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Accepted Manuscript
Preparation and performance of modified calcium chloride
hexahydrate composite phase change material for air-conditioning
cold storage
Ting Zou , Wanwan Fu , Xianghui Liang , Shuangfeng Wang ,
Xuenong Gao , Zhengguo Zhang , Yutang Fang
PII:
DOI:
Reference:
S0140-7007(18)30288-3
https://doi.org/10.1016/j.ijrefrig.2018.08.001
JIJR 4068
To appear in:
International Journal of Refrigeration
Received date:
Revised date:
Accepted date:
14 May 2018
31 July 2018
4 August 2018
Please cite this article as: Ting Zou , Wanwan Fu , Xianghui Liang , Shuangfeng Wang ,
Xuenong Gao , Zhengguo Zhang , Yutang Fang , Preparation and performance of modified calcium
chloride hexahydrate composite phase change material for air-conditioning cold storage, International
Journal of Refrigeration (2018), doi: https://doi.org/10.1016/j.ijrefrig.2018.08.001
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ACCEPTED MANUSCRIPT
Preparation and performance of modified calcium chloride
hexahydrate composite phase change material for air-conditioning
cold storage
Ting Zoua, Wanwan Fua, Xianghui Lianga, Shuangfeng Wanga, Xuenong Gaoa,
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Zhengguo Zhanga, Yutang Fanga,*
a
Key Laboratory of Enhanced Heat Transfer and Energy Conservation of the Ministry of Education,
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South China University of Technology, Guangzhou, 510640, China
*Corresponding author. Tel. /fax: +86-20-87113870
E-mail
address:
ppytfang@scut.edu.cn
(Y.
T.
Fang),
wtingzou@126.com
(T.
Zou),
819355206@qq.com (W. W. Fu) , liangxh@scut.edu.cn (X. H. Liang), sfwang@scut.edu.cn (S. F.
Wang), cexngao@scut.edu.cn (X. N. Gao), cezhang@scut.edu.cn ?Z. G. Zhang?
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Abstract
Cold energy storage technology with phase change materials (PCMs) has been receiving
increasing attention due to the ability to effectively alleviate the electricity load. The modified
CaCl2�2O composite PCM for cold energy storage was prepared using urea and ethanol as
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thermoregulation additives, strontium chloride hexahydrate (SrCl2�2O) and methyl cellulose
(MC) as nucleating agent and thickening agent respectively. The results manifested that
CaCl2�2O composite PCM containing 15 wt.% urea and 5.0 wt.% ethanol had a promising
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potential in air-conditioning application with phase change temperature of 11.62 癈 and phase
change enthalpy of 127.2 J/g. Fourier transform infrared spectroscopy (FT-IR) verified the
composition of the composite PCM. With the addition of 2.0 wt.% SrCl2�2O, the degree of
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supercooling of CaCl2�2O composite PCM could be reduced to 0.95 癈, and its enhanced
crystallization behavior was confirmed by optical microscope. The modified composite PCM also
Modified CaCl2�2O; Composite PCM; Cold energy storage; Supercooling;
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Key words:
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showed an excellent thermal reliability after 50 heating-cooling cycles.
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Thermal reliability
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Nomenclature
Abbreviations
phase change materials
DSC
differential scanning calorimetry
FT-IR
Fourier transform infrared spectroscopy
MC
methyl cellulose
HEC
hydroxyethyl cellulose
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PCMs
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Parameters
phase change temperature(癈)
?H
phase change enthalpy(J/g)
?T
supercooling degree(癈)
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diameter
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Tm
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1. Introduction
Currently the rapid growth of energy demand along with social economy boosting has
brought serious challenge for sustainable energy development and ecological environment. As one
of the key energy consumption equipment in civil applications, air-conditioning system consumes
[1]
. With the popularity of air-conditioning
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more than 50% of total electricity power consumption
system in large buildings, the consumption of electricity power and peak-valley difference in
electricity system are increasing gradually, further resulting in the growing demand for energy.
