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ZSM-5 composite phase change materials for latent heat storage
Accepted Manuscript Synthesis and characterization of PEG/ZSM-5 composite phase change materials for latent heat storage
Chaoen Li, Hang Yu, Yuan Song, Mei Zhao PII:
S0960-1481(17)31293-4
DOI:
10.1016/j.renene.2017.12.089
Reference:
RENE 9586
To appear in:
Renewable Energy
Received Date:
12 May 2017
Revised Date:
02 December 2017
Accepted Date:
26 December 2017
Please cite this article as: Chaoen Li, Hang Yu, Yuan Song, Mei Zhao, Synthesis and characterization of PEG/ZSM-5 composite phase change materials for latent heat storage, Renewable Energy (2017), doi: 10.1016/j.renene.2017.12.089
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ACCEPTED MANUSCRIPT Graphical Abstract
ACCEPTED MANUSCRIPT 1
Synthesis and characterization of PEG/ZSM-5 composite phase change
2
materials for latent heat storage
3
Chaoen LI, Hang YU*, Yuan SONG, Mei ZHAO
4
School of Mechanical Engineering Tongji University, Tongji University, Shanghai
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201804, China
6 7 8
Corresponding author
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Hang YU
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Mobile: 13621894175
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E-mail Address: [email protected]
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Postal Address: Tongji University, 1239 Siping Road, Shanghai, P.R. China
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Abstract:
15
A novel poly (ethylene glycol)/ZSM-5 composite was fabricated by a simple way,
16
and it was characterized and evaluated for heat energy storage. The composite PCMs
17
was fabricated through introducing PEG to mesoporous pores of ZSM-5 assisted with
18
a vacuum impregnation system. XRD and FTIR analysis results showed that there was
19
no chemical reaction happened between PEG4000 and ZSM-5. DSC and TGA analysis
20
results indicated that the as-prepared PCMs has outstanding latent heats and superb
21
thermal stability. The latent heat of the as-prepared PCMs achieves to 76.37 J/g without
22
any leakiness. Compare with pure PEG, the thermal conductivity of as-prepared PCMs
23
was improved 200 %, which means selecting ZSM-5 serve as the supporting material
24
was an effective way to enhance the thermal conductivity. Exudation stability tests
25
results showed that the doping of PEG can reach up 50 wt% without any leakage when
26
the composite undergoing the melting process. According to the above analysis, the
27
prepared composite with good thermal stability, high latent heats and excellent thermal
28
conductivity is very suitable for heat energy storage.
29
Keywords: PEG, Mesoporous ZSM-5, Phase change materials, Thermal properties,
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Heat energy storage
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Introduction
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With the rapid economic and technological development, fossil fuels are
34
continuously to run out, energy shortage and environmental pollution problem
35
becoming more and more urgently
36
paying more and more attention to the utilization of renewable-energy [2]. Solar energy
37
is a clean, non-polluting, promising renewable-energy sources, and the technology has
38
used widely in modern life. However, sunlight is not available during the night.
39
Therefore, energy storage technique is required to store energy at daytime and release
40
energy at night. It is well known that phase change materials are regarded as a promising
41
energy saving, which for storage and release latent heat by phase transition [3]. PCMs
42
can be used in many fields such as solar-energy utilization
43
building techniques [5], HVAC system [6], heat management of electronics [7], waste heat
44
recovery
45
energy storage has more advantages like huge latent heat density, keeping constant
46
temperature when undergoing the melting/solidification process.
[8],
[1].
Therefore, researchers around the world are
[4],
eco-energy-saving
etc. Comparing with sensible energy storage, PCMs used in latent heat
47
Typically, based on chemical composition, there are two principal types of PCMs:
48
inorganic and organic [9, 10]. To compare with inorganic PCMs, organic PCMs are more
49
chemical and thermal stable, they melt congruently, and fewer super-cooling problems
50
[11, 12].
51
energy storage, because of its nice properties like appropriate phase change temperature,
52
large phase change enthalpy, no phase segregation, no or slight super cooling, non-
Among various PCMs, poly (ethylene glycol) has pleasant prospects for thermal
3
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toxicity, good thermal and chemical stability and repeatable utilization [13]. Furthermore,
54
many inorganic and organic supporting materials can be compatible with PEG
55
molecules
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utilization of organic PEG is also subject to some restrictions such as heat insulation
57
and poor interfacial force between the supporting carrier, which restrict its further
58
applications [14].
[14].
Unfortunately, like most organic phase change materials, the direct
59
Concerning above issues, it is wisely to bond PEG with porous materials to form
60
shape-stabilized PCMs. PEG combining with the inorganic porous supporting materials
61
cannot only prevent leakage, but also enhance the thermal conductivity of PCMs during
62
the melting and solidifying processes
63
exhibits better thermal conductivity, flame retardant, chemical and thermal stability as
64
compared to organic carrier materials
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phenomenon of PCMs happened in the carrier pores is various which other than that of
66
the pure PCMs. To be specific, the porous structure often has different pore size
67
distribution and geometric structure, which can greatly influence the crystalline
68
behavior in the small pores [17]. When the pore of supporting materials too narrow, the
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crystalline behavior of PCM will be inhibited, which make decreased the latent heat
70
enthalpy. On the contrary, too large pores can’t afford enough capillary force to catch
71
the PCM when it undergoing the melting process. Some studies suggested that using
72
mesoporous materials as supporting materials can greatly to raise the thermal
73
performance of PCMs [16]. Li Hong He et al. [18] dispersed PEG into mesoporous silica
[15].
[16].
4
Because the inorganic supporting matrix
It is worth noting that, the phase change
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structure and prepared shapes-stabilized PEG/SiO2 composites by Sol-gel methods.
75
There was no leakage of melted PEG after numerous thermal cycles. Takahiro Nomura
76
et al.
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diatomaceous earth, aluminium oxide) with a phase change material. They found that
78
the melting temperature of the PCM will be decreased because of the narrow pore
79
structure. Yong Deng et al. [20] encapsulated poly (ethylene glycol) into flower-like TiO2
80
nanostructure for thermal energy storage, and they found that the weak physical
81
interaction between the PEG and surface of FLN-TiO2 led to the lower phase change
82
temperatures of PEG/FLN-TiO2 (ss-CPCM). Di Suet et al.
83
octadecane/stearic acid eutectic mixtures with hexagonal boron nitride as phase change
84
materials. Mahmoud A. Hussein et al. [22] synthesized ZSM-5/PEG composites, and the
85
author introduced the ZSM-5/PEG as composites and test the electronic conductance
86
during the heating. However, in practical heat storage application, overcoming the
87
leakage defects of the PCMs is very important. It is necessary to study more detail about
88
the ZSM-5/PEG composites like thermal conduction, leakage test for practical heat
89
storage application.
[19]
studied the impregnation of a serious of porous materials (expanded perlite,
[21]
prepared n-
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In this study, we selected ZSM-5 as the porous supporting materials. Its chemical
91
formula is NanAlnSi96-nO192·16H2O (0
92
units linked together by oxygen bridges to form pentasil chains. A pentasil unit consists
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of eight five-membered rings. In these rings, the vertices are Al or Si and an O is
94
assumed to be bonded between the vertices [23]. ZSM-5 is regard as one of the excellent 5
ACCEPTED MANUSCRIPT [24],
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mesoporous materials
which can be used the carrier matrix to absorb PEG by
96
interfacial tension and capillary force. At the same time, ZSM-5 can also provide
97
thermal transfer pathways for PEG, which improve the thermal conductivity. Thus,
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ZSM-5 is an ideally supporting material to overcome the defects of PEG.
