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Enhanced mechanical and thermal properties of hollow wood composites filled with phase-change material
Journal Pre-proof Enhanced mechanical and thermal properties of hollow wood composites filled with phase-change material Chusheng Qi, Feng Zhang, Jun Mu, Yang Zhang, Zhiming Yu PII:
S0959-6526(20)30420-0
DOI:
https://doi.org/10.1016/j.jclepro.2020.120373
Reference:
JCLP 120373
To appear in:
Journal of Cleaner Production
Received Date: 18 November 2019 Revised Date:
20 January 2020
Accepted Date: 31 January 2020
Please cite this article as: Qi C, Zhang F, Mu J, Zhang Y, Yu Z, Enhanced mechanical and thermal properties of hollow wood composites filled with phase-change material, Journal of Cleaner Production (2020), doi: https://doi.org/10.1016/j.jclepro.2020.120373. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier Ltd.
Credit Author Statement Chusheng Qi: Conceptualization, methodology, investigation, writing-original draft, writingreview & Editing Feng Zhang: Formal analysis, investigation, writing-original draft Jun Mu: Funding acquisition, methodology, supervision Yang Zhang: Resources, validation Zhiming Yu: Funding acquisition, resources
Enhanced Mechanical and Thermal Properties of Hollow Wood Composites Filled with Phase-Change Material
Chusheng Qi 1,2, Feng Zhang 1,2, Jun Mu* 1,2, Yang Zhang 1,2, Zhiming Yu 1,2
1. Key Laboratory of Wood Material Science and Utilization of Ministry of Education, Beijing Forestry University, Beijing, 100083, P. R. China. 2. Beijing City Key Laboratory of Wood Science and Engineering, Beijing Forestry University, Beijing, 100083, P. R. China.
Corresponding author: Dr. Jun Mu Professor Key Laboratory of Wood Material Science and Utilization of Ministry of Education, Beijing Forestry University, Beijing, 100083, P. R. China. Email: [email protected]
1
Graphica abstract
1
Enhanced Mechanical and Thermal Properties of Hollow Wood Composites Filled
2
with Phase-Change Material
3 4
Wordcount: 5153 words with 28 pages
5
Abstract
6
To obtain lightweight wood building materials with good thermal insulation,
7
energy-saving properties, and satisfying mechanical properties, low-density fiberboard
8
and hollow wood composites (HWC) embedded polyvinyl chloride tubes were
9
fabricated by hot-pressing. Polyethylene glycol was used as the phase-change material
10
to fill polyvinyl chloride tubes and obtain phase-change hollow wood composite
11
(PHWC). The physical and mechanical properties of HWC and PHWC were tested, and
12
their thermal properties were analyzed and simulated. The results showed that the
13
thermal conductivities of low-density fiberboard, HWC, and PHWC ranged from
14
0.06-0.07 W/(m·K), indicating they had sufficient physical and mechanical properties to
15
be used as thermal insulation building materials. The combination of series and parallel
16
models accurately predicted the thermal conductivity of HWC and PHWC, whose
17
structures were similar to a series structure. The addition of polyethylene glycol into
18
HWC allowed the PHWC to store latent heat and reduce indoor temperature fluctuations.
19
Heat transfer simulations showed that when used as a non-structural building wall
20
material, the PHWC wall had a better energy efficiency compared with a concrete wall.
21
Thus, PHWC has potential applications as thermal insulation and phase-change building
22
material.
23
Keywords: Hollow wood composites; Lightweight wood composites; Phase-change
24
material; Mechanical properties; Thermal conductivity 2
25 26
1. Introduction The energy consumption of buildings accounts for more than 30% of total global
27
energy usage (Berardi, 2017), and thermal insulation of building envelopes is one of the
28
main techniques to reduce energy consumed by air conditioning. Thermal insulation
29
products such as fiberglass, mineral wool, and polyurethane foams have low thermal
30
conductivities in the range 0.02-0.05 W/(m·K), but they pose environmental and health
31
hazards (Corporation, 2004). Additionally, polyurethane foams are highly flammable.
32
Building materials should meet relevant requirements for structural safety, quality of life,
33
energy efficiency, cost, fire resistance, and durability (Matalkah et al., 2017). Wood is a
34
natural and renewable material that has be used in many applications, such as bioenergy
35
(Mardoyan and Braun, 2015; Pradhan et al., 2018), biorefining (Akim, 2016; MarouÅ
36
and Žák, 2015) and biochar (Agegnehu et al., 2017; Maroušek et al., 2019).
37
Wood-based composites with simple processing technologies and low energy and
38
financial requirements are one of the most important uses of wood. Lightweight wood
39
composites (LWC) with thermal and sound insulation properties have great application
40
prospects for use in building envelopes. Xie et al. (2011) found that ultra-low density
41
fiberboard had a very low thermal conductivity and a high sound reduction coefficient.
42
Many other studies have shown that thermal conductivity decreases with the density of
43
natural fiber composites because gases have a lower thermal conductivity and far slower
44
heat transfer than solids (Binici et al., 2012). Additionally, the shortage of forestry
45
resources (Li et al., 2017), forest destruction due to wildfires and the prohibition of the
46
logging of natural forests in some countries have made it necessary to develop LWC
47
which consuming fewer wood resources.
48
Much research has been devoted to developing many types of LWC. For example, 3
49
Hussain et al. (2019) fabricated wood-based sandwich panels with wood-based cores
50
and face sheets composed of glass-fiber-reinforced polymer with a density of 0.21
51
g/cm3. Monteiro et al. (2019) produced particleboards with a density range of 0.32-0.54
52
g/cm3 using sour cassava starch as the adhesive and foam. The formation of hollow
53
wood composites can also reduce the density of a product while retaining its satisfying
54
performance. For example, Voth et al. (2015) fabricated wood-stand sandwich panels
55
with hollow-core interiors and a density of around 0.3 g/cm3. The interior cavity of
56
hollow wood composites can be used to house thermal and sound insulation materials,
57
but they must be further developed for filling with phase-change materials (PCM) due
58
to PCM leakage after melting.
59
Energy demand today is rapidly increasing around the world (Dag et al., 2019), and
60
global buildings were responsible for about 32% of energy consumption and 19% of
61
energy-related greenhouse gases emission in 2010 (Abanda and Byers, 2016). This has
62
caused the various governments to focus on the research and development of
63
energy-efficient buildings. Phase-change materials are used to reduce energy
64
consumption by decreasing indoor temperatures, decreasing indoor temperature
65
fluctuations, and shifting loads away from peak usage times. The incorporation of PCM
66
in building components may reduce indoor temperature fluctuations because PCM can
67
store large amounts of latent heat within a small temperature range associated with a
68
phase change. The application of PCM in buildings has been conducted to reduce
69
energy usage and increase energy efficiency (Ascione et al., 2014; Li et al., 2015;
70
Shafie-khah et al., 2016). The high economic value of PCM for reducing energy
71
consumption in a typical multistory office building was demonstrated by Mi et al.
72
(2016). Potential leakage is the primary problem with the use of solid-to-liquid PCM 4
73
because they exist as liquids above their melting temperature. An inexpensive and
74
effective solution to this problem is to contain PCM in hollow plastic tubes.
75
The objective of this research is to develop lightweight wood composites with
76
hollow thermoplastic tubes, which were then filled with PCM to obtain final composites
77
with thermal insulation and thermal absorption properties. The physical and mechanical
78
properties and the thermal performance of the obtained composites were analyzed and
79
simulated.
