy storage phase change materials.
Energy Convers Manage 2013;76(430–43):201–5.
[26] Hong H, Kim SK, Kim YS. Accuracy improvement of T-history method for
measuring heat of fusion of various materials. Int J Refrig 2004;27(4):360–6.
[27] Marín JM, Zalba B, Cabeza LF, Mehling H. Determination of enthalpytemperature curves of phase change materials with the temperature-history
method: improvement to temperature dependent properties. Meas Sci
Technol 2003;14(2):184–9.
[28] Rubitherm, Technologies. Innovative PCM’s and thermal technology. Product
Manual RUBITHERMÒ 2009; SP22A17.
329
[29] Günther E, Mehling H, Hiebler S. Modelling of under-cooling and solidification
of phase change materials. Modell Simul Mater Sci Eng 2007;15(8):879–93.
[30] Günther E, Mehling H, Werner M. Melting and nucleation temperatures of
three salt hydrate phase change materials under static pressures up to
800 MPa. J Phys: D Appl Phys 2007;40(15):4636–42.
[31] Tyagi VV, Buddhi D. Thermal cycle testing of calcium chloride hexahydrate as a
possible PCM for latent heat storage. Sol Energy Mater Sol Cells
2008;92(8):891–9.
[32] Cabeza LF, Roca J, Nogués M, Mehling H, Hiebler S. Immersion corrosion tests
on metal-salt hydrate pairs used for latent heat storage in the 48–58 °C
temperature range. Mater Corros 2002;53(12):902–7.
[33] Farrell AJ, Norton B, Kennedy DM. Corrosive effects of salt hydrate phase
change materials used with aluminium and copper. J Mater Process Technol
2006;175(1–3):198–205.
[34] El-Sebaii AA, Al-Amir S, Al-Marzouki FM, Faidah AS, Al-Ghamdi AA, Al-Heniti S.
Fast thermal cycling of acetanilide and magnesium chloride hexahydrate for
indoor solar cooking. Energy Convers Manage 2009;50(12):3104–11.
[35] Sari A, Karaipekli A. Preparation and thermal properties of capric acid/palmitic
acid eutectic mixture as a phase change energy storage material. Mater Lett
2008;62(6–7):903–6.
[36] Sedat K, Kamil K, Ahmet S. Lauric and myristic acids eutectic mixture as phase
change material for low-temperature heating applications. Int J Energy Res
2005;29(9):857–70.
[37] Shilei LV, Neng Z, Guohui F. Eutectic mixtures of capric acid and lauric acid
applied in building wallboards for heat energy storage. Energy Build
2006;38(6):708–11.
[38] Sharma SD, Sagara K. Latent heat storage materials and systems: a review. Int J
Green Energy 2005;2(1):1–56.
[39] Karaipekli A, Sari A. Capric acid and palmitic acid eutectic mixture applied in
building wallboard for latent heat thermal energy storage. J Sci Ind Res (JSIR)
2007;66(06):470–6.
[40] Sari A, Kaygusuz K. Thermal performance of a eutectic mixture of lauric and
stearic acids as PCM encapsulated in the annulus of two concentric pipes. Sol
Energy 2002;72(6):493–504.
[41] Zalba B, Marín JM, Cabeza LF, Mehling H. Review on thermal energy storage
with phase change: materials, heat transfer analysis and applications. Appl
Therm Eng 2003;23(3):251–83.
[42] Hasan A, McCormack SJ, Huang MJ, Norton B. Phase change materials for
thermal regulation of building integrated photovoltaics. PhD thesis. Dublin
Institute of Technology; 2010, p. 71–88.