99
In this work, we applied a simple method to prepare PEG/ZSM-5 PCMS with
100
vacuum impregnation assistance. Then, XRD (X-ray diffraction), FT-IR (Fourier
101
transform infrared spectroscope), DSC (Differential scanning calorimeter), TGA
102
(Thermogravimetric analyser) was adopted to characterize the structure and thermal
103
properties. The probable mechanism of preparing PCMs with ZSM-5 was proposed.
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Based on the experimental and chemical analysis results showed that the as-prepared
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composite PCMs is suitable to application on energy saving buildings [5], solar energy
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storage [25]，waste heat recovering [8] et al.
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1. Experimental
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1.1 Materials and reagents
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Poly (ethylene glycol) (PEG 4000), absolute ethanol, were supplied by Sinopharm
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Chemical Reagents Company. ZSM-5 was supplied by Catalyst Plant of Nankai
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University. All chemicals were used as received without further purification.
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1.2 Preparation of composite PCMs
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According to the literatures
[26, 27],
comparing with natural impregnation, vacuum
115
treatment can help to improve the doping rate of PEG. In this manuscript, form-stable
116
PCMs were prepared through a simple physical blending method assist with a vacuum
117
impregnation system. Fig. 1 shows the whole steps for preparing composite PCMs.
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Briefly, amount of PEG was dissolved in 80 ml absolute ethanol then heated to 70 ℃
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to form an even transparent solution. Then, the ZSM-5 powder was added into the
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above solution and stirred by a magnetic stirrer for 2 h with assist with a vacuum
121
impregnation system. Eventually, the composites were dried in an oven maintain 60 ℃
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for 24 h. Here we call composite PCMs with various PEG contents form-stable PCMs,
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called for short FS-CPCMs. FS-CPCMs-1, FS-CPCMs-2, FS-CPCMs-3, FS-CPCMs-4
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represent PEG content 30 %, 40 %, 50 %, 60 % respectively in the composite.
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1.3 Characterization of composite PCMs
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The morphology of form-stable composite PCMs was determined by a scanning
127
electron microscope (SEM, Nova NanoSEM 450). Infrared spectra of the PCMs was
128
obtained by FT-IR (Jasco 430 model). FT-IR spectra was obtained in the range of 40007
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400 cm-1 and the test was resolution 4.0 cm-1. Powder XRD patterns were taken on a
130
BRUKER D8 ADVANCE Diffractometer. N2 physisorption isotherms were
131
determined by a Micromeritics ASAP 2020 instrument. The thermal stability and the
132
weight loss of the composite PCMs were determined by a thermogravimetric analyser
133
(Perkin Elmer TGA7 model) at a heating rate of 10℃/min and temperature of 25-600℃.
134
Thermal properties of the composite PCMs were measured by a DSC instrument (Q100
135
V9.9 Build 303 model). All measurements were carried out at 5℃/min constant heating
136
and cooling rate in purified nitrogen atmosphere. Thermal conductivity was measured
137
by a Hot Disk Thermal Constant Analysor (TPS2500, Sweden).
138
2. Test results and discussions
139
2.1 Pore structures of the ZSM-5 and the prepared FS-CPCMs
140
Crystalline behaviors of PCM and thermal stabilization ability of porous materials
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were greatly influenced by different pore structures [16]. Fig. 2 demonstrates that the N2
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adsorption/desorption isotherms of ZSM-5 and prepared FS-CPCMs-3. Table 1
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summarized N2 physisorption data from the ZSM-5 and prepared FS-CPCMs. ZSM-5
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could be classified as Type Ⅳ isotherm in accordance with IUPAC (mesopores are
145
between 2 and 50 nm) convention for adsorption isotherms. The relative pressure (p/p0)
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range of 0.4~0.8 is attributed to capillary condensation of mesoporous adsorption [26].
147
Pore size distribution is one of the important factors for ZSM-5 as the PCM supporting
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material. As the pore size distribution shown in Fig. 2, the BET surface area of ZSM-5
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is 238 m2/g, average pore diameter is 3.4 nm and total pore volume is 280 mm3/g. 8
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According to the literature [20], the large specific surface area and abundant mesoporous
151
structure are favorable for the adsorption of large amounts of PEG and enhance the
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thermal reliability of the PEG/ZSM-5 composites during the phase change cycles of
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melting and solidification. For the FS-CPCMs samples, the BET surface area and pore
154
volume are much smaller than that of ZSM-5, even can’t be determined by the
155
instrument, due to much PEG impregnation make the composites severe aggregation.
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2.2 Chemical compatibility of FS-CPCMs
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XRD patterns of both ZSM-5 and prepared form-stable composite phase change
158
materials are presented in Fig. 3. As shown in Fig. 3, ZSM-5 exhibited intense specific
159
peaks in the range of 2θ=22-25°, which could be observed over all the samples,
160
indicating that the crystal formation of the PEG keep the same with that of the
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composites PCM after the impregnation process. The X-ray intensity of PEG in FS-
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CPCM-3 composite is lower than that of pure PEG. This demonstrates that the pores of
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ZSM-5 limited the crystals of the PEG which lead to the decrease the crystallite size of
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PEG in the composites
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PEG in the composites by peak resolution
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crystallinity of PEG4000 was 0.76 and the crystallinities of the FS-CPCM-3 was 1.45.
167
This indicates that the crystallinities of the PEG/ZSM-5 PCMs decreased with the
168
increasing of ZSM-5 content. It can be deduced that the ZSM-5 acted as an impurity in
169
FS-CPCM-3 composites affect PEG crystal growth during the crystallization process.
170
[28].
We calculated the crystallinities of pure PEG and that of [29].
According to the calculation, the
The FTIR spectra of ZSM-5, PEG, FS-PCMs-3 were shown in Fig. 4 and the relevant 9
ACCEPTED MANUSCRIPT 171
data of which are given in Table 2. The absorption bands at 545 and 450 cm-1 stand for
172
the characteristics of the ZSM-5 crystalline structure [30]. From the FT-IR spectrum of
173
PEG/ZSM-5, there were no new characteristic peaks were found except the
174
characteristic peaks from PEG and ZSM-5. The typical peaks at 1060, 1105 and 1145
175
cm-1 which caused by stretching vibration of the functional group of C–O from the
176
PEG4000 were disappeared in FS-PCMs-3. However, the peak at 795 cm-1 represents
177
the SiO-H vibration from ZSM-5 were enhanced in FS-PCMs-3. That means hydrogen
178
bonds were formed between PEG ether groups and OH group from ZSM-5. It is also
179
obvious that the peaks at 1467, 963 and 842 cm-1 which caused by stretching vibration
180
of the functional group of C–H from the FS-PCMs-3 were weakened compared with
181
that of PEG4000. That means PEG molecules were maintained by ZSM-5’s pores by
182
capillary and surface tension forces which prevented the PEG leak from ZSM-5 when
183
the composites were undergoing melting[31]. There was no shift in the above main
184
absorption peaks and in FS-PCMs-3, PEG, ZSM-5 absorption peaks could be clearly
185
seen.
186
2.3 Micro-morphological analysis
187
As shown in Fig. 5(a), ZSM-5 consists of numerous rectangular particles with fairly
188
smooth and angular surface and the average size is nearly 1μm. As Fig. 5(b, c, d) shown,
189
the morphology of the composite phase change materials with various PEG contents
190
ratios showed no significant difference compared with the ZSM-5, which means that
191
PEG was adsorbed consistently by ZSM-5 and the composite keeps its shape after 10
ACCEPTED MANUSCRIPT 192
absorb the melted PEG. Under the assistance of vacuum impregnation, the PEG
193
impregnated into the nanoparticles of ZSM-5 were uniformly dispersed. PEG is easily
194
soluble in anhydrous ethanol, so the PEG molecules can be adsorbed by the fluffy
195
network structure of ZSM-5. With the evaporation of anhydrous ethanol, PEG stay
196
dispersed uniformly in the pores of ZSM-5 structure. However, as shown in Fig. 5(e),
197
with too much PEG doped, the composites were severe agglomeration.
