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Preparation, thermal properties and thermal reliability of a novel mid-temperature composite phase change material for energy conservation
Accepted Manuscript Preparation, thermal properties and thermal reliability of a novel mid-temperature composite phase change material for energy conservation
Suling Zhang, Wei Wu, Shuangfeng Wang PII:
S0360-5442(17)30653-9
DOI:
10.1016/j.energy.2017.04.087
Reference:
EGY 10727
To appear in:
Energy
Received Date:
06 January 2017
Revised Date:
30 March 2017
Accepted Date:
15 April 2017
Please cite this article as: Suling Zhang, Wei Wu, Shuangfeng Wang, Preparation, thermal properties and thermal reliability of a novel mid-temperature composite phase change material for energy conservation, Energy (2017), doi: 10.1016/j.energy.2017.04.087
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ACCEPTED MANUSCRIPT Highlight 1. SA-AC/EG CPCM was obtained with optimum mass ratio of 90wt.% SA-AC eutectic mixture. 2. The intrinsic latent heat of SA-AC was enhanced due to the EG porous filler. 3. The thermal conductivity of CPCM was significantly enhanced by a factor of 17.59. 4. CPCM possessed favorable heat storage capacity with appropriate melting temperature. 5. The second law of thermodynamics was used to explain phase change characteristics.
ACCEPTED MANUSCRIPT 1
Preparation, thermal properties and thermal reliability of a novel mid-
2
temperature composite phase change material for energy conservation
3 4
Suling Zhang, Wei Wu, Shuangfeng Wang*
5
Key Laboratory of Enhanced Heat Transfer and Energy Conservation of the Ministry
6
of Education, School of Chemistry and Chemical Engineering, South China
7
University of Technology, Guangzhou 510640, China
8 9 10
* Corresponding author. Tel.:+86 20 22236929 E-mail address: [email protected]
11 12
Abstract:
13
Condensation heat of air-conditioner in household and public is a kind of
14
indispensable waste heat, which is necessary to recover and reuse it. Herein, phase
15
change material is widely used in exhaust heat recover and storage. In the present
16
work, expanded graphite(EG) was introduced to stearic acid-acetamide(SA-AC)
17
eutectic mixture, aiming at obtaining composite phase change material(CPCM) with
18
high thermal conductivity, large heat storage capacity and favorable thermal
19
repeatability for efficient heat recover. DSC results exhibited its remarkable energy
20
storage capacity with a latent heat of CPCM of 186.8 J·g-1 compared to most of the
21
organic eutectic composite. The second law of thermodynamics was used to
22
explain
the
phase
change
characteristics 1
of
the
SA-AC/EG
CPCMs
ACCEPTED MANUSCRIPT 23
corresponding to the pristine SA-AC eutectic mixture. The thermal conductivity of
24
the CPCM was enhanced by 17.59 times comparing to pristine SA-AC. The results of
25
thermal conductivity and infrared thermal images confirmed the CPCM
26
possessed prominent heat storage efficiency. The thermo-physical properties of the
27
SA-AC/EG CPCM after 500 accelerated thermal cycles were slightly decreased which
28
did no distinct influence on heat storage. Due to the low cost and remarkable
29
properties, the SA-AC/EG CPCM was a promising candidate for energy conservation
30
by condensation heat recovery of air-conditioner.
31 32
Key words:
33
Composite
34
thermodynamics; Thermal conductivity; Heat recovery
phase
change
material;
Thermal
properties;
Second
law
of
35 36
1. Introduction
37
The global warming brought about the increase of the temperature so that the
38
demand for air-conditioner was significantly enhanced. The usage of air-conditioner
39
provided a comfortable environment for users while the condensation heat was
40
discharged into atmosphere without any dispose. This behavior not only created air
41
pollution and noise pollution, but also ulteriorly improved the environmental
42
temperature. Herein, according to the standard outlined in Ref.[1], the temperature of
43
refrigerant at the outlet of compressor can be up to 145°C while R22 was served as
44
refrigerant. Consequently, the necessarity to recycle and reuse the low-grade 2
ACCEPTED MANUSCRIPT 45
condensation heat is evident. A literature survey indicates that the temperature of
46
domestic hot water was between 35°C and 50°C for immediate usage[2]. Generally
47
speaking, daily hot water was gained by consuming fossil fuels at low energy
48
efficiency, leading to not only environment problems but also lack of energy sources.
49
Therefore, the condensation heat of air-conditioner can be used to produce hot water
50
for domestic usage. However, the usage of air-conditioner mainly focused on the
51
daytime while the hot water usage more concentrated on night. Thus, the mismatch
52
of time and quantity between condensation heat production and hot water
53
supply happened. Heat storage becomes essential to solve this problem.
54
Phase change material (PCM) was a potential candidate for heat storage[3-5].
55
Due to large latent heat of fusion, high energy storage capacity and isothermal
56
behavior while endothermic/exothermic heat, phase change material attracted
57
considerable interests[6-12]. Herein, owing to the extensive source, low operating
58
cost, appropriate phase change temperature, favorable latent heat of phase change and
59
good thermal cycle stability, fatty acid(such as stearic acid)[13] attracted enormous
60
interest in the last few years. As for sole fatty acid, the phase change temperature
61
was constant which cannot satisfy the requirement for application in some case.
62
Therefore, the eutectic mixtures[8, 14] were utilized to obtain the material in various
63
phase change temperatures. Yuan et al. [13] paid much attention to the eutectic
64
mixtures and developed CA-PA-MA eutectic mixture with the melting temperature of
65
18.98°C and enthalpy of 135.6J·g-1[14], PA-SA eutectic mixture of 53.95°C for phase
66
change temperature and 177.67 J·g-1 for enthalpy[8] and LA-PA-SA eutectic mixture 3
ACCEPTED MANUSCRIPT 67
with the endothermic temperature of 32.1°C and enthalpy of 151.6 J·g-1[15].
68
However, the resultant organic fatty acid eutectic mixture with low phase change
69
temperature and relatively low latent heat of fusion cannot meet the requirement
70
for air-conditioner condensation heat recovery. Therefore, the objective of this
71
study was to develop a new kind of organic eutectic mixture with higher melting
72
temperature for wider applications such as recovering the condensation heat of
73
air-conditioner.
74
However, the bulk PCMs suffer from low thermal conductivity and small
75
viscosity in the molten state[16], which significantly restricted their wide applications.
76
The heat transfer rate is dominated by the thermal conductivity[17], which
77
governs the energy storage capacity, heat storage efficiency and service life.
78
Consequently, the modification of the pristine phase change material was an effective
79
method to expand functions. Thereinto, impregnating PCM into inorganic materials
80
with a porous and layered structure, such as metal foam[18], expanded
81
graphite(EG)[8, 19-21], graphite foam[22], was the popular way to overcome the
82
aforementioned disadvantages. Due to the low cost, low density and high thermal
83
conductivity, expanded graphite was the optimal filler for PCMs[23]. Zhang et al.[20]
84
prepared paraffin/EG composite phase change material(CPCM) and the optimum
85
adsorption capacity was 85.6wt.% of paraffin with no leakage for thermal energy
86
storage. With the addition of expanded graphite, the thermal conductivity of the
87
PEG/EG CPCM containing 10wt.% EG was up to 6.11W·m-1·K-1 with the melting
88
latent heat of 108.1kJ·kg and no seepage was observed while it melted to liquid[21]. 4
ACCEPTED MANUSCRIPT 89
The
enhancement
of
thermal
conductivity
90
storage/retrieval rate so that a larger amount of thermal energy will be
91
charging/discharging in finite time. The SA/EG composite was obtained via adsorbing
92
liquid stearic acid(SA) into the expanded graphite with the optimal mass ratio of
93
83wt.% in Ref.[24]. The thermal diffusivity of the composite was enhanced by a
94
factor of 10.27 with respect to pristine SA. Lee et al.[19] developed the erythritol/EG
95
CPCM with various interlayer distances via simple blending and impregnating
96
method. The thermal conductivity was increased with the interlayer distances.
97
Consequently, with regard to thermal energy storage material, EG-based CPCM
98
exhibited good application prospects. However, the phase change characteristics of
99
the resultant EG-based organic eutectic mixtures were with relatively low latent
100
heat, which can not satisfy the requirement for condensation heat effective
101
recover of air-conditioner. Due to the wide source and low cost of stearic acid, in
102
the present work, a novel EG-based CPCM with stearic acid-acetamide(SA-AC)
103
eutectic mixture as phase change material was obtained for air-conditioner
104
condensation heat recover. With a melting temperature in the desired operating
105
temperature range, a high latent heat of fusion per unit mass and low sub-cooling
106
temperature, the SA-AC/EG CPCM was a promising candidate for recovering the air-
107
conditioner condensation heat. By taking into account the literature survey, it can be
108
noted that this is the first time to adsorb relatively high phase change
109
temperature organic eutectic mixture into expanded graphite. Moreover, SA-AC
110
eutectic mixture, as a relatively high melting temperature phase change material, has 5
can
effectively
increase
heat
ACCEPTED MANUSCRIPT 111
not been studied and reported in detail, till date. Thus, it is necessary to investigate the
112
thermo-physical properties of the CPCM.