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The cold energy storage technology in air-conditioning has attracted attention widely due to the
fact that it can balance the contradiction between electricity supply and demand, thereby relieving
the pressure of energy demand
[2-4]
. In air-conditioning system, the cold energy storage medium
[5]
. The cold energy
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commonly used water and ice is a key factor in cold storage technology
storage system using water can be accomplished by sensible heat. Because of the low cool storage
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density for water medium, the large volume of reservoir cooling tank is needed, restricting its
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application. Although the ice cold storage in the form of latent heat can greatly reduce the volume
of reservoir cooling tank, it has drawbacks of the low evaporation temperatures in range from -10
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to -5 癈 and complicated work conditions. Consequently, it is necessary to develop an ideal cold
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storage material with the ability of handling the problems mentioned above.
Phase change materials (PCMs) are regarded as a new type of energy-storing material, which
can effectively release or store a great amount of latent heat during the melting or solidification
process
[6]
. Based on the advantages of high storage energy density, small volume change and
nearly constant temperature during the phase change process, PCMs are the promising medium of
cold energy storage, which are widely applied in many fields. Qi et al.
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[7]
proposed a portable
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solar-powered air-cooling system for vehicle cabins based on PCMs, and concluded that this
cooling system was effective and significantly beneficial to control the temperature inside a
vehicle cabin. Xia et al.
[8]
designed and analyzed a cold storage condensation heat recovery
system with PCMs. Huo et al.
[9]
constructed the lattice Boltzmann model of for PCMs based the
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battery thermal management cold temperature. PCMs are mainly divided into two categories
according to the composition: organic and inorganic. With superiority in good thermal stability,
melting congruency, a wide range of melting temperatures and low toxicity, paraffinic waxes and
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fatty acids as organic compounds used in cold energy storage system have been investigated
widely by many researchers [10]. He et al. [11] investigated the thermal properties of technical grade
paraffin waxes as PCM, indicating that paraffin waxes used in the cold storage system were able
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to conserve energy and reduce the operation cost. Cho et al. [12] studied the thermal characteristics
of paraffin in a spherical capsule during freezing and melting processes. Dimaano et al.
[13]
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experimentally analyzed the heat transfer characteristics of the mixture of 65 wt.% capric acid and
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35 wt.% lauric acid for its cooling capacity. Nevertheless, the intrinsic shortcomings of the low
thermal conductivity and flammability are inevitable, which have the limitations in its practical
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application. Inorganic PCMs have higher volumetric thermal storage capacity and lower cost as
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compared to organic materials, which are superior choices in thermal energy storage. Salt hydrates
are the most commonly studied among inorganic PCMs because of their excellent characteristics.
Zhu et al. [14] investigated the performance and economic analysis of ground source heat pump
(GSHP) integrated with PCM cooling storage system for an office building, and found that the
combined system with Na2SO4�H2O as PCM and NH4Cl/KCl as additives had the phase change
melting temperature of 8.3 癈, which was capable of improving the reliability of the operation
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performance and reducing the energy consumption and operation cost. Gracia et al.
[15]
analyzed
the environmental impact of integrating phase change materials with experimental buildings, and
found that salt hydrates had an advantage over paraffin in reducing the manufacturing/disposal
impact.
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Calcium chloride hexahydrate (CaCl2�2O) is considered as a representative salt hydrate
PCM owing to the merits of high fusion enthalpy, non-poisonous and accessibility. In common
with most salt hydrates, CaCl2�2O has a tendency toward supercooling and phase separation in
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practical application. The supercooling belongs to a metastable state of PCMs, and mainly reflects
in the delayed solidification below the freezing temperature, which is attributed to poor capability
of nucleation. The majority of salt hydrates experience the melting incongruently process where
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partial salt dissolved in the hydration water, thus causing phase separation [16]. The solution to the
problem discussed above is to use nucleating agents and thickening agent. The nucleating agents
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can be divided into isomorphous nucleators and non-isostructural nucleator by comparing the
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similarity in the crystal structure and lattice parameters of additive and its attached material. Li
and Zhang et al.