80
2 Materials and methods
81
2.1 Materials
82
Poplar wood fibers were obtained from a local market in China and then ground to
83
80 mesh with a moisture content of 6.0 %. Polyvinyl chloride (PVC) tubes with an outer
84
diameter of 7.0 mm, a wall thickness of 0.5 mm, and a solid density of 1.2 g/cm3 were
85
purchased from Dongguan Taolue Electronic Products Co., Ltd. Liquid isocyanate
86
adhesive (MDI, PM200) with an NCO content of 30.5 % and a viscosity of 250 MPa·s
87
(25oC) was obtained from Wanhua Chemicals. Acetone purchased from Beijing
88
Chemicals was used as a diluent for MDI. Polyethylene glycol (PEG) from Jiangsu
89
Haian Chemical Plant was used as the PCM. PEG-800 with a molecular weight of
90
760-840, a density of 1.27 g/cm3, and a viscosity of 2.3 MPa·s at 25 oC was used in this
91
study.
92
2.2 Fabrication of hollow wood composites
93
Hollow wood composites (HWC) with dimensions of 350 × 350 × 20 mm3 were
94
fabricated with a target density of 0.3 g/cm3. Four percent (based on the oven-dried
95
weight of wood fibers) of MDI was used as the adhesive, and it was diluted with 5
96
acetone in a ratio of 1:1 to reduce its viscosity before spraying onto wood fiber in a
97
roller. The wood fiber containing MDI was divided into three portions in a mass ratio of
98
3:4:3. The first 30% of the wood fiber was evenly placed on a caul in a frame with a
99
size of 350× 350 mm2. Parallel hollow PVC tubes settled by a self-made clamp (Fig.
100
1(a)) were placed on the first layer of wood fiber. Then, 40% of wood fiber was added,
101
and another layer of parallel hollow PVC tubes was placed on the second layer of the
102
wood fiber. Finally, the last 30% of the wood fiber was evenly placed on top. The
103
formed mat was placed between two aluminum cauls, and silicon paper was placed
104
between the caul and mat to prevent adhesion between the caul and the final HWC. The
105
mat was pressed for 8 min at 180 oC under a pressure of 1 MPa in a hot press machine,
106
and the thickness of final products was controlled by a thickness gauge. Figure 1(b)
107
shows the final HWC. The low-density fiberboard (LDF) without hollow PVC tubes
108
was also fabricated using the same hot press schedule as the reference material. All
109
samples were equilibrated at 25 oC and a humidity of 65% for at least one week before
110
performance testing, and six replicates of each test were performed.
111
The air volume fraction of HWC was adjusted by changing the number of PVC
112
tubes at a constant HWC bulk density of 0.3 g/cm3. The relationship between the
113
number of tubes and the volume ratios are listed in Eq. 1 - Eq. 4: λ +λ +λ +λ =1 λ = λ = λ =
V × 100% = V
V /
(1)
× 100%
(2)
× 100%
(3)
× 100%
(4)
114 115
where λ , λ , λ , and λ are the volume ratios of the PVC tube cavity, solid PVC, 6
116
wood fiber cell wall, and air inside the wood fiber, respectively; V,
117
the volumes (cm3) of HWC, the PVC tube cavity, solid PVC, and wood fiber cell wall,
118
respectively; n is the number of PVC tubes,
119
fiber (g); and
120
λ and λ is the air volume ratio of HWC (λ ). Different λ and λ values were
121
obtained by changing the number of PVC tubes, and Table 1 shows the experimental
122
design and corresponding parameters.
123
Table 1 Experimental design and parameters of hollow wood composites
,
, and V
are
represents the dry weight of wood
is the density of the wood fiber cell wall (1.5 g/cm3). The sum of
Sample
Number of PVC tubes
λ
λ
λ
(%)
(%)
HWC-8
19
7.67
HWC-9
22
HWC-10
25
Design density (g/cm3)
Average HWC density (g/cm3)
λ
λ
(%)
(%)
(%)
2.77
17.78
71.78
79.45
0.289
8.88
3.21
17.43
70.48
79.36
0.274
10.09
3.65
17.08
69.18
79.27
0.308 0.3
HWC-11
28
11.30
4.08
16.73
67.89
79.19
0.292
HWC-12
31
12.52
4.52
16.38
66.58
79.10
0.289
HWC-13
34
13.73
4.96
16.03
65.28
79.01
0.280
124 125 126
2.3 Preparation of phase-change hollow wood composites Polyethylene glycol was first melted at 50oC in a water bath, and then 7.7 g of PEG
127
was injected via syringe into each hollow PVC tube inside the HWC. Then, the two
128
sides of the hollow PVC tube were sealed using a thermosetting resin to obtain
129
phase-change hollow wood composite (PHWC) (Fig. 1(c)). The melt PEG was cooled to
130
20oC before testing, and six replicates were performed for each test.
131
2.4 Physical and mechanical property evaluation 7
132
The modulus of rupture (MOR), modulus of elasticity (MOE), internal bond
133
strength (IB), thickness swelling (TS), and water absorption (WA) of the samples were
134
tested based on the Chinese Standard GB/T17657-2013. Nine specimens were tested for
135
each measurement.
136
The thermal conductivity and thermal resistance of HWC and PHWC were
137
measured using a thermal conductivity analyzer (DRH-300) according to standard GB/T
138
10294-2008. The cold plate and hot plate temperatures were set to 23oC and 43oC,
139
respectively, with an ambient temperature of 21oC.
140
A self-made temperature detector (Fig. 2) was developed and used to test the heat
141
transfer characteristics of LDF, HWC, and PHWC. Samples were placed on a hot plate
142
set to 50oC and surrounded and covered by a 5 mm thick polystyrene foam board to
143
prevent heat exchange with the environment. Nine thermocouples were placed on the
144
top surface to monitor the temperature changes over time. The samples were first heated
145
at 50oC for 1 h and then cooled naturally for another 1 h.
146
2.5 Vertical density profile analysis
147
The vertical density profile (VDP) of lightweight wood composites, HWC, and
148
PHWC was measured by an X-ray profile density analyzer (GreCon DAX 6000) with a
149
scan speed of 0.5 mm/s and a specimen size of 50 × 50 × 20 mm3.
150
2.6 DSC analysis
151
Differential scanning calorimetry (DSC) was used to evaluate the thermal
152
properties and energy storage of PEG from -20 oC - 100 oC at a heating rate of 5 oC /min
153
under a nitrogen (N2) atmosphere. The temperature and enthalpy accuracies of the DSC
154
were 0.1oC and 1%, respectively. The measured heat enthalpy and specific heat of PEG 8
155
were used as the material parameters for heat transfer simulations.
156
2.7 Heat transfer simulations
157
The heat transfer of HWC and PHWC was simulated using ANSYS Workbench
158
software with a simplified model size of 50 × 50 × 20 mm3. The heat transfer of a
159
building wall made of PHWC with a thickness of 240 mm was also simulated and
160
compared with a building wall made of concrete. An outdoor temperature of 40 oC, a
161
convective heat transfer coefficient of 20 W/(m2·K) for the inner surface, and an initial
162
environmental temperature of 20 oC were used to simulate a typical summer
163
environment in southern China. The material properties used in the simulation were
164
experimentally determined and are listed in Table 2.