113
In this study, a novel composite phase change material was obtained via
114
simple blending and impregnating method. The thermal energy storage capacity was
115
measured by the DSC. The second law of thermodynamics was used to explain the
116
phase change characteristics of the SA-AC/EG CPCMs corresponding to the
117
pristine SA-AC eutectic mixture. The heat storage efficiency, thermal reliability
118
and thermal stability of SA-AC/EG CPCM with optimal mass ratio were
119
characterized along with those of SA-AC eutectic mixture for comparison
120
propose.
121
2. Experimental details and characterization
122
2.1 Preparation of SA-AC eutectic mixture
123
According to the previous study, the mass ratio of the eutectic mixture of stearic
124
acid (98%, Tianjin Damao reagent Co.Ltd, China) and acetamide (98%, Tianjin
125
Damao reagent Co.Ltd, China) was 83:17. 1.7000g AC and 8.3000g SA were
126
weighed by an analytical balance (precision: 0.1mg) and was mixed in a beaker. The
127
mixture was put into oven with temperature of 85°C until the mixture was completely
128
fused. The molten mixture was magnetic stirring at 300rpm for 2h in order to insure
129
the homogeneous mixing of two components. Finally, the mixture was cooled down
130
to ambient temperature and the eutectic mixture of SA and AC was obtained.
131
2.2 Preparation of SA-AC/EG CPCM
132
The EG was obtained from expandable graphite (mesh 50, expandable volume: 6
ACCEPTED MANUSCRIPT 133
300 mL/g, Qingdao Graphite Co. Ltd., China) via microwave treatment at a power of
134
700W for 40s. The SA-AC/EG CPCM was fabricated through simple blending and
135
physical adsorption method. The as-prepared SA-AC eutectic mixture was put in the
136
oven of 85°C to melt them. 1.0000g EG was put in a beaker and the corresponding
137
mass of molten SA-AC eutectic mixture was added to the EG with stirring by a glass
138
rod. The mass fraction of SA-AC/EG CPCM obtained in this paper was listed in
139
Fig.1. The mixture of SA-AC and EG was put into oven with temperature at 85°C for
140
8h and stirred per 30min to insure the uniform mixing. Ultimately, the mixture was
141
chilled at room temperature and the SA-AC/EG CPCM was obtained.
142
2.3 Characterization and Uncertainty Analysis
143
To determine the optimal mass ratio of the resultant SA-AC/EG CPCM, the
144
seepage tests were undergone. 0.2000g CPCMs were put in filter papers and heated in
145
the oven at 85°C for 2h to melt the composite thoroughly. Then, the CPCMs were
146
taken out and cooled until solidified. The filter papers were observed carefully and the
147
mass of the CPCMs were weighed again.
148
The microstructures of EG and SA-AC/EG CPCM were observed using a
149
scanning electron microscope (SEM, S-3700N, Japan). Chemical compatibilities of
150
PCMs were characterized by Fourier transformation infrared spectra (FT-IR, Tensor
151
27, Bruke) at 26°C. Thermal properties of the composites were determined by
152
differential scanning calorimeter (DSC, DSC Q20) under nitrogen atmosphere with
153
flow rate of 50 ml·min-1 and the heating/cooling rate was set as 5°C·min-1. The
154
transient temperature response behavior was measured by means of infrared 7
ACCEPTED MANUSCRIPT 155
thermography by thermacam (FLIR, SC 3000). The composite and SA-AC circle
156
slices were placed on a thermostatic bath at 50°C. The accelerated thermal cycle tests
157
were carried out by an oven. The as-prepared SA-AC/EG CPCM was placed in a 10
158
ml aluminum specimen box and heated in the oven at 85°C for 30 min, and then
159
cooled at room temperature for 30 min. After 500 accelerated thermal cycles, the SA-
160
AC/EG CPCM was tested by DSC and FTIR. The thermal reliability of the SA-
161
AC/EG CPCM was observed by comparing the DSC and FTIR results before and
162
after 500 accelerated thermal cycles. The serving duration of the CPCM was
163
undergone by put the SA-AC/EG CPCM blocks on filter papers and heated the
164
CPCMs at 85°C in an oven for one month and the thermal properties were
165
characterized and compared before and after serving duration tests. The thermal
166
stability of the CPCM can also be characterized by the serving duration tests. In the
167
present work, the measurements were repeated for three times to ensure accuracy and
168
repeatability of the results.
169
The thermal conductivity was characterized by transient plane source
170
method(TPS 2500S, Sweden Hot Disk) with NO.7577 probe (Radius=2.001mm).
171
Bouguerra A et al.[25] measured the thermal conductivity and thermal diffusivity of
172
the highly porous materials with the transient plane source technique at room
173
temperature and normal pressure. Comparing to the theoretical thermal conductivity,
174
the measurement result was in complete agreement with the deviation in the range of
175
0.5% and 4.1%. Meanwhile, the TPS technique was validated for solid materials such
176
as perspex, alumina and extruded polystyrene. The thermal conductivities were 8
ACCEPTED MANUSCRIPT 177
reproducible to within 5% and the values were within 4% comparing to the NPL's
178
guarded hot-plate and axial heat-flow techniques[26]. Consequently, TPS technique
179
was an effective method for the measurement of solid material. Before measurement,
180
the CPCM powders were formed into cylindrical blocks by dry pressed method using
181
home-made mould with the size of 4.0cm in diameter and 1.0cm in height. Then, the
182
surface of the CPCM cylindrical blocks were polished by 3000-mesh sandpaper
183
which make sure the well contact between CPCM block and the probe to minimize
184
the contact thermal resistance. When the minimum distance from the border of Hot
185
Disk probe to the free surface of the sample greater than the detecting depth, the
186
influence of the size of the sample to the measurement thermal conductivity would be
187
negligible with transient plane source method[27]. During the experiment, the
188
appropriate heating time and heating power were chosen. Afterwards, 200 data points
189
were generated containing the physical properties of the measured medium between
190
the temperature and characteristic time. During the analysis of those data points, the
191
"Time Correction" and "Calibrated Specific Heat of Sample" were chosen so that the
192
influences of the heat capacity of the Hot Disk probe and slower running of other
193
hardware and software would be revised effectively. Furthermore, more than 100
194
continuous data points were used and the ratio of "Total to Characteristic Time" was
195
in the range of 0.5 and 1 while analyzing. Eventually, the measurement was repeated
196
for three times for every sample and the time interval was no less than 30 minutes.
197
Because the experiment was carried out by the above criterion strictly, the
198
repeatability of the thermal conductivity was ±2% and the accuracy would be higher 9
ACCEPTED MANUSCRIPT 199
than 5%.
200
3. Results and discussions
201
3.1 Determine the optimal mass ratio of SA-AC/EG CPCM
202
The results of leakage test were shown in Fig.1 and Fig.2. It can be seen in Fig.1,
203
there was no obvious seepage appearing in the mass fraction of 90wt.% SA-AC
204
eutectic mixture. The mass losses of the CPCMs were exhibited in Fig.2 and it is
205
0.56wt.% at the mass fraction of 90wt.%. That is to say no obvious leakage of the SA-
206
AC/EG CPCM containing 90wt.% SA-AC eutectic mixture. Among the
207
aforementioned results, the optimal mass ratio of the SA-AC/EG CPCM was 90wt.%
208
of SA-AC eutectic mixture.
209
3.2 Characterize the microstructure of the EG and SA-AC/EG CPCM
210
The microstructures of EG and SA-AC/EG CPCMs containing 95wt.%, 90wt.%
211
and 85wt.% SA-AC eutectic mixture were observed in Fig.3. Herein, Fig.3b, Fig.3c
212
and Fig.3d were corresponding to the SA-AC/EG CPCM containing 95wt.%, 90wt.%
213
and 85wt.% of SA-AC eutectic mixture, respectively. In Fig.3a, the microstructure of
214
EG revealed that EG has irregularly worm-like porous structure constructed by curved
215
flakes and smooth surface. The microstructure of SA-AC/EG CPCM containing
216
95wt.% SA-AC eutectic mixture was exhibited in Fig.3b, the SA-AC eutectic mixture
217
was fulfilling in the gap of EG flake while some of them was overflowed to the
218
surface of the EG. The CPCM of this mass ratio was excessive. Whereas, the
219
microstructure revealed in Fig.3c of the CPCM containing 90wt.% SA-AC eutectic
220
mixture was full of SA-AC eutectic mixture and less space was rest in the EG. As for 10
ACCEPTED MANUSCRIPT 221
CPCM containing 85wt.% SA-AC eutectic mixture, the flake of EG became thicker
222
and much space was left without SA-AC eutectic mixture in Fig.3d. On account of the
223
result of the leakage tests and the microstructures, the SA-AC/EG CPCM containing
224
90wt.% SA-AC eutectic mixture was the optimal mass proportion of the CPCM. The
225
CPCM containing 90wt.% SA-AC eutectic mixture was investigated in the following
226
parts without additional statement.
227
3.3 The thermo-physical properties of SA-AC and SA-AC/EG CPCM
228
With respect to PCM, the latent heat is a key parameter that determines the
229
total amount of heat recovery in thermal energy storage systems. The DSC
230
thermograms of SA, AC and SA-AC eutectic mixture were shown in Fig.4. The
231
endothermic/exothermic temperature of SA, AC and SA-AC eutectic mixture were
232
70.45/64.79°C, 68.76/54.34°C and 66.35/56.45°C, respectively. The corresponding
233
latent heat were 204.7/196.3J/g, 226.2/227.3 J/g and 202.5/201.1 J/g, respectively,
234
which was much higher than the resultant eutectic mixtures[13].