198
2.4 Thermal energy storage performance of FS-CPCMs
199
Thermal energy storage performance and phase changes temperature were
200
determined by DSC. Table 3 summarizes the phase change parameters from the DSC
201
results, including onset melting temperature ( Tom ), melting latent enthalpy ( H m ), onset
202
solidification temperature ( Tos ), and solidification latent enthalpy ( H s ). As shown in
203
Fig. 6(a), we can determine several characteristic temperatures by deal with heat flow
204
signal [32]. As reported by the references [33], the theoretical latent enthalpy ( H c ) of the
205
composites PCMs can be explicit by Eq. (1):
206
H c 1 H m
(1)
207
Here H c denotes the theoretical enthalpy of the composites PCMs. H m denotes the
208
enthalpy of PEG4000, and denotes the content of ZSM-5. From Table 3 and Fig. 6b,
209
we found that the composite PCMs have ideal phase change temperatures and perfectly
210
high phase change enthalpies. The latent heat of FS-CPCMs increased from 41.55 J/g
211
to 115.56 J/g with the increasing doping of PEG. However, according to Eq. (1), the
212
measured value always inferior the theoretical value. The reason is that hydroxyl groups 11
ACCEPTED MANUSCRIPT 213
of PEG interact with Si-OH of ZSM-5 and form hydrogen bonds which influence the
214
behavior crystallization
215
PEG molecular chain’s crystal arrangement and orientation, which contribute to
216
decrease the regularity of crystal line regions and the increase of lattice defects [26].
217 218
[34].
In the meanwhile, the mesopores structure also influence
The super cooling can be explicit by Eq. (2):
T Tm - Ts
(2)
219
In practical application, we should consider the super cooling of phase change
220
materials [35]. As Fig. 6c shown, the super cooling of composites PCMs were reduced
221
to 66.3 %, 57.0 %, and 56.3 %, 38.1 % respectively. That’s means ZSM-5 as a
222
supporting material can greatly narrow the super cooling degree of PEG. When the
223
doping amount of ZSM-5 is low (FS-CPCMs-4), the ZSM-5 act as the crystalline
224
nucleus to decrease the overcooling. When the adoption become high, the formation of
225
the effective thermally conductive pathway composed of ZSM-5 is primary
226
contribution to the enhancement of thermal conductivity, which to be another reason to
227
decrease the super cooling. The detail will discuss in the following. As Table 3 shown
228
that Tm and Ts of the composite PCMs decrease as 3 °C and 8 °C respectively. The
229
reason is that the interaction between the PEG and ZSM-5 influence the shift direction
230
of melting and solidification point [36]. In Fig. 6b, the melting enthalpy are always higher
231
than the solidifying enthalpy value, which was consistent with other researchers [37, 38].
232
The reason may be attributed to mass loss during melting by DSC, and the choice of
233
the base line during the calculation of the enthalpy value. According to previous 12
ACCEPTED MANUSCRIPT 234
comparison studies on the thermal properties of the prepared composite PCM with that
235
of some composite PCM (Table 4), the phase change latent heat of the prepared
236
composite PCM in the paper was slightly lower than the previous result.
237
2.5 Thermal stability and exudation stability of the composites PCMs
238
In practical heat storage application, thermal stability of PCM is another important [44].
239
index
240
stability below 155 ℃ which much higher than its phase transition temperature, and the
241
weight loss curve of all samples exhibited in only one step. Meanwhile, the sharp
242
weight loss happened at temperature of about 250 ℃ is due to the disintegration of the
243
PEG. By calculating the residual masses of all samples, the result shown that the PEG
244
doping ratios of all as-prepared samples are in good agreement with the default value.
245
Fig. 8 shows the shape-stabilized effect of pure PEG and as-prepared PCMs by
246
increasing working temperature. First, we pressed the samples into wafers respectively
247
by tablet machine. The size of each wafer maintained at the diameter 13 mm, and height
248
2 mm
249
80 ℃ to explore their exudation stabilities for 30 min. From the picture, we can see that
250
the wafers of all the as-prepared sample were stable, except the pure PEG. The wafer
251
of FS-CPCMs-4 is stable, slight leakage was observed due to too much PEG was
252
impregnated into the ZSM-5. Consequently, pores structure network of ZSM-5 and
253
adsorption comprehensive effect the stability of companies of PCMs.
254
[15].
As shown in Fig. 7, the as-prepared PCMs exhibited exceptional thermal
All the samples were put on a thermostatic heating plate which heated at
The synthesis mechanism of PEG/ZSM-5 phase change materials with various 13
ACCEPTED MANUSCRIPT 255
weight percentages was proposed and simply described in Fig. 9. When the contents of
256
Peg were low, the crystallization behave of PEG is affected by ZSM-5. PEG molecules
257
adsorbed by ZSM-5 by surface tension and capillary, and the movement of the polymer
258
chains were confined. When PEG content is low, PEG in composites is limited by
259
excessive ZSM-5. On the contrary, when the PEG content exceeds the ZSM-5 pore
260
structure accommodation, a part of PEG was adsorbed on the surface ZSM-5 and could
261
crystallize freely. The more content of PEG, the more materials can be crystallized with
262
larger thermal enthalpy. Although the movement of PEG was restricted by the pores of
263
ZSM-5, the stabilization of PEG/ZSM-5 PCMs also improve with the ZSM-5.
264
2.6 Thermal conductivity of composites PCMs
265
Fig. 10 shows the thermal conductivity of all the as-prepared samples. As shown in
266
Fig.10, the thermal conductivity of the pure PEG, and the composites PCMs were 0.22,
267
0.66, 0.61, 0.57 and 0.54 W/(m·K), respectively. Due to the ZSM-5 as supporting
268
materials, the thermal conductivities of all the samples were enhanced. In this work, the
269
improvement of thermal conductivity of composite PCMs is not ascribed to the
270
decrease of the interfacial thermal resistance and the increase of crystallinity of the PEG
271
matrix, because ZSM-5 was directly used without any surface treatment, and the fillers
272
had a little effect on the crystallization properties of the matrix discussed later in this
273
paper, respectively. Therefore, the formation of the effective thermally conductive
274
pathway composed of ZSM-5 is primary contribution to the enhancement of thermal
275
conductivity [45]. That indicates ZSM-5 serving as a supporting material greatly enhance 14
ACCEPTED MANUSCRIPT 276
the thermal conductivity of composites PCMs.
277
2.7 Melting and solidifying process of composites PCMs
278
The schematic of the experimental system which was employed for study the thermal
279
property of the as-prepared PCMs from the research [38]. The first step is putting 3-4 g
280
of pure PEG4000 and 3-4 g of FS-CPCMs-3 into two small glass tubes, respectively.
281
Then two T-type thermocouples were placed in the center of the two tubes, respectively.
282
Finally, two constant temperature water baths were used to supplied different
283
conditions to complete melting and solidification process. When doing the melting test,
284
the two testing tubes with thermocouples were placed into one water bath which
285
keeping the water at 80 ℃, and the results were record every 1 s until the temperature
286
rich the maximum value. After the melting process, the two testing tubes were quickly
287
placed into another water bath which keeping the water at 20 ℃. From the Fig. 11(a),
288
we can see the melting time will be 432 s for PEG and 275 s for FS-CPCMs-3 from 20
289
℃ to 75 ℃. The melting time was reduced by 57 %. Solidification time also shown in
290
Fig. 11(b), the solidification time will be 519 s for PEG and 247 s for FS-CPCMs-3
291
from 75 ℃ to 25 ℃. The solidification time was reduced by 110 %. These results
292
indicate that the thermal response ratio of composite PCM obviously increased for
293
practical application.