165
Table 2 Material properties for heat transfer simulation Properties Density (g/cm3) Specific heat (J/(kg·K)) Thermal conductivity（W/ （m·K））
Wood fiber
PVC plastic
PEG
Concrete (Zhao et al., 2011)
0.3
1.2
1.27
2.4
1173 (Guo et al., 2013)
895.6
3393
960
0.061
0.14 (Zhang et al., 2015)
0.24 (Wu et al., 2019)
1.28
166 167
3. Results and discussion
168
3.1 Physical and mechanical properties of HWC
169 170
Figure 3 (a) shows the MOR and MOE values of LDF and HWC. The low-density fiberboard without added PVC hollow tubes had a MOR of 1.8 MPa and a MOE of 9
171
196.1 MPa. These values were much lower than medium-density fiberboard (MDF),
172
which has a MOR of around 28.0 MPa and MOE of 1.4 GPa because they were much
173
less dense than MDF (around 0.65 g/cm3) (Hussain et al., 2019), but these mechanical
174
properties can support the transportation, hoisting, and installation of LDF and HWC.
175
Panyakaew and Fotios (2011) reported that the MOR and MOE of binderless bagasse
176
boards with a density of 0.25 g/cm3 were 0.43 MPa and 102 MPa, respectively, and
177
these values increased with the density. The MOR and MOE values obtained in this
178
study were higher than those previously reported, which was attributed to the addition
179
of the MDI resin. The isocyanate group of MDI reacted with the hydroxyl group of the
180
wood fiber to form a strong covalent bond that contributed to the good mechanical
181
properties of LDF and HWC. There was an obvious increase in the MOR and MOE
182
when hollow PVC tubes were added into LDF (Fig. 3 (a)), which was attributed to the
183
toughness and stiffness of PVC tubes. As λ increased, the MOR and MOE of HWC
184
slightly fluctuated. A higher λ indicates hollower PVC tubes and less wood fiber in
185
HWC. The PVC tubes had positive effects on MOR and MOE, but decreasing the
186
wood fiber content had an inverse effect.
187
The internal bond strengths of LDF and HWC are given in Fig. 3 (b). LDF had an
188
IB of 0.06 MPa, which is within the range of conventional thermal insulation materials,
189
which typically have IB values of 0.03 - 0.08 MPa (Pfundstein et al., 2012). Kawasaki
190
et al. (1998) reported that low-density fiberboard with a density of 0.3 g/cm3 had an IB
191
of around 0.1 MPa. These results demonstrate that low-density fiberboard has a low IB
192
because the fibers are not close enough to be bonded. However, the IB of LDF was
193
much higher than that of binderless bagasse board (0.01 MPa) with a density of 0.35
194
g/cm3 (Panyakaew and Fotios, 2011), showing that the MDI resin improves the 10
195
mechanical properties. The addition of hollow PVC tubes did not improve the IB,
196
possibly because no interfacial bond was formed between the hydrophobic PVC and
197
hydrophilic wood fiber. This should be improved in future work.
198
The thickness swelling and water absorption of LDF and HWC are shown in Fig. 3
199
(c) and Fig. 3 (d). The thickness swelling of LDF after 2 h and 24 h were 4.4% and
200
9.3%, respectively. For comparison, a 24 h thickness swelling of 13.2% was observed in
201
particleboard with a density of 0.4 g/cm3 (Monteiro et al., 2019), and LDF in this study
202
had better water resistance when considering its low density. Thickness swelling slightly
203
increased when hollow PVC tubes were added due to increased water passage and
204
contact surface since the water can fill the gaps between the hollow PVC tubes and
205
fibers. The water absorption rates of LDF after 2 h and 24 h were 39.2% and 104.5%,
206
respectively, and the 24 h water absorption of HWC had a trend of decrease as λ
207
increased. The water penetrated the voids between the wood fibers and lumen, and the
208
chemical components of the wood cell walls absorbed water and swelled. Therefore,
209
when more voids and lumen are present inside the HWC, and the thickness swelling and
210
water absorption rate are both higher. λ , which indicates the void fraction of lumen
211
inside HWC, decreased as λ increased (Table 1), causing the 24 h thickness swelling
212
and water absorption to decrease with λ .
213
3.2 Vertical density profile analysis
214
Figure 4 (a) presents the typical vertical density profiles of LDF, HWC, and
215
PHWC in the thickness direction, and the results show that the VDP for LWC was
216
nearly constant with an average of 0.3 g/cm3. However, there were large fluctuations for
217
HWC when hollow PVC tubes were present, and a U-type shape was observed. This
218
mainly occurred because the hollow PVC tube had a U-type shape density profile (Fig. 11
219
4 (b)). The density of PVC is 1.2 g/cm3, which is much higher than LWC, and the
220
vertical density of hollow PVC tube ranged from 0.2 - 1.2 g/cm3 (Fig. 4 (b)), resulting
221
in a lower density (around 0.2 g/cm3) of wood fiber in HWC when the overall density
222
was the same. This low density resulted in a low internal bond strength, as shown in Fig.
223
3 (b). When the hollow PVC tubes inside the HWC were filled with PEG (ρ = 1.27
224
g/cm3), the corresponding VDP of PVC tubes increased, as shown in Fig. 4 (a).
225
3.3 Thermal conductivity and resistance
226
The thermal conductivity (k) is inversely proportional to the thermal resistance (R),
227
and their values of HWC and PHWC are shown in Fig. 5(a). The k and R of LDF were
228
0.06 W/(m·K) and 0.33 m2·K/W at 33 oC, respectively. The thermal conductivity of
229
HWC and PHWC ranged from 0.06-0.07 W/(m·K), indicating that the lightweight wood
230
composites with a density of 0.3 g/cm3 could be used as a thermally insulating building
231
material since its thermal conductivity was below 0.12 W /(m·K). The thermal
232
conductivities of binderless coconut husk and bagasse insulation boards with densities
233
of 250-350 kg/m3 have been reported to rang from 0.046 to 0.068 W/(m·K) (Panyakaew
234
and Fotios, 2011), which are consistent with the results reported here. The addition of
235
hollow PVC tubes into the LWC did not significantly influence their thermal
236
conductivity or resistance since the thermal conductivity of solid PVC (
237
W/(m·K) (Zhang et al., 2015) and higher than that of LWC. This was mainly because
238
both λ
239
had the lowest thermal conductivity of 0.026 W/(m·K) at 33 oC. PHWC had a slightly
240
higher thermal conductivity than HWC since the PEG-800 had a thermal conductivity
241
of 0.24 W/(m·K) (Wu et al., 2019), showing that the addition of PEG-800 increased the
242
thermal conductivity of PHWC. High thermal conductivity helped improve the energy
and λ
) is 0.14
decreased with an increase in λ (Table 1), and air inside the HWC
12
243
change efficiency of phase-change materials.
244
Series model, parallel model, as well as a combined series and parallel model for
245
three phases were employed to predict the thermal conductivity of HWC and PHWC.
