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.

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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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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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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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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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