Applied Energy 154 (2015) 92–101
Contents lists available at ScienceDirect
Applied Energy
journal homepage: www.elsevier.com/locate/apenergy
Macroencapsulation and characterization of phase change materials for
latent heat thermal energy storage systems
Tanvir E. Alam a, Jaspreet S. Dhau b,c,e,⇑, D. Yogi Goswami b,d, Elias Stefanakos b,c
a
Department of Mechanical Engineering, University of South Florida, 4202 E. Fowler Ave., ENB 118, Tampa, FL 33620, USA
Clean Energy Research Center, University of South Florida, 4202 E. Fowler Ave., ENB 118, Tampa, FL 33620, USA
c
Department of Electrical Engineering, University of South Florida, 4202 E. Fowler Ave., ENB 118, Tampa, FL 33620, USA
d
Department of Chemical and Biomedical Engineering, University of South Florida, 4202 E. Fowler Ave., ENB 118, Tampa, FL 33620, USA
e
Florida Polytechnic University, Lakeland, FL 33805, USA
b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
An innovative technique to
encapsulate high temperature PCM is
presented.
No need of a sacrificial layer to
accommodate the expansion of the
PCM on melting.
Non-vacuum metal deposition
process for large-scale fabrication of
capsules.
Capsules survived thermal cycles,
equivalent to seven years of power
plant service.
No degradation in thermophysical
properties of the capsules and PCM
on cycling.
a r t i c l e
i n f o
Article history:
Received 24 July 2014
Received in revised form 19 April 2015
Accepted 20 April 2015
Keywords:
Thermal energy storage
Latent heat storage
Phase change materials
Encapsulation
a b s t r a c t
An innovative technique to encapsulate PCMs that melt in the 120–350 °C temperature range is presented. The developed technique does not require a sacrificial layer to accommodate the volumetric
expansion of the PCMs on melting. The encapsulation consists of coating a non-reactive polymer over
the PCM pellet followed by deposition of a metal layer by a novel non-vacuum metal deposition technique. The fabricated capsules have survived more than 2200 thermal cycles, which is equivalent to about
seven years of service in a thermal energy storage system. Thermophysical properties of the PCMs were
investigated by DSC/TGA, IR and weight change analysis. Thermal cycling test showed no significant
degradation in these properties at any stage of testing.
Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction
⇑ Corresponding author at: Florida Polytechnic University, 4700 Research Way,
Lakeland, FL 33805, USA. Tel.: +1 8638748518; fax: +1 8635839070.
E-mail addresses: jdhau@flpoly.org, jaspreetdhau@usf.edu, jassiv02@yahoo.co.in
(J.S. Dhau).
http://dx.doi.org/10.1016/j.apenergy.2015.04.086
0306-2619/Ó 2015 Elsevier Ltd. All rights reserved.
Latent heat storage (LHS) using phase change materials (PCMs)
can be designed to have much higher energy storage density than
the sensible heat storage (SHS) [1]. However, the charging and
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T.E. Alam et al. / Applied Energy 154 (2015) 92–101
Nomenclature
LHS
PCMs
SHS
PTFE
FEP
NaNO3
PVDF
LiNO3
KNO3
PIF
PI-84
FTIR
TGA
Uc
rrandom
rsystematic systematic error
latent heat storage
phase change materials
sensible heat storage
polytetrafluoroethylene
fluorinated ethylene propylene
sodium nitrate
polyvinylidene fluoride
lithium nitrate
potassium nitrate
polyimide film
polyimide 84
Fourier transform infrared spectroscopy
thermogravimetric analysis
combined standard uncertainty
random error
Q
m
Cp
T
L
energy stored in the capsule (J)
weight (Kg)
specific heat of PCM (J/kg K)
temperature (°C)
latent heat of fusion (J/Kg)
Subscripts
i
initial
f
final
s
solid
l
liquid
m
melting
pcm
phase change material
poly
polymer
discharging is a major concern for LHS systems since most of the
PCMs have very low thermal conductivity [2]. A number of methods have been proposed to increase the thermal conductivity of
PCMs [3–9]. The finned tube configuration [9–12], metal structure
insertion into the PCMs [13,14], and dispersion of thermally conductive nano or micro particles (graphite flakes, CNTs, graphene,
etc.) within the PCM matrix [15–19] are some of the widely studied
approaches. Apart from being expensive, all these techniques suffer from certain drawbacks that include reduction in the latent
heat of fusion [8], dampening of the natural convection
process [19–21], and settling/floating of the additives in the liquid
PCMs.