246
The HWC was assumed to be composed of three phases: wood fiber (including wood
247
fiber lumen and voids between wood fibers), solid PVC, and cavity of the hollow PVC
248
tubes. For PHWC, the PEG phase was instead of the PVC tube cavity. The series model
249
(Eq. 5), parallel model (Eq. 6), and their combined model (Eq. 7) can be described as
250
follows: 1
= ( = ε
= ( 251
where
+
+ (1 − ε +
+ +
+
+
(5) ）
+
(6)
+
(7)
）
is the thermal conductivity of the final HWC or PHWC;
is the
252
thermal conductivity of the wood fiber phase, and its experimentally determined value
253
was 0.163 W/(m·K);
254
hollow PVC tube for HWC and PEG (0.24 W/(m·K)) in PHWC;
255
and λ in Table 1, and the sum of
256
factor of the series model to the parallel model and is decided by the material structure
257
and can be calculated by linear fitting.
is the thermal conductivity of air (0.026 W/(m·K)) inside the
,
, and
is the sum of λ
is one, and ε signifies the ratio
258
Figure 5 (b) and (c) compare the experimental data and predicted results of HWC
259
and PHWC. The parallel model predicted the maximum thermal conductivity when the
260
series model predicted the minimum level, and the experimental data fall between these
261
two extremes. ε was found to be 0.79 for HWC and 0.94 for PHWC with an R2 of 0.99, 13
262
indicating that the structures of HWC and PHWC were very similar to a serial structure,
263
especially for PHWC with PEG-filled PVC tubes, which can be further shown in Fig. 1
264
(b) and Fig. 1 (c). The combined series and parallel model accurately predicted the
265
thermal conductivity of HWC and PHWC.
266
3.4 The thermal properties of polyethylene glycol
267
The heat flow of PEG during heating and cooling is shown in Fig. 6 (a). The results
268
indicated that the onset melting temperature was 23.4oC, with a melting peak at 29.6oC,
269
which was consistent with previous research (Sánchez et al., 2007) and was in an ideal
270
temperature range for a phase change material for building insulation. The PEG began
271
to release heat when the temperature decreased to 22.0oC with a heat release peak at
272
17.3oC, which will help keep a building warm. The latent heat of PEG was calculated as
273
125.7 J/g, and its heat enthalpy at 0oC, 20oC, 28oC, and 35oC were 3.18× 10! ,
274
1.60× 10" , 2.49× 10" , and 2.76× 10" J/m3, respectively. These values were used for
275
later heat transfer simulations.
276
3.5 Heat transfer characteristics
277
Figure 6 (b) and Fig. 6 (c) present the top surface temperatures of HWC and
278
PHWC when their bottom temperatures were remained at 50oC for 1 hour, followed by
279
natural cooling for another 1 h. The top surface temperatures of HWC with different λ
280
were similar (Fig. 6 (b)), mainly because the HWC density was the same (0.3 g/cm3) as
281
λ and λ changed, and λ
282
the addition of hollow PVC tubes had almost no effect on heat transfer, and their
283
thermal conductivities showed the same trend (Fig. 5(a)). The top surface temperatures
284
of PHWC at different λ were notably different, and a lower top surface temperature
and λ decreased when λ and λ increased. Therefore,
14
285 286
was observed at higher λ since more heat was absorbed by the PEG during melting. Figure 6 (c) compares the top surface temperature of LDF, HWC, and PHWC, and
287
the results show that LDF and HWC had similar temperatures, but PHWC had a much
288
lower temperature than LDF and HWC during the heating stage and a higher
289
temperature during the cooling stage. This indicates that the addition of PEG into the
290
hollow PVC tubes improved the energy efficiency of HWC, which has also been
291
reported by other researchers (Li et al., 2015).
292
Figure 7 (a) compares the simulated temperature distributions at the same heat
293
transfer experimental testing conditions. The simulation results show that PHWC had a
294
notably lower temperature than HWC, and PEG delayed temperature increases, which
295
was consistent with the experimental data.
296
3.6 Thermal transition simulation of a building wall
297
Figure 7(b) and (c) compare the indoor temperatures of a building wall with a
298
thickness of 240 mm made of PHWC and concrete. The results show that the indoor
299
temperature of the room with a PHWC wall was 23.0 oC, which was 3.5 oC lower than
300
the concrete wall after 5 h insulation, indicating that lightweight wood building
301
materials with good thermal insulation, energy-saving properties, and satisfying
302
mechanical properties were successfully developed.
303
The research performed by Yun et al. (2020) showed that the addition of 0.1%-0.9%
304
building volume fraction of PCM with a phase change temperature of 20-28 oC could
305
save up to 3.19 kWh/m2 energy per year. Sharma and Rai (2020) concluded that
306
PCM-enhanced walls could reduce heat gain by 10.4%-26.6%, while also reducing
307
annual electricity consumption and greenhouse gas emissions. The phase-change hollow 15
308
wood composites with a PCM volume fraction of up to 13.7% in this study can
309
potentially be applied as non-structural thermal insulation and phase-change wall
310
materials. Future efforts will attempt to further reduce the density and increase the PCM
311
volume fraction and mechanical properties of PHWC.
312
4 Conclusions
313
Lightweight wood building composites filled with phase-change material were
314
successfully developed and
315
temperature fluctuations. The addition of hollow PVC tubes into LDF increased its
316
MOR and MOE, and the internal bond strength, 24 h thickness swelling, and 24 h water
317
absorption of HWC decreased with an increasing number of hollow PVC tubes. A
318
U-shaped vertical profile in the HWC was observed where the hollow PVC tubes were
319
present, and it fluctuated less in the PEG-filled tube. The thermal conductivities of LDF,
320
HWC, and PHWC ranged from 0.06-0.07 W/(m·K) at 33 oC, and the addition of hollow
321
PVC tubes and PEG filling only slightly affected the thermal conductivity. Combined
322
series and parallel model predicted the thermal conductivity of HWC and PHWC, and
323
their structures were very similar to the series structure. Both experimental data and
324
ANSYS simulation results showed that the addition of PEG decreased the top surface
325
temperature of PHWC when the bottom surface was continuously heated. Compared
326
with concrete, the use of PHWC as a non-structural wall material resulted in a much
327
lower indoor temperature, demonstrating its promising potential for use as a thermal
328
insulation and phase-change building material.
329
Acknowledgment
330
were able to store latent heat and reduce indoor
This work was supported by the Fundamental Research Funds for the Central
16
331
Universities (NO. 2016ZCQ01).
332
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412
Figure Captions
413 414
Figure 1 (a) Hollow PVC tubes orientation clamp, (b) hollow wood composites, and (c)
415
hollow wood composites filled with polyethylene glycol
416
Figure 2 Schematic diagram of heat transfer testing
417
Figure 3 Physical and mechanical properties of HWC, (a) MOE, and MOR, (b) internal
418
bond strength, (c) thickness swelling, and (d) water absorption
419
Figure 4 (a) Typical vertical density profiles of LDF, HWC, and PHWC, and (b) density
420
distribution of a hollow PVC tube
421
Figure 5 (a) Thermal conductivity and resistance of LDF, HWC, and PHWC, and
422
thermal conductivity comparison of predicted values with experimental data of (b)
423
HWC and (c) PHWC
424
Figure 6 (a) Heat flow of PEG, top surface temperature of (b) HWC, (c) PHWC, and (d)
425
comparison of LDF, HWC, and PHWC with a bottom hot plate temperature of 50 oC.