[17]
found that SrCl2�2O and SrCO3 as nucleating agents could reduce the
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supercooling of CaCl2�2O system to 2 癈, and hydroxyethyl cellulose (HEC) as thickener could
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improve the phase separation. Bilen and Takgil et al. [18] reported that 3 wt.% KNO3 as nucleating
agent could suppress the supercooling degree of CaCl2�2O. Shahbaz et al.
[19]
prepared a novel
calcium chloride hexahydrate-based deep eutectic solvent as a phase change materials, and
showed that 2 wt.% of SrCl2�2O as nucleating agent and fumed silica as thickener could avoid
the supercooling and phase segregation. Li and Zhou et al.
[20]
investigated that CaCl2�2O
containing ?-Al2O3 nanoparticles exhibited low supercooling and excellent thermal cycling
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stability. In these previous works, the supercooling and phase separation of CaCl2�2O could be
relieved by the addition of nucleating agent and thickener respectively. The melting temperature of
CaCl2�2O (29 癈) is not suitable for the application of air-conditioning cold storage, which
could be adjusted with additives. Based on the researches above, SrCl2�2O and SrCO3 are
could effectively suppress the supercooling of CaCl2�2O.
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attributed to isomorphous nucleators for CaCl2�2O due to their similar lattice parameters, which
Currently, inorganic materials which can be used for air conditioning mostly belong to
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sodium sulfate decahydrate system. Nevertheless, researches about the synthesis, characterization
and modification of CaCl2�2O used in the air-conditioning cold storage are rarely reported. In
the present study, a novel composite PCM based on CaCl2�2O was developed for
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air-conditioning system, which was expected to provide more choices for cool storage material. In
terms of the air-conditioning system, phase change temperature of materials should be in the range
[21-23]
. To make the melting of CaCl2�2O appropriate for practical application, urea
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of 5?12 癈
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and ethanol based on the advantages of the low cost, low toxicity and easily available were used as
additives to modify PCM. SrCl2�2O as nucleating agent and methyl cellulose as thickener are
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able to effectively mend problems inherent in CaCl2�2O composite system, including the
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supercooling and phase separation. Meanwhile, the effect of SrCl2�2O nucleating agent on the
crystal morphology of composite PCMs was verified by optical microscope. Thermophysical
properties of the modified composite including phase change temperature, latent heat, thermal
cycle reliability and supercooling degree, were experimentally investigated.
2. Experimental
2.1 Materials
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Calcium chloride hexahydrate (CaCl2�2O, AR), carbamide (Urea, AR), ethanol (AR) and
strontium chloride hexahydrate (SrCl2�2O, AR) were provided by Guangzhou chemical reagent
factory. Methyl cellulose (MC, AR) was purchased from Tianjin Fuchen chemical reagent factory.
All reagents and deionized water were used in the whole experiment directly.
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2.2 Preparation
Using CaCl2�2O as PCM main component, the modified composite PCM for
air-conditioning cold storage was prepared by the addition of additives. In a typical process, 80
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wt.% of CaCl2�2O and 15 wt.% of urea were mixed and vigorously stirred at 40 癈 until
dissolved. Subsequently, 5 wt.% of ethanol was added into the above-mentioned mixed solution
and vigorously stirred at room temperature for 1 h. The obtained PCMs with different amount of
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SrCl2�2O and 2.0 wt.% of MC were prepared to examine the effects on supercooling and phase
separation.