Another method to enhance the heat transfer rate is by utilizing
micro- (capsule size 1–1000 lm) or macroencapsulated (capsule
size above 1000 lm) PCMs [22]. Considerable work has been carried out on the microencapsulation of the low melting (50–
120 °C) inorganic salt hydrates and organic materials such as
waxes, terpenes, and low molecular weight polymers [23–27,6].
Compared to macroencapsulation, the microencapsulation of
PCMs provides faster charging and discharging rates because of
the smaller distance for heat transfer. However, the lower
PCM-to-coating mass ratio (1:1) greatly reduces the energy storage density of the storage media and increases the storage capital
cost [28]. Recently, Zhang et al. [29] encapsulated NaNO3/KNO3
PCM in AISI 321 tubular capsules. Zhang et al. [30] fabricated
spherical capsules with copper as the PCM and chromium-nickel
as the shell material. The fabricated capsules have been shown to
withstand 1000 thermal cycles. Vicente and Silva [31] tested parafin wax in rectangular steel shell in horizontally hollow brick
for 8 days. Zhao [32] and Zheng et al. [33] have reported an
encapsulation technique that uses stainless steel/carbon steel as
the shell material. The process follows a post-formed approach
where cylindrical steel capsules are fabricated first and then they
are filled with the PCM followed by welding a cap at the top. The
major challenge in this approach is countering the corrosion of
the metal cans from the molten salt at high temperatures.
Mathur and Kasetty [34] have demonstrated a ceramic based
macroencapsulation technique for the sodium nitrate pellets (5–
15 mm in diameter). The technique involves the decomposition
of a sacrificial polymer layer to provide a void in between the coating and core PCM, which is needed for accommodating expansion
of the PCM during the phase change process. Some of the other
techniques [35,36] reported in the literature are tabulated in
Table 1.
The present study was undertaken to fabricate encapsulated
PCMs that can withstand the highly corrosive environment of molten alkali metal nitrate based salts and their eutectic. We report
herein, an innovative approach to encapsulate salts and eutectics
in the temperature range of 120–350 °C [37, patent pending]. The
developed encapsulation technique does not require a sacrificial
layer to accommodate the volumetric expansion of the PCMs on
melting and reduces the chance of metal corrosion inside the
capsule.
2. Encapsulation of PCM
There are three major concerns in the encapsulation of PCMs:
(i) The first concern is about how to accommodate a large volumetric expansion of the PCM on melting.
Table 1
Macroencapsulation techniques and materials.
S. no.
Core (PCM) material
Shell material
Core to
shell ratio
Temperature of
operation (°C)
Geometry of
capsules
Average size of
capsules
Thermal
cycles
Reference
1.
2.
3.
4.
NaNO3–KNO3
Copper
Paraffin wax
NaNO3, NaCl–MgCl2, MgCl2,
Aluminum
NaNO3, molten salt
Hydrated salts, paraffin, fatty
acids, bio PCM
Paraffin, salt hydrate
AISI 321
Chromium–Nickel
Steel
Stainless steel,
carbon steel
Ceramic–metallic
Polyolefin
–
4:1
–
–
160–270
1050–1150
0–36
300–450/300–750
Cylindrical
Spherical
Rectangular
Cylindrical
27.3/39/75 mma
2 mm
(30 18 2.8 cm)
[29]
[30]
[31]
[32,33]
–
–
300–550
(64)–120
Spherical
Spherical
5–15 mm
98 mm
5000
1000
8 days
60
(480 h)
2500
–
Aluminum,
plastics
PTFE–Nickel
–
(10)–100
Box, bag
–
–
[36]
8:1/12:1
120–350
Spherical
27.43 mm
2200
(5133 h)
Present
work
5.
