Materials
and processes for energy: communicating current research and technological developments (A. Méndez-Vilas, Ed.)
____________________________________________________________________________________________________
Energy storage: Preparations and physicochemical properties of solidliquid Phase change materials for thermal energy storage
Daolin Gao and Tianlong Deng*
Tianjin Key Laboratory of Marine Resources and Chemistry
College of Marine Science and Engineering at Tianjin University of Science and Technology, Tianjin, 300457, PRC
* Corresponding author, email: [email protected]
Abstracts: Using phase change materials (PCMs) to store and release latent heat is essential to develop the renewable
energy, improve the energy efficiency and relieve the conflict of energy between supply and demand. The aim of this
study is to prepare novel inorganic PCMs for thermal energy storage with phase change temperatures at room temperature
(18-25ºC), middle temperature (40-50ºC) and medium-high temperature (60-80ºC). In this chapter, on the basis of a brief
introduction for the basic principle of PCMs and the progress on the available thermal energy storage technology, authors
mainly focused on our newest research results on preparations and thermal chemical properties on magnesium nitrate
hexahydrate as a basic substance and calcium chloride solution, ammonium nitrate or lithium nitrate as additions to
modulate the phase change temperatures. After a series of thermal stability, supercooling, phase separating and recycle
application studies, three kinds of PCMs were successful established. The experimental results indicated that: (i) 50%
calcium chloride solution containing 5% magnesium nitrate hexahydrate formed a room temperature composite PCM-A
with a phase change temperature of 22.6ºC and latent heat values of more than 160 kJ/kg; (ii) magnesium nitrate
hexahydrate mixed with 38.8% ammonium nitrate is as a middle temperature composite PCM-B with a phase change
temperature of 44.8ºC and latent heat values of about 155 kJ/kg; (iii) magnesium nitrate hexahydrate blended 14% lithium
nitrate only can be formed as a medium-high temperature PCM-C with phase change temperature of 72.1ºC, and the latent
heat is more than 165 kJ/kg. It is worthy saying that the re-heating and cooling-recycle tests for three PCMs showed that
the maximum deviations of melting temperature and latent heat after thirty recycles for PCM-A, after 100 recycles for
PCM-B and PCM-C are only 5.6% and 4.1%, 2.1% and 2.0%, 2.4% and 1.7%, respectively. More parameters on
thermodynamics and thermal chemistry of the three PCMs of PCM-A, PCM-B and PCM-C were reported in the first time.
Keywords: phase change materials; magnesium nitrate hexahydrate; latent heat; phase change temperature
1. Introduction
Energy is essential for human being’s survival and development. Due to the mineral and fossil energy resources are
gradually exhausted with the increasing of global economy development let alone the serious environmental problems,
scientists are paying more attention to improve the energy efficiency and develop the renewable energy. It has being
widely realized that the mineral energy and nuclear power have to be replaced by renewable energy sources. However,
thermal energy storage using PCMs as the latent heat storage media is an effective means to improve energy utilization.
PCMs with high latent heat of fusion can store and release large amounts of energy when the phase transition
happens. Using PCMs to store and retrieve thermal energy as latent heat is one of the most important techniques to
develop the renewable energy, improve the energy efficiency and relieve the conflict of energy between supply and
demand [1-3]. The principle of PCMs was illustrated in Fig. 1. Using the latent heat to store or release thermal energy
of PCMs and the temperature can stay nearly constant during the process of phase change to effectively solve the
imbalance of energy supply and demand in time and space. Therefore, PCMs can be widely applied in solar energy
utilization, heat exchanger, building energy-saving, electric peak-shaving, textiles and so on [4-8].
2. Phase change materials
2.1 Classification
According to phase change behaviour of PCMs, PCMs can be generally divided into four categories: solid-solid, solidliquid, solid-gas and liquid-gas PCMs [9]. As to the four categories, the latter two kinds of PCMs are never adopted due
to the large volume variations or high pressure with occurrence of the gas phase. Moreover, solid-solid PCMs have a
rather low heat of transformation. Hence, only the solid-liquid PCMs have widely application prospects.
According to the solid-liquid type of PCMs, PCMs can be divided into organic PCMs, inorganic PCMs and eutectic
PCMs [10-12]. A comparison of these different kinds of PCMs is listed in Table 1 [13-14]. Organic PCMs can be
further described as paraffin and non-paraffin without phase separation and supercooling but volumetric latent heat
storage capacity and thermal conductivity is low[15]. Inorganic PCMs mainly include salt hydrate, molten salts, metals
and alloys, and the most typical is hydrated salt with high latent heat and thermal conductivity but severe supercooling
and phase separation [16]. In order to overcome the shortcoming of single PCM can not control the melting
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temperature, eutectic PCMs arises, and eutectic PCMs included organic-organic, organic-inorganic, inorganic-inorganic
[10].
Table 1 Comparison of different kinds of PCMs.