426
Figure 7 (a) Heat flux and temperature cloud field of HWC and PHWC, (b) indoor
427
surface temperature simulation results, and (c) temperature distribution simulation
428
results of a 240 mm wall made of PHWC and concrete
21
Figure 1 a
c
b
22
Figure 2
23
Figure 3 b
a
d
c
24
Figure 4 a
b
25
Figure 5 a
b
c
26
Figure 6 a
b
c
d
27
Figure 7 b
a
c
28
1
Highlights:
2
Lightweight hollow wood composites with a density of 0.3 g/cm3 were fabricated
3
The thermal conductivity of lightweight hollow wood composites is 0.06~0.07 W/(m·K)
4
Polyethylene glycol as a phase change material was added into hollow wood composites
5
Phase-change lightweight wood composites were better insulators than concrete
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0959-6526(20)30420-0
DOI:
https://doi.org/10.1016/j.jclepro.2020.120373
Reference:
JCLP 120373
To appear in:
Journal of Cleaner Production
Received Date: 18 November 2019 Revised Date:
20 January 2020
Accepted Date: 31 January 2020
Please cite this article as: Qi C, Zhang F, Mu J, Zhang Y, Yu Z, Enhanced mechanical and thermal properties of hollow wood composites filled with phase-change material, Journal of Cleaner Production (2020), doi: https://doi.org/10.1016/j.jclepro.2020.120373. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier Ltd.
Credit Author Statement Chusheng Qi: Conceptualization, methodology, investigation, writing-original draft, writingreview & Editing Feng Zhang: Formal analysis, investigation, writing-original draft Jun Mu: Funding acquisition, methodology, supervision Yang Zhang: Resources, validation Zhiming Yu: Funding acquisition, resources
Enhanced Mechanical and Thermal Properties of Hollow Wood Composites Filled with Phase-Change Material
Chusheng Qi 1,2, Feng Zhang 1,2, Jun Mu* 1,2, Yang Zhang 1,2, Zhiming Yu 1,2
1. Key Laboratory of Wood Material Science and Utilization of Ministry of Education, Beijing Forestry University, Beijing, 100083, P. R. China. 2. Beijing City Key Laboratory of Wood Science and Engineering, Beijing Forestry University, Beijing, 100083, P. R. China.
Corresponding author: Dr. Jun Mu Professor Key Laboratory of Wood Material Science and Utilization of Ministry of Education, Beijing Forestry University, Beijing, 100083, P. R. China. Email: [email protected]
1
Graphica abstract
1
Enhanced Mechanical and Thermal Properties of Hollow Wood Composites Filled
2
with Phase-Change Material
3 4
Wordcount: 5153 words with 28 pages
5
Abstract
6
To obtain lightweight wood building materials with good thermal insulation,
7
energy-saving properties, and satisfying mechanical properties, low-density fiberboard
8
and hollow wood composites (HWC) embedded polyvinyl chloride tubes were
9
fabricated by hot-pressing. Polyethylene glycol was used as the phase-change material
10
to fill polyvinyl chloride tubes and obtain phase-change hollow wood composite
11
(PHWC). The physical and mechanical properties of HWC and PHWC were tested, and
12
their thermal properties were analyzed and simulated. The results showed that the
13
thermal conductivities of low-density fiberboard, HWC, and PHWC ranged from
14
0.06-0.07 W/(m·K), indicating they had sufficient physical and mechanical properties to
15
be used as thermal insulation building materials. The combination of series and parallel
16
models accurately predicted the thermal conductivity of HWC and PHWC, whose
17
structures were similar to a series structure. The addition of polyethylene glycol into
18
HWC allowed the PHWC to store latent heat and reduce indoor temperature fluctuations.
19
Heat transfer simulations showed that when used as a non-structural building wall
20
material, the PHWC wall had a better energy efficiency compared with a concrete wall.
21
Thus, PHWC has potential applications as thermal insulation and phase-change building
22
material.
23
Keywords: Hollow wood composites; Lightweight wood composites; Phase-change
24
material; Mechanical properties; Thermal conductivity 2
25 26
1. Introduction The energy consumption of buildings accounts for more than 30% of total global
27
energy usage (Berardi, 2017), and thermal insulation of building envelopes is one of the
28
main techniques to reduce energy consumed by air conditioning. Thermal insulation
29
products such as fiberglass, mineral wool, and polyurethane foams have low thermal
30
conductivities in the range 0.02-0.05 W/(m·K), but they pose environmental and health
31
hazards (Corporation, 2004). Additionally, polyurethane foams are highly flammable.
32
Building materials should meet relevant requirements for structural safety, quality of life,
33
energy efficiency, cost, fire resistance, and durability (Matalkah et al., 2017). Wood is a
34
natural and renewable material that has be used in many applications, such as bioenergy
35
(Mardoyan and Braun, 2015; Pradhan et al., 2018), biorefining (Akim, 2016; MarouÅ
36
and Žák, 2015) and biochar (Agegnehu et al., 2017; Maroušek et al., 2019).
37
Wood-based composites with simple processing technologies and low energy and
38
financial requirements are one of the most important uses of wood. Lightweight wood
39
composites (LWC) with thermal and sound insulation properties have great application
40
prospects for use in building envelopes. Xie et al. (2011) found that ultra-low density
41
fiberboard had a very low thermal conductivity and a high sound reduction coefficient.
42
Many other studies have shown that thermal conductivity decreases with the density of
43
natural fiber composites because gases have a lower thermal conductivity and far slower
44
heat transfer than solids (Binici et al., 2012). Additionally, the shortage of forestry
45
resources (Li et al., 2017), forest destruction due to wildfires and the prohibition of the
46
logging of natural forests in some countries have made it necessary to develop LWC
47
which consuming fewer wood resources.
48
Much research has been devoted to developing many types of LWC. For example, 3
49
Hussain et al. (2019) fabricated wood-based sandwich panels with wood-based cores
50
and face sheets composed of glass-fiber-reinforced polymer with a density of 0.21
51
g/cm3. Monteiro et al. (2019) produced particleboards with a density range of 0.32-0.54
52
g/cm3 using sour cassava starch as the adhesive and foam. The formation of hollow
53
wood composites can also reduce the density of a product while retaining its satisfying
54
performance. For example, Voth et al. (2015) fabricated wood-stand sandwich panels
55
with hollow-core interiors and a density of around 0.3 g/cm3. The interior cavity of
56
hollow wood composites can be used to house thermal and sound insulation materials,
57
but they must be further developed for filling with phase-change materials (PCM) due
58
to PCM leakage after melting.
59
Energy demand today is rapidly increasing around the world (Dag et al., 2019), and
60
global buildings were responsible for about 32% of energy consumption and 19% of
61
energy-related greenhouse gases emission in 2010 (Abanda and Byers, 2016). This has
62
caused the various governments to focus on the research and development of
63
energy-efficient buildings. Phase-change materials are used to reduce energy
64
consumption by decreasing indoor temperatures, decreasing indoor temperature
65
fluctuations, and shifting loads away from peak usage times. The incorporation of PCM
66
in building components may reduce indoor temperature fluctuations because PCM can
67
store large amounts of latent heat within a small temperature range associated with a
68
phase change. The application of PCM in buildings has been conducted to reduce
69
energy usage and increase energy efficiency (Ascione et al., 2014; Li et al., 2015;
70
Shafie-khah et al., 2016). The high economic value of PCM for reducing energy
71
consumption in a typical multistory office building was demonstrated by Mi et al.
72
(2016). Potential leakage is the primary problem with the use of solid-to-liquid PCM 4
73
because they exist as liquids above their melting temperature. An inexpensive and
74
effective solution to this problem is to contain PCM in hollow plastic tubes.
75
The objective of this research is to develop lightweight wood composites with
76
hollow thermoplastic tubes, which were then filled with PCM to obtain final composites
77
with thermal insulation and thermal absorption properties. The physical and mechanical
78
properties and the thermal performance of the obtained composites were analyzed and
79
simulated.