6.
7.
8.
a
NaNO3, KNO3, NaNO3–KNO3,
NaNO3–KNO3–LiNO3
Same diameter and height.
[34]
[35]
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T.E. Alam et al. / Applied Energy 154 (2015) 92–101
(ii) The second concern is the pressure build-up due to the
expansion of air at elevated temperatures and during charging of the TES system.
(iii) The third concern is the reactivity of the molten PCM with
the encapsulant materials. The salts used in the present
study are alkali metal nitrates, which are powerful oxidizers,
especially in the molten state. These salts are highly reactive
with a variety of metal, organic and inorganic materials
[38–42].
expansion of the PCM on melting. Therefore, a polymer coating
which is both flexible and selectively permeable in nature was conceived to address the first concern.
2.1. Material compatibility study
In order to study the compatibility of the PCMs with the encapsulating material (third concern), a systematic study on the thermal and chemical behavior of the selected polymers with molten
PCMs was conducted by thermal gravimetric analysis (TGA). The
PCMs selected for the present study include sodium-, potassiumand lithium nitrate, and their eutectics. Three sets of polymers;
non-fluorinated, partly-fluorinated and fully-fluorinated, were
selected for the present study (Table 2).
The TG analysis was performed at a ramp rate of 10 °C/min
under an inert (Argon) atmosphere. As evident from Fig. 2,
polyimide-film (PIF) has the highest, and PVDF the lowest thermal
A selectively permeable coating that lets the hot air diffuse out
but not the molten PCM will overcome the problem of pressure
buildup due to the expansion of air on heating. Since the PCM
solidifies from the outside-in (during the cooling process), it is postulated that an impervious solid layer will be formed that would
prevent the air to diffuse back-in (Fig. 1). A flexible coating that
can expand and contract would accommodate the large volumetric
Fig. 1. PCM encapsulation model.
Table 2
Effect of alkali metal nitrates on the thermal and chemical stability of the selected polymers.
Onset
decomp.
temp. (°C)
Onset decomp.b
temp. with NaNO3 (°C)
Latent heat (KJ/g)
PIF
569
467
PI-84 (Resin)
–
Polymers
PVDF
FEP
PTFE
a
b
Monomer unit
F
H
C
C
F
H
F F F F
C C C C
F F F CF3
F
F
C
C
F
F
KNO3 (92)
a
Molten nitrate salts (testing temperature)
a
a
NaNO3 (172)
LiNO3 (362)
KNO3 (354 °C)
NaNO3
(326 °C)
LiNO3 (275 °C)
91
157
343
Reactive
Reactive
Reactive
–
86
146
341
Reactive
Reactive
Reactive
441
436
86
173
363
Reactive
Reactive
Non-reactive
470
470
93
174
375
Non-reactive
Non-reactive
Non-reactive
534
53
92
174
368
Non-reactive
Non-reactive
Non-reactive
Values in the parentheses represent the latent heat of the as-received salts.
Decomp. = Decomposition.
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T.E. Alam et al. / Applied Energy 154 (2015) 92–101
110
110
350
100
105
325
90
100
300
275
95
80
225
Weight (%)
Weight (%)
70
60
50
40
85
200
80
175
75
150
125
70
30
100
PTFE
65
PTFE
PIF
PIF
20
60
50
PVDF
55
FEP
75
PI-84
PVDF
10
25
FEP
50
0
0
50
0
0
100 150 200 250 300 350 400 450 500 550 600 650 700 750
Temperature ( oC)
250
90
25
50
75
100
125
150
175
200
225
250
275
Time (min)
Temperature ( oC)
Fig. 4. Isothermal TGA (4 h at 326 °C) of as-received polymers.