Classification
Organic PCMs
Advantages
Available in a large temperature range
Low or non supercooling
Chemically stable and recyclable
Good compatibility with other materials
High heat of fusion
High thermal conductivity
Low volume change
Availability and cheapness
Sharp melting temperature
High volumetric thermal storage density
Inorganic PCMs
Eutectics
Disadvantages
Low volumetric latent heat storage capacity
Low thermal conductivity
Relative large volume change
Flammability
Supercooling
Phase separation
Lack of thermal stability
Corrosion
Lack of test datum of thermophysical properties
2.2 Criteria of PCMs selection
ib le
Liquid
latent
sens
Phase change
se n
si b l
e
Solid
melting point
enthalpy of fusion, /(kJ/kg)
energy storage
The melting temperature and phase change enthalpy of several low-temperature PCMs are shown in Fig. 2. From the
point of melting temperature it can be seen that for latent heat storage in middle-high applications, the potential PCMs
are salt hydrates and eutectics.
500
300
water
salt hydrate
and eutectics
sugar
alcohol
200
paraffin
100
0
-100
0
100
200
Temperature, /℃
temperature
Fig. 1 Principle of phase change materials
salts
400
Fig. 2 Melting temperature and latent heat of several PCMs
PCMs are latent heat storage materials, thermal energy transfer occurs when materials change from solid to liquid, or
liquid to solid. A kind of ideal PCM must melt with large mounts of heat of fusion and with little or no supercooling.
However, for their employment as latent heat storage materials it must exhibit certain desirable thermodynamic, kinetic,
physical properties and chemical properties. Moreover, economic considerations and easy availability of these materials
has to be kept in mind. Selection criteria of PCMs are listed in Table 2 [10, 12, 15, 17].
2.3 Typical material problems and possible solutions
Usually, a candidate material as PCM does not meet up all the requirements. Nevertheless, it is often still choose such
as a potential PCM if some of the strategy developments to solve or avoid the potential problems.
Some PCMs start crystallization only after a temperature below the melting temperature is reached, this phenomenon
is called supercooling. Supercooling is a serious problem in the application of PCMs. The phenomenon of supercooling
makes it necessary to reduce the well temperature below the phase change temperature to start crystallization and to
release the latent heat stored in the material. If nucleation does not happen, the latent heat is not released at all and the
material only stores sensible heat. The problem of supercooling can be tackled by one of the following means: (i)
adding the nucleating agent, (ii) mechanical stirring, (iii) adding of some impurity method, (iv) cold finger technique,
(v) encapsulating the PCM to reduce supercooling [15, 18].
Phase separation is an unstable tendency of PCMs as environmental conditions change. The high storage density of
PCMs with phase separation is difficult to maintain and usually decreases with cycling. Salt hydrates may be regarded
as consist of a salt and water in a discrete mixing ratio which are usually with the phenomenon of phase segregation as
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PCMs. This is because most hydrated salts melt congruently with the formation of the lower hydrated salt, making the
process irreversible and leading to the continuous decline in their storage efficiency. The main approach to get rid of
phase separation can be listed as follows: (i) adding of the thickening agents, (ii) using excess of water, (iii) mechanical
stirring, (iv) encapsulating PCM to reduce separation, (v) modifying the chemical composition of the system and
making incongruent material congruent [15, 18-21].
Most PCMs have unacceptably low thermal conductivity, leading to slow charging and discharging rates. Since PCM
store large amounts of heat or cold in a small volume to transfer this heat or cold energy to outside of the storage to use,
the low thermal conductivity could be a problem. Good thermal conductivity is able to store or release the latent heat in
a given volume of the storage material in a short time. Hence, heat transfer enhancement techniques are required for
most latent heat thermal energy applications. The feasible approach under investigation to increase thermal conductivity
in PCMs include finned tube of different configuration, bubble agitation, insertion of a metal matrix into the PCMs,
using PCMs dispersed with high conductivity particles, micro-encapsulation of the PCM or shell and tube [22-28].
Table 2 Selection criteria of phase change materials.
Properties
Thermal properties
Kinetic properties
Physical properties
Chemical properties
Economic properties
Characteristics of each property
Phase change temperature in desired range
High latent heat of fusion
High thermal conductivity
Good heat transfer
High nucleation rate to avoid supercooling
High rate of crystal growth to meet demands of heat recovery
Favorable phase equilibrium
High specific heat and high density
Small volume change on phase transformation
Small vapour pressure at operating temperatures
Long-term chemical stability
Congruent melting
No toxic and no corrosiveness
No flammable and no explosive material
Abundant raw materials and Low-cost
3. Experimental studies
In this chapter, three novel solid-liquid inorganic eutectic PCMs of magnesium nitrate hexahydrate as a base material
and calcium chloride solution, ammonium nitrate or lithium nitrate as additions to modulate the phase change
temperatures were prepared and characterized, and the eutectic ratio of every binary systems of composite PCMs were
optimized and discussed, respectively. The physicochemical properties of latent heat, specific heat, thermal
conductivity, density, phase change temperature, supercooling degree and thermal stability were investigated by using
differential scanning calorimetry (DSC 200F3, Netzsch Instrument Inc., Germany), simultaneous thermal analyzer
(Labsys Evo TG-DSC, Setaram Instrument Inc., France), thermal conductivity analyzer (DRE-2B, Xiangtan Instrument
Co. Ltd, China), density meter (DMA 4500M, Anton Paar GmbH, Austria), melting point apparatus (SGWX-4B,
Shanghai precision scientific instrument Co. Ltd, China) and temperature recorder (VX2103R/C2/U/TP1, Shanghai
Yadu electronic technology Co. Ltd, China). In addition, Thermal stability of the every optimized composite PCM as a
potential PCM for repeated melting and crystallization recycles were also performed and investigated.