235
The DSC thermograms of SA-AC eutectic mixture and SA-AC/EG CPCMs were
236
observed in Fig.5. The thermal properties were summarized in Table1. As listed in
237
Table1, the melting temperatures of SA-AC/EG CPCMs were closed to that of
238
pristine SA-AC eutectic mixture, while the freezing temperatures were much higher
239
than that of raw SA-AC eutectic mixture. The latent heat of fusion of SA-AC/EG
240
CPCMs were ranged from 178.5J/g to 186.8J/g at the mass ratios of 85wt%~90wt%.
241
Meanwhile, as exhibited in Table2, the as-prepared SA-AC/EG CPCM had a higher
242
enthalpy value, compared to the other organic EG-based CPCMs. The results 11
ACCEPTED MANUSCRIPT 243
showed that the resultant SA-AC/EG CPCM possessed great thermal energy
244
storage capacity for condensation heat recover. Hence, the prospect of improving
245
thermal conductivity was promising.
246
As stated in Table1, the endothermic/exothermic latent heat of the SA-AC/EG
247
CPCMs was slightly decreased compared to SA-AC eutectic mixture. It was owing to
248
that the EG had no phase change phenomenon occurred at this operation temperature.
249
However, the addition of EG did no passive effect on the phase change characteristic
250
of SA-AC eutectic mixture, the intrinsic phase transition latent heat of SA-AC
251
eutectic mixture of the SA-AC/EG CPCM can be calculated from equation(1).
252
SA AC
253
where SA AC was the intrinsic phase change latent heat of SA-AC eutectic
254
mixture(J/g), com was the phase change latent heat of SA-AC/EG CPCMs
255
measuring by DSC(J/g), SA AC was the mass fraction of SA-AC eutectic mixture
256
among CPCMs(%).
com
SA AC
(1)
257
The desirably theoretical results were listed in Table1. On account of the
258
integration of EG, the phase change latent heat of SA-AC eutectic mixture was
259
increased with respect to the pristine SA-AC eutectic mixture. The reason would be
260
explained by the second law of thermodynamics applying in the phase change
261
process(2).
262
H T S
(2)
263
where H is the phase change heat per unit mass, T is the phase change temperatures
264
of the SA-AC/EG CPCM or pristine SA-AC eutectic mixture, S is the entropy 12
ACCEPTED MANUSCRIPT 265
change during phase change. As for SA-AC/EG CPCM obtained in this work which
266
can be seen in Table1, the melting temperatures of the CPCMs were all around that of
267
the original SA-AC eutectic mixture. Therefore, the melting temperatures of the
268
CPCMs were regarded as the melting temperature of pristine SA-AC eutectic mixture.
269
According to the entropy formula, the entropy of the composition of the SA-AC/EG
270
CPCM will be larger than that in their primary substances. In consequence, the
271
melting enthalpies of SA-AC in the CPCMs will become larger. Therefore, the
272
existence of EG can not only decrease the subcooling temperature, but also
273
enhance the heat storage performance of SA-AC eutectic mixture.
274
3.4 The chemical compatibility of SA-AC/EG CPCM
275
The FTIR curves of SA-AC eutectic mixture, EG and SA-AC/EG CPCM were
276
exhibited in Fig.6. As for the FTIR curve of SA-AC eutectic mixture, the absorption
277
peaks at 3190cm-1 and 1592.18cm-1 are attributed to the stretching vibration and
278
bending deformation of N-H of AC. The asymmetric and symmetric vibrations of C-
279
H appear at 2919.77cm-1 and 2849.89cm-1. The peak located at 1703.05cm-1 proves
280
the presence of O=C-OH stretching vibration of SA. The stretching vibration of C-N
281
and bending vibration of N-H are both at 1411.74cm-1. The characteristic peaks at
282
1465.32cm-1 and 947.08cm-1 are assigned to the bending vibration of C-H and O-H,
283
respectively. The peaks at 1298.81cm-1, 591.23cm-1 and 470.10cm-1 are the
284
characteristic peak for stretching vibration of C-O, bending vibration of N-C=O and
285
C-C=O, respectively. Moreover, it can be seen that the CPCM includes all the
286
observed characteristic absorption peaks of both the SA-AC eutectic mixture and EG, 13
ACCEPTED MANUSCRIPT 287
but with no additional new peaks formed. Consequently, the integration SA-AC
288
eutectic mixture with EG attributed to the capillary and surface tension forces as well
289
as chemically inert to each other. In other words, the incorporation of SA-AC eutectic
290
mixture and EG did no influence of the chemical structure of both of them.
291
3.5 Thermal conductivity of SA-AC/EG CPCM
292
Thermal conductivity played a vital role in heat storage system, as it may
293
influence the energy storage efficiency during charge and discharge of latent
294
heat. Thermal conductivities of SA-AC/EG CPCM with various packing densities
295
were shown in Fig.7. The thermal conductivity of SA-AC/EG CPCM blocks at the
296
packing densities of 404.5, 500.2, 601.7, 700.9 and 801.1kg·m-3 were 1.849, 3.101,
297
4.228, 4.979 and 5.909W·m-1·K-1, respectively. However, the thermal conductivity of
298
SA-AC eutectic mixture at the packing density of 1255.1kg·m-3 was equivalent to
299
0.336W·m-1·K-1. As a result, thermal conductivity of SA-AC/EG composite was
300
significantly enhanced by a factor of 17.59, which can be attributed to network
301
structure and the constructed thermal conductive pathway. As shown in Table3,
302
the thermal conductivity of the resultant SA-AC/EG CPCM was much higher than the
303
other composite PCM, which elaborated the CPCM obtained in the present study
304
possessed great heat storage efficiency. Consequently, the heat charging/discharging
305
rates were accelerated and more thermal energy can be stored/released in limit time.
306
In the present work, an infrared thermal imager was employed to record the
307
temperature response during endothermic/exothermic process and the thermal images
308
were shown in Fig.8. During heating, the temperature distribution graphs of SA-AC 14
ACCEPTED MANUSCRIPT 309
and SA-AC/EG CPCM at 5s, 60s, 240s and 600s were observed in Fig.8a. From
310
Fig.8a, the temperature of SA-AC/EG composite was increased in a higher rate,
311
which confirmed its higher heat transfer efficiency with respect to pristine SA-
312
AC eutectic mixture. During cooling, the surface temperature distribution images at
313
5s, 30s, 60s and 180s were shown in Fig.8b. The temperature of SA-AC/EG
314
composite was decreased monotonously in a higher rate so that it kept lower
315
temperature in the whole process. That is to say that the SA-AC/EG composite
316
possessed much higher heat release efficiency corresponding to SA-AC. Therefore,
317
higher efficiency can be achieved during energy storage and release for SA-AC/EG
318
composite.
319
3.6 Accelerated thermal cycle tests of SA-AC/EG CPCM
320
For a CPCM, it is essential to have good thermal reliabilities over a number of
321
thermal cycles. 500 accelerated thermal cycles were undergone of the SA-AC/EG
322
CPCM and the FTIR curve and DSC thermogram were demonstrated in Fig.6 and
323
Fig.9. There was no obvious discrepancy in the FTIR curve of SA-AC/EG CPCM
324
before and after 500 accelerated thermal cycle tests. The result of FTIR curve
325
illustrated that the chemical structure between SA-AC eutectic mixture and EG stayed
326
invariable after the accelerated thermal cycle tests. The characteristic thermal
327
properties of SA-AC/EG CPCM were listed in Table4. The endothermic/exothermic
328
temperatures of SA-AC/EG CPCM before and after thermal cycle tests were
329
66.94/58.02°C
330
endothermic/exothermic
and
66.91/57.59°C, enthalpies
respectively, were 15
while
186.8/187.8J/g
the and
corresponding 171.7/168.6J/g,
ACCEPTED MANUSCRIPT 331
respectively. As a result, with the slightly decrease of the phase change latent heat, the
332
SA-AC/EG CPCM exhibited favorable chemical and thermal stability after 500
333
accelerated thermal cycle tests.
334
3.7 Serving duration of SA-AC/EG CPCM
335
Serving duration is an important property for a CPCM for thermal energy storage
336
applications. In the present work, the serving duration was validated by heating the
337
as-prepared SA-AC/EG CPCM with temperature at 85°C for a month and the thermal
338
properties were characterized by DSC analysis and TPS method. The thermograph of
339
SA-AC/EG CPCM before and after heat treatment was exhibited in Fig.10 and the
340
thermal conductivities in various densities were shown in Fig.7. As can be seen in
341
Fig.10, the endothermic/exothermic temperatures and enthalpies of SA-AC/EG
342
CPCM after heat treatment were 66.46/58.09°C and 173.9/171.6J/g, respectively. The
343
results indicated that no distinct discrepancy occurred in the comparison of thermo-
344
properties of SA-AC/EG CPCM before and after heat treatment. The thermal
345
conductivity of SA-AC/EG CPCM after heat treatment were 1.528, 2.893, 3.879,
346
4.723 and 5.687W·m-1·K-1 at the densities of 404.5, 500.2, 601.7, 700.9 and
347
801.1kg·m-3, respectively, which were slightly decreased compared to that without
348
serving duration tests. The variation of the thermal conductivity before and after
349
serving duration did no distinct influence for heat charging/discharging. In a word, the
350
SA-AC/EG CPCM possessed remarkable thermal stability in the molten state.