Fig. 2. TGA of the selected polymers alone.
PIF > PTFE > FEP > PVDF As-received
melting point of the nitrate salts (326 °C in the case of NaNO3).
No significant weight change was noticed for PTFE and FEP,
whereas PI-84 and PVDF showed substantial weight loss (Figs. 4
and 5). Although no significant weight change was noticed in the
PI-film there was substantial reduction in the latent heat value of
NaNO3 (152 kJ/g). The TGA of the polymers with KNO3 gave results
similar to that of NaNO3 (Fig. 6). Contrary to NaNO3 and KNO3, the
PI-film showed substantial weight loss in the presence of molten
LiNO3. PI-84 showed an even more severe reaction, whereas
PTFE, FEP and PVDF practically remained unreactive with the molten LiNO3 (Fig. 7). It is evident from the later discussion that PIF,
PI-84 and PVDF are not suitable for the encapsulation of the nitrate
based PCMs. The fully fluorinated polymers, PTFE and FEP, are the
best materials to encapsulate PCMs as they showed no sign of reaction with the molten salts.
PTFE > FEP > PIF > PVDF In the presence of molten NaNO3
2.2. Encapsulation procedure
stability among the as-received polymers. However, their thermal
behavior changed in the presence of molten sodium nitrate
(NaNO3). The onset decomposition temperature of the PIF
decreased by more than 100 °C (Table 2, Fig. 3). In addition, an
abrupt weight change was noticed at 466 °C that signals the
decomposition of NaNO3 (onset of decomposition for the
as-received NaNO3 starts at 626 °C). There is practically no change
in the onset decomposition temperature of PTFE and FEP. PVDF
shows a small decrease in the onset decomposition temperature
and an additional step corresponding to the decomposition of
NaNO3 at 470 °C. Based on these results, the thermal and chemical
stability of the studied polymers is found to be as follows:
The selected polymers were also subjected to the isothermal TG
analysis in order to examine the suitability of these polymers over
a long duration of usage at a temperature of 20 °C above the
2.2.1. Step 1: polymer coating
NaNO3 powder was pressed in a hydraulic press at 980 N of
force to form hemispherical pellets of 12.5 to 25.5 mm diameter.
350
110
110
325
100
105
300
90
275
100
80
225
95
Weight (%)
Weight (%)
70
60
50
40
PTFE with NaNO3
200
175
90
150
85
125
PTFE with NaNO3
PIF with NaNO3
30
PVDF with NaNO3
20
PIF with NaNO3
80
PI-84 with NaNO3
FEP with NaNO3
75
10
NaNO3
0
50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800
Temperature ( oC)
Fig. 3. TGA of the selected polymers with NaNO3.
0
25
50
75
100
125
150
175
100
75
FEP with NaNO3
50
PVDF with NaNO3
25
70
0
Temperature ( oC)
250
200
225
250
0
275
Time (min)
Fig. 5. Isothermal TGA (4 h at 326 °C) of polymers in the presence of NaNO3.
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T.E. Alam et al. / Applied Energy 154 (2015) 92–101
400
110
375
350
105
325
100
300
Weight (%)
250
225
90
200
175
85
150
Temperature ( oC)
275
95
125
PTFE with KNO3
80
100
PVDF with KNO3
75
FEP with KNO3
75
PIF with KNO3
50
PI-84 with KNO3
25
70
0
0
25
50
75
100
125
150
175
200
225
250
275
300
Time (min)
110
300
105
275
100
250
95
225
90
200
85
175
80
150
75
125
70
Temperature (oC)
Weight (%)
Fig. 6. Isothermal TGA (4 h at 350 °C) of polymers in the presence of KNO3.
Fig. 8. Bisected PCM capsule; (a) optical microscope picture, (b) showing voids
created after thermal cycling.