3.1 Performance of room temperature composite PCM (PCM-A)
With the fast economic development, the global energy demand is quickly increasing. However, the energy
consumption of building industry has become the dominant with 28% amounts in overall industrial energy consumption
around the world [29]. As the demand for thermal comfort of buildings rises increasingly, the energy consumption is
also increasing correspondingly both in the domestic and commercial buildings. To overcome this challenging situation,
energy resources need to be used more efficiently. PCMs have been considered as thermal energy storage materials in
buildings since 1980, and with PCMs implemented in gypsum board, plaster, concrete or other wallboard material,
thermal energy storage can be part of the building structure. The latent heat storage by incorporating PCMs into
building structure is an attractive mean to compensate for the small storage capacity and increase thermal comfort of
buildings. Hence, PCMs with room temperature between 18 and 25ºC adopted in building is an effective to reduce
building energy consumption.
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3.1.1 Preparation of PCM-A
Calcium chloride solution was prepared with the mass ratio of calcium chloride anhydrous and doubly deionized water
by 1:1, calcium chloride anhydrous and doubly deionized water were weighted and uniformly mixed in the test
container and heating in a water bath until completely dissolved. Then magnesium nitrate hexahydrate was added into
calcium chloride solution with heating and stirring until it was completely homogeneous. After that, strontium chloride
hexahydrate was added as nucleating agent. Finally, the composite PCM was obtained by cooled down to room
temperature.
Calcium chloride solution as a candidate PCM was also measured and showed that phase change temperature of
calcium chloride solution was 28.7ºC. That’s to say, calcium chloride solution is not suitable for building energy-saving
applications because of the high temperature. Hence, we need to drop phase change temperature of calcium chloride
solution. Since the target temperature range for building energy-saving to be handled by the mixture based on calcium
chloride solution is 18~25ºC, we sought a PCM with a phase change temperature within that range by adding
magnesium nitrate hexahydrate as an addition to modulate the temperature, as illustrated in Fig. 3.
30
3
20
DSC, /(mW/mg)
Temperature, /℃
2
10
0
0
5
Mg(NO
)
3 2
10
·6H O,
2
/wt%
15
20
Fig. 3 Effect of Mg(NO3)·6H2O on phase change temperature
1
PCM-A
Tm = 22.8℃
Hm = 161.5 kJ/kg
Hs = 157.2 kJ/kg
0
Calcium
-1
chloride only
Tm = 28.8℃
-2
Hm = 193.4 kJ/kg
Hs = 193.0 kJ/kg
-3
0
10
20
30
Temperature, /℃
40
50
Fig. 4 Comparisons of DSC curves of PCM-A
and blank with calcium chloride only
In order to study the influence of phase change temperature of magnesium nitrate hexahydrate at different mass
fractions, five groups was adopted by the mass fraction of magnesium nitrate hexahydrate from 0 to 20%. From Fig. 3,
the result showed that the temperature of phase change of the composite PCM is sharply decreased with the increasing
of magnesium nitrate hexahydrate content. Without adding magnesium nitrate hexahydrate, the temperature of phase
change of the composite PCM is 28.7ºC, when adding 20% magnesium nitrate hexahydrate, the temperature of phase
change of the composite PCM is 7.9ºC, the melting point of the composite PCM decreases 20.8ºC. It also can be clearly
see while adding 15% magnesium nitrate hexahydrate, phase change temperature of the composite PCM is 14.3ºC with
14.4ºC decrease. As mentioned before, PCMs with phase change temperature range from 18 to 25ºC is suitable for
building energy-saving application. That’s to say, only when the mass fraction of magnesium nitrate hexahydrate is not
more than 10%. However, the more content of magnesium nitrate hexahydrate in the binary system, the less latent heat
of the composite PCM was found in our research. Comprehensive consideration, 5% magnesium nitrate hexahydrate
was chosen to adjust the temperature of phase change of calcium chloride solution. In such case, the optimized
composite 5% magnesium nitrate hexahydrate named PCM-A has a good performance properties and suitable phase
change temperature for building energy-saving.
3.1.2 DSC analysis of PCM-A
The heat of fusion and the melting point of PCM-A were determined using a differential scanning calorimeter calibrated
with an indium standard in the range from -30 to 120ºC, and the scanning rate was at 5ºC/min in a nitrogen atmosphere
from 0 to 40ºC. Fig. 4 shows the comparison of DSC curves of blank with calcium chloride only and PCM-A in the
process of both heating and cooling. It can be obtained that the melting temperature of PCM-A is 22.8ºC which are
decrease 6.0ºC after adding 5% magnesium nitrate hexahydrate, the absorption and release latent heats of PCM-A in the
process of fusing and crystallizing were 161.5 kJ/kg and 157.2 kJ/kg, respectively, which were decreased 31.9 kJ/kg
and 35.8 kJ/kg compared with the blank of calcium chloride only, respectively. In addition, more than 97% of the total
absorbed thermal energy of PCM-A in the melting process was released in the crystallizing process.
From the DSC thermal analysis it can be clearly indicated that magnesium nitrate hexahydrate has remarkable effect
on the phase-transition temperature of calcium chloride solution, and the value of the latent heat of PCM-A shows that
it can be used as a potential PCM for building energy-saving applications with respect to the climate requirement.