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2.3 Characterization of the modified CaCl2?6H2O composite PCM
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Q20 di?erential scanning calorimeter (DSC, TA Instrument, USA) was carried out to analyze
the thermal properties under a nitrogen atmosphere using an empty sealed alumina crucible
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reference in the range of -30?40癈 at a heating/cooling rate of 5 癈/min. The phase change
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temperature (Tm) of PCM was determined by drawing a line at the point of maximum slope of the
leading edge of the peak, and extrapolating to the baseline [24]. The phase change enthalpy (?H) of
PCM was determined as total by numerical integration of the peak areas. Each sample was tested
three times to ensure accuracy and repeatability of the results.
Agilent 34970 data acquisition instrument system consisting of a computer, a data collector, a
T-type thermocouple and low-temperature thermostat bath (10% glycol aqueous solution as
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cooling agent) was used to test the supercooling degree (?T) of composite PCMs under the
condition of temperature from 18 to 3 癈, as shown in Figure 1. For cooling test, the platform or
inflection point of curve corresponded to phase change temperature of material. 20g PCM was
filled in 20 ml test-tube of during the cooling teat. In this work, the testes were repeated three
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times for each sample.
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Figure 1 Schematic of the experimental apparatus for cooling curve of composite PCM
A Bruker Tensor 27 Fourier transform infrared spectrometer (FT-IR, Bruker Instrument Co.,
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Germany) was conducted to the chemical characterization of the composite PCM, CaCl2�2O,
urea and ethanol, respectively.
The microscopic manipulating apparatus (Carl Zeiss Shanghai Co., Ltd) were used to observe
the morphology of the composite PCM. The apparatus mainly include four parts: (1) an inverted
optical microscope (Carl Zeiss Axio Observer A1), (2) a low temperature cell (? 50� mm, made
of stainless steel), (3) two micromanipulators, (4) a digital recording system.
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3. Results and discussion
3.1 Determination of the urea content
The ideal PCMs should satisfy a number of criteria such as high energy storage density,
appropriate phase change temperature, congruent melting, low supercooling and so on
[25]
. In this
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study, owing to the inherent advantages of the low cost, low toxicity and easily available, urea and
ethanol were chosen to adjust the melting temperature of CaCl2�2O. The urea content has effect
on the thermodynamic property of CaCl2�2O to some extent. The different contents of urea
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were added into the composite PCM to confirm the optimal content respectively, where the mass
ratio of ethanol was fixed at 5 wt.%. Figure 2 shows the DSC curves of a series of composite PCM
prepared with the different addition amounts of urea. The related thermal parameters are listed in
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Table 1.
Table 1 Effect of the urea and ethanol content on the thermal properties of composite PCM
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Urea content (5% ethanol fixed)
Additives content
14%
15%
16%
2%
3%
4%
5%
6%
11.00
11.20
11.62
9.54
13.88
12.72
11.58
11.62
7.98
134.9
120.1
127.2
104.8
145.5
133.6
113.3
127.2
96.31
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?H (J/g)
13%
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Tm (癈)
Ethanol content (15% urea fixed)
The phase change temperatures (Tm) of the composite PCMs were found to be 11.00, 11.20,
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11.62, 9.54 癈 when the addition amounts of urea were 13, 14, 15, 16 wt.%, respectively, with the
corresponding phase change enthalpies (?H) of 134.9, 120.1, 127.2, 104.8 J/g, respectively. The
phase change temperatures for composite PCMs containing different urea contents were in the
range from 9-12癈, which was in accordance with application in air-conditioning
[26]
. It is
conclude that urea has the capacity to regulate the phase change behavior of CaCl2�2O. The
reason for this is that the amide group of urea can form the hydrogen bond with crystal water of
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CaCl2�2O, weakening the force between CaCl2 and crystal water. Though composite PCM with
13 wt.% urea had the highest phase change enthalpy among the investigated systems, the division
of the melting peak appeared, indicating the incongruent melting of the PCM. The interpretation is
that the amount of released water is obviously inadequate to dissolve the salt at the melting
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temperature, and higher temperature is required to dissolve the remainder, leading to
inconvenience in practical application. When the urea content increased, the division of the
melting peak is gradually weakened. As observed in Figure 2, beside being congruent melting, the
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composite PCM with 15 wt.% urea exhibited a relatively high enthalpy. Considering the criteria
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required for the ideal PCM analysis, 15 wt.% urea was regarded as the optimal.