294
3. Conclusions
295
1） PEG/ZSM-5 composite PCMs as form-stable PCMs were prepared in a simple way
296
by incorporating PEG in the mesoporous pores of ZSM-5 assist with vacuum 15
ACCEPTED MANUSCRIPT 297
impregnation system.
298
2） The maximum content of PEG can reach up to 50 % in the composite without
299
leakage. XRD and FTIR analysis results showed that there was not any chemical
300
reaction happened between PEG4000 and ZSM-5. The latent heat of the as-prepared
301
PCMs achieves to 76.37 J/g.
302
3） Compare with pure PEG, the thermal conductivity of as-prepared PCMs was
303
improved 200 %, which means selecting ZSM-5 serve as the supporting material
304
was an effective way to enhance the thermal conductivity. Exudation stability tests
305
results showed that the doping of PEG can reach up 50 wt% without any leakage
306
when the composite undergoing the melting process.
307 308 309
Acknowledgements
310
The study has been supported by the China National Key R&D Program "Solutions
311
to heating and cooling of building in the Yangtze river region" (Grant
312
No.2016YFC0700305-02)
313
16
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Figure：
433
Fig. 1. N2 adsorption/desorption isotherms of (a) ZSM-5; (b) FS-CPCMs-3
434
Fig. 2. XRD spectra of ZSM-5, PEG-4000, FS-CPCM-3
435
Fig. 3. FT-IR spectra of ZSM-5, PEG-4000, FS-CPCM-3
436
Fig. 4. SEM images of (a) ZSM-5, (b) FS-CPCM-1, (c) FS-CPCM-2, (d) FS-CPCM-3,
437
(e) FS-CPCM-4
438
Fig. 6. PEG and FS-CPCMs: (a) DSC curves of pure PEG and FS-CPCMs-3 during
439
melting and freezing phases (b) DSC curves; (c) Phase change temperatures; (d)
440
Enthalpies of melting and solidifying
441
Fig. 7. TGA curves of pristine PEG and the FS-PCMs
442
Fig. 8. Macroscopic images (a) wafers of PCMs and pure PEG at room temperature;
443
(b) exudation stability of PCMs and pure PEG at 80 ℃ after 10 min (c) exudation
444
stability of PCMs and pure PEG at 80 ℃ after 30 min
445
Fig. 9. Schematic illustration showing the interaction of PEG with ZSM-5 in the
446
PEG/ZSM-5 PCMs with various weight percentages.
447
Fig. 10. thermal conductivities of the prepared PEG, and FS-CPCMs.
448
Fig. 11. (a) Melting process curves of pristine PEG and FS-CPCMs-3, and (b)
449
Solidifying process curves of pristine PEG and FS-CPCMs-3.
450
20
ACCEPTED MANUSCRIPT 451
Table ：
452
Table 1 The compositions of the FS-CPCMs
453
Table 2 Textural properties of ZSM-5 and FS-CPCMs.
454
Table 3 FTIR absorption features and their assignment for ZSM-5 and PEG.
455
Table 4 DSC results of pure PEG and PEG/ZSM-5 composite PCMs
456
21
ACCEPTED MANUSCRIPT Figure：
Fig. 1. Schematic of the preparation of FS-PCMs.
Fig. 2 N2 adsorption/desorption isotherms of (a) ZSM-5; (b) FS-CPCMs-3
Fig. 3 XRD spectra of ZSM-5, PEG-4000, FS-CPCM-3
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Fig. 4 FT-IR spectra of ZSM-5, PEG-4000, FS-CPCM-3
Fig. 5 SEM images of (a) ZSM-5, (b) FS-CPCM-1, (c) FS-CPCM-2, (d) FS-CPCM-3, (e) FS-CPCM-4
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Fig. 6 PEG and FS-CPCMs: (a) DSC curves of pure PEG and FS-CPCMs-3 during melting and freezing phases (b) DSC curves; (c) Phase change temperatures; (d) Enthalpies of melting and solidifying
Fig. 7. TGA curves of pristine PEG and the FS-PCMs
ACCEPTED MANUSCRIPT
Fig. 8. Macroscopic images (a) wafers of PCMs and pure PEG at room temperature; (b) exudation stability of PCMs and pure PEG at 80 ℃ after 10 min (c) exudation stability of PCMs and pure PEG at 80 ℃ after 30 min
Fig. 9. Schematic illustration showing the interaction of PEG with ZSM-5 in the PEG/ZSM-5 PCMs with various weight percentages.
Fig. 10. thermal conductivities of the prepared PEG, and FS-CPCMs.
ACCEPTED MANUSCRIPT
Fig. 11. (a) Melting process curves of pristine PEG and FS-CPCMs-3, and (b) Solidifying process curves of pristine PEG and FS-CPCMs-3.
ACCEPTED MANUSCRIPT Highlights: 1）Mesoporous ZSM-5 supported PEG phase change materials was compound by a simple way and the synthesis mechanism was proposed. 2）PEG/ZSM-5 has good thermal stability, high latent heat and excellent thermal conductivity.
ACCEPTED MANUSCRIPT Table： Table 1 Textural properties of ZSM-5 and FS-CPCMs.
SBETa (m2/g) 238.07 3.54 1.75 0.33 -
Samples ZSM-5 FS-CPCMs-1 FS-CPCMs-2 FS-CPCMs-3 FS-CPCMs-4
Vtb (cm3/g) 0.03 0.01 0.22 0.21 -
DBJHc (nm) 3.43 2.46 1.80 49.03 -
a SBET, BET surface area calculated by the adsorption branch of N2 isotherm. b Vt, pore volume. c DBJH, pore diameter determined by BJH model. Table 2 FTIR absorption features and their assignment for ZSM-5 and PEG.