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3.1.3 Specific heat capacity of PCM-A
The purpose of the experiment is to measure the specific heat capacity of PCM-A by differential scanning calorimeter.
Samples to be tested are heated at temperature increasing linearly as consistent with a program control procedures, the
heat input of samples is measured continuously, which is equal to the heat input of the standard sample. The specific
heat capacity as a function of the heat input can be expressed as follows:
Cp =
dH
1
×
dT m sample
(1)
Where Cp is the specific heat capacity of the sample; dH and dT are the difference of the heat input and the
temperature between the sample and the standard, respectively; and msample is the mass of the sample tested [30].
Actually, it is difficult to detect absolute value of dH/dT accurately, thus the indirect method of measurement is
adopted. Specific heat capacity test of PCM-A was showed in Fig. 5. It shows that the specific heat capacity of PCM-A
was estimate as 3.5357 J/(g·K) in solid state and 2.7379 J/(g·K) in liquid state. However, the value of specific heat
capacity was significant increase in the process of phase change and get a maximum value of 23.8812 J/(g·K). That’s to
say, PCM-A is suitable for thermal energy storage at room temperature.
1.0
Cp, /(J/(g•K))
λ, /(W/(m•K))
20
0.9
15
10
0.8
5
0
10
20
30
Temperature, /℃
0.7
40
Fig. 5 Specific heat capacity test of PCM-A
10
20
Temperature, /℃
30
Fig. 6 Thermal conductivities of PCM-A
3.1.4 Thermal conductivity of PCM-A
Thermal conductivity of PCM is the important gauge of the rates of heat storage and release during crystallizing and
fusing process. Since PCM store large amounts of heat or cold in a small volume, the low thermal conductivity can be a
problem. In the liquid phase, convection can significantly enhance heat transfer, however often this is not sufficient. In
the solid phase, there is no convection. Low thermal conductivity not only reduced the rate of heat storage and
extraction during the melting and solidification recycles but also restricted their wide applications. In order to increase
thermal conductivity of PCM, some additions such as graphite powder and metal particles can be added [31]. In this
study, thermal conductivities of PCM-A were measured in the range of 10~35ºC by thermal conductivity analyzer.
Thermal conductivities of PCM-A at different temperatures was showed in Fig. 6. Experimental results illustrated that
thermal conductivity of PCM-A in the solid state was estimated as 0.9112 W·m-1·K-1 while in the liquid state was
approximate to 0.7519 W·m-1·K-1, but while in the process of phase change, thermal conductivity was significantly
increases to 1.0067 W·m-1·K-1. In other words, thermal conductivity of PCM-A was not low as a PCM for building
energy-saving application and suitable for heat storage at about 21ºC.
3.1.5 Reduction of supercooling degree of PCM-A
Supercooling is a serious problem associated with almost all inorganic PCMs. When environmental temperature reaches
to the theory freeze point, PCMs are still not crystallization, and it begins to crystallize until the temperature of
environment decreases below the freeze point. In order to overcome the problem, researchers have adopted a lot of
methods to reach a reasonable rate of nucleation. One promising solution is to add a nucleating agent to provide the
crystal nucleon. Another possibility is to keep some crystals in a small cold region to serve as nuclei. In addition,
mechanical stirring, impurity and ultrasonic vibrator can also decrease the supercooling degree [18].
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In our research, the supercooling degree of PCM-A was
found to be more than 20ºC. However, experimental results
indicated that strontium chloride hexahydrate was a very highefficient nucleating agent for PCM-A. Supercooling test through
temperature recorder with the biochemistry incubators in a
cooling process of constant temperature at 5ºC for PCM-A after
2% strontium chloride hexahydrate added as a nucleating agent
is presented in Fig. 7. It indicates that supercooling degree of
PCM-A in the cooling process was significantly reduced. Within
ten times recycle-used, the maximum degree of supercooling of
PCM-A is 1.8ºC and the average degree of supercooling is about
0.8ºC.
Supercooling degree, /℃
1.8
1.5
1.2
0.9
0.6
0.3
0.0
0
2
4
6
Recycle-used times
8
10
Fig. 7 Supercooling test of PCM-A added
SrCl2·6H2O as a nucleating agent
3.1.6 Thermal stability of PCM-A
Thermal stability parameters refer to thermodynamics properties of PCMs including phase change temperature, latent
heat, supercooling degree changes and so on before and after a series of repeated melting and crystallization recycles. In
this study, the test was performed consecutively up to 30 times thermal cycling using biochemistry incubators as the
melting and crystallization facility. According to experimental results, repeatability properties tests of PCM-A shows in
Fig. 8, and a comparison of DSC curves of PCM-A before and after 30 times recycle-used shows in Fig. 9. It can be see
that PCM-A can be keep good store thermal performance, the phase change temperature is varied between 23.5ºC and
22.2ºC, and the supercooling degree is in the range of 0 ~ 1.8ºC. Before recycle-used of PCM-A, the absorbed heat,
released heat, and melting point are 161.5 kJ/kg, 157.2 kJ/kg and 22.8ºC, respectively. And after 30th recycle-used of
PCM-A, they are 154.9 kJ/kg, 152.2 kJ/kg and 22.7ºC, respectively. Within thirty times recycling, the maximum
deviations of the phase change temperature and the latent heat of PCM-A are 5.6% and 4.1%, respectively.