80
2 Materials and methods
81
2.1 Materials
82
Poplar wood fibers were obtained from a local market in China and then ground to
83
80 mesh with a moisture content of 6.0 %. Polyvinyl chloride (PVC) tubes with an outer
84
diameter of 7.0 mm, a wall thickness of 0.5 mm, and a solid density of 1.2 g/cm3 were
85
purchased from Dongguan Taolue Electronic Products Co., Ltd. Liquid isocyanate
86
adhesive (MDI, PM200) with an NCO content of 30.5 % and a viscosity of 250 MPa·s
87
(25oC) was obtained from Wanhua Chemicals. Acetone purchased from Beijing
88
Chemicals was used as a diluent for MDI. Polyethylene glycol (PEG) from Jiangsu
89
Haian Chemical Plant was used as the PCM. PEG-800 with a molecular weight of
90
760-840, a density of 1.27 g/cm3, and a viscosity of 2.3 MPa·s at 25 oC was used in this
91
study.
92
2.2 Fabrication of hollow wood composites
93
Hollow wood composites (HWC) with dimensions of 350 × 350 × 20 mm3 were
94
fabricated with a target density of 0.3 g/cm3. Four percent (based on the oven-dried
95
weight of wood fibers) of MDI was used as the adhesive, and it was diluted with 5
96
acetone in a ratio of 1:1 to reduce its viscosity before spraying onto wood fiber in a
97
roller. The wood fiber containing MDI was divided into three portions in a mass ratio of
98
3:4:3. The first 30% of the wood fiber was evenly placed on a caul in a frame with a
99
size of 350× 350 mm2. Parallel hollow PVC tubes settled by a self-made clamp (Fig.
100
1(a)) were placed on the first layer of wood fiber. Then, 40% of wood fiber was added,
101
and another layer of parallel hollow PVC tubes was placed on the second layer of the
102
wood fiber. Finally, the last 30% of the wood fiber was evenly placed on top. The
103
formed mat was placed between two aluminum cauls, and silicon paper was placed
104
between the caul and mat to prevent adhesion between the caul and the final HWC. The
105
mat was pressed for 8 min at 180 oC under a pressure of 1 MPa in a hot press machine,
106
and the thickness of final products was controlled by a thickness gauge. Figure 1(b)
107
shows the final HWC. The low-density fiberboard (LDF) without hollow PVC tubes
108
was also fabricated using the same hot press schedule as the reference material. All
109
samples were equilibrated at 25 oC and a humidity of 65% for at least one week before
110
performance testing, and six replicates of each test were performed.
111
The air volume fraction of HWC was adjusted by changing the number of PVC
112
tubes at a constant HWC bulk density of 0.3 g/cm3. The relationship between the
113
number of tubes and the volume ratios are listed in Eq. 1 - Eq. 4: λ +λ +λ +λ =1 λ = λ = λ =
V × 100% = V
V /
(1)
× 100%
(2)
× 100%
(3)
× 100%
(4)
114 115
where λ , λ , λ , and λ are the volume ratios of the PVC tube cavity, solid PVC, 6
116
wood fiber cell wall, and air inside the wood fiber, respectively; V,
117
the volumes (cm3) of HWC, the PVC tube cavity, solid PVC, and wood fiber cell wall,
118
respectively; n is the number of PVC tubes,
119
fiber (g); and
120
λ and λ is the air volume ratio of HWC (λ ). Different λ and λ values were
121
obtained by changing the number of PVC tubes, and Table 1 shows the experimental
122
design and corresponding parameters.
123
Table 1 Experimental design and parameters of hollow wood composites
,
, and V
are
represents the dry weight of wood
is the density of the wood fiber cell wall (1.5 g/cm3). The sum of
Sample
Number of PVC tubes
λ
λ
λ
(%)
(%)
HWC-8
19
7.67
HWC-9
22
HWC-10
25
Design density (g/cm3)
Average HWC density (g/cm3)
λ
λ
(%)
(%)
(%)
2.77
17.78
71.78
79.45
0.289
8.88
3.21
17.43
70.48
79.36
0.274
10.09
3.65
17.08
69.18
79.27
0.308 0.3
HWC-11
28
11.30
4.08
16.73
67.89
79.19
0.292
HWC-12
31
12.52
4.52
16.38
66.58
79.10
0.289
HWC-13
34
13.73
4.96
16.03
65.28
79.01
0.280
124 125 126
2.3 Preparation of phase-change hollow wood composites Polyethylene glycol was first melted at 50oC in a water bath, and then 7.7 g of PEG
127
was injected via syringe into each hollow PVC tube inside the HWC. Then, the two
128
sides of the hollow PVC tube were sealed using a thermosetting resin to obtain
129
phase-change hollow wood composite (PHWC) (Fig. 1(c)). The melt PEG was cooled to
130
20oC before testing, and six replicates were performed for each test.
131
2.4 Physical and mechanical property evaluation 7
132
The modulus of rupture (MOR), modulus of elasticity (MOE), internal bond
133
strength (IB), thickness swelling (TS), and water absorption (WA) of the samples were
134
tested based on the Chinese Standard GB/T17657-2013. Nine specimens were tested for
135
each measurement.
136
The thermal conductivity and thermal resistance of HWC and PHWC were
137
measured using a thermal conductivity analyzer (DRH-300) according to standard GB/T
138
10294-2008. The cold plate and hot plate temperatures were set to 23oC and 43oC,
139
respectively, with an ambient temperature of 21oC.
140
A self-made temperature detector (Fig. 2) was developed and used to test the heat
141
transfer characteristics of LDF, HWC, and PHWC. Samples were placed on a hot plate
142
set to 50oC and surrounded and covered by a 5 mm thick polystyrene foam board to
143
prevent heat exchange with the environment. Nine thermocouples were placed on the
144
top surface to monitor the temperature changes over time. The samples were first heated
145
at 50oC for 1 h and then cooled naturally for another 1 h.
146
2.5 Vertical density profile analysis
147
The vertical density profile (VDP) of lightweight wood composites, HWC, and
148
PHWC was measured by an X-ray profile density analyzer (GreCon DAX 6000) with a
149
scan speed of 0.5 mm/s and a specimen size of 50 × 50 × 20 mm3.
150
2.6 DSC analysis
151
Differential scanning calorimetry (DSC) was used to evaluate the thermal
152
properties and energy storage of PEG from -20 oC - 100 oC at a heating rate of 5 oC /min
153
under a nitrogen (N2) atmosphere. The temperature and enthalpy accuracies of the DSC
154
were 0.1oC and 1%, respectively. The measured heat enthalpy and specific heat of PEG 8
155
were used as the material parameters for heat transfer simulations.
156
2.7 Heat transfer simulations
157
The heat transfer of HWC and PHWC was simulated using ANSYS Workbench
158
software with a simplified model size of 50 × 50 × 20 mm3. The heat transfer of a
159
building wall made of PHWC with a thickness of 240 mm was also simulated and
160
compared with a building wall made of concrete. An outdoor temperature of 40 oC, a
161
convective heat transfer coefficient of 20 W/(m2·K) for the inner surface, and an initial
162
environmental temperature of 20 oC were used to simulate a typical summer
163
environment in southern China. The material properties used in the simulation were
164
experimentally determined and are listed in Table 2.