351
Therefore, with prominent serving durability and thermal stability of molten SA-
352
AC/EG CPCM, the resultant SA-AC/EG CPCM was appropriate for heat storage of 16
ACCEPTED MANUSCRIPT 353
condensation heat of air-conditioner.
354
3.8 Application prospect
355
With the increasing requirement of the air-conditioner, the producing waste heat
356
was gone up year by year. As for traditional air-conditioner, with the temperature of
357
refrigerating fluid in the inlet of the condenser more than 100°C, the thermal energy
358
was discharged to the atmosphere through fans. This behavior not only resulted in the
359
waste heat pollution, but also gave rise to the noise pollution by the fans. Besides, it is
360
the main reason for the low coefficient of performance (COP) of air-conditioner.
361
According to the viewpoint mentioned above, it is necessary to recycle and reuse the
362
condensation heat of air-conditioner with the obtained SA-AC/EG CPCM. There were
363
two advantages to use the SA-AC/EG CPCM. Firstly, with the addition of CPCM, the
364
refrigerating fluid can be condensed with no noise production while the storage of the
365
condensation heat. Moreover, with the relatively lower temperature of the
366
refrigerating fluid entering into the cooling process by fans, the energy consumption
367
will be decreased by a large margin and the noise produced by the fans will also
368
decline. Secondly, the thermal energy storage in the CPCM can be used to obtain the
369
hot water for household which can further improve the efficiency of the air-
370
conditioner system. Furthermore, the SA-AC/EG CPCM can be applied to waste heat
371
recover in the industry and solar energy heat storage.
372
4. Conclusion
373
In the present work, a novel composite phase change material was proposed to
374
recover condensation heat of air-conditioner. The following conclusions were 17
ACCEPTED MANUSCRIPT 375
obtained:
376
(1) The melting/freezing temperatures and enthalpies of SA-AC/EG CPCM
377
containing 90wt.% SA-AC eutectic mixture were 66.94/58.02°C and 186.8/187.8J/g,
378
respectively, which exhibited the CPCM possessed excellent energy storage capacity
379
per unit mass. Meanwhile, the intrinsic latent heat of fusion of SA-AC eutectic
380
mixture was enhanced due to the EG porous filler corresponding to the pristine SA-
381
AC eutectic mixture which can be well explained by the second law of
382
thermodynamics.
383
(2) The thermal conductivity of SA-AC/EG CPCM was 5.909W·m-1·K-1, which was
384
enhanced by a factor of 17.59. The results of thermal conductivity and infrared
385
thermal images illustrated the outstanding heat storage efficiency of the resultant
386
CPCM.
387
(3) 500 accelerated thermal cycle tests indicated that the SA-AC/EG CPCM possessed
388
excellent circulating stability. One month serving duration tests demonstrated that the
389
SA-AC/EG CPCM presented prominent thermal durability and thermal stability in the
390
molten state.
391
Consequently, SA-AC/EG CPCM exhibited a good prospect in air-conditioner
392
condensation heat and industrial waste heat recovery to achieve energy conversation
393
and emission reduction.
394
Acknowledgements
395
This work was supported by the International Cooperation Project (Grant
396
No.S2016G6199) and the National Natural Science Foundation of China (Granted No. 18
ACCEPTED MANUSCRIPT 397
51536003).
398
References
399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438
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21
ACCEPTED MANUSCRIPT 499
Figure Captions
500
Fig.1. Images of SA-AC/EG CPCMs (a) before and (b) after the leakage tests
501
Fig.2. Mass loss of SA-AC/EG CPCMs after leakage test
502
Fig.3. SEM images of EG(a) and SA-AC/EG CPCMs(b, c and d)
503
Fig.4. DSC curves of SA, AC and SA-AC eutectic mixture
504
Fig.5. DSC curves of SA-AC eutectic mixture and SA-AC CPCMs (90wt%~85wt%
505
SA-AC eutectic mixture marks as M1~M6)
506
Fig.6. FTIR curves of EG, SA-AC and SA-AC/EG CPCMs
507
Fig.7. Thermal conductivity of SA-AC/EG CPCM blocks with different packing
508
densities
509
Fig.8 Thermal images of SA-AC and SA-AC/EG CPCM during heating (a) and
510
cooling (b)
511
Fig.9. DSC curves of SA-AC/EG CPCMs before and after heat treatment
512
Fig.10. DSC curves of SA-AC/EG CPCMs before and after serving duration
513
22
ACCEPTED MANUSCRIPT 515
516 517
Fig.1. Images of SA-AC/EG CPCMs (a) before and (b) after the leakage tests
518
23
ACCEPTED MANUSCRIPT
Fig.2. Mass loss of SA-AC/EG CPCMs
Fig.4. DSC curves of SA, AC and SA-
after leakage test
AC eutectic mixture
519
24
ACCEPTED MANUSCRIPT
520
Fig.3. SEM images of EG(a) and SA-AC/EG CPCMs(b, c and d)
521
25
ACCEPTED MANUSCRIPT
522
523 524 525
Fig.5. DSC curves of SA-AC eutectic mixture and SA-AC CPCMs (90wt%~85wt% SA-AC eutectic mixture marks as M1~M6)
26
ACCEPTED MANUSCRIPT
527 528
Fig.6. FTIR curves of EG, SA-AC and SA-AC/EG CPCMs
529 530
27
ACCEPTED MANUSCRIPT
531 532
Fig.7. Thermal conductivity of SA-AC/EG CPCM blocks with different packing
533
densities
28
ACCEPTED MANUSCRIPT
535 536
Fig.8 Thermal images of SA-AC and SA-AC/EG CPCM during heating (a) and
537
cooling (b)
538 539
29
ACCEPTED MANUSCRIPT
Fig.9. DSC curves of SA-AC/EG
Fig.10. DSC curves of SA-AC/EG
CPCMs before and after heat treatment
CPCMs before and after serving duration
540 541
30
ACCEPTED MANUSCRIPT 543
Table Captions
544
Table.1 The thermal properties of SA-SA, SA-AC/EG CPCMs with various mass
545
ratios
546
Table. 2 Comparison of thermal properties of CPCMs
547
Table.3 Comparison of thermal conductivities of CPCMs
548
Table.4 DSC data of SA-AC and SA-AC/EG CPCMs before and after heat treatment
31
ACCEPTED MANUSCRIPT 550 551
Table.1 The thermal properties of SA-SA, SA-AC/EG CPCMs with various mass ratios Samples
Melting point (°C)
552 553 554 555
Melting
Melting
enthalpy of
enthalpy
composites
of SA-AC
(J/g)
(J/g)
Freezing point (°C)
Freezing
Freezing
enthalpy of
enthalpy
composites
of SA-AC
(J/g)
(J/g)
neat
66.35
202.5
202.5
56.45
201.1
201.1
M1
66.94
186.8
207.6
58.02
187.8
208.7
M2
66.66
184.0
206.7
57.97
183.1
205.7
M3
66.71
185.3
210.6
57.63
185.4
210.7
M4
67.46
185.8
213.3
57.65
183.5
210.9
M5
66.08
178.8
207.9
58.16
176.0
204.7
M6
66.32
178.5
210.0
58.10
176.5
207.6
*The SA-AC/EG CPCMs containing 90wt%~85wt% SA-AC eutectic mixture were marked as M1~M6, respectively.
32
ACCEPTED MANUSCRIPT 557
Table.2 Comparison of thermal properties of CPCMs Composite PCM Tm(°C) Hm(J/g) Tf(°C) Carnauba wax /EG 81.98 150.9 80.43 Palmitic-stearic acid /EG 53.89 166.27 54.37 Acetamide/EG 65.91 163.71 65.52 Rt100/EG 84.62 177.3 105.90 lauric–myristic–stearic 29.05 137.1 29.38 acid /EG Polyethylene glycol /EG 61.46 161.2 46.91 SA-AC/EG 66.94 186.8 58.02
558
33
Hf(J/g) 142.6 166.13 173.3 131.3
References [2] [8] [28] [29] [30]
187.8
[31] Present work
ACCEPTED MANUSCRIPT 560
Table.3 Comparison of thermal conductivities of CPCMs Composite PCM Thermal conductivity Enhancement (W·m-1·K-1) factor Palmitic-stearic acid /EG 2.51 9.65 Acetamide /EG 2.61 6 Polyethylene glycol /EG 1.324 4.44 Capric–palmitic–stearic 5.225 15.34 acid /EG Palmitic acid /EG 0.6 2.5 Methyl stearate /EG 3.6 SA-AC/EG 5.909 17.59
561
34
References [8] [28] [31] [32] [33] [34] Present work
ACCEPTED MANUSCRIPT 563
Table.4 DSC data of SA-AC and SA-AC/EG CPCMs before and after heat treatment Samples
Melting
Melting
Freezing
Freezing
temperature(℃)
enthalpy(J/g)
temperature(℃)
enthalpy(J/g)
neat
66.35
202.5
56.45
201.1
M1
66.94
186.8
58.02
187.8
M2
66.91
171.7
57.59
168.6
M3
66.46
173.9
58.09
171.6
564
*SA-AC/EG CPCM, SA-AC/EG CPCM after 500 accelerated thermal cycles and SA-
565
AC/EG CPCM after one month serving duration were marked as M1~M3,
566
respectively.
567
35
Suling Zhang, Wei Wu, Shuangfeng Wang PII:
S0360-5442(17)30653-9
DOI:
10.1016/j.energy.2017.04.087
Reference:
EGY 10727
To appear in:
Energy
Received Date:
06 January 2017
Revised Date:
30 March 2017
Accepted Date:
15 April 2017
Please cite this article as: Suling Zhang, Wei Wu, Shuangfeng Wang, Preparation, thermal properties and thermal reliability of a novel mid-temperature composite phase change material for energy conservation, Energy (2017), doi: 10.1016/j.energy.2017.04.087
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Highlight 1. SA-AC/EG CPCM was obtained with optimum mass ratio of 90wt.% SA-AC eutectic mixture. 2. The intrinsic latent heat of SA-AC was enhanced due to the EG porous filler. 3. The thermal conductivity of CPCM was significantly enhanced by a factor of 17.59. 4. CPCM possessed favorable heat storage capacity with appropriate melting temperature. 5. The second law of thermodynamics was used to explain phase change characteristics.
ACCEPTED MANUSCRIPT 1
Preparation, thermal properties and thermal reliability of a novel mid-
2
temperature composite phase change material for energy conservation
3 4
Suling Zhang, Wei Wu, Shuangfeng Wang*
5
Key Laboratory of Enhanced Heat Transfer and Energy Conservation of the Ministry
6
of Education, School of Chemistry and Chemical Engineering, South China
7
University of Technology, Guangzhou 510640, China
8 9 10
* Corresponding author. Tel.:+86 20 22236929 E-mail address: [email protected]
11 12
Abstract:
13
Condensation heat of air-conditioner in household and public is a kind of
14
indispensable waste heat, which is necessary to recover and reuse it. Herein, phase
15
change material is widely used in exhaust heat recover and storage. In the present
16
work, expanded graphite(EG) was introduced to stearic acid-acetamide(SA-AC)
17
eutectic mixture, aiming at obtaining composite phase change material(CPCM) with
18
high thermal conductivity, large heat storage capacity and favorable thermal
19
repeatability for efficient heat recover. DSC results exhibited its remarkable energy
20
storage capacity with a latent heat of CPCM of 186.8 J·g-1 compared to most of the
21
organic eutectic composite. The second law of thermodynamics was used to
22
explain
the
phase
change
characteristics 1
of
the
SA-AC/EG
CPCMs
ACCEPTED MANUSCRIPT 23
corresponding to the pristine SA-AC eutectic mixture. The thermal conductivity of
24
the CPCM was enhanced by 17.59 times comparing to pristine SA-AC. The results of
25
thermal conductivity and infrared thermal images confirmed the CPCM
26
possessed prominent heat storage efficiency. The thermo-physical properties of the
27
SA-AC/EG CPCM after 500 accelerated thermal cycles were slightly decreased which
28
did no distinct influence on heat storage. Due to the low cost and remarkable
29
properties, the SA-AC/EG CPCM was a promising candidate for energy conservation
30
by condensation heat recovery of air-conditioner.
31 32
Key words:
33
Composite
34
thermodynamics; Thermal conductivity; Heat recovery
phase
change
material;
Thermal
properties;
Second
law
of
35 36
1. Introduction
37
The global warming brought about the increase of the temperature so that the
38
demand for air-conditioner was significantly enhanced. The usage of air-conditioner
39
provided a comfortable environment for users while the condensation heat was
40
discharged into atmosphere without any dispose. This behavior not only created air
41
pollution and noise pollution, but also ulteriorly improved the environmental
42
temperature. Herein, according to the standard outlined in Ref.[1], the temperature of
43
refrigerant at the outlet of compressor can be up to 145°C while R22 was served as
44
refrigerant. Consequently, the necessarity to recycle and reuse the low-grade 2
ACCEPTED MANUSCRIPT 45
condensation heat is evident. A literature survey indicates that the temperature of
46
domestic hot water was between 35°C and 50°C for immediate usage[2]. Generally
47
speaking, daily hot water was gained by consuming fossil fuels at low energy
48
efficiency, leading to not only environment problems but also lack of energy sources.
49
Therefore, the condensation heat of air-conditioner can be used to produce hot water
50
for domestic usage. However, the usage of air-conditioner mainly focused on the
51
daytime while the hot water usage more concentrated on night. Thus, the mismatch
52
of time and quantity between condensation heat production and hot water
53
supply happened. Heat storage becomes essential to solve this problem.
54
Phase change material (PCM) was a potential candidate for heat storage[3-5].
55
Due to large latent heat of fusion, high energy storage capacity and isothermal
56
behavior while endothermic/exothermic heat, phase change material attracted
57
considerable interests[6-12]. Herein, owing to the extensive source, low operating
58
cost, appropriate phase change temperature, favorable latent heat of phase change and
59
good thermal cycle stability, fatty acid(such as stearic acid)[13] attracted enormous
60
interest in the last few years. As for sole fatty acid, the phase change temperature
61
was constant which cannot satisfy the requirement for application in some case.
62
Therefore, the eutectic mixtures[8, 14] were utilized to obtain the material in various
63
phase change temperatures. Yuan et al. [13] paid much attention to the eutectic
64
mixtures and developed CA-PA-MA eutectic mixture with the melting temperature of
65
18.98°C and enthalpy of 135.6J·g-1[14], PA-SA eutectic mixture of 53.95°C for phase
66
change temperature and 177.67 J·g-1 for enthalpy[8] and LA-PA-SA eutectic mixture 3
ACCEPTED MANUSCRIPT 67
with the endothermic temperature of 32.1°C and enthalpy of 151.6 J·g-1[15].
68
However, the resultant organic fatty acid eutectic mixture with low phase change
69
temperature and relatively low latent heat of fusion cannot meet the requirement
70
for air-conditioner condensation heat recovery. Therefore, the objective of this
71
study was to develop a new kind of organic eutectic mixture with higher melting
72
temperature for wider applications such as recovering the condensation heat of
73
air-conditioner.
74
However, the bulk PCMs suffer from low thermal conductivity and small
75
viscosity in the molten state[16], which significantly restricted their wide applications.
76
The heat transfer rate is dominated by the thermal conductivity[17], which
77
governs the energy storage capacity, heat storage efficiency and service life.
78
Consequently, the modification of the pristine phase change material was an effective
79
method to expand functions. Thereinto, impregnating PCM into inorganic materials
80
with a porous and layered structure, such as metal foam[18], expanded
81
graphite(EG)[8, 19-21], graphite foam[22], was the popular way to overcome the
82
aforementioned disadvantages. Due to the low cost, low density and high thermal
83
conductivity, expanded graphite was the optimal filler for PCMs[23]. Zhang et al.[20]
84
prepared paraffin/EG composite phase change material(CPCM) and the optimum
85
adsorption capacity was 85.6wt.% of paraffin with no leakage for thermal energy
86
storage. With the addition of expanded graphite, the thermal conductivity of the
87
PEG/EG CPCM containing 10wt.% EG was up to 6.11W·m-1·K-1 with the melting
88
latent heat of 108.1kJ·kg and no seepage was observed while it melted to liquid[21]. 4
ACCEPTED MANUSCRIPT 89
The
enhancement
of
thermal
conductivity
90
storage/retrieval rate so that a larger amount of thermal energy will be
91
charging/discharging in finite time. The SA/EG composite was obtained via adsorbing
92
liquid stearic acid(SA) into the expanded graphite with the optimal mass ratio of
93
83wt.% in Ref.[24]. The thermal diffusivity of the composite was enhanced by a
94
factor of 10.27 with respect to pristine SA. Lee et al.[19] developed the erythritol/EG
95
CPCM with various interlayer distances via simple blending and impregnating
96
method. The thermal conductivity was increased with the interlayer distances.
97
Consequently, with regard to thermal energy storage material, EG-based CPCM
98
exhibited good application prospects. However, the phase change characteristics of
99
the resultant EG-based organic eutectic mixtures were with relatively low latent
100
heat, which can not satisfy the requirement for condensation heat effective
101
recover of air-conditioner. Due to the wide source and low cost of stearic acid, in
102
the present work, a novel EG-based CPCM with stearic acid-acetamide(SA-AC)
103
eutectic mixture as phase change material was obtained for air-conditioner
104
condensation heat recover. With a melting temperature in the desired operating
105
temperature range, a high latent heat of fusion per unit mass and low sub-cooling
106
temperature, the SA-AC/EG CPCM was a promising candidate for recovering the air-
107
conditioner condensation heat. By taking into account the literature survey, it can be
108
noted that this is the first time to adsorb relatively high phase change
109
temperature organic eutectic mixture into expanded graphite. Moreover, SA-AC
110
eutectic mixture, as a relatively high melting temperature phase change material, has 5
can
effectively
increase
heat
ACCEPTED MANUSCRIPT 111
not been studied and reported in detail, till date. Thus, it is necessary to investigate the
112
thermo-physical properties of the CPCM.