30
3
20
15
Phase change temperature, /℃
10
Supercooling degree, /℃
DSC, /(mW/mg)
Temperature, /℃
25
5
0
-5
0
10
20
30
Recycle-used times
Fig. 8 Repeatability properties tests of PCM-A
2
Before :
Tm = 22.8℃
1
Hm = 161.5 kJ/kg
Hs = 157.2 kJ/kg
0
-1
After :
Tm = 22.7℃
-2
Hm = 154.9 kJ/kg
Hs = 152.2 kJ/kg
-3
0
10
20
Temperature, /℃
30
40
Fig. 9 Comparison of DSC curves of PCM-A before
and after 30 times recycle-used
Based on the experimental results of thermal properties analysis, the binary system of calcium chloride solution and
magnesium nitrate hexahydrate in the weight ratio of 19: 1 forms PCM-A with a good characters of a large enthalpy of
161.5 kJ/kg, a suitable phase change temperature of 22.6ºC, a high thermal conductivity of 1.0067 W·m-1·K-1 and a high
density of 1.4812×103 kg/m3. Although PCM-A has a serious problem of supercooling, a nucleating agent of 2%
strontium chloride hexahydrate plays an important role to decrease the supercooling degree within 1.8ºC. Therefore, the
low-cost and non-toxic advantage of PCM-A can be used as a room temperature PCM in building energy-saving.
3.2 Performance of middle temperature composite PCM (PCM-B)
In urban energy supply systems, how to effective utilization of waste heat from cogeneration systems, metallurgy and so
on has become an increasing factor for energy conservation, and economize investment in new urban infrastructure.
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One of the effective methods is to utilize the heat source of waste heat in passive solar energy house and civilian
industry products by heat storage systems. From this point of view, utilization of middle temperature PCMs as a media
applied in such heat storage systems will be a high-efficient method, and PCMs with the temperature of phase change in
the range of 40 to 50ºC should be more practical value.
3.2.1 Preparation of PCM-B
Since the target temperature range for the waste heat storage system to be handled is 40~50ºC, we sought a PCM with a
phase change temperature within this range by a binary mixture of magnesium nitrate hexahydrate as an based material
and ammonium nitrate as an addition. We were looking for the minimization of the factors such as possible toxicity and
cost. Fig. 10 shows the binary eutectic phase diagram of ammonium nitrate and magnesium nitrate hexahydrate.
According to Fig. 10, it can be see that the phase change temperature varies sharply drop down with the increasing of
the concentration of ammonium nitrate between 0 and 38.8% in mass, and increase up linearly with the increasing of
ammonium nitrate content between 38.8% and 100%. The lowest temperature is about 44.8ºC with the composition of
38.8% ammonium nitrate and 61.2% magnesium nitrate hexahydrate. In order to effectively use in the waste heat
storage systems, the melting point needs to be modulated to about 40~50ºC. During experimental process, we also
found that the binary mixture has no phase separation phenomenon and phase transition process rapidly in the eutectic
point. Hence, we modulated the melting point by eutectic means, and we mainly discuss the eutectic composite PCM
(PCM-B) which containing 38.8% ammonium nitrate and 61.2% magnesium nitrate hexahydrate in this work.
5
150
4
DSC, /(mW/mg)
Temperature, /℃
120
90
60
E=0.3880
Tm = 48.2℃
Hm = 154.8 kJ/kg
3
2
1
30
0
0
0
20
Mg(NO3)2·6H2O
40
60
Component, /wt%
80
100
NH4NO3
Fig. 10 Binary phase diagram of NH4NO3 and Mg(NO3)·6H2O
-1
30
40
50
60
Temperature, /℃
70
80
Fig. 11 DSC curves of PCM-B
3.2.2 DSC analysis of PCM-B
The DSC curve of PCM-B was measured by a differential scanning calorimeter at the scanning rate of 5ºC/min in a
nitrogen atmosphere from 30 to 80ºC as shown in Fig. 11. It shows that the melting temperature of PCM-B is 48.2ºC,
and the latent heat value of phase change is 154.8 kJ/kg. Compared with the physical parameters of single magnesium
nitrate hexahydrate [17], the melting point and the latent heat of phase change of PCM-B are lower than that of single
magnesium nitrate hexahydrate, and the reduction amplitude of PCM-B are 40.8oC and 8.0 kJ/kg, respectively. From
the DSC thermal analysis it can be clearly indicated that ammonium nitrate has remarkable effect on the phasetransition temperature of magnesium nitrate hexahydrate and slight influence on the value of the latent heat. The
suitable melting temperature and the latent heat of PCM-B also display that it is a potential PCM for waste heat storage
systems applications.
3.2.3 Specific heat capacity of PCM-B
The specific heat capacity of PCM-B was determined by the differential scanning calorimeter meter, the result shows in
Fig. 12. It can be obtained from Fig. 12 that the specific heat capacity of PCM-B is 3.0216 J/(g·K) in solid state and
3.5564 J/(g·K) in liquid state. However, the value of specific heat capacity has a significant increase in the process of
phase change and get a maximum value of 58.0544 J/(g·K). That’s to say, PCM-B has a big specific heat capacity for
thermal energy storage at middle temperature.