165
Table 2 Material properties for heat transfer simulation Properties Density (g/cm3) Specific heat (J/(kg·K)) Thermal conductivity（W/ （m·K））
Wood fiber
PVC plastic
PEG
Concrete (Zhao et al., 2011)
0.3
1.2
1.27
2.4
1173 (Guo et al., 2013)
895.6
3393
960
0.061
0.14 (Zhang et al., 2015)
0.24 (Wu et al., 2019)
1.28
166 167
3. Results and discussion
168
3.1 Physical and mechanical properties of HWC
169 170
Figure 3 (a) shows the MOR and MOE values of LDF and HWC. The low-density fiberboard without added PVC hollow tubes had a MOR of 1.8 MPa and a MOE of 9
171
196.1 MPa. These values were much lower than medium-density fiberboard (MDF),
172
which has a MOR of around 28.0 MPa and MOE of 1.4 GPa because they were much
173
less dense than MDF (around 0.65 g/cm3) (Hussain et al., 2019), but these mechanical
174
properties can support the transportation, hoisting, and installation of LDF and HWC.
175
Panyakaew and Fotios (2011) reported that the MOR and MOE of binderless bagasse
176
boards with a density of 0.25 g/cm3 were 0.43 MPa and 102 MPa, respectively, and
177
these values increased with the density. The MOR and MOE values obtained in this
178
study were higher than those previously reported, which was attributed to the addition
179
of the MDI resin. The isocyanate group of MDI reacted with the hydroxyl group of the
180
wood fiber to form a strong covalent bond that contributed to the good mechanical
181
properties of LDF and HWC. There was an obvious increase in the MOR and MOE
182
when hollow PVC tubes were added into LDF (Fig. 3 (a)), which was attributed to the
183
toughness and stiffness of PVC tubes. As λ increased, the MOR and MOE of HWC
184
slightly fluctuated. A higher λ indicates hollower PVC tubes and less wood fiber in
185
HWC. The PVC tubes had positive effects on MOR and MOE, but decreasing the
186
wood fiber content had an inverse effect.
187
The internal bond strengths of LDF and HWC are given in Fig. 3 (b). LDF had an
188
IB of 0.06 MPa, which is within the range of conventional thermal insulation materials,
189
which typically have IB values of 0.03 - 0.08 MPa (Pfundstein et al., 2012). Kawasaki
190
et al. (1998) reported that low-density fiberboard with a density of 0.3 g/cm3 had an IB
191
of around 0.1 MPa. These results demonstrate that low-density fiberboard has a low IB
192
because the fibers are not close enough to be bonded. However, the IB of LDF was
193
much higher than that of binderless bagasse board (0.01 MPa) with a density of 0.35
194
g/cm3 (Panyakaew and Fotios, 2011), showing that the MDI resin improves the 10
195
mechanical properties. The addition of hollow PVC tubes did not improve the IB,
196
possibly because no interfacial bond was formed between the hydrophobic PVC and
197
hydrophilic wood fiber. This should be improved in future work.
198
The thickness swelling and water absorption of LDF and HWC are shown in Fig. 3
199
(c) and Fig. 3 (d). The thickness swelling of LDF after 2 h and 24 h were 4.4% and
200
9.3%, respectively. For comparison, a 24 h thickness swelling of 13.2% was observed in
201
particleboard with a density of 0.4 g/cm3 (Monteiro et al., 2019), and LDF in this study
202
had better water resistance when considering its low density. Thickness swelling slightly
203
increased when hollow PVC tubes were added due to increased water passage and
204
contact surface since the water can fill the gaps between the hollow PVC tubes and
205
fibers. The water absorption rates of LDF after 2 h and 24 h were 39.2% and 104.5%,
206
respectively, and the 24 h water absorption of HWC had a trend of decrease as λ
207
increased. The water penetrated the voids between the wood fibers and lumen, and the
208
chemical components of the wood cell walls absorbed water and swelled. Therefore,
209
when more voids and lumen are present inside the HWC, and the thickness swelling and
210
water absorption rate are both higher. λ , which indicates the void fraction of lumen
211
inside HWC, decreased as λ increased (Table 1), causing the 24 h thickness swelling
212
and water absorption to decrease with λ .
213
3.2 Vertical density profile analysis
214
Figure 4 (a) presents the typical vertical density profiles of LDF, HWC, and
215
PHWC in the thickness direction, and the results show that the VDP for LWC was
216
nearly constant with an average of 0.3 g/cm3. However, there were large fluctuations for
217
HWC when hollow PVC tubes were present, and a U-type shape was observed. This
218
mainly occurred because the hollow PVC tube had a U-type shape density profile (Fig. 11
219
4 (b)). The density of PVC is 1.2 g/cm3, which is much higher than LWC, and the
220
vertical density of hollow PVC tube ranged from 0.2 - 1.2 g/cm3 (Fig. 4 (b)), resulting
221
in a lower density (around 0.2 g/cm3) of wood fiber in HWC when the overall density
222
was the same. This low density resulted in a low internal bond strength, as shown in Fig.
223
3 (b). When the hollow PVC tubes inside the HWC were filled with PEG (ρ = 1.27
224
g/cm3), the corresponding VDP of PVC tubes increased, as shown in Fig. 4 (a).
225
3.3 Thermal conductivity and resistance
226
The thermal conductivity (k) is inversely proportional to the thermal resistance (R),
227
and their values of HWC and PHWC are shown in Fig. 5(a). The k and R of LDF were
228
0.06 W/(m·K) and 0.33 m2·K/W at 33 oC, respectively. The thermal conductivity of
229
HWC and PHWC ranged from 0.06-0.07 W/(m·K), indicating that the lightweight wood
230
composites with a density of 0.3 g/cm3 could be used as a thermally insulating building
231
material since its thermal conductivity was below 0.12 W /(m·K). The thermal
232
conductivities of binderless coconut husk and bagasse insulation boards with densities
233
of 250-350 kg/m3 have been reported to rang from 0.046 to 0.068 W/(m·K) (Panyakaew
234
and Fotios, 2011), which are consistent with the results reported here. The addition of
235
hollow PVC tubes into the LWC did not significantly influence their thermal
236
conductivity or resistance since the thermal conductivity of solid PVC (
237
W/(m·K) (Zhang et al., 2015) and higher than that of LWC. This was mainly because
238
both λ
239
had the lowest thermal conductivity of 0.026 W/(m·K) at 33 oC. PHWC had a slightly
240
higher thermal conductivity than HWC since the PEG-800 had a thermal conductivity
241
of 0.24 W/(m·K) (Wu et al., 2019), showing that the addition of PEG-800 increased the
242
thermal conductivity of PHWC. High thermal conductivity helped improve the energy
and λ
) is 0.14
decreased with an increase in λ (Table 1), and air inside the HWC
12
243
change efficiency of phase-change materials.
244
Series model, parallel model, as well as a combined series and parallel model for
245
three phases were employed to predict the thermal conductivity of HWC and PHWC.