100
PTFE with LiNO3
65
75
PIF with LiNO3
PI-84 with LiNO3
60
50
FEP with LiNO3
55
25
PVDF with LiNO3
50
solidifies. This void provides space for the expansion of the PCM
when it melts again in the capsule. In another variation, PTFE–
FEP composite material was used to encapsulate the NaNO3–
KNO3 eutectic, whereas FEP alone was used to encapsulate the
eutectic salts that melt below 200 °C (Table 3).
0
0
25
50
75
100
125
150
175
200
225
250
275
300
Time (min)
Fig. 7. Isothermal TGA (4 h at 280 °C) of polymers in the presence of LiNO3.
This size range was chosen as optimum based on earlier theoretical
modeling studies by Ramos-Archibold et al. [43,44]. The pressed
pellets were then coated with a layer of polymer by using the
jar-milling technique. The loosely held polymer particles were
pressed in a hydraulic press at 980 N of force to form a thin polymeric film over the pellet. In another variation, a PTFE film was
wrapped around the pellet and the whole pellet was pressed in
the hydraulic press to form a PTFE layer. It is desirable to have
the PCM to polymer shell mass ratio as large as possible.
However, practical fabrication of a uniform layer of polymer that
would hold intact during cycling limited the thickness to 0.5–
0.7 mm (Fig. 8a) which gave the PCM-to-polymer mass ratio below
12:1. The coated capsules were heated to a temperature beyond
the melting point of the PCM and then cooled to below the melting
point to solidify the PCM. As postulated (Fig. 1), the PCM solidifies
from outside-in, and in the process the increased size of the capsule was maintained. As evident from Fig. 8b, a void zone is naturally formed within the capsule when the remainder of the PCM
2.2.2. Step 2: metal coating
A thin layer of metal is needed over the top of the PTFE layer to
maintain its structural integrity in the packed bed environment.
For this, it is desirable to develop a process which could be used
to metalize polymer coated capsules on a commercial scale. The
use of a vacuum based metallization technique is practically and
economically not feasible for this application. We have developed
a fully manufacturable proprietary method to metalize polymer
coated capsules by utilizing commercially available electroless
and electroplating chemistry. The method involves coating of a
PTFE layer with proprietary particles [37, patent pending] that
make it hydrophilic and solvophilic in nature (Fig. 9). This is followed by deposition of a palladium catalyst that catalyzes the
deposition of nickel. The initial electroless deposition is 1–4 micro
inch, which is enough to make the PTFE layer conductive for the
subsequent electrolytic deposition of nickel or other metals and
metal alloys. The capsules have been electroplated with a nickel,
zinc, tin or zinc-nickel/iron alloy by the rack and barrel plating
technique. The barrel plating technique was developed for plating
a large number of capsules in a single step. The average thickness
of the plated metal was measured to be in the 10–80 lm range.
Fig. 10 depicts all the steps involved in the encapsulation of the
PCMs.
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T.E. Alam et al. / Applied Energy 154 (2015) 92–101
Table 3
Performance evaluation of encapsulated capsules.
S. no.
PCM (M.p.)
Polymer
Metal coating
PCM-tocoating
mass ratio
Max. temp.
(charging temp.)
(°C)
Min. temp.
(discharging temp.)
(°C)
No. of
cycles
passed
DHf after
thermal
cycling (kJ/g)
1.
2.
3.
4.
5.
6.
NaNO3 (306 °C)
NaNO3
NaNO3
NaNO3
KNO3 (334 °C)
50NaNO3–50KNO3
(222 °C)
NaNO3–KNO3–LiNO3
(122 °C)
PTFE
PTFE
PTFE
PTFE
PTFE
PTFE–
FEP
FEP
Nickel (10–80 lm)
–
–
–
Nickel (50–80 lm)
–
8:1 and 12:1
12:1
8:1
20:1
8:1
12:1
326
326
326
326
350
242
250
250
250
250
280
180
2200*
1000*
1000*
5
110*
1000*
170 (172)**
172
172
172
92 (92)**
117 (120) **
–
10:1
144
100
440*
140 (140)
7.