113
In this study, a novel composite phase change material was obtained via
114
simple blending and impregnating method. The thermal energy storage capacity was
115
measured by the DSC. The second law of thermodynamics was used to explain the
116
phase change characteristics of the SA-AC/EG CPCMs corresponding to the
117
pristine SA-AC eutectic mixture. The heat storage efficiency, thermal reliability
118
and thermal stability of SA-AC/EG CPCM with optimal mass ratio were
119
characterized along with those of SA-AC eutectic mixture for comparison
120
propose.
121
2. Experimental details and characterization
122
2.1 Preparation of SA-AC eutectic mixture
123
According to the previous study, the mass ratio of the eutectic mixture of stearic
124
acid (98%, Tianjin Damao reagent Co.Ltd, China) and acetamide (98%, Tianjin
125
Damao reagent Co.Ltd, China) was 83:17. 1.7000g AC and 8.3000g SA were
126
weighed by an analytical balance (precision: 0.1mg) and was mixed in a beaker. The
127
mixture was put into oven with temperature of 85°C until the mixture was completely
128
fused. The molten mixture was magnetic stirring at 300rpm for 2h in order to insure
129
the homogeneous mixing of two components. Finally, the mixture was cooled down
130
to ambient temperature and the eutectic mixture of SA and AC was obtained.
131
2.2 Preparation of SA-AC/EG CPCM
132
The EG was obtained from expandable graphite (mesh 50, expandable volume: 6
ACCEPTED MANUSCRIPT 133
300 mL/g, Qingdao Graphite Co. Ltd., China) via microwave treatment at a power of
134
700W for 40s. The SA-AC/EG CPCM was fabricated through simple blending and
135
physical adsorption method. The as-prepared SA-AC eutectic mixture was put in the
136
oven of 85°C to melt them. 1.0000g EG was put in a beaker and the corresponding
137
mass of molten SA-AC eutectic mixture was added to the EG with stirring by a glass
138
rod. The mass fraction of SA-AC/EG CPCM obtained in this paper was listed in
139
Fig.1. The mixture of SA-AC and EG was put into oven with temperature at 85°C for
140
8h and stirred per 30min to insure the uniform mixing. Ultimately, the mixture was
141
chilled at room temperature and the SA-AC/EG CPCM was obtained.
142
2.3 Characterization and Uncertainty Analysis
143
To determine the optimal mass ratio of the resultant SA-AC/EG CPCM, the
144
seepage tests were undergone. 0.2000g CPCMs were put in filter papers and heated in
145
the oven at 85°C for 2h to melt the composite thoroughly. Then, the CPCMs were
146
taken out and cooled until solidified. The filter papers were observed carefully and the
147
mass of the CPCMs were weighed again.
148
The microstructures of EG and SA-AC/EG CPCM were observed using a
149
scanning electron microscope (SEM, S-3700N, Japan). Chemical compatibilities of
150
PCMs were characterized by Fourier transformation infrared spectra (FT-IR, Tensor
151
27, Bruke) at 26°C. Thermal properties of the composites were determined by
152
differential scanning calorimeter (DSC, DSC Q20) under nitrogen atmosphere with
153
flow rate of 50 ml·min-1 and the heating/cooling rate was set as 5°C·min-1. The
154
transient temperature response behavior was measured by means of infrared 7
ACCEPTED MANUSCRIPT 155
thermography by thermacam (FLIR, SC 3000). The composite and SA-AC circle
156
slices were placed on a thermostatic bath at 50°C. The accelerated thermal cycle tests
157
were carried out by an oven. The as-prepared SA-AC/EG CPCM was placed in a 10
158
ml aluminum specimen box and heated in the oven at 85°C for 30 min, and then
159
cooled at room temperature for 30 min. After 500 accelerated thermal cycles, the SA-
160
AC/EG CPCM was tested by DSC and FTIR. The thermal reliability of the SA-
161
AC/EG CPCM was observed by comparing the DSC and FTIR results before and
162
after 500 accelerated thermal cycles. The serving duration of the CPCM was
163
undergone by put the SA-AC/EG CPCM blocks on filter papers and heated the
164
CPCMs at 85°C in an oven for one month and the thermal properties were
165
characterized and compared before and after serving duration tests. The thermal
166
stability of the CPCM can also be characterized by the serving duration tests. In the
167
present work, the measurements were repeated for three times to ensure accuracy and
168
repeatability of the results.
169
The thermal conductivity was characterized by transient plane source
170
method(TPS 2500S, Sweden Hot Disk) with NO.7577 probe (Radius=2.001mm).
171
Bouguerra A et al.[25] measured the thermal conductivity and thermal diffusivity of
172
the highly porous materials with the transient plane source technique at room
173
temperature and normal pressure. Comparing to the theoretical thermal conductivity,
174
the measurement result was in complete agreement with the deviation in the range of
175
0.5% and 4.1%. Meanwhile, the TPS technique was validated for solid materials such
176
as perspex, alumina and extruded polystyrene. The thermal conductivities were 8
ACCEPTED MANUSCRIPT 177
reproducible to within 5% and the values were within 4% comparing to the NPL's
178
guarded hot-plate and axial heat-flow techniques[26]. Consequently, TPS technique
179
was an effective method for the measurement of solid material. Before measurement,
180
the CPCM powders were formed into cylindrical blocks by dry pressed method using
181
home-made mould with the size of 4.0cm in diameter and 1.0cm in height. Then, the
182
surface of the CPCM cylindrical blocks were polished by 3000-mesh sandpaper
183
which make sure the well contact between CPCM block and the probe to minimize
184
the contact thermal resistance. When the minimum distance from the border of Hot
185
Disk probe to the free surface of the sample greater than the detecting depth, the
186
influence of the size of the sample to the measurement thermal conductivity would be
187
negligible with transient plane source method[27]. During the experiment, the
188
appropriate heating time and heating power were chosen. Afterwards, 200 data points
189
were generated containing the physical properties of the measured medium between
190
the temperature and characteristic time. During the analysis of those data points, the
191
"Time Correction" and "Calibrated Specific Heat of Sample" were chosen so that the
192
influences of the heat capacity of the Hot Disk probe and slower running of other
193
hardware and software would be revised effectively. Furthermore, more than 100
194
continuous data points were used and the ratio of "Total to Characteristic Time" was
195
in the range of 0.5 and 1 while analyzing. Eventually, the measurement was repeated
196
for three times for every sample and the time interval was no less than 30 minutes.
197
Because the experiment was carried out by the above criterion strictly, the
198
repeatability of the thermal conductivity was ±2% and the accuracy would be higher 9
ACCEPTED MANUSCRIPT 199
than 5%.
200
3. Results and discussions
201
3.1 Determine the optimal mass ratio of SA-AC/EG CPCM
202
The results of leakage test were shown in Fig.1 and Fig.2. It can be seen in Fig.1,
203
there was no obvious seepage appearing in the mass fraction of 90wt.% SA-AC
204
eutectic mixture. The mass losses of the CPCMs were exhibited in Fig.2 and it is
205
0.56wt.% at the mass fraction of 90wt.%. That is to say no obvious leakage of the SA-
206
AC/EG CPCM containing 90wt.% SA-AC eutectic mixture. Among the
207
aforementioned results, the optimal mass ratio of the SA-AC/EG CPCM was 90wt.%
208
of SA-AC eutectic mixture.
209
3.2 Characterize the microstructure of the EG and SA-AC/EG CPCM
210
The microstructures of EG and SA-AC/EG CPCMs containing 95wt.%, 90wt.%
211
and 85wt.% SA-AC eutectic mixture were observed in Fig.3. Herein, Fig.3b, Fig.3c
212
and Fig.3d were corresponding to the SA-AC/EG CPCM containing 95wt.%, 90wt.%
213
and 85wt.% of SA-AC eutectic mixture, respectively. In Fig.3a, the microstructure of
214
EG revealed that EG has irregularly worm-like porous structure constructed by curved
215
flakes and smooth surface. The microstructure of SA-AC/EG CPCM containing
216
95wt.% SA-AC eutectic mixture was exhibited in Fig.3b, the SA-AC eutectic mixture
217
was fulfilling in the gap of EG flake while some of them was overflowed to the
218
surface of the EG. The CPCM of this mass ratio was excessive. Whereas, the
219
microstructure revealed in Fig.3c of the CPCM containing 90wt.% SA-AC eutectic
220
mixture was full of SA-AC eutectic mixture and less space was rest in the EG. As for 10
ACCEPTED MANUSCRIPT 221
CPCM containing 85wt.% SA-AC eutectic mixture, the flake of EG became thicker
222
and much space was left without SA-AC eutectic mixture in Fig.3d. On account of the
223
result of the leakage tests and the microstructures, the SA-AC/EG CPCM containing
224
90wt.% SA-AC eutectic mixture was the optimal mass proportion of the CPCM. The
225
CPCM containing 90wt.% SA-AC eutectic mixture was investigated in the following
226
parts without additional statement.