Wavenumber(cm-1) 2888 1105,1060,1145 1467, 963, 842 795 3200-3600 545 ,450
Assignment C–H stretching vibration of CH2 in PEG C–O stretching vibration in PEG C–H bending vibration of CH2 in PEG O-H vibration of SiO–H (ZSM-5) O-H stretching vibration in PEG characteristics of the ZSM-5 crystalline structure
Table 3 DSC results of pure PEG and PEG/ZSM-5 composite PCMs
Samples
Tm (℃)
PEG4000 FS-CPCMs-1 FS-CPCMs-2 FS-CPCMs-3 FS-CPCMs-4
Melting process Tpeak-m (℃) ΔHm (J/g)
Solidification process T s (℃ ) Tpeak-s (℃) ΔHs (J/g)
59.5
61.6
192.6
37.4
35.2
163.2
53.1
58.2
41.5
45.7
43.9
33.9
56.4
59.9
56.4
46.9
45.4
47.1
56.3
60.5
76.4
46.7
44.9
64.3
59.9
61.7
115.6
46.2
44.9
96.8
Table 4 Comparison of thermal properties of the form-stable SA/ZSM-5 composite PCM with that of different composite PCMs in literature. PCM
Paraffin/diatomite Capric-palmitic acid/attapulgite Paraffin/expanded perlite Stearic acid/silica fume paraffin/expanded vermiculite PEG/ZSM-5
Phase change temperature（℃）
Latent heat（J/g）
References
36.5
53.1
[39]
21.7
48.2
[40]
42.3
87.4
[41]
59.9
84.5
[42]
48.8
103
[43]
56.3
76.4
This study
Chaoen Li, Hang Yu, Yuan Song, Mei Zhao PII:
S0960-1481(17)31293-4
DOI:
10.1016/j.renene.2017.12.089
Reference:
RENE 9586
To appear in:
Renewable Energy
Received Date:
12 May 2017
Revised Date:
02 December 2017
Accepted Date:
26 December 2017
Please cite this article as: Chaoen Li, Hang Yu, Yuan Song, Mei Zhao, Synthesis and characterization of PEG/ZSM-5 composite phase change materials for latent heat storage, Renewable Energy (2017), doi: 10.1016/j.renene.2017.12.089
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ACCEPTED MANUSCRIPT Graphical Abstract
ACCEPTED MANUSCRIPT 1
Synthesis and characterization of PEG/ZSM-5 composite phase change
2
materials for latent heat storage
3
Chaoen LI, Hang YU*, Yuan SONG, Mei ZHAO
4
School of Mechanical Engineering Tongji University, Tongji University, Shanghai
5
201804, China
6 7 8
Corresponding author
9
Hang YU
10
Mobile: 13621894175
11
E-mail Address: [email protected]
12
Postal Address: Tongji University, 1239 Siping Road, Shanghai, P.R. China
13
1
ACCEPTED MANUSCRIPT 14
Abstract:
15
A novel poly (ethylene glycol)/ZSM-5 composite was fabricated by a simple way,
16
and it was characterized and evaluated for heat energy storage. The composite PCMs
17
was fabricated through introducing PEG to mesoporous pores of ZSM-5 assisted with
18
a vacuum impregnation system. XRD and FTIR analysis results showed that there was
19
no chemical reaction happened between PEG4000 and ZSM-5. DSC and TGA analysis
20
results indicated that the as-prepared PCMs has outstanding latent heats and superb
21
thermal stability. The latent heat of the as-prepared PCMs achieves to 76.37 J/g without
22
any leakiness. Compare with pure PEG, the thermal conductivity of as-prepared PCMs
23
was improved 200 %, which means selecting ZSM-5 serve as the supporting material
24
was an effective way to enhance the thermal conductivity. Exudation stability tests
25
results showed that the doping of PEG can reach up 50 wt% without any leakage when
26
the composite undergoing the melting process. According to the above analysis, the
27
prepared composite with good thermal stability, high latent heats and excellent thermal
28
conductivity is very suitable for heat energy storage.
29
Keywords: PEG, Mesoporous ZSM-5, Phase change materials, Thermal properties,
30
Heat energy storage
2
ACCEPTED MANUSCRIPT 32
Introduction
33
With the rapid economic and technological development, fossil fuels are
34
continuously to run out, energy shortage and environmental pollution problem
35
becoming more and more urgently
36
paying more and more attention to the utilization of renewable-energy [2]. Solar energy
37
is a clean, non-polluting, promising renewable-energy sources, and the technology has
38
used widely in modern life. However, sunlight is not available during the night.
39
Therefore, energy storage technique is required to store energy at daytime and release
40
energy at night. It is well known that phase change materials are regarded as a promising
41
energy saving, which for storage and release latent heat by phase transition [3]. PCMs
42
can be used in many fields such as solar-energy utilization
43
building techniques [5], HVAC system [6], heat management of electronics [7], waste heat
44
recovery
45
energy storage has more advantages like huge latent heat density, keeping constant
46
temperature when undergoing the melting/solidification process.
[8],
[1].
Therefore, researchers around the world are
[4],
eco-energy-saving
etc. Comparing with sensible energy storage, PCMs used in latent heat
47
Typically, based on chemical composition, there are two principal types of PCMs:
48
inorganic and organic [9, 10]. To compare with inorganic PCMs, organic PCMs are more
49
chemical and thermal stable, they melt congruently, and fewer super-cooling problems
50
[11, 12].
51
energy storage, because of its nice properties like appropriate phase change temperature,
52
large phase change enthalpy, no phase segregation, no or slight super cooling, non-
Among various PCMs, poly (ethylene glycol) has pleasant prospects for thermal
3
ACCEPTED MANUSCRIPT 53
toxicity, good thermal and chemical stability and repeatable utilization [13]. Furthermore,
54
many inorganic and organic supporting materials can be compatible with PEG
55
molecules
56
utilization of organic PEG is also subject to some restrictions such as heat insulation
57
and poor interfacial force between the supporting carrier, which restrict its further
58
applications [14].
[14].
Unfortunately, like most organic phase change materials, the direct
59
Concerning above issues, it is wisely to bond PEG with porous materials to form
60
shape-stabilized PCMs. PEG combining with the inorganic porous supporting materials
61
cannot only prevent leakage, but also enhance the thermal conductivity of PCMs during
62
the melting and solidifying processes
63
exhibits better thermal conductivity, flame retardant, chemical and thermal stability as
64
compared to organic carrier materials
65
phenomenon of PCMs happened in the carrier pores is various which other than that of
66
the pure PCMs. To be specific, the porous structure often has different pore size
67
distribution and geometric structure, which can greatly influence the crystalline
68
behavior in the small pores [17]. When the pore of supporting materials too narrow, the
69
crystalline behavior of PCM will be inhibited, which make decreased the latent heat
70
enthalpy. On the contrary, too large pores can’t afford enough capillary force to catch
71
the PCM when it undergoing the melting process. Some studies suggested that using
72
mesoporous materials as supporting materials can greatly to raise the thermal
73
performance of PCMs [16]. Li Hong He et al. [18] dispersed PEG into mesoporous silica
[15].
[16].
4
Because the inorganic supporting matrix
It is worth noting that, the phase change
ACCEPTED MANUSCRIPT 74
structure and prepared shapes-stabilized PEG/SiO2 composites by Sol-gel methods.
75
There was no leakage of melted PEG after numerous thermal cycles. Takahiro Nomura
76
et al.
77
diatomaceous earth, aluminium oxide) with a phase change material. They found that
78
the melting temperature of the PCM will be decreased because of the narrow pore
79
structure. Yong Deng et al. [20] encapsulated poly (ethylene glycol) into flower-like TiO2
80
nanostructure for thermal energy storage, and they found that the weak physical
81
interaction between the PEG and surface of FLN-TiO2 led to the lower phase change
82
temperatures of PEG/FLN-TiO2 (ss-CPCM). Di Suet et al.
83
octadecane/stearic acid eutectic mixtures with hexagonal boron nitride as phase change
84
materials. Mahmoud A. Hussein et al. [22] synthesized ZSM-5/PEG composites, and the
85
author introduced the ZSM-5/PEG as composites and test the electronic conductance
86
during the heating. However, in practical heat storage application, overcoming the
87
leakage defects of the PCMs is very important. It is necessary to study more detail about
88
the ZSM-5/PEG composites like thermal conduction, leakage test for practical heat
89
storage application.
[19]
studied the impregnation of a serious of porous materials (expanded perlite,
[21]
prepared n-
90
In this study, we selected ZSM-5 as the porous supporting materials. Its chemical
91
formula is NanAlnSi96-nO192·16H2O (0
92
units linked together by oxygen bridges to form pentasil chains. A pentasil unit consists
93
of eight five-membered rings. In these rings, the vertices are Al or Si and an O is
94
assumed to be bonded between the vertices [23]. ZSM-5 is regard as one of the excellent 5
ACCEPTED MANUSCRIPT [24],
95
mesoporous materials
which can be used the carrier matrix to absorb PEG by
96
interfacial tension and capillary force. At the same time, ZSM-5 can also provide
97
thermal transfer pathways for PEG, which improve the thermal conductivity. Thus,
98
ZSM-5 is an ideally supporting material to overcome the defects of PEG.