*
**
**
Continuing.
As-received.
Left hand side angel=108.040
o
Left hand side angel=95.977 o
(a)
(b)
Fig. 9. Contact angle measurement with DI water on (a) as-received PTFE, (b) coated PTFE.
Fig. 10. Procedure for the encapsulation of PCMs.
3. Experimental measurements
3.1. Materials
Polytetrafluoroethylene (PTFE) films and fluorinated ethylene
propylene (FEP) were obtained from McMASTER-CARR, USA.
Sodium nitrate (NaNO3), polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE) powder were purchased from Sigma–
Aldrich, USA. Lithium nitrate (LiNO3) and potassium nitrate
(KNO3) was obtained from Alfa Aesar, USA. Polyimide sheets were
purchased from HD Microsystems, USA. PI-84 sample was provided by EVONIK, Austria. Electroless nickel solution
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T.E. Alam et al. / Applied Energy 154 (2015) 92–101
(Mid-phosphorus, 6–10% by weight) and Nickel sulfamate electroplating solution (Macdermid Inc., USA) were procured from
Transene Company Inc., USA, and Allied Plating, USA, respectively.
330
325
320
3.2. Characterization
Melting
315
Solidification
Temperature ( oC)
The DSC/DTA/TGA analysis was carried out using the SDT-Q 600
by TA instrument. This machine can simultaneously perform differential scanning calorimetry and thermogravimetric analysis.
Heat flow, temperature and weight accuracy of this device are
±2% (based on metal melting standards), ±1 °C (based on metal
melting standards) and ±1%, respectively. All the TG analyses were
performed at a ramp rate of 10 °C/min under an inert (Argon)
atmosphere The FTIR spectra were taken by using a JASCO 6300
Fourier transform infrared spectroscopy (FTIR) instrument. The
thickness of the dissected capsules was measured with a Leitz
Optical Microscope (5 to 100). Contact angles on the polymer
surface were measured by a Ramé-hart Contact Angle
Goniometer, retrofitted with a digital camera. A K-type thermocouple was used to measure the temperature profile at the center of
the capsule and the temperature was recorded with the aid of
Labview.
310
305
300
295
290
o
285
Uncertainty of the thermocouple measurement is ± 1 C
Melting time, tm= 22 min
280
Solidification time, ts= 29 min
275
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200
Time (min)
Fig. 12. Average temperature profile inside NaNO3 capsule thermal cycled between
280 and 326 °C.
3.3. Uncertainty analysis
250
TGA, weight and temperature measurements were conducted
several times to observe the repeatability of the measured data.
The Root-sum-square method was employed to evaluate the
uncertainty of the measurements [45] with a 95% confidence level.
240
Melting
ð1Þ
3.4. Determination of the temperature profile inside the capsule
Before thermal cycling, we investigated the temperature distribution inside a single NaNO3 capsule during the charging and discharging process. A K-type thermocouple was implanted at the
center of the capsule (Fig. 11). The capsule was placed in a furnace
for thermal cycling from 280 °C to 326 °C. During this procedure,
the temperature inside the capsule was monitored with the help
of LabVIEW. From Fig. 12, it is clear that the melting took about
22 min to complete. Further, it took about 70 min for the whole
capsule to reach the temperature of the furnace (326 °C).
Expectedly, the solidification took longer time (29 min) than the
melting process. The experiment was performed 6 times to observe
its repeatability. Similarly, the temperature distribution inside a
KNO3–NaNO3 capsule was investigated between 202 °C and
242 °C. The melting took 20 min, whereas the solidification took
25 min for completion (Fig. 13). The uncertainties in the measurement of temperature in NaNO3 and KNO3–NaNO3 capsules are
235
Temperature ( oC)
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
U c ¼ r2random þ r2systematic
245
Solidification
230
225
220
215
o
Uncertainty of the thermocouple measurement is ± 1.65 C
Melting time, t m= 20 min
210
Solidification time, t s= 25 min
205
200
0
20
40
60
80
100
120
140
160
180
200
Time (min)
Fig. 13. Average temperature profile inside KNO3–NaNO3 capsule thermal cycled
between 202 and 242 °C.