227
3.3 The thermo-physical properties of SA-AC and SA-AC/EG CPCM
228
With respect to PCM, the latent heat is a key parameter that determines the
229
total amount of heat recovery in thermal energy storage systems. The DSC
230
thermograms of SA, AC and SA-AC eutectic mixture were shown in Fig.4. The
231
endothermic/exothermic temperature of SA, AC and SA-AC eutectic mixture were
232
70.45/64.79°C, 68.76/54.34°C and 66.35/56.45°C, respectively. The corresponding
233
latent heat were 204.7/196.3J/g, 226.2/227.3 J/g and 202.5/201.1 J/g, respectively,
234
which was much higher than the resultant eutectic mixtures[13].
235
The DSC thermograms of SA-AC eutectic mixture and SA-AC/EG CPCMs were
236
observed in Fig.5. The thermal properties were summarized in Table1. As listed in
237
Table1, the melting temperatures of SA-AC/EG CPCMs were closed to that of
238
pristine SA-AC eutectic mixture, while the freezing temperatures were much higher
239
than that of raw SA-AC eutectic mixture. The latent heat of fusion of SA-AC/EG
240
CPCMs were ranged from 178.5J/g to 186.8J/g at the mass ratios of 85wt%~90wt%.
241
Meanwhile, as exhibited in Table2, the as-prepared SA-AC/EG CPCM had a higher
242
enthalpy value, compared to the other organic EG-based CPCMs. The results 11
ACCEPTED MANUSCRIPT 243
showed that the resultant SA-AC/EG CPCM possessed great thermal energy
244
storage capacity for condensation heat recover. Hence, the prospect of improving
245
thermal conductivity was promising.
246
As stated in Table1, the endothermic/exothermic latent heat of the SA-AC/EG
247
CPCMs was slightly decreased compared to SA-AC eutectic mixture. It was owing to
248
that the EG had no phase change phenomenon occurred at this operation temperature.
249
However, the addition of EG did no passive effect on the phase change characteristic
250
of SA-AC eutectic mixture, the intrinsic phase transition latent heat of SA-AC
251
eutectic mixture of the SA-AC/EG CPCM can be calculated from equation(1).
252
SA AC
253
where SA AC was the intrinsic phase change latent heat of SA-AC eutectic
254
mixture(J/g), com was the phase change latent heat of SA-AC/EG CPCMs
255
measuring by DSC(J/g), SA AC was the mass fraction of SA-AC eutectic mixture
256
among CPCMs(%).
com
SA AC
(1)
257
The desirably theoretical results were listed in Table1. On account of the
258
integration of EG, the phase change latent heat of SA-AC eutectic mixture was
259
increased with respect to the pristine SA-AC eutectic mixture. The reason would be
260
explained by the second law of thermodynamics applying in the phase change
261
process(2).
262
H T S
(2)
263
where H is the phase change heat per unit mass, T is the phase change temperatures
264
of the SA-AC/EG CPCM or pristine SA-AC eutectic mixture, S is the entropy 12
ACCEPTED MANUSCRIPT 265
change during phase change. As for SA-AC/EG CPCM obtained in this work which
266
can be seen in Table1, the melting temperatures of the CPCMs were all around that of
267
the original SA-AC eutectic mixture. Therefore, the melting temperatures of the
268
CPCMs were regarded as the melting temperature of pristine SA-AC eutectic mixture.
269
According to the entropy formula, the entropy of the composition of the SA-AC/EG
270
CPCM will be larger than that in their primary substances. In consequence, the
271
melting enthalpies of SA-AC in the CPCMs will become larger. Therefore, the
272
existence of EG can not only decrease the subcooling temperature, but also
273
enhance the heat storage performance of SA-AC eutectic mixture.
274
3.4 The chemical compatibility of SA-AC/EG CPCM
275
The FTIR curves of SA-AC eutectic mixture, EG and SA-AC/EG CPCM were
276
exhibited in Fig.6. As for the FTIR curve of SA-AC eutectic mixture, the absorption
277
peaks at 3190cm-1 and 1592.18cm-1 are attributed to the stretching vibration and
278
bending deformation of N-H of AC. The asymmetric and symmetric vibrations of C-
279
H appear at 2919.77cm-1 and 2849.89cm-1. The peak located at 1703.05cm-1 proves
280
the presence of O=C-OH stretching vibration of SA. The stretching vibration of C-N
281
and bending vibration of N-H are both at 1411.74cm-1. The characteristic peaks at
282
1465.32cm-1 and 947.08cm-1 are assigned to the bending vibration of C-H and O-H,
283
respectively. The peaks at 1298.81cm-1, 591.23cm-1 and 470.10cm-1 are the
284
characteristic peak for stretching vibration of C-O, bending vibration of N-C=O and
285
C-C=O, respectively. Moreover, it can be seen that the CPCM includes all the
286
observed characteristic absorption peaks of both the SA-AC eutectic mixture and EG, 13
ACCEPTED MANUSCRIPT 287
but with no additional new peaks formed. Consequently, the integration SA-AC
288
eutectic mixture with EG attributed to the capillary and surface tension forces as well
289
as chemically inert to each other. In other words, the incorporation of SA-AC eutectic
290
mixture and EG did no influence of the chemical structure of both of them.
291
3.5 Thermal conductivity of SA-AC/EG CPCM
292
Thermal conductivity played a vital role in heat storage system, as it may
293
influence the energy storage efficiency during charge and discharge of latent
294
heat. Thermal conductivities of SA-AC/EG CPCM with various packing densities
295
were shown in Fig.7. The thermal conductivity of SA-AC/EG CPCM blocks at the
296
packing densities of 404.5, 500.2, 601.7, 700.9 and 801.1kg·m-3 were 1.849, 3.101,
297
4.228, 4.979 and 5.909W·m-1·K-1, respectively. However, the thermal conductivity of
298
SA-AC eutectic mixture at the packing density of 1255.1kg·m-3 was equivalent to
299
0.336W·m-1·K-1. As a result, thermal conductivity of SA-AC/EG composite was
300
significantly enhanced by a factor of 17.59, which can be attributed to network
301
structure and the constructed thermal conductive pathway. As shown in Table3,
302
the thermal conductivity of the resultant SA-AC/EG CPCM was much higher than the
303
other composite PCM, which elaborated the CPCM obtained in the present study
304
possessed great heat storage efficiency. Consequently, the heat charging/discharging
305
rates were accelerated and more thermal energy can be stored/released in limit time.
306
In the present work, an infrared thermal imager was employed to record the
307
temperature response during endothermic/exothermic process and the thermal images
308
were shown in Fig.8. During heating, the temperature distribution graphs of SA-AC 14
ACCEPTED MANUSCRIPT 309
and SA-AC/EG CPCM at 5s, 60s, 240s and 600s were observed in Fig.8a. From
310
Fig.8a, the temperature of SA-AC/EG composite was increased in a higher rate,
311
which confirmed its higher heat transfer efficiency with respect to pristine SA-
312
AC eutectic mixture. During cooling, the surface temperature distribution images at
313
5s, 30s, 60s and 180s were shown in Fig.8b. The temperature of SA-AC/EG
314
composite was decreased monotonously in a higher rate so that it kept lower
315
temperature in the whole process. That is to say that the SA-AC/EG composite
316
possessed much higher heat release efficiency corresponding to SA-AC. Therefore,
317
higher efficiency can be achieved during energy storage and release for SA-AC/EG
318
composite.
319
3.6 Accelerated thermal cycle tests of SA-AC/EG CPCM
320
For a CPCM, it is essential to have good thermal reliabilities over a number of
321
thermal cycles. 500 accelerated thermal cycles were undergone of the SA-AC/EG
322
CPCM and the FTIR curve and DSC thermogram were demonstrated in Fig.6 and
323
Fig.9. There was no obvious discrepancy in the FTIR curve of SA-AC/EG CPCM
324
before and after 500 accelerated thermal cycle tests. The result of FTIR curve
325
illustrated that the chemical structure between SA-AC eutectic mixture and EG stayed
326
invariable after the accelerated thermal cycle tests. The characteristic thermal
327
properties of SA-AC/EG CPCM were listed in Table4. The endothermic/exothermic
328
temperatures of SA-AC/EG CPCM before and after thermal cycle tests were
329
66.94/58.02°C
330
endothermic/exothermic
and
66.91/57.59°C, enthalpies
respectively, were 15
while
186.8/187.8J/g
the and
corresponding 171.7/168.6J/g,
ACCEPTED MANUSCRIPT 331
respectively. As a result, with the slightly decrease of the phase change latent heat, the
332
SA-AC/EG CPCM exhibited favorable chemical and thermal stability after 500
333
accelerated thermal cycle tests.
334
3.7 Serving duration of SA-AC/EG CPCM
335
Serving duration is an important property for a CPCM for thermal energy storage
336
applications. In the present work, the serving duration was validated by heating the
337
as-prepared SA-AC/EG CPCM with temperature at 85°C for a month and the thermal
338
properties were characterized by DSC analysis and TPS method. The thermograph of
339
SA-AC/EG CPCM before and after heat treatment was exhibited in Fig.10 and the
340
thermal conductivities in various densities were shown in Fig.7. As can be seen in
341
Fig.10, the endothermic/exothermic temperatures and enthalpies of SA-AC/EG
342
CPCM after heat treatment were 66.46/58.09°C and 173.9/171.6J/g, respectively. The
343
results indicated that no distinct discrepancy occurred in the comparison of thermo-
344
properties of SA-AC/EG CPCM before and after heat treatment. The thermal
345
conductivity of SA-AC/EG CPCM after heat treatment were 1.528, 2.893, 3.879,
346
4.723 and 5.687W·m-1·K-1 at the densities of 404.5, 500.2, 601.7, 700.9 and
347
801.1kg·m-3, respectively, which were slightly decreased compared to that without
348
serving duration tests. The variation of the thermal conductivity before and after
349
serving duration did no distinct influence for heat charging/discharging. In a word, the
350
SA-AC/EG CPCM possessed remarkable thermal stability in the molten state.