3.2.4 Thermal conductivity of PCM-B
By using the thermal conductivity apparatus, the thermal conductivity of PCM-B at different temperatures is presented
in Fig. 13. In our research, thermal conductivities of PCM-B were measured at the range of 35~65ºC by use a thermal
conductivity analyzer. Experimental results indicated that thermal conductivity of PCM-B in solid and liquid states is
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0.7594 W·m-1·K-1 and 0.6329 W·m-1·K-1, while in the process of phase change, thermal conductivity is significantly
increases to 1.0094 W·m-1·K-1. In other words, thermal conductivity of PCM-B is suitable for waste heat storage
systems at about 49ºC.
3.2.5 Reduction of supercooling degree of PCM-B
PCM-B is colorless and transparent crystal, and it has a melting point of 48.2ºC and latent heat values of 154.8 kJ/kg,
which may be a promising middle PCM used in thermal energy storage systems. But it has a serious disadvantage of
supercooling as a thermal energy storage material. Supercooling degree of PCM-B was more than 10ºC. However, after
a series of experimental studies, the mixture agent of 0.1% strontium hydroxide and 0.5% carbon was found to be the
high-efficient nucleating agent for PCM-B because the two materials have the similar crystal structure with magnesium
nitrate hexahydrate. Fig. 14 illustrates supercooling degree of PCM-B within ten times recycle-used after the mixture
nucleating agent added. It was found that supercooling degree of PCM-B in the cooling process is significantly reduced
in nature cooling. It can be seen the maximum degree of supercooling of PCM-B is 3.8ºC and the average degree of
supercooling is about 3.45ºC.
60
1.0
λ, /(W/(mK))
50
Cp, /(J/(g•K))
40
30
0.9
0.8
20
0.7
10
40
50
60
Temperature, /℃
0.6
70
Fig. 12 Specific heat capacity test of PCM-B
3.2.6
40
50
Temperature, /℃
60
Fig. 13 Thermal conductivities of PCM-B
Thermal stability of PCM-B
A fine composite PCM must be thermally stability. Therefore,
there should be no or little change in its thermal properties after
long-term utility period [32]. In order to measure the thermal
stability of PCM-B, a hundred recycle tests of heating and
cooling was conducted in nature environment. Repeatability
properties tests of PCM-B and comparison of DSC curves for
the PCM-B before and after 100 times recycle-used are
summarized in Figs. 15 and 16. It can be see that PCM-B can be
keep good store thermal performance, and the highest and
lowest temperatures of phase change are 44.8ºC and 42.5ºC with
supercooling degree within 0 ~ 4.0ºC. The absorbed heat and
melting point before and after 100 times recycles of PCM-B are
154.8 kJ/kg and 48.2ºC, 157.9 kJ/kg and 48.2ºC, respectively.
Within a hundred times repeated melting and crystallization
recycles, the phase change temperature, supercooling degree
were varied by 2.1%, 1.9% and 0.4%. Therefore, PCM-B
prepared in this study had a good thermal stability for latent heat
thermal energy storage applications.
©FORMATEX 2013
Supercooling degree, /℃
0
4
3
2
1
0
0
2
4
6
Recycle-used times
8
10
Fig. 14 Supercooling test of PCM-B added
mixed nucleating agent
39
60
5
50
4
40
30
Phase change temperature, /℃
20
Supercooling degree, /℃
DSC, /(mW/mg)
Temperature, /℃
Materials
and processes for energy: communicating current research and technological developments (A. Méndez-Vilas, Ed.)
____________________________________________________________________________________________________
3
2
After :
Tm = 48.4℃
Before :
Tm = 48.2℃
Hm = 157.9 kJ/kg
Hm = 154.8 kJ/kg
1
10
0
0
0
20
40
60
80
100
-1
30
40
Recycle-used times
Fig. 15 Repeatability properties tests of PCM-B
50
60
Temperature, /℃
70
80
Fig. 16 Comparison of DSC curves of PCM-B before
and after 100 times recycle-used
3.2.7 Density and expansion coefficient of PCM-B
The density of PCM will be change in the process of phase change, and for a certain quality of the samples, the density
is inversely proportional to the volume. That's to say, we can calculate the changes of the volume if the density of PCM
is known. The density at a specific temperature as a function can be expressed as follows:
ρs ,t = m2 ⋅ ρw,t / (m1 + m2 − m3 )
(2)
Where ρs,t and ρw,t are the densities of the sample and the doubly deionized water at t ºC; m1, m2, and m3 are the
weight of empty density bottle, the weight of the density bottle with the sample, and the weight of the density bottle
with the doubly deionized water, respectively. The density of PCM-B at different temperatures is listed in Table 3.
Table 3 The density of PCM-B at different temperatures.
Temperature, /ºC
solid
liquid
Measurement data, /(103kg/m3)
1
1.595
1.515
2
1.596
1.515
3
1.596
1.516
Average density,
/(103kg/m3)
1.596
1.515
According to Table 3, the density of PCM-B was 1.596×103 kg/m3 in the solid state and 1.515×103 kg/m3 in the
liquid state. The volume of PCM-B increased 5.35% from solid to liquid and decreased 5.08% from liquid to solid. It
can be concluded that PCM-B as a potential PCM meets the demand of heavy density and small expansion coefficient.