99
In this work, we applied a simple method to prepare PEG/ZSM-5 PCMS with
100
vacuum impregnation assistance. Then, XRD (X-ray diffraction), FT-IR (Fourier
101
transform infrared spectroscope), DSC (Differential scanning calorimeter), TGA
102
(Thermogravimetric analyser) was adopted to characterize the structure and thermal
103
properties. The probable mechanism of preparing PCMs with ZSM-5 was proposed.
104
Based on the experimental and chemical analysis results showed that the as-prepared
105
composite PCMs is suitable to application on energy saving buildings [5], solar energy
106
storage [25]，waste heat recovering [8] et al.
107
6
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1. Experimental
109
1.1 Materials and reagents
110
Poly (ethylene glycol) (PEG 4000), absolute ethanol, were supplied by Sinopharm
111
Chemical Reagents Company. ZSM-5 was supplied by Catalyst Plant of Nankai
112
University. All chemicals were used as received without further purification.
113
1.2 Preparation of composite PCMs
114
According to the literatures
[26, 27],
comparing with natural impregnation, vacuum
115
treatment can help to improve the doping rate of PEG. In this manuscript, form-stable
116
PCMs were prepared through a simple physical blending method assist with a vacuum
117
impregnation system. Fig. 1 shows the whole steps for preparing composite PCMs.
118
Briefly, amount of PEG was dissolved in 80 ml absolute ethanol then heated to 70 ℃
119
to form an even transparent solution. Then, the ZSM-5 powder was added into the
120
above solution and stirred by a magnetic stirrer for 2 h with assist with a vacuum
121
impregnation system. Eventually, the composites were dried in an oven maintain 60 ℃
122
for 24 h. Here we call composite PCMs with various PEG contents form-stable PCMs,
123
called for short FS-CPCMs. FS-CPCMs-1, FS-CPCMs-2, FS-CPCMs-3, FS-CPCMs-4
124
represent PEG content 30 %, 40 %, 50 %, 60 % respectively in the composite.
125
1.3 Characterization of composite PCMs
126
The morphology of form-stable composite PCMs was determined by a scanning
127
electron microscope (SEM, Nova NanoSEM 450). Infrared spectra of the PCMs was
128
obtained by FT-IR (Jasco 430 model). FT-IR spectra was obtained in the range of 40007
ACCEPTED MANUSCRIPT 129
400 cm-1 and the test was resolution 4.0 cm-1. Powder XRD patterns were taken on a
130
BRUKER D8 ADVANCE Diffractometer. N2 physisorption isotherms were
131
determined by a Micromeritics ASAP 2020 instrument. The thermal stability and the
132
weight loss of the composite PCMs were determined by a thermogravimetric analyser
133
(Perkin Elmer TGA7 model) at a heating rate of 10℃/min and temperature of 25-600℃.
134
Thermal properties of the composite PCMs were measured by a DSC instrument (Q100
135
V9.9 Build 303 model). All measurements were carried out at 5℃/min constant heating
136
and cooling rate in purified nitrogen atmosphere. Thermal conductivity was measured
137
by a Hot Disk Thermal Constant Analysor (TPS2500, Sweden).
138
2. Test results and discussions
139
2.1 Pore structures of the ZSM-5 and the prepared FS-CPCMs
140
Crystalline behaviors of PCM and thermal stabilization ability of porous materials
141
were greatly influenced by different pore structures [16]. Fig. 2 demonstrates that the N2
142
adsorption/desorption isotherms of ZSM-5 and prepared FS-CPCMs-3. Table 1
143
summarized N2 physisorption data from the ZSM-5 and prepared FS-CPCMs. ZSM-5
144
could be classified as Type Ⅳ isotherm in accordance with IUPAC (mesopores are
145
between 2 and 50 nm) convention for adsorption isotherms. The relative pressure (p/p0)
146
range of 0.4~0.8 is attributed to capillary condensation of mesoporous adsorption [26].
147
Pore size distribution is one of the important factors for ZSM-5 as the PCM supporting
148
material. As the pore size distribution shown in Fig. 2, the BET surface area of ZSM-5
149
is 238 m2/g, average pore diameter is 3.4 nm and total pore volume is 280 mm3/g. 8
ACCEPTED MANUSCRIPT 150
According to the literature [20], the large specific surface area and abundant mesoporous
151
structure are favorable for the adsorption of large amounts of PEG and enhance the
152
thermal reliability of the PEG/ZSM-5 composites during the phase change cycles of
153
melting and solidification. For the FS-CPCMs samples, the BET surface area and pore
154
volume are much smaller than that of ZSM-5, even can’t be determined by the
155
instrument, due to much PEG impregnation make the composites severe aggregation.
156
2.2 Chemical compatibility of FS-CPCMs
157
XRD patterns of both ZSM-5 and prepared form-stable composite phase change
158
materials are presented in Fig. 3. As shown in Fig. 3, ZSM-5 exhibited intense specific
159
peaks in the range of 2θ=22-25°, which could be observed over all the samples,
160
indicating that the crystal formation of the PEG keep the same with that of the
161
composites PCM after the impregnation process. The X-ray intensity of PEG in FS-
162
CPCM-3 composite is lower than that of pure PEG. This demonstrates that the pores of
163
ZSM-5 limited the crystals of the PEG which lead to the decrease the crystallite size of
164
PEG in the composites
165
PEG in the composites by peak resolution
166
crystallinity of PEG4000 was 0.76 and the crystallinities of the FS-CPCM-3 was 1.45.
167
This indicates that the crystallinities of the PEG/ZSM-5 PCMs decreased with the
168
increasing of ZSM-5 content. It can be deduced that the ZSM-5 acted as an impurity in
169
FS-CPCM-3 composites affect PEG crystal growth during the crystallization process.
170
[28].
We calculated the crystallinities of pure PEG and that of [29].
According to the calculation, the
The FTIR spectra of ZSM-5, PEG, FS-PCMs-3 were shown in Fig. 4 and the relevant 9
ACCEPTED MANUSCRIPT 171
data of which are given in Table 2. The absorption bands at 545 and 450 cm-1 stand for
172
the characteristics of the ZSM-5 crystalline structure [30]. From the FT-IR spectrum of
173
PEG/ZSM-5, there were no new characteristic peaks were found except the
174
characteristic peaks from PEG and ZSM-5. The typical peaks at 1060, 1105 and 1145
175
cm-1 which caused by stretching vibration of the functional group of C–O from the
176
PEG4000 were disappeared in FS-PCMs-3. However, the peak at 795 cm-1 represents
177
the SiO-H vibration from ZSM-5 were enhanced in FS-PCMs-3. That means hydrogen
178
bonds were formed between PEG ether groups and OH group from ZSM-5. It is also
179
obvious that the peaks at 1467, 963 and 842 cm-1 which caused by stretching vibration
180
of the functional group of C–H from the FS-PCMs-3 were weakened compared with
181
that of PEG4000. That means PEG molecules were maintained by ZSM-5’s pores by
182
capillary and surface tension forces which prevented the PEG leak from ZSM-5 when
183
the composites were undergoing melting[31]. There was no shift in the above main
184
absorption peaks and in FS-PCMs-3, PEG, ZSM-5 absorption peaks could be clearly
185
seen.
186
2.3 Micro-morphological analysis
187
As shown in Fig. 5(a), ZSM-5 consists of numerous rectangular particles with fairly
188
smooth and angular surface and the average size is nearly 1μm. As Fig. 5(b, c, d) shown,
189
the morphology of the composite phase change materials with various PEG contents
190
ratios showed no significant difference compared with the ZSM-5, which means that
191
PEG was adsorbed consistently by ZSM-5 and the composite keeps its shape after 10
ACCEPTED MANUSCRIPT 192
absorb the melted PEG. Under the assistance of vacuum impregnation, the PEG
193
impregnated into the nanoparticles of ZSM-5 were uniformly dispersed. PEG is easily
194
soluble in anhydrous ethanol, so the PEG molecules can be adsorbed by the fluffy
195
network structure of ZSM-5. With the evaporation of anhydrous ethanol, PEG stay
196
dispersed uniformly in the pores of ZSM-5 structure. However, as shown in Fig. 5(e),
197
with too much PEG doped, the composites were severe agglomeration.