Figure 2 Effect of the urea content on the thermal properties of composite PCM
3.2 Determination of the ethanol content
Ethanol as additive cooperating with urea was added to the binary mixture with the object of
adjusting the phase behavior of composite PCMs.
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Figure 3 Effect of the ethanol content on the thermal properties of composite PCM
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As was mentioned previously, the addition amount of urea with 15 wt.% could make
composite PCMs possessed a comparatively ideal phase change behavior. In the case of the fixed
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mass ratio of urea, the effect of the ethanol contents on phase transition behavior of composite
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PCMs is presented in Figure 3, its corresponding experimental results were listed in Table 1. The
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phase change temperatures (Tm) of composite PCMs at the ethanol contents of 2.0, 3.0, 4.0, 5.0,
6.0 wt.% were 13.88, 12.72, 11.58, 11.62, 7.98 癈, respectively. Accordingly, the phase change
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enthalpies (?H) were 145.5, 133.6, 113.3, 127.2, 96.31 J/g, respectively. It means that the ethanol
has an effect on the phase change temperature and phase change enthalpy of composite PCMs,
which is attributed to the hydrogen-bond interaction between the hydroxyl group of ethanol and
crystal water of CaCl2?6H2O. Remarkably, the melting peaks under the ethanol contents of 4.0
wt.%, 6.0 wt.% were split into two sections, which wasn't fit for the standard of ideal PCMs.
Considering the problems in actual application for air-conditioning system, the ethanol content of
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5.0 wt.% giving suitable phase transition temperature and relatively high enthalpy was chosen for
composite PCM in further study.
3.3 Determination of the nucleating agent content
Almost all inorganic PCMs are subjected to a prevalent problem, namely supercooling,
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restricting its further application [27]. It is a state in which the onset solidification of PCM is below
the melting temperature due to the poor nuclear capability of materials, which is not conductive to
releasing the latent heat in time. Based on the theory of heterogeneous nucleation, adding a
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suitable nucleating agent is considered to be a solution to the supercooling degree (?T).
Figure 4 Effect of strontium chloride hexahydrate content on the supercooling of composite PCM
Based on some researches, SrCl2�2O as isomorphous nucleators can function well in
effectively suppressing the supercooling degree of CaCl2�2O composite PCM
[28, 29]
. The
content of nucleating agent should be kept in an appropriate scope during the process of reducing
the supercooling of PCM, which can be determined by experiments. Figure 4 presents the
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influence of SrCl2�2O concentration from 1.0 wt.% to 3.0 wt.% on the supercooling of
CaCl2�2O-urea-ethanol PCM.
In terms of CaCl2�2O-urea-ethanol PCM without nucleating agent, there was no
crystallization under experimental conditions, which is primarily caused by its high degree of
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supercooling. The supercooling degrees (?T) of the composite PCMs with different SrCl2�2O
contents of 1.0, 1.5, 2.0, 2.5, 3.0 wt.% were 4.23, 3.23, 0.95, 1.25, 1.46 癈, respectively, appearing
decreased firstly and then increased tendency. Therefore, it was easy to conclude that 2.0 wt.%
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SrCl2�2O content could reduce the supercooling degree to a great extent. The results above
indicated that the supercooling degree of CaCl2�2O-urea-ethanol composite PCM tended to
increase when the nucleator contents are more than 2.0 wt.%, for which there are several possible
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reasons: (1) part of the nucleating agent plays a key role in heterogeneous nucleation, the
remainder of that get deposited in the surface of PCM, inhibiting the combination of salt and
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crystal water in solution. (2) crystal nucleus growth can be regarded as the stacking process of
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large amounts of atom, during which crystal nucleus receives the force from the atom of nucleator.