±1.0 °C and ±1.65 °C, respectively. It is pertinent to mention that
the numerical analysis of a similar system was solved by our group
[43,44], with the heat transfer process during the melting of NaNO3
in an encapsulated spherical shell modeled. A mathematical correlation of the heat transfer rate and melting was developed from the
numerical results [43,44].
3.5. Thermal performance evaluation of PCM capsules
Fig. 11. Schematic of the thermocouple setup inside the capsule.
Performance of the NaNO3 capsules was evaluated by thermal
cycling according to the profile shown in Fig. 14a. The capsules
were dwelled at 326 °C for 80 min and then cooled to 280 °C and
then dwelled for 1 h. Fig. 14b shows a set of capsules, which have
successfully passed 2200 thermal cycles and still continuing. The
discoloration of the metal coat is due to the oxidation of the outer
metal layer into metal oxide. Capsules with different polymer
thicknesses were fabricated to determine the minimum polymer
T.E. Alam et al. / Applied Energy 154 (2015) 92–101
400
350
326 oC
326 oC
326 oC
300
Temperature ( o C)
286 oC
286 oC
286 oC
250
200
150
100
50
0
0
40
80
120 160 200 240 280 320 360 400 440 480 520 560
Time (min)
99
8:1 and 12:1 PCM-to-coating mass ratio have not shown any visible degradation in 1000 thermal cycles completed so far. Cycling of
these capsules is continuing. The capsule with 20:1 mass ratio
failed after only few cycles. The performance of the fabricated capsules has been tabulated in Table 3. Fig. 15a and b shows two sets
of capsules after 50 cycles, one set is polymer coated and the other
is metal coated on polymer. Fig. 15c shows a set of capsules, which
have successfully passed 2200 thermal cycles and still continuing.
At various stages of thermal cycling, capsules were dissected to
analyze their thermophysical properties. DSC analysis of the thermal cycled capsules of NaNO3, KNO3 and their eutectic showed no
significant change in their thermophysical properties (Fig. 16 and
Table 3). In addition, the weight change analysis showed no substantial weight change after thermal cycling (Fig. 17). FTIR was also
used to characterize NaNO3 before and after thermal cycling. The IR
spectra of ‘‘as-received’’ and thermal cycled (after 2000 thermal
cycles, Fig. 18) perfectly matched with each other. All these tests
indicate a perfect compatibility of the coating material with the
nitrate based salts over a long period of usage.
Fig. 14. Thermal cycling profile.
3.6. Energy stored in a single capsule
thickness needed to maintain the integrity of the film and achieve
a PCM to coating ratio as high as possible. The PCM-to-coating
mass ratio was varied from 8:1 to 20:1 (Table 3). The capsules with
The average theoretical energy stored in a single capsule over
the temperature range of 286–326 °C was 4.27 ± 0.33 kJ. This was
computed by the following equation:
Fig. 15. (a) Only polymer coated capsules, (b)metal coated capsules tested at 326 °C after 50 cycles, (b) capsules tested at 326 °C for 2200 thermal cycles.
100
T.E. Alam et al. / Applied Energy 154 (2015) 92–101
100
6
296.23 oC
296.16 oC
90
5
170.8 J/g
299.15 oC
80
170.4 J/g
298.52 oC
70
Transmittance (%)
Heat Flow (W/g)
4
3
2
1
50
1383.67
40
After 2000 cycle
10
After 2000 Cycle
303.22 oC
1789.61
20
As Received
-1
60
30
168.3 J/g
298.59 oC
167.8 J/g
298.87 oC
0
1789.61
1384.63
As Received PCM
303.66 oC
0
-2
750
0
50
100
150
200
250
300
350
400
850
950
1050 1150 1250 1350 1450 1550 1650 1750 1850 1950
450
Wavelength (cm -1)
Temperature ( oC)
Fig. 18. FTIR of as received and thermal cycled NaNO3.