198
2.4 Thermal energy storage performance of FS-CPCMs
199
Thermal energy storage performance and phase changes temperature were
200
determined by DSC. Table 3 summarizes the phase change parameters from the DSC
201
results, including onset melting temperature ( Tom ), melting latent enthalpy ( H m ), onset
202
solidification temperature ( Tos ), and solidification latent enthalpy ( H s ). As shown in
203
Fig. 6(a), we can determine several characteristic temperatures by deal with heat flow
204
signal [32]. As reported by the references [33], the theoretical latent enthalpy ( H c ) of the
205
composites PCMs can be explicit by Eq. (1):
206
H c 1 H m
(1)
207
Here H c denotes the theoretical enthalpy of the composites PCMs. H m denotes the
208
enthalpy of PEG4000, and denotes the content of ZSM-5. From Table 3 and Fig. 6b,
209
we found that the composite PCMs have ideal phase change temperatures and perfectly
210
high phase change enthalpies. The latent heat of FS-CPCMs increased from 41.55 J/g
211
to 115.56 J/g with the increasing doping of PEG. However, according to Eq. (1), the
212
measured value always inferior the theoretical value. The reason is that hydroxyl groups 11
ACCEPTED MANUSCRIPT 213
of PEG interact with Si-OH of ZSM-5 and form hydrogen bonds which influence the
214
behavior crystallization
215
PEG molecular chain’s crystal arrangement and orientation, which contribute to
216
decrease the regularity of crystal line regions and the increase of lattice defects [26].
217 218
[34].
In the meanwhile, the mesopores structure also influence
The super cooling can be explicit by Eq. (2):
T Tm - Ts
(2)
219
In practical application, we should consider the super cooling of phase change
220
materials [35]. As Fig. 6c shown, the super cooling of composites PCMs were reduced
221
to 66.3 %, 57.0 %, and 56.3 %, 38.1 % respectively. That’s means ZSM-5 as a
222
supporting material can greatly narrow the super cooling degree of PEG. When the
223
doping amount of ZSM-5 is low (FS-CPCMs-4), the ZSM-5 act as the crystalline
224
nucleus to decrease the overcooling. When the adoption become high, the formation of
225
the effective thermally conductive pathway composed of ZSM-5 is primary
226
contribution to the enhancement of thermal conductivity, which to be another reason to
227
decrease the super cooling. The detail will discuss in the following. As Table 3 shown
228
that Tm and Ts of the composite PCMs decrease as 3 °C and 8 °C respectively. The
229
reason is that the interaction between the PEG and ZSM-5 influence the shift direction
230
of melting and solidification point [36]. In Fig. 6b, the melting enthalpy are always higher
231
than the solidifying enthalpy value, which was consistent with other researchers [37, 38].
232
The reason may be attributed to mass loss during melting by DSC, and the choice of
233
the base line during the calculation of the enthalpy value. According to previous 12
ACCEPTED MANUSCRIPT 234
comparison studies on the thermal properties of the prepared composite PCM with that
235
of some composite PCM (Table 4), the phase change latent heat of the prepared
236
composite PCM in the paper was slightly lower than the previous result.
237
2.5 Thermal stability and exudation stability of the composites PCMs
238
In practical heat storage application, thermal stability of PCM is another important [44].
239
index
240
stability below 155 ℃ which much higher than its phase transition temperature, and the
241
weight loss curve of all samples exhibited in only one step. Meanwhile, the sharp
242
weight loss happened at temperature of about 250 ℃ is due to the disintegration of the
243
PEG. By calculating the residual masses of all samples, the result shown that the PEG
244
doping ratios of all as-prepared samples are in good agreement with the default value.
245
Fig. 8 shows the shape-stabilized effect of pure PEG and as-prepared PCMs by
246
increasing working temperature. First, we pressed the samples into wafers respectively
247
by tablet machine. The size of each wafer maintained at the diameter 13 mm, and height
248
2 mm
249
80 ℃ to explore their exudation stabilities for 30 min. From the picture, we can see that
250
the wafers of all the as-prepared sample were stable, except the pure PEG. The wafer
251
of FS-CPCMs-4 is stable, slight leakage was observed due to too much PEG was
252
impregnated into the ZSM-5. Consequently, pores structure network of ZSM-5 and
253
adsorption comprehensive effect the stability of companies of PCMs.
254
[15].
As shown in Fig. 7, the as-prepared PCMs exhibited exceptional thermal
All the samples were put on a thermostatic heating plate which heated at
The synthesis mechanism of PEG/ZSM-5 phase change materials with various 13
ACCEPTED MANUSCRIPT 255
weight percentages was proposed and simply described in Fig. 9. When the contents of
256
Peg were low, the crystallization behave of PEG is affected by ZSM-5. PEG molecules
257
adsorbed by ZSM-5 by surface tension and capillary, and the movement of the polymer
258
chains were confined. When PEG content is low, PEG in composites is limited by
259
excessive ZSM-5. On the contrary, when the PEG content exceeds the ZSM-5 pore
260
structure accommodation, a part of PEG was adsorbed on the surface ZSM-5 and could
261
crystallize freely. The more content of PEG, the more materials can be crystallized with
262
larger thermal enthalpy. Although the movement of PEG was restricted by the pores of
263
ZSM-5, the stabilization of PEG/ZSM-5 PCMs also improve with the ZSM-5.
264
2.6 Thermal conductivity of composites PCMs
265
Fig. 10 shows the thermal conductivity of all the as-prepared samples. As shown in
266
Fig.10, the thermal conductivity of the pure PEG, and the composites PCMs were 0.22,
267
0.66, 0.61, 0.57 and 0.54 W/(m·K), respectively. Due to the ZSM-5 as supporting
268
materials, the thermal conductivities of all the samples were enhanced. In this work, the
269
improvement of thermal conductivity of composite PCMs is not ascribed to the
270
decrease of the interfacial thermal resistance and the increase of crystallinity of the PEG
271
matrix, because ZSM-5 was directly used without any surface treatment, and the fillers
272
had a little effect on the crystallization properties of the matrix discussed later in this
273
paper, respectively. Therefore, the formation of the effective thermally conductive
274
pathway composed of ZSM-5 is primary contribution to the enhancement of thermal
275
conductivity [45]. That indicates ZSM-5 serving as a supporting material greatly enhance 14
ACCEPTED MANUSCRIPT 276
the thermal conductivity of composites PCMs.
277
2.7 Melting and solidifying process of composites PCMs
278
The schematic of the experimental system which was employed for study the thermal
279
property of the as-prepared PCMs from the research [38]. The first step is putting 3-4 g
280
of pure PEG4000 and 3-4 g of FS-CPCMs-3 into two small glass tubes, respectively.
281
Then two T-type thermocouples were placed in the center of the two tubes, respectively.
282
Finally, two constant temperature water baths were used to supplied different
283
conditions to complete melting and solidification process. When doing the melting test,
284
the two testing tubes with thermocouples were placed into one water bath which
285
keeping the water at 80 ℃, and the results were record every 1 s until the temperature
286
rich the maximum value. After the melting process, the two testing tubes were quickly
287
placed into another water bath which keeping the water at 20 ℃. From the Fig. 11(a),
288
we can see the melting time will be 432 s for PEG and 275 s for FS-CPCMs-3 from 20
289
℃ to 75 ℃. The melting time was reduced by 57 %. Solidification time also shown in
290
Fig. 11(b), the solidification time will be 519 s for PEG and 247 s for FS-CPCMs-3
291
from 75 ℃ to 25 ℃. The solidification time was reduced by 110 %. These results
292
indicate that the thermal response ratio of composite PCM obviously increased for
293
practical application.