Since then, a new inorganic PCM-B combined with magnesium nitrate hexahydrate and ammonium nitrate is
proposed. Thermal properties of PCM-B can be conducted that the weight ration of magnesium nitrate hexahydrate and
ammonium nitrate in 61.2: 38.8 forms the optimum middle composite. It has characters of large heat enthalpy, a
suitable phase change temperature, a high thermal conductivity and a high density. In addition, low cost is also the
advantage of PCM-B, there is no legal restriction with using these substances as a potential PCM for latent heat thermal
energy storage. Hence, PCM-B is an ideal PCM for waste heat energy storage systems.
3.3 Performance of middle-high temperature composite PCM (PCM-C)
As many energy sources are intermittent in nature, and solar energy is one of the most promising renewable energy
sources. In order to meet the life demand of people, more and more buildings were equipped with solar energy water
heaters. Solar energy water heater convert the sun light energy into heat energy, and then water from low temperature
heating to high temperature for application. But solar radiation is only available in sunny day, and its application require
a high-efficient thermal energy storage system so that the excess heat energy can be collected in sunshine hours and
released when solar radiation is not available. Hence, PCMs with the temperature of phase change in the range of 6080ºC would be very practical in the application of solar energy water heater.
3.3.1 Preparation of PCM-C
By taking into consideration of the above predominant characteristics of salt hydrate as PCM, a large amount of their
binary eutectics may be tailored. Amongst the studied crystalline hydrate salt, the magnesium nitrate hexahydrate is an
ideal phase change material for heat storage in solar space and water heating systems, latent heat of fusion and thermal
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Materials
and processes for energy: communicating current research and technological developments (A. Méndez-Vilas, Ed.)
____________________________________________________________________________________________________
performance. However, the melting temperature of 89ºC is high for hot water requirements under the climate conditions
of some residential and agricultural regions. Therefore, the melting temperature of magnesium nitrate hexahydrate must
be decrease for the applications of solar energy water heater. And after a series experimental, lithium nitrate as an
addition to decrease the melting temperature of magnesium nitrate hexahydrate to 72.1ºC was found. Fig. 17 shows the
binary eutectic phase diagram of lithium nitrate and magnesium nitrate hexahydrate.
In Fig. 17, the phase change temperature drops down sharply with the content of lithium nitrate within 0~14%, and
the lowest eutectic temperature is 72.1ºC with the composition of 14% lithium nitrate and 86% magnesium nitrate
hexahydrate. Therefore, the composition at lowest eutectic is a suitable material for solar thermal energy storage with
respect to solar energy water heater systems. Hence, we named the eutectic composite material as PCM-C to discuss.
3.3.2 DSC analysis of PCM-C
The latent heat and melting temperature of the eutectic mixture of magnesium nitrate hexahydrate and lithium nitrate
determined by DSC analysis are presented in Fig. 18. It can be concluded that the phase change latent heat and melting
point of PCM-C are 167 kJ/kg and 72.1ºC. Compared with the physical parameters of single magnesium nitrate
hexahydrate [17], the melting point of PCM-C is lower than that of single magnesium nitrate hexahydrate while the
latent heat of phase change is slightly increased. From the DSC thermal analysis it can be clearly indicated that lithium
nitrate has significant influence on the phase transition temperature of magnesium nitrate hexahydrate and small effect
on the value of the latent heat. The suitable melting temperature and the latent heat of PCM-C also reveal that it can be
used as a potential PCM for solar energy water heater systems applications.
3.3.3 Specific heat capacity of PCM-C
Specific heat capacity of PCM-C within 50-100ºC was measured by using comparison method is illustrated in Fig. 19.
Experimental result shows that specific heat capacity of PCM-C in solid and liquid states is about 3.0329 J/(g·K) and
2.8307 J/(g·K), and the specific heat capacity value is significantly increased in the process of phase change with
maximum value of 60.6203 J/(g·K). That’s to say PCM-C has a large specific heat capacity for heat storage at middlehigh temperature.
240
3
DSC, /(mW/mg)
Temperature, /℃
180
120
60
Tm = 72.1℃
Hm = 167 kJ/kg
2
1
E=0.14
0
0
0
20
Mg(NO3)2·6H2O
40
60
Component, /wt%
80
100
LiNO3
50
Fig. 17 Binary phase diagram of LiNO3 and Mg(NO3)·6H2O
60
70
80
Temperature, /℃
90
100
Fig. 18 DSC curves of PCM-C
2.1
λ, /(W/(mK))
50
Cp, /(J/(g•K))
40
30
1.8
1.5
20
10
1.2
0
50
60
70
80
Temperature, /℃
90
100
Fig. 19 Specific heat capacity test of PCM-C
50
60
70
Temperature, /℃
80
Fig. 20 Thermal conductivities of PCM-C
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Materials
and processes for energy: communicating current research and technological developments (A. Méndez-Vilas, Ed.)
____________________________________________________________________________________________________
3.3.4 Thermal conductivity of PCM-C
The thermal conductivity of PCM-C was measured by a thermal conductivity apparatus by using the transient heat
probe method. The thermal conductivity data of PCM-C is determined within 50-85ºC as in Fig. 20. Experimental
results indicated that thermal conductivity of PCM-C in solid and liquid states was estimated as 1.3206 W·m-1·K-1 and
1.1062 W·m-1·K-1, and thermal conductivity at phase change point was significantly increases to 2.0857 W·m-1·K-1. In
other words, thermal conductivity of PCM-C was not low as a PCM for solar energy water heater systems application,
and suitable for heat storage at about 70ºC.