The force increases with the content of nucleation agents increasing, which can restrain the crystal
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nucleus growth, eventually reducing the nucleation effect. (3) as the content of nucleation agents
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increases, the surface free energy of nucleation increases, hindering the formation of nucleation to
some degree, thus increasing the supercooling degree.
Generally, the additive, namely SrCl2�2O, has certain effects on the thermophysical
properties of CaCl2�2O-urea-ethanol PCM, which is due to the fact that the additive cannot
release the latent heat as much as PCM in the tested temperature range of -40?40 癈. As shown in
Figure 5, all of melting peaks corresponding to positions were almost identical, and the phase
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change enthalpies (?H) with different SrCl2�2O contents of 1.0, 1.5, 2.0, 2.5, 3.0 wt.% were
121.5, 103.6, 123.1, 121.7, 130.8 J/g, respectively, with the corresponding phase transition
temperature of 11.33, 11.94, 11.54, 11.93, 11.45 癈, respectively, which showed a negligible
change as compared to CaCl2�2O-urea-ethanol composite PCM without nucleation additive.
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The optical content of SrCl2�2O was 2.0 wt.% for the following experiment, taking into account
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both the supercooling and enthalpy of CaCl2�2O-urea-ethanol composite PCM.
Figure 5 Effect of strontium chloride hexahydrate content on the enthalpies and phase change temperature of
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composite PCM
3.4 Effect of the nucleating agent on crystalline behavior of composite PCM
The effect of the nucleating agent on the crystallization of composite PCM was analyzed by
optical microscope. As shown in Figure 6(a), composite PCM without SrCl2�2O displayed a
loose and needle-like crystal morphology with the length of about 200 ?m, which may be
attributed to the low crystallization rate in one direction for composite PCM. By contrast, with the
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addition of 2.0 wt.% SrCl2�2O nucleating agent, the crystal morphology of composite PCMs in
Figure 6(b) possessed more compact surface and smaller spindle-like crystal. The reasonable
interpretation is that the addition of nucleating agent can speed up the crystallization of composite
PCM, namely enhancing the crystallization ability. Consequently, SrCl2�2O as nucleating agent
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which is consistent with the above results about supercooling.
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is effective in improving the crystallization behavior of CaCl2�2O-urea-ethanol composite PCM,
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Figure 6 Effect of SrCl2�2O on the crystal morphology of composite PCM: (a) 0 wt.%; (b) 2.0 wt.%
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3.5 FTIR analysis
The chemical composition for composite PCM was analyzed by FTIR spectra. Figure 7
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shows the FTIR spectra of composite PCM, CaCl2�2O, urea and ethanol, respectively. The -OH
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group of ethanol and -NH group of urea possessed characteristic peaks at 3436 cm-1. In the
spectrum of urea, the peaks at 1629 cm-1 and 1462 cm-1 corresponded to C=O stretching vibration
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and C-N stretching vibration, respectively. Additionally, the peak at 1157 cm-1 was assigned to
N-H bending vibration of urea. For CaCl2�2O, the spectra showed the wade bands from 3000 to
3600 cm-1, which was due to O-H stretching vibration of crystal water. The characteristic peaks at
1675 and 602 cm-1 were assigned to H-O-H bending vibration and rotational motions of crystal
water, respectively. The peak at 2154 cm-1 was attributed to the characteristic band of CaCl2.
Obviously, the characteristic peaks of CaCl2�2O, urea and ethanol were observed in the
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spectrum of composite PCM, indicating the successful preparation of composite PCM.