294
3. Conclusions
295
1） PEG/ZSM-5 composite PCMs as form-stable PCMs were prepared in a simple way
296
by incorporating PEG in the mesoporous pores of ZSM-5 assist with vacuum 15
ACCEPTED MANUSCRIPT 297
impregnation system.
298
2） The maximum content of PEG can reach up to 50 % in the composite without
299
leakage. XRD and FTIR analysis results showed that there was not any chemical
300
reaction happened between PEG4000 and ZSM-5. The latent heat of the as-prepared
301
PCMs achieves to 76.37 J/g.
302
3） Compare with pure PEG, the thermal conductivity of as-prepared PCMs was
303
improved 200 %, which means selecting ZSM-5 serve as the supporting material
304
was an effective way to enhance the thermal conductivity. Exudation stability tests
305
results showed that the doping of PEG can reach up 50 wt% without any leakage
306
when the composite undergoing the melting process.
307 308 309
Acknowledgements
310
The study has been supported by the China National Key R&D Program "Solutions
311
to heating and cooling of building in the Yangtze river region" (Grant
312
No.2016YFC0700305-02)
313
16
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References
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Figure：
433
Fig. 1. N2 adsorption/desorption isotherms of (a) ZSM-5; (b) FS-CPCMs-3
434
Fig. 2. XRD spectra of ZSM-5, PEG-4000, FS-CPCM-3
435
Fig. 3. FT-IR spectra of ZSM-5, PEG-4000, FS-CPCM-3
436
Fig. 4. SEM images of (a) ZSM-5, (b) FS-CPCM-1, (c) FS-CPCM-2, (d) FS-CPCM-3,
437
(e) FS-CPCM-4
438
Fig. 6. PEG and FS-CPCMs: (a) DSC curves of pure PEG and FS-CPCMs-3 during
439
melting and freezing phases (b) DSC curves; (c) Phase change temperatures; (d)
440
Enthalpies of melting and solidifying
441
Fig. 7. TGA curves of pristine PEG and the FS-PCMs
442
Fig. 8. Macroscopic images (a) wafers of PCMs and pure PEG at room temperature;
443
(b) exudation stability of PCMs and pure PEG at 80 ℃ after 10 min (c) exudation
444
stability of PCMs and pure PEG at 80 ℃ after 30 min
445
Fig. 9. Schematic illustration showing the interaction of PEG with ZSM-5 in the
446
PEG/ZSM-5 PCMs with various weight percentages.
447
Fig. 10. thermal conductivities of the prepared PEG, and FS-CPCMs.
448
Fig. 11. (a) Melting process curves of pristine PEG and FS-CPCMs-3, and (b)
449
Solidifying process curves of pristine PEG and FS-CPCMs-3.
450
20
ACCEPTED MANUSCRIPT 451
Table ：
452
Table 1 The compositions of the FS-CPCMs
453
Table 2 Textural properties of ZSM-5 and FS-CPCMs.
454
Table 3 FTIR absorption features and their assignment for ZSM-5 and PEG.
455
Table 4 DSC results of pure PEG and PEG/ZSM-5 composite PCMs
456
21
ACCEPTED MANUSCRIPT Figure：
Fig. 1. Schematic of the preparation of FS-PCMs.
Fig. 2 N2 adsorption/desorption isotherms of (a) ZSM-5; (b) FS-CPCMs-3
Fig. 3 XRD spectra of ZSM-5, PEG-4000, FS-CPCM-3
ACCEPTED MANUSCRIPT
Fig. 4 FT-IR spectra of ZSM-5, PEG-4000, FS-CPCM-3
Fig. 5 SEM images of (a) ZSM-5, (b) FS-CPCM-1, (c) FS-CPCM-2, (d) FS-CPCM-3, (e) FS-CPCM-4
ACCEPTED MANUSCRIPT
Fig. 6 PEG and FS-CPCMs: (a) DSC curves of pure PEG and FS-CPCMs-3 during melting and freezing phases (b) DSC curves; (c) Phase change temperatures; (d) Enthalpies of melting and solidifying
Fig. 7. TGA curves of pristine PEG and the FS-PCMs
ACCEPTED MANUSCRIPT
Fig. 8. Macroscopic images (a) wafers of PCMs and pure PEG at room temperature; (b) exudation stability of PCMs and pure PEG at 80 ℃ after 10 min (c) exudation stability of PCMs and pure PEG at 80 ℃ after 30 min
Fig. 9. Schematic illustration showing the interaction of PEG with ZSM-5 in the PEG/ZSM-5 PCMs with various weight percentages.
Fig. 10. thermal conductivities of the prepared PEG, and FS-CPCMs.
ACCEPTED MANUSCRIPT
Fig. 11. (a) Melting process curves of pristine PEG and FS-CPCMs-3, and (b) Solidifying process curves of pristine PEG and FS-CPCMs-3.
ACCEPTED MANUSCRIPT Highlights: 1）Mesoporous ZSM-5 supported PEG phase change materials was compound by a simple way and the synthesis mechanism was proposed. 2）PEG/ZSM-5 has good thermal stability, high latent heat and excellent thermal conductivity.
ACCEPTED MANUSCRIPT Table： Table 1 Textural properties of ZSM-5 and FS-CPCMs.
SBETa (m2/g) 238.07 3.54 1.75 0.33 -
Samples ZSM-5 FS-CPCMs-1 FS-CPCMs-2 FS-CPCMs-3 FS-CPCMs-4
Vtb (cm3/g) 0.03 0.01 0.22 0.21 -
DBJHc (nm) 3.43 2.46 1.80 49.03 -
a SBET, BET surface area calculated by the adsorption branch of N2 isotherm. b Vt, pore volume. c DBJH, pore diameter determined by BJH model. Table 2 FTIR absorption features and their assignment for ZSM-5 and PEG.
Wavenumber(cm-1) 2888 1105,1060,1145 1467, 963, 842 795 3200-3600 545 ,450
Assignment C–H stretching vibration of CH2 in PEG C–O stretching vibration in PEG C–H bending vibration of CH2 in PEG O-H vibration of SiO–H (ZSM-5) O-H stretching vibration in PEG characteristics of the ZSM-5 crystalline structure
Table 3 DSC results of pure PEG and PEG/ZSM-5 composite PCMs
Samples
Tm (℃)
PEG4000 FS-CPCMs-1 FS-CPCMs-2 FS-CPCMs-3 FS-CPCMs-4
Melting process Tpeak-m (℃) ΔHm (J/g)
Solidification process T s (℃ ) Tpeak-s (℃) ΔHs (J/g)
59.5
61.6
192.6
37.4
35.2
163.2
53.1
58.2
41.5
45.7
43.9
33.9
56.4
59.9
56.4
46.9
45.4
47.1
56.3
60.5
76.4
46.7
44.9
64.3
59.9
61.7
115.6
46.2
44.9
96.8
Table 4 Comparison of thermal properties of the form-stable SA/ZSM-5 composite PCM with that of different composite PCMs in literature. PCM
Paraffin/diatomite Capric-palmitic acid/attapulgite Paraffin/expanded perlite Stearic acid/silica fume paraffin/expanded vermiculite PEG/ZSM-5
Phase change temperature（℃）
Latent heat（J/g）
References
36.5
53.1
[39]
21.7
48.2
[40]
42.3
87.4
[41]
59.9
84.5
[42]
48.8
103
[43]
56.3
76.4
This study