3.3.5 Thermal stability of PCM-C
The thermal stability of PCM-C was evaluated by using temperature recorder in nature environment. Repeatability
properties tests of PCM-C and comparison of DSC curves for the PCM-C after 0 and 100 times recycle-used are given
in Fig. 21 and Fig. 22, respectively. It could be found from the above figures that the datum of PCM-C before recycleused of initial crystallization temperatures, finial crystallization temperature and enthalpy of melting are 73.4ºC, 63.7ºC
and 167 kJ/kg, respectively. And after 100th recycle-used the datum of initial crystallization temperatures, finial
crystallization temperature and enthalpy of melting are 71.6ºC, 63.1ºC and 164.2 kJ/kg, respectively. Within a hundred
times repeatedly melting and crystallization recycles, initial crystallization temperatures and finial crystallization
temperature changed by 2.7% and 0.9% for the freezing process, enthalpy of melting changed by 1.7% for the melting
process. The results intensely indicate that PCM-C can be keep good store thermal performance for latent heat thermal
energy storage applications.
4
3
DSC, /(mW/mg)
Temperature, /℃
80
70
Initial crystallization temperature, /℃
2
Before :
Tm = 72.1℃
After :
Tm = 71.6℃
Hm = 167 kJ/kg
Hm = 164.2 kJ/kg
1
Finial crystallization temperature, /℃
60
0
0
20
40
60
80
100
50
60
Recycle-used times
Fig. 21 Repeatability properties tests of PCM-C
70
80
Temperature, /℃
90
100
Fig. 22 Comparison of DSC curves of PCM-C before
and after 100 times recycle-used
3.3.6 Density and expansion coefficient of PCM-C
The density of PCM-C at different temperatures is listed in Table 4. Table 4 shows that the density of PCM-C was
1.610×103 kg/m3 in the solid state and 1.590×103 kg/m3 in the liquid state. The volume of PCM-B increased 1.26%
from solid to liquid and decreased 1.24% from liquid to solid.
Table 4 The density of PCM-C at different temperatures.
Temperature, /ºC
solid
liquid
Measurement data, /(103kg/m3)
1
1.610
1.590
2
1.609
1.590
3
1.610
1.590
Average density,
/(103kg/m3)
1.610
1.590
Based on the experimental results of thermal properties analysis, it can be conducted that the binary system of
magnesium nitrate hexahydrate and lithium nitrate in the weight ration of 86: 14 forms the optimum composite PCM-C
without supercooling, a suitable phase change temperature, a high thermal conductivity and a heavy density. In
addition, low cost and no legal restriction with using these substances as a potential PCM for latent heat thermal energy
storage is also the advantage of PCM-C. Hence, PCM-C is an ideal PCM for the application of solar energy water
heater.
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Materials
and processes for energy: communicating current research and technological developments (A. Méndez-Vilas, Ed.)
____________________________________________________________________________________________________
4. Conclusion
In this chapter, the basic principle and progresses on the available thermal energy storage technology and requirements
of PCMs were summarized briefly. Then, three novel inorganic eutectic materials named as PCM-A, PCM-B and PCMC using magnesium nitrate hexahydrate as basic materials of PCMs were reported. Their preparations, thermodynamics
and physicochemical properties of the three solid-liquid PCMs were determined in the first time. PCM-A as a room
temperature PCM has a suitable phase change temperature of 22.6ºC and a large enthalpy of 161.5 kJ/kg for building
energy-saving, PCM-B as a potential middle temperature PCM with a phase change temperature of 44.8ºC and latent
heat values of about 155 kJ/kg for waste heat storage systems, PCM-C as a middle-high temperature with a phase
change temperature of 72.1ºC and latent heat values greater than 165 kJ/kg for solar water heaters application.
Compared with sensible storage materials, PCMs has a lot of advantages such as high volumetric storage density,
high thermal conductivity and the energy is stored at a relatively constant temperature and energy losses to surroundings
are lower than with conventional systems. We believe that further research should obtain from the following several
aspects: (i) Further select full aspects of environmental friendly and low-cost PCMs such as hydrated salts, fatty acid
and its derivatives. Study on binary and multi-component eutectic mixtures to make use of the advantage of multicomponent PCMs. (ii) According to different environment conditions and purposes, prepared PCMs with appropriate
phase change temperature, heat enthalpy, structural strength, physical and chemical properties stable in the process of
long-term use. (iii) Improved the coefficient of thermal conductivity and increasing heat transfer rate. Improve the
performance of heat conducting properties by adding modifier of graphite, SiO2, metal powder and so on. (iv) Research
mechanical properties and durability of PCMs, develop numerical simulation software, increase the circulation of the
heat storage experiment, provided a basis prediction.
Acknowledgment Financial support from the State Surface Project of National Natural Science of China (Grant 21276194), the
Specialized Research Fund for the Doctoral Program of Chinese Higher Education (Grant 20101208110003), and the Key Pillar
Program of Tianjin Municipal Science and Technology (Grant 11ZCKGX02800) is gratefully acknowledged. Authors also hope to
appreciate Drs Y.F, Guo, S.Q. Wang and D.J. Yan for their active help and a part of research works in those projects.
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