351
Therefore, with prominent serving durability and thermal stability of molten SA-
352
AC/EG CPCM, the resultant SA-AC/EG CPCM was appropriate for heat storage of 16
ACCEPTED MANUSCRIPT 353
condensation heat of air-conditioner.
354
3.8 Application prospect
355
With the increasing requirement of the air-conditioner, the producing waste heat
356
was gone up year by year. As for traditional air-conditioner, with the temperature of
357
refrigerating fluid in the inlet of the condenser more than 100°C, the thermal energy
358
was discharged to the atmosphere through fans. This behavior not only resulted in the
359
waste heat pollution, but also gave rise to the noise pollution by the fans. Besides, it is
360
the main reason for the low coefficient of performance (COP) of air-conditioner.
361
According to the viewpoint mentioned above, it is necessary to recycle and reuse the
362
condensation heat of air-conditioner with the obtained SA-AC/EG CPCM. There were
363
two advantages to use the SA-AC/EG CPCM. Firstly, with the addition of CPCM, the
364
refrigerating fluid can be condensed with no noise production while the storage of the
365
condensation heat. Moreover, with the relatively lower temperature of the
366
refrigerating fluid entering into the cooling process by fans, the energy consumption
367
will be decreased by a large margin and the noise produced by the fans will also
368
decline. Secondly, the thermal energy storage in the CPCM can be used to obtain the
369
hot water for household which can further improve the efficiency of the air-
370
conditioner system. Furthermore, the SA-AC/EG CPCM can be applied to waste heat
371
recover in the industry and solar energy heat storage.
372
4. Conclusion
373
In the present work, a novel composite phase change material was proposed to
374
recover condensation heat of air-conditioner. The following conclusions were 17
ACCEPTED MANUSCRIPT 375
obtained:
376
(1) The melting/freezing temperatures and enthalpies of SA-AC/EG CPCM
377
containing 90wt.% SA-AC eutectic mixture were 66.94/58.02°C and 186.8/187.8J/g,
378
respectively, which exhibited the CPCM possessed excellent energy storage capacity
379
per unit mass. Meanwhile, the intrinsic latent heat of fusion of SA-AC eutectic
380
mixture was enhanced due to the EG porous filler corresponding to the pristine SA-
381
AC eutectic mixture which can be well explained by the second law of
382
thermodynamics.
383
(2) The thermal conductivity of SA-AC/EG CPCM was 5.909W·m-1·K-1, which was
384
enhanced by a factor of 17.59. The results of thermal conductivity and infrared
385
thermal images illustrated the outstanding heat storage efficiency of the resultant
386
CPCM.
387
(3) 500 accelerated thermal cycle tests indicated that the SA-AC/EG CPCM possessed
388
excellent circulating stability. One month serving duration tests demonstrated that the
389
SA-AC/EG CPCM presented prominent thermal durability and thermal stability in the
390
molten state.
391
Consequently, SA-AC/EG CPCM exhibited a good prospect in air-conditioner
392
condensation heat and industrial waste heat recovery to achieve energy conversation
393
and emission reduction.
394
Acknowledgements
395
This work was supported by the International Cooperation Project (Grant
396
No.S2016G6199) and the National Natural Science Foundation of China (Granted No. 18
ACCEPTED MANUSCRIPT 397
51536003).
398
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399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438
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Figure Captions
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Fig.1. Images of SA-AC/EG CPCMs (a) before and (b) after the leakage tests
501
Fig.2. Mass loss of SA-AC/EG CPCMs after leakage test
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Fig.3. SEM images of EG(a) and SA-AC/EG CPCMs(b, c and d)
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Fig.4. DSC curves of SA, AC and SA-AC eutectic mixture
504
Fig.5. DSC curves of SA-AC eutectic mixture and SA-AC CPCMs (90wt%~85wt%
505
SA-AC eutectic mixture marks as M1~M6)
506
Fig.6. FTIR curves of EG, SA-AC and SA-AC/EG CPCMs
507
Fig.7. Thermal conductivity of SA-AC/EG CPCM blocks with different packing
508
densities
509
Fig.8 Thermal images of SA-AC and SA-AC/EG CPCM during heating (a) and
510
cooling (b)
511
Fig.9. DSC curves of SA-AC/EG CPCMs before and after heat treatment
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Fig.10. DSC curves of SA-AC/EG CPCMs before and after serving duration
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516 517
Fig.1. Images of SA-AC/EG CPCMs (a) before and (b) after the leakage tests
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Fig.2. Mass loss of SA-AC/EG CPCMs
Fig.4. DSC curves of SA, AC and SA-
after leakage test
AC eutectic mixture
519
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Fig.3. SEM images of EG(a) and SA-AC/EG CPCMs(b, c and d)
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523 524 525
Fig.5. DSC curves of SA-AC eutectic mixture and SA-AC CPCMs (90wt%~85wt% SA-AC eutectic mixture marks as M1~M6)
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527 528
Fig.6. FTIR curves of EG, SA-AC and SA-AC/EG CPCMs
529 530
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531 532
Fig.7. Thermal conductivity of SA-AC/EG CPCM blocks with different packing
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densities
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535 536
Fig.8 Thermal images of SA-AC and SA-AC/EG CPCM during heating (a) and
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cooling (b)
538 539
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Fig.9. DSC curves of SA-AC/EG
Fig.10. DSC curves of SA-AC/EG
CPCMs before and after heat treatment
CPCMs before and after serving duration
540 541
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Table Captions
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Table.1 The thermal properties of SA-SA, SA-AC/EG CPCMs with various mass
545
ratios
546
Table. 2 Comparison of thermal properties of CPCMs
547
Table.3 Comparison of thermal conductivities of CPCMs
548
Table.4 DSC data of SA-AC and SA-AC/EG CPCMs before and after heat treatment
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Table.1 The thermal properties of SA-SA, SA-AC/EG CPCMs with various mass ratios Samples
Melting point (°C)
552 553 554 555
Melting
Melting
enthalpy of
enthalpy
composites
of SA-AC
(J/g)
(J/g)
Freezing point (°C)
Freezing
Freezing
enthalpy of
enthalpy
composites
of SA-AC
(J/g)
(J/g)
neat
66.35
202.5
202.5
56.45
201.1
201.1
M1
66.94
186.8
207.6
58.02
187.8
208.7
M2
66.66
184.0
206.7
57.97
183.1
205.7
M3
66.71
185.3
210.6
57.63
185.4
210.7
M4
67.46
185.8
213.3
57.65
183.5
210.9
M5
66.08
178.8
207.9
58.16
176.0
204.7
M6
66.32
178.5
210.0
58.10
176.5
207.6
*The SA-AC/EG CPCMs containing 90wt%~85wt% SA-AC eutectic mixture were marked as M1~M6, respectively.
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Table.2 Comparison of thermal properties of CPCMs Composite PCM Tm(°C) Hm(J/g) Tf(°C) Carnauba wax /EG 81.98 150.9 80.43 Palmitic-stearic acid /EG 53.89 166.27 54.37 Acetamide/EG 65.91 163.71 65.52 Rt100/EG 84.62 177.3 105.90 lauric–myristic–stearic 29.05 137.1 29.38 acid /EG Polyethylene glycol /EG 61.46 161.2 46.91 SA-AC/EG 66.94 186.8 58.02
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Hf(J/g) 142.6 166.13 173.3 131.3
References [2] [8] [28] [29] [30]
187.8
[31] Present work
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Table.3 Comparison of thermal conductivities of CPCMs Composite PCM Thermal conductivity Enhancement (W·m-1·K-1) factor Palmitic-stearic acid /EG 2.51 9.65 Acetamide /EG 2.61 6 Polyethylene glycol /EG 1.324 4.44 Capric–palmitic–stearic 5.225 15.34 acid /EG Palmitic acid /EG 0.6 2.5 Methyl stearate /EG 3.6 SA-AC/EG 5.909 17.59
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References [8] [28] [31] [32] [33] [34] Present work
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Table.4 DSC data of SA-AC and SA-AC/EG CPCMs before and after heat treatment Samples
Melting
Melting
Freezing
Freezing
temperature(℃)
enthalpy(J/g)
temperature(℃)
enthalpy(J/g)
neat
66.35
202.5
56.45
201.1
M1
66.94
186.8
58.02
187.8
M2
66.91
171.7
57.59
168.6
M3
66.46
173.9
58.09
171.6
564
*SA-AC/EG CPCM, SA-AC/EG CPCM after 500 accelerated thermal cycles and SA-
565
AC/EG CPCM after one month serving duration were marked as M1~M3,
566
respectively.
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