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Figure 7. FTIR spectra of composite PCM, CaCl2�2O, urea and ethanol
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3.6 Thermal reliability
The most important criterion that has limited wide applications of PCMs is the thermal
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reliability of thermal energy storage materials. When heated to the melting point, salt hydrates are
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only partially soluble in the hydration water, the remainder of this section is deposited in the
bottom of the container, which is called phase separation. With the iterative process of
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heat-cooling cycle, the deposition of salt become increasingly serious, resulting in the poor heat
storage capacity. As a consequence, phase separation is the main factor closely related to thermal
reliability. As phase separation can reduce the energy storage density of PCM through several
heat-cooling cycles, necessary measures should be taken to address these problems.
Generally, adding thickening agent is one of the simplest and the most effective approaches
to get rid of phase separation, thus enhancing the thermal stability. The thickness agent can
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increase the viscosity of solution, making precipitated crystals distribute uniformly rather than
sinking to the bottom in the solution, thereby preventing the phase separation. In order to improve
the phase separation, methyl cellulose (MC) as thickening agent was added into composite PCM.
The heat-cooling cycle test was conducted to testify the thermal reliability of PCM.
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The cooling curves of the composite PCM containing 2.0 wt.% MC at different cycle
numbers were presented in Figure 8. The supercooling degree (?T) with different cycle numbers
(5, 10, 20, 30, 40, 50 cycles) were 0.28, 0.98, 0.91, 1.02, 1.60, 3.12 癈, respectively. Obviously, as
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the numbers of cycles increased, the degree of supercooling increased to some extent. Though
composite PCM containing 2.0 wt.% MC had undergone several thermal cycles, the degree of
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supercooling degree stayed below 3.5 癈.
Figure 8 Cooling curves of composite PCM containing 2.0 wt.% MC at different cycle numbers
DSC curves of composite PCM containing 2.0 wt.% MC at different cycle numbers are
shown in Figure 9, and the related results are listed in Table 2. It is clear that the position of
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melting peaks remains unchanged basically. The onset melting temperatures of PCMs under
different cycle numbers were 12.42, 12.53, 12.99, 11.76, 11.20, 12.58 癈, corresponding to the
phase change enthalpies (?H) of 112.0, 112.4, 107.0, 105.6, 110.7, 102.8 J/g, respectively, which
indicated that thermal properties almost remained the same after several heat-cooling cycles.
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Based on the results of cooling test and DSC, the composite PCM containing 2.0 wt.% MC
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exhibits an excellent thermal reliability.
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Figure 9 DSC curves of composite PCM containing 2.0 wt.% MC at different cycle numbers
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Table 2 Effect of thermal cycling on thermal properties of the composite PCM
Number of cycles
5
10
20
30
40
50
Tm (癈)
12.42
12.53
12.99
11.76
11.20
12.58
?H (J/g)
112.0
112.4
107.0
105.6
110.7
102.8
?T (癈)
0.28
0.98
0.91
1.02
1.60
3.12
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4. Conclusions
Novel modified CaCl2�2O composite PCM containing 15 wt.% urea and 5.0 wt.% ethanol
possessed an appropriate phase change temperature (Tm=11.62 癈) and relatively high enthalpy
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(?H=127.2 J/g) for application in air-conditioning cold storage. The composition of the composite
PCM could be verified by the FTIR spectra. To address the problem of supercooling and phase
separation of composite PCM, SrCl2�2O was proposed as a nucleating agent and methylated
M
cellulose (MC) was selected as a thickening agent. The 2.0 wt.% SrCl2�2O was the optimal
ED
content, which could make the supercooling of the composite PCM reduce to 0.95 癈, and also
could improve the crystalline ability of the composite PCM, which was confirmed by optical
PT
microscope. With the addition of 2.0 wt.% MC, the phase separation of composite PCM could be
CE
improved, thus increasing the thermal reliability. With regard to composite PCM, the supercooling
degree increased from 0.95 to 3.20 癈 as the number of cycle increased from 0 to 50, and the
AC
phase change temperature and phase change enthalpy had little change during the 50 cycles of
heat-cooling test.
Acknowledgment
This work was supported by National Natural Science Foundation of China (No.51536003,
No.21471059).
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