Fig. 16. DSC of NaNO3 before and after thermal cycling (>2200 cycles).
3.7. Testing under packed-bed system
PTFE coated capsules were also tested under a packed-bed environment for more than 50 cycles where each cycle constituted
approximately 6 h of continuous operation [48]. The packed-bed
thermal storage tank contained randomly packed 770 encapsulated spherical NaNO3 capsules stacked one over the other. The
average diameter of each capsule was 2.743 ± 0.038 cm. The capsule contained an average weight of 17.4 ± 1.6 g PCM [48]. The capsules inside the packed-bed were observed after 50 cycles (300 h
of operation). All of the capsules survived thermal cycling without
any leakage. There were nine layers of capsules lying on the top of
the bottom layer capsules that is approximately 12 kg of weight on
the capsules at the bottom layer. This demonstrates the mechanical stability of the capsules both during melting and solidification
process.
17.9
17.8
17.7
Weight (gm)
17.6
17.5
17.4
17.3
17.2
SL100
17.1
4. Summary and conclusions
SL13
17
0
100
200
300
400
500
600
700
800
900
1000
1100
Cycle
Fig. 17. Weight of the PCM capsules after thermal cycling (uncertainty in the
weight measurement is ±0.006 g).
Q ¼ mpcm ½C ps;pcm ðT m;pcm T i;pcm Þ þ Lpcm þ C pl;pcm ðT f ;pcm T m;pcm Þ
þmpoly C ps;poly ðT f ;poly T i;poly Þ
ð2Þ
The data used for the calculation is given in Table 4.
Table 4
Physical properties of NaNO3 and PTFE used for the calculation of energy stored in a
single capsule.
NaNO3 (PCM)
mpcm
Cps,pcm
Tm,pcm
Ti,pcm
Lpcm
Tf,pcm
Cpl,pcm
PTFE (coating)
17.4 ± 1.6 g
1.655 J/g K [46]
306 °C
286 °C
172 J/g
326 °C
1.655 J/g K [46]
mpoly
Cps,poly
Tf,poly
Ti,poly
1.70 ± 0.40 g
1.500 J/g K [47]
326 °C
286 °C
An innovative PCM encapsulation technique has been developed
that does not require a sacrificial layer to accommodate the volumetric expansion of the PCM on melting. From this research and
development, PTFE and FEP were found to be appropriate coating
materials for encapsulating nitrate based PCMs for the temperature
range of 120–350 °C. A non-vacuum based technique was developed
to coat a metal on the polymer layer that provides sufficient strength
to stack the capsules in a thermal storage tank. The developed process reduces the chance of metal corrosion due to molten salts as
there is a polymer layer in-between the PCM and the metal coating.
The flexible coating over the capsule has allowed the use of a very
thin coating layer that has significantly increased the
PCM-to-coating ratio. In addition, the PCM in the macro-capsules
melts and solidifies in a significantly shorter time to satisfy the need
for a quick response time for generating power on demand. Thermal
cycling tests have shown that the encapsulated nitrate based materials have excellent thermal and chemical stability even after more
than 2200 thermal cycles. Based on these results, it can be concluded
that the developed materials have good potential for use in LHS systems in renewable energy and conventional power plants.
Acknowledgements
The authors gratefully acknowledge the financial support for
this research received from the U.S. Department of Energy
T.E. Alam et al. / Applied Energy 154 (2015) 92–101
(#DEEE0003590) and E-On Corporation (#CC - EIRI - 14 – 2010).
The authors also like to thank Qi Ni from Micro Integration
Laboratory (University of South Florida) for aiding us with the contact angle measurements. Thanks are also acknowledged to
EVONIK, Austria, for providing free PI-84 samples.
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