246
The HWC was assumed to be composed of three phases: wood fiber (including wood
247
fiber lumen and voids between wood fibers), solid PVC, and cavity of the hollow PVC
248
tubes. For PHWC, the PEG phase was instead of the PVC tube cavity. The series model
249
(Eq. 5), parallel model (Eq. 6), and their combined model (Eq. 7) can be described as
250
follows: 1
= ( = ε
= ( 251
where
+
+ (1 − ε +
+ +
+
+
(5) ）
+
(6)
+
(7)
）
is the thermal conductivity of the final HWC or PHWC;
is the
252
thermal conductivity of the wood fiber phase, and its experimentally determined value
253
was 0.163 W/(m·K);
254
hollow PVC tube for HWC and PEG (0.24 W/(m·K)) in PHWC;
255
and λ in Table 1, and the sum of
256
factor of the series model to the parallel model and is decided by the material structure
257
and can be calculated by linear fitting.
is the thermal conductivity of air (0.026 W/(m·K)) inside the
,
, and
is the sum of λ
is one, and ε signifies the ratio
258
Figure 5 (b) and (c) compare the experimental data and predicted results of HWC
259
and PHWC. The parallel model predicted the maximum thermal conductivity when the
260
series model predicted the minimum level, and the experimental data fall between these
261
two extremes. ε was found to be 0.79 for HWC and 0.94 for PHWC with an R2 of 0.99, 13
262
indicating that the structures of HWC and PHWC were very similar to a serial structure,
263
especially for PHWC with PEG-filled PVC tubes, which can be further shown in Fig. 1
264
(b) and Fig. 1 (c). The combined series and parallel model accurately predicted the
265
thermal conductivity of HWC and PHWC.
266
3.4 The thermal properties of polyethylene glycol
267
The heat flow of PEG during heating and cooling is shown in Fig. 6 (a). The results
268
indicated that the onset melting temperature was 23.4oC, with a melting peak at 29.6oC,
269
which was consistent with previous research (Sánchez et al., 2007) and was in an ideal
270
temperature range for a phase change material for building insulation. The PEG began
271
to release heat when the temperature decreased to 22.0oC with a heat release peak at
272
17.3oC, which will help keep a building warm. The latent heat of PEG was calculated as
273
125.7 J/g, and its heat enthalpy at 0oC, 20oC, 28oC, and 35oC were 3.18× 10! ,
274
1.60× 10" , 2.49× 10" , and 2.76× 10" J/m3, respectively. These values were used for
275
later heat transfer simulations.
276
3.5 Heat transfer characteristics
277
Figure 6 (b) and Fig. 6 (c) present the top surface temperatures of HWC and
278
PHWC when their bottom temperatures were remained at 50oC for 1 hour, followed by
279
natural cooling for another 1 h. The top surface temperatures of HWC with different λ
280
were similar (Fig. 6 (b)), mainly because the HWC density was the same (0.3 g/cm3) as
281
λ and λ changed, and λ
282
the addition of hollow PVC tubes had almost no effect on heat transfer, and their
283
thermal conductivities showed the same trend (Fig. 5(a)). The top surface temperatures
284
of PHWC at different λ were notably different, and a lower top surface temperature
and λ decreased when λ and λ increased. Therefore,
14
285 286
was observed at higher λ since more heat was absorbed by the PEG during melting. Figure 6 (c) compares the top surface temperature of LDF, HWC, and PHWC, and
287
the results show that LDF and HWC had similar temperatures, but PHWC had a much
288
lower temperature than LDF and HWC during the heating stage and a higher
289
temperature during the cooling stage. This indicates that the addition of PEG into the
290
hollow PVC tubes improved the energy efficiency of HWC, which has also been
291
reported by other researchers (Li et al., 2015).
292
Figure 7 (a) compares the simulated temperature distributions at the same heat
293
transfer experimental testing conditions. The simulation results show that PHWC had a
294
notably lower temperature than HWC, and PEG delayed temperature increases, which
295
was consistent with the experimental data.
296
3.6 Thermal transition simulation of a building wall
297
Figure 7(b) and (c) compare the indoor temperatures of a building wall with a
298
thickness of 240 mm made of PHWC and concrete. The results show that the indoor
299
temperature of the room with a PHWC wall was 23.0 oC, which was 3.5 oC lower than
300
the concrete wall after 5 h insulation, indicating that lightweight wood building
301
materials with good thermal insulation, energy-saving properties, and satisfying
302
mechanical properties were successfully developed.
303
The research performed by Yun et al. (2020) showed that the addition of 0.1%-0.9%
304
building volume fraction of PCM with a phase change temperature of 20-28 oC could
305
save up to 3.19 kWh/m2 energy per year. Sharma and Rai (2020) concluded that
306
PCM-enhanced walls could reduce heat gain by 10.4%-26.6%, while also reducing
307
annual electricity consumption and greenhouse gas emissions. The phase-change hollow 15
308
wood composites with a PCM volume fraction of up to 13.7% in this study can
309
potentially be applied as non-structural thermal insulation and phase-change wall
310
materials. Future efforts will attempt to further reduce the density and increase the PCM
311
volume fraction and mechanical properties of PHWC.
312
4 Conclusions
313
Lightweight wood building composites filled with phase-change material were
314
successfully developed and
315
temperature fluctuations. The addition of hollow PVC tubes into LDF increased its
316
MOR and MOE, and the internal bond strength, 24 h thickness swelling, and 24 h water
317
absorption of HWC decreased with an increasing number of hollow PVC tubes. A
318
U-shaped vertical profile in the HWC was observed where the hollow PVC tubes were
319
present, and it fluctuated less in the PEG-filled tube. The thermal conductivities of LDF,
320
HWC, and PHWC ranged from 0.06-0.07 W/(m·K) at 33 oC, and the addition of hollow
321
PVC tubes and PEG filling only slightly affected the thermal conductivity. Combined
322
series and parallel model predicted the thermal conductivity of HWC and PHWC, and
323
their structures were very similar to the series structure. Both experimental data and
324
ANSYS simulation results showed that the addition of PEG decreased the top surface
325
temperature of PHWC when the bottom surface was continuously heated. Compared
326
with concrete, the use of PHWC as a non-structural wall material resulted in a much
327
lower indoor temperature, demonstrating its promising potential for use as a thermal
328
insulation and phase-change building material.
329
Acknowledgment
330
were able to store latent heat and reduce indoor
This work was supported by the Fundamental Research Funds for the Central
16
331
Universities (NO. 2016ZCQ01).
332
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Figure Captions
413 414
Figure 1 (a) Hollow PVC tubes orientation clamp, (b) hollow wood composites, and (c)
415
hollow wood composites filled with polyethylene glycol
416
Figure 2 Schematic diagram of heat transfer testing
417
Figure 3 Physical and mechanical properties of HWC, (a) MOE, and MOR, (b) internal
418
bond strength, (c) thickness swelling, and (d) water absorption
419
Figure 4 (a) Typical vertical density profiles of LDF, HWC, and PHWC, and (b) density
420
distribution of a hollow PVC tube
421
Figure 5 (a) Thermal conductivity and resistance of LDF, HWC, and PHWC, and
422
thermal conductivity comparison of predicted values with experimental data of (b)
423
HWC and (c) PHWC
424
Figure 6 (a) Heat flow of PEG, top surface temperature of (b) HWC, (c) PHWC, and (d)
425
comparison of LDF, HWC, and PHWC with a bottom hot plate temperature of 50 oC.
426
Figure 7 (a) Heat flux and temperature cloud field of HWC and PHWC, (b) indoor
427
surface temperature simulation results, and (c) temperature distribution simulation
428
results of a 240 mm wall made of PHWC and concrete
21
Figure 1 a
c
b
22
Figure 2
23
Figure 3 b
a
d
c
24
Figure 4 a
b
25
Figure 5 a
b
c
26
Figure 6 a
b
c
d
27
Figure 7 b
a
c
28
1
Highlights:
2
Lightweight hollow wood composites with a density of 0.3 g/cm3 were fabricated
3
The thermal conductivity of lightweight hollow wood composites is 0.06~0.07 W/(m·K)
4
Polyethylene glycol as a phase change material was added into hollow wood composites
5
Phase-change lightweight wood composites were better insulators than concrete
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: