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Thermophysical properties and thermal characteristics of phase change emulsion for thermal energy storage media
Energy xxx (2016) 1e7
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Thermophysical properties and thermal characteristics of phase change emulsion for thermal energy storage media Tsuyoshi Kawanami a, *, Kenichi Togashi b, Koji Fumoto c, Shigeki Hirano d, Peng Zhang e, Katsuaki Shirai a, Shigeki Hirasawa a a
Department of Mechanical Engineering, Kobe University, 1-1 Rokkodai-cho, Nada-ku, Kobe, 657-8501, Japan Department of Mechanical Engineering, Aoyama Gakuin University, 5-10-1, Fuchinobe, Chuo-ku, Sagamihara, 252-5258, Japan Department of Intelligent Machines and System Engineering, Hirosaki University, 3 Bunkyo-cho, Hirosaki, 036-8561, Japan d Industrial Research Institute, Hokkaido Research Organization, N19-W11, Kita-ku, Sapporo, 060-0819, Japan e School of Mechanical Engineering, Shanghai Jiao Tong University, 800 Dong Chuan Rd., Shanghai 200240, PR China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 October 2015 Received in revised form 10 February 2016 Accepted 6 April 2016 Available online xxx
A great deal of attention has been paid to energy saving devices in place of conventional air-cooled and water-cooled devices. The thermal energy storage system that uses the latent heat of a PCM (phase change material) for air-conditioning or heating has recently become popular because it does not require high electric power and it saves energy. An emulsion dispersed nano-size particles of phase change material is produced. We discuss with the thermophysical properties, the stability of emulsion, and the heat transport characteristics as a thermal functional fluid. The testing emulsion, which has nano-size particles as the discrete phase, is produced with a D-phase emulsification method. The diameter of discrete phase in the emulsion is measured for evaluation of the long-term stability of emulsion. In addition, the DSC (differential scanning calorimetry) curve of emulsion is determined. Thermophysical properties such as viscosity and thermal conductivity of emulsions were studied in this work, and was compared with that of the base fluid. The results reveal that the emulsion with the D-phase emulsification method has the superior stability. From the differential thermal analysis, the DSC curve of present emulsion indicates a discontinuous change at the phase change temperature of phase change material due to its latent heat. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Thermal storage Emulsion Phase change material Thermophysical property
1. Introduction A dispersed system which contains phase change materials for the dispersed phase attracts increasing attentions as one kind of thermal storage media [1e4]. A microcapsule system [5e8] has a reputation as a thermofunctional fluid. However, the microcapsule fluid has a bunch of very real problems such as a lack of long-term stability and durability for the practical usage of thermal storage devices. An emulsion is a type of dispersed system [9]. In the emulsion, one of two liquids which will not mix with each other is dispersed as particles (the dispersed phase) in the other (the continuous phase). Especially, the emulsion which contains nano-size phase change materials for the dispersed phase has great advantages
* Corresponding author. E-mail address: [email protected] (T. Kawanami).
against other type of phase change materials. For instance, the nanoemulsion indicates a low viscosity, a high fluidity, and a longterm stability comparison with the microemulsion [10]. Furthermore, its heat transport characteristics as a thermofunctional fluid is superior to a single phase fluid system [11e15]. The aim of our research group is to propose more sophisticated latent heat storage technologies and new thermofunctional fluids. As part of that effort, we have replaced the dispersed phase in emulsions with alkane-based phase change materials, and by reducing the size of those materials to nano-size, we have developed phase change emulsions and also examined the thermal properties and stability of these emulsions [16]. On the other hand, a diverse variety of techniques for preparing emulsions have been proposed in the field of cosmetics science, but these techniques have not been described based on their long-term stability and convenience from the perspective of use as thermofunctional fluids, and that research amounts to nothing more than selecting preparation techniques which have empirically been
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Nomenclature
Abbreviations TD tetradecane HD hexadecane OD octadecane Roman letter symbols n empirical shape factor T temperature, C Greek letters b sphericity h viscosity, Pa s f volume fraction l thermal conductivity, W m1 K1 j mass fraction Subscripts e emulsion in inlet out outlet p dispersed particle in emulsion w water
successful in many cases. There, in this research, an emulsion was prepared using the D-phase emulsification method, which is easy and requires little energy compared to the phase inversion emulsification method which has previously been the most common technique for emulsion preparation [17e19]. Then the stability of that emulsion was evaluated, and its thermal properties were measured. Heat storage and radiation characteristics were also experimentally examined.
2. Phase change emulsion Generally speaking, emulsion preparation methods can be divided into two types: mechanical techniques and surface chemistry techniques. With mechanical techniques, the dispersed particles are reduced to microparticles using a homogenizer with high shearing force or high pressure, and this technique is suitable for large-scale mass production. With surface chemistry techniques, on the other hand, the HLB value (hydrophilicelipophilic balance) of the emulsifier (surfactant) is adjusted, and emulsification is achieved by obtaining the D-phase, which is a surfactant associate, through adjustment of the temperature or adding an additive. This method does not require large-scale equipment, and energy consumption for producing the emulsion can be reduced. In this research, the emulsion was prepared using a surface chemistry technique called the D-phase emulsification method, and the particle size distribution and characteristics of the obtained emulsion were examined. Also, the emulsion which was the subject of this research was the O/W type in which the continuous phase is water. The D-phase emulsification method is a technique developed by Sagitani et al. [20]. With this method, an emulsion is produced by adding water-soluble polyalcohol to a nonionic surfactant, oil and water system, adjusting the HLB value, and obtaining a D-phase and O/D gel. The details of the preparation procedure are given in the references [10], but since energy consumed in emulsion production can be reduced because the method requires no heating or cooling
as with the PIT method, and no great mechanical force for agitation, it was thought to be suitable for the application which was the subject of this research. Furthermore, the method can use a watersoluble surfactant with a comparatively wide range of HLB values, and thus the impact on the environment is low and the amount which can be prepared at one time is larger than with the PIT method. Fig. 1 shows an overview of the method of emulsion production. For the detailed procedure of D-phase emulsification and information on characteristics of the produced emulsion, please refer our previous study [16]. The D-phase emulsification needs the following steps: preparing surfactant-polyalcohol water solution with surfactant; formation of clear gel emulsion; formation of oilin-water emulsion. For process of making this O/W emulsion, the addition of polyalcohol water to the usual emulsion components (oil, water, and surfactant) is necessary since the addition of polyalcohol water changes the liquid crystalline phase to the surfactant isotropic solution. Fig. 2 shows the phase diagram of emulsification process of the D-phase emulsification method. The mixture rates are indicated by taking each vertex as 100mass%, and the opposing side as 0mass%. For example, a solution at point A in the diagram indicates a blend of a mixed liquid of polyalcohol water (50mass%) and surfactant (50mass%). Moving along the arrow from point (a) to point (b) means that oil is added until it accounts for approximately 80mass% of the mixed liquid. In the D-phase emulsification method, the first step is formation of an O/D gel emulsion by dispersing an oil phase (photo (b) in Fig. 3) into a solution containing water, polyalcohol and surfactant (point A). This is then made into an O/W solution by diluting the gel emulsion with water (point C). Photographs of the appearance at points (a), (b) and (c) in Fig. 2 are shown, respectively, in Fig. 3 (a), (b) and (c). For preparing an emulsion using the D-phase emulsification method in this research, polyoxyethylene sorbitan monooleate (HLB value 15.0) was selected as the nonionic surfactant. This surfactant is highly hydrophilic, and thus the cloud point which indicates hydrophobicity as the temperature rises is not near the ordinary use temperature and it is thought that the emulsion can be kept in a stable condition even through repeated cycles of heating and cooling. Regarding the substances used as emulsion materials in this research, purified water was used as the continuous phase. Polyoxyethylene sorbitan monooleate (C64H124O26) was used as the surfactant, and 1,3-Butane diol (C4H10O2) and ethylene glycol (C2H6O2) were adopted as the polyalcohol. In addition, tetradecane (C14H30, melting point 5.9 C), hexadecane (C16H34, melting point 18.2 C), and octadecane (C18H38, melting point 27.0 C) as the dispersed phase which are the phase change materials. In this experiment, an emulsion blended
Polyol Water
+
O/D emulsion Clear gel
Non-ionic surfactant
Isotropic solution
Oil phase (with stirring, dropwise)
Water phase (with stirring) O/W emulsion Fig. 1. Process for production of emulsion by D-phase emulsification method.
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Water Surfactant
50
0
Table 2 Composition of testing emulsions.
(c)
100
D (a)
3
50
Emulsions
Mass fraction of PCM alkanes, j ()
TD10 e EG: Tetradecane base TD40 e EG: Tetradecane base HD10 e 1e3,BD: Hexadecane base OD10 e EG: Tetradecane base
10 40 10 0.1e20
D+O (b)
100 0
100
50
Diol water
0
Liquid paraffin
Gel
Fig. 2. Phase diagram for production of emulsion by D-phase emulsification method.
This section describes the results of those measurements. A laser diffraction particle size analyzer (Shimadzu, SALD-3000) was used to measure particle size. Fig. 4 shows the particle size distribution of the hexadecanebased emulsion immediately after preparation. The horizontal
Fig. 3. Photos of emulsion process: (a) polyalcohol water; (b) O/D phase emulsion; (c) diluted with water.
produced by in the ratios indicates the and Table 2
3. Physical properties of emulsion Thermophysical properties of slurry systems and microcapsule fluids have been studied widely [21e23]. In this section, important properties of nanoemulsions with PCM as thermofunctional fluids are evaluated. 3.1. Stability There is a correlation between emulsion particle size and emulsion stability, and in general the smaller the particle size, the higher the stability of the emulsion. Therefore, it is crucial to ascertain the particle size of the prepared emulsion. In order to determine the size of the hexadecane particles dispersed in the emulsion, and their changes over time, the particle size and particle size distribution of the prepared emulsion were measured from immediately after the emulsion was finished until 65 days later. Table 1 Composition of polyalcohol water with surfactant (mass ratio of surfactant: polyalcohol: water). Surfactants
Ethylene glycol
1,3-Butane diol
POE(20) sorbitan monostearate [10] POE(10) oleyl ether [10] POE(20) sorbitan monooleate
4:3:1
4:2:2
4:2.8:1.2 4:3:1
4:2.4:1.6 4:2:2
axis of the graph indicates particle size, and the vertical axis indicates the number of particles of a certain size as a percentage of the total number of particles. Immediately after preparing the emulsion, the minimum particle size was 0.333 mm, maximum particle size 0.604 mm, and median diameter 0.422 mm. On the other hand, Fig. 5 shows changes in the most common particle size over the 65-day interval. It can be confirmed from the figure that a highly stable emulsion was prepared, with almost no evident changes in particle diameter even after 65 days had elapsed. 3.2. Viscosity In this research, a rotating viscometer (Brookfield, DV-II Pro) was used to measure the viscosity of the prepared hexadecanebased emulsion with the uncertainty of ±1.02%. In the
30 Repetition rate (%)
with the liquid PCM (phase change material) was mixing purified water, surfactant and polyalcohol given in the table to yield a total of 100 g. Table 1 composition of polyalcohol water with surfactant, shows the composition of testing emulsions.
25 20 15 10 5 0 0.1
1 Diameter [ m]
Fig. 4. Repetition rate of diameter of dispersed phase in emulsion.
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0.45 0.4
(-)
1
0.9
e/ w
Diameter ( m)
0.5
0.8
H-C model by Eq.(1)
0.35 0.7
0.3
0
10
20
30
40
50
60
0
70
Elapsed days (days)
5 10 15 20 Concentration of octadecane (mass%)
Fig. 7. Relationship between thermal conductivity and concentration of emulsion. Fig. 5. Stability of emulsion.
experiment, temperature control was carried out by connecting the rotating viscometer with a thermostatic chamber, and the emulsion viscosity was measured from 25 C to 0 C. In addition, the rotation speed of the viscometer was set to 200 rpm. Fig. 6 shows the relationship between temperature and viscosity h. From the figure, it was found that viscosity of the prepared emulsion is 2.2 times that of water at 10 C, and 2.1 times that of water at 20 C.
3.3. Thermal conductivity Thermal conductivity tester (Decagon, KD2 Pro) was used to measure thermal conductivity. During measurement, the sample temperature was kept constant using a thermostatic bath. Measurement was repeated 5 times at the same temperature, and then the average value was found. The uncertainty of each measurement is estimated as ±5.0%. Fig. 7 shows the relationship between the dispersed particle concentration and thermal conductivity ratio for the octadecanebased emulsions. The test liquid temperature was set at 20 C. The thermal conductivity ratio is the value obtained by dividing the thermal conductivity of the sample emulsion le by the thermal conductivity of the base liquid of water lw. The solid line indicates the values obtained using the HamiltonCrosser model [24].
le lp þ ðn 1Þlw ðn 1Þf lw lp ¼ lw lp þ ðn 1Þlw þ f lw lp
(1)
n¼
3
(2)
b
Here, f indicates the volume fraction. The measured value was used as the thermal conductivity of water lw and the literature value [25] was used as the thermal conductivity of the dispersed particles lp. b and n are the sphericity and the empirical shape factor, respectively. In this research, b ¼ 1 based on the assumption that the dispersed particles have a perfectly spherical shape in the emulsion. From Fig. 7 that the thermal conductivity ratio of the emulsion decreases monotonically as the dispersed particle concentration rises. This is attributable to the fact that thermal conductivity of octadecane (the dispersed particle) is lower than thermal conductivity of water (the dispersion medium). In addition, if the measurements of the emulsion are compared against theoretical values, they are found to match over the entire range within ±4%. Fig. 8 shows the relationship between temperature and thermal conductivity of the octadecane-based emulsion. The sample concentration is 10mass% and 20mass%. Measurement was conducted while continuously raising the sample temperature from 10 C to 40 C. The solid line in the figure indicates theoretical values obtained from the HeC model. The graph shows that thermal conductivity of the emulsion increases with rising temperature. On the other hand, a large drop in thermal conductivity was seen near 27 C regardless of the emulsion concentration. This factor appears because, as the temperature rises, the dispersed nano-particle (octadecane) changes from a solid phase with high thermal conductivity to a liquid phase with low thermal conductivity. Also, when the measurements and theoretical values were compared, with a 10mass% emulsion a large difference was seen at and above
5
0.75 (W m-1 K-1)
4 3 2
e
(mPa s)
0.8
Emulsion Water (reference value)
1
0.7 0.65 0.6
10mass% Measurement H-C model by Eq.(1) 20mass% Measurement H-C model by Eq.(1)
Melting point of octadecane
0.55 0.5 0.45
0
0
5
10
15
Temperature Fig. 6. Viscosity of emulsion.
20
25
0.4
10
15
20
25
30
35
40
Temperature Fig. 8. Temperature dependence of thermal conductivity.
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the melting point of octadecane, compared with a good match below the melting point. Due to the above results from Figs. 6 and 7, the HeC model was shown to be valid for predicting thermal conductivity of the emulsion within the range of dispersed particle concentrations in this research. 3.4. Thermal characteristics The melting point of the hexadecane dispersed in the prepared emulsion is 18.2 C. On the other hand, it is known that liquid phase change materials dispersed in an emulsion exhibit lower crystallization temperature with smaller particles sizes [26e30], and to enable industrial use, it is necessary to know the solidifying point of the hexadecane dispersed in the emulsion. In this research, differential scanning calorimetry (DSC; Rigaku, Thermo plus EVO2) was used to measure the melting point and solidifying point of the prepared emulsion. Fig. 9 shows the DSC measurement results and their relationship with heat flow and temperature. The vertical axis of the figure indicates heat flow, and the horizontal axis temperature. The line at the top of the figure is the DSC curve during cooling process, and the line at the bottom is the DSC curve during heating process. The rate of temperature rise was set to 2 K/min. From the graph it is observed that, in the cooling process, there is a peak accompanying latent heat absorption near 1 C. Since latent heat is released as the sample material solidifies, this peak is thought to be due to the solidification heat of the hexadecane contained in the emulsion. In addition, a peak is evident at approximately 18 C in measurement of the heating process, and it is inferred that release of latent heat occurred here. Due to the above results, it was found that the degree of supercooling of the hexadecane dispersed in the emulsion prepared in this research was approximately 17 K. These results are almost the same as the results of Eric Dickerson et al. [31]. In addition to evaluate the phase change temperature, we consider the amount of latent heat of tetradecane-based emulsion using DSC. The results are indicated in Table 3. It is found that the latent heat is measured to be 19.3 kJ/kg for 10mass emulsion and 73.3 kJ/kg for 40mass% one at 5.9 C. 3.5. Heat transport characteristics In order to ascertain the heat exchange characteristics of the phase change emulsion in this research, a convection heat dissipation experiment was carried out using a parallel flow type double-pipe heat exchanger.
Heat flow (mW)
10 Cooling
5 0 -5 Heating
-10 -15 -10
-5
0
5
10
15
20
25
Temperature Fig. 9. Heat flow measurement of emulsion with DSC.
30
5
Table 3 Amount of latent heat of emulsion. Emulsions
Amount of latent heat (kJ/kg of emulsion)
Pure tetradecane TD10 TD40
229 19.3 (8.4% of tetradecene) 73.3 (32.0% of tetradecene)
Figs. 10 and 11 show a schematic drawing and a setup of the experimental apparatus for the heat exchange test, respectively. The apparatus is comprised of a cooling liquid circulation system and emulsion circulation system. The cooling liquid circulation system is comprised of a cooling water pipe, low-temperature thermostatic chamber, testing section, and temperature measurement system. The emulsion circulation system is comprised of an emulsion storage tank, emulsion transport pipe, pump, flow control valve, test section, and temperature measurement system. The test section as a flow channel is a double-pipe heat exchanger having 2 m length, and it has a structure in which cooling liquid and emulsion flow in the same direction. In addition, thermocouples are provided at the inlet and outlet of each unit, so that inlet and outlet temperature can be measured for each channel. Fig. 12 depicts the cross-sectional view of the flow channel. The outer tube is made of transparent acrylic. Its inner and outer diameter are 15 mm and 25 mm, respectively. On the other hand, a copper tube having 12 mm outer diameter is adopted as the inner flow tube. A thermal insulator is installed along the outer circumference of the outer tube. Table 4 shows the experiment conditions. In this experiment, an aqueous solution of propylene glycol with adjusted concentration (approximately 33 mass%) was used as the cooling liquid. In all experiment conditions, the temperature difference was set to 25 C at the test section inlet for the sample liquid and cooling liquid. This was done in order to set the difference in temperature of each sample fluid between the test section inlet and outlet to approximately 10 C, and ensure an adequate temperature difference between the inlet and outlet. For example, in experiment Condition 1, the emulsion inlet temperature was 30 C, and the outlet temperature was approximately 20 C. In order to ascertain the effects on heat exchange duty due to the phase change of hexadecane dispersed in the emulsion, experiments were conducted under a condition (Condition 1) not straddling the 18.2 C phase change temperature of hexadecane, and under conditions (Condition 2 and 3) straddling the phase change temperature of hexadecane where there is a possibility of a phase change by hexadecane. Also, it was possible to estimate through a preliminary experiment that heat loss when the cooling liquid inlet temperature is 5 C is 5% or less of the quantity of heat obtained by the cooling liquid. Similarly, heat loss at 5 C was estimated to be 8% or less, and at 10 C to be 10% or less. Fig. 13 shows the emulsion temperature at the outlet of the test section. The horizontal axis of the figure indicates the numbers of the experiment conditions given in Table 1, and the vertical axis indicates the water temperature at the test section outlet. It can be seen from the graph that the outlet temperature is approximately 18 C in Condition 1, approximately 10 C in Condition 2, and approximately 5 C in Condition 3. On the other hand, Fig. 14 shows the emulsion temperature differences between the test section inlet and outlet. The horizontal axis in the figure indicates the experiment condition number, and the vertical axis indicates the emulsion temperature difference TinTout between the test section inlet and outlet. It is evident from the figure that the temperature difference is approximately 11 K for Condition 1, approximately 10 K for
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Table 4 Experimental condition. Inlet temperature of emulsion, Tin ( C)
Inlet temperature of coolant ( C)
15
1 2 3
30 20 15
5 5 10
10
Tout
Condition no.
5 Coolant flow
P
0
Thermostatic bath
Tin
Test section
2
3
Condition No. Fig. 13. Outlet temperature of emulsion as a function of experimental condition.
2000mm
15
Emulsion flow
P
1
Tout
Thermocouples
10
Fig. 10. Schematic drawing of experimental apparatus.
Tin - Tout
Emulsion tank
5
0
1
2
3
Condition No. Fig. 14. Temperature difference between Tin and Tout of emulsion.
4. Conclusions
Fig. 11. Photo of experimental setup.
Condition 2, and approximately 8 K for Condition 3. These results show that the temperature difference is smaller to the extent that a condition has a lower set temperature. As is evident from the value for the drop in solidification temperature shown in Fig. 9, it is likely that the actual solidifying point of hexadecane is about 1 C, and this is believed to be due to the fact that the effects of latent heat absorption were seen in the experiment for Condition 3 where that temperature was passed through.
In order to ascertain the thermophysical properties and heat transport characteristics of a phase change emulsion, the particle size distribution, phase change point, viscosity, and thermal conductivity were measured for n-alkanes contained in the phase change emulsion prepared in this experiment. It is found that the particle diameter in the emulsion prepared using the D-phase emulsification method was in the range 0.333e0.604 mm. When the viscosity of emulsion prepared in the research was compared with the viscosity of water, it exhibited a value roughly twice as high. In addition, through an experiment using a DSC to measure the degree of supercooling, it was found that the degree of supercooling of hexadecane-based emulsion was 17 K. The latent heat of tetradecane-based emulsion is indicated to be 19.3 kJ/kg for 10mass emulsion and 73.3 kJ/kg for 40mass% one at 5.9 C in this research. As regard the thermal conductivity of octadecane-based emulsion, a large change in thermal conductivity of emulsion was evident accompanying the phase change near the melting point of the dispersed particles. Furthermore, the heat transport characteristics of the phase change emulsion were experimentally ascertained using a parallel flow double-pipe heat exchanger. From the experiment, it is thought that, in the low-temperature condition, there was a phase change of the hexadecane phase change material contained in the emulsion, and an accompanying release of latent heat. Acknowledgments
Fig. 12. Cross-sectional view of the flow channel.
This work was partially supported by the Strategic Basic Research Programs (Advanced Low Carbon Technology Research
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Energy journal homepage: www.elsevier.com/locate/energy
Thermophysical properties and thermal characteristics of phase change emulsion for thermal energy storage media Tsuyoshi Kawanami a, *, Kenichi Togashi b, Koji Fumoto c, Shigeki Hirano d, Peng Zhang e, Katsuaki Shirai a, Shigeki Hirasawa a a
Department of Mechanical Engineering, Kobe University, 1-1 Rokkodai-cho, Nada-ku, Kobe, 657-8501, Japan Department of Mechanical Engineering, Aoyama Gakuin University, 5-10-1, Fuchinobe, Chuo-ku, Sagamihara, 252-5258, Japan Department of Intelligent Machines and System Engineering, Hirosaki University, 3 Bunkyo-cho, Hirosaki, 036-8561, Japan d Industrial Research Institute, Hokkaido Research Organization, N19-W11, Kita-ku, Sapporo, 060-0819, Japan e School of Mechanical Engineering, Shanghai Jiao Tong University, 800 Dong Chuan Rd., Shanghai 200240, PR China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 October 2015 Received in revised form 10 February 2016 Accepted 6 April 2016 Available online xxx
A great deal of attention has been paid to energy saving devices in place of conventional air-cooled and water-cooled devices. The thermal energy storage system that uses the latent heat of a PCM (phase change material) for air-conditioning or heating has recently become popular because it does not require high electric power and it saves energy. An emulsion dispersed nano-size particles of phase change material is produced. We discuss with the thermophysical properties, the stability of emulsion, and the heat transport characteristics as a thermal functional fluid. The testing emulsion, which has nano-size particles as the discrete phase, is produced with a D-phase emulsification method. The diameter of discrete phase in the emulsion is measured for evaluation of the long-term stability of emulsion. In addition, the DSC (differential scanning calorimetry) curve of emulsion is determined. Thermophysical properties such as viscosity and thermal conductivity of emulsions were studied in this work, and was compared with that of the base fluid. The results reveal that the emulsion with the D-phase emulsification method has the superior stability. From the differential thermal analysis, the DSC curve of present emulsion indicates a discontinuous change at the phase change temperature of phase change material due to its latent heat. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Thermal storage Emulsion Phase change material Thermophysical property
1. Introduction A dispersed system which contains phase change materials for the dispersed phase attracts increasing attentions as one kind of thermal storage media [1e4]. A microcapsule system [5e8] has a reputation as a thermofunctional fluid. However, the microcapsule fluid has a bunch of very real problems such as a lack of long-term stability and durability for the practical usage of thermal storage devices. An emulsion is a type of dispersed system [9]. In the emulsion, one of two liquids which will not mix with each other is dispersed as particles (the dispersed phase) in the other (the continuous phase). Especially, the emulsion which contains nano-size phase change materials for the dispersed phase has great advantages
* Corresponding author. E-mail address: [email protected] (T. Kawanami).
against other type of phase change materials. For instance, the nanoemulsion indicates a low viscosity, a high fluidity, and a longterm stability comparison with the microemulsion [10]. Furthermore, its heat transport characteristics as a thermofunctional fluid is superior to a single phase fluid system [11e15]. The aim of our research group is to propose more sophisticated latent heat storage technologies and new thermofunctional fluids. As part of that effort, we have replaced the dispersed phase in emulsions with alkane-based phase change materials, and by reducing the size of those materials to nano-size, we have developed phase change emulsions and also examined the thermal properties and stability of these emulsions [16]. On the other hand, a diverse variety of techniques for preparing emulsions have been proposed in the field of cosmetics science, but these techniques have not been described based on their long-term stability and convenience from the perspective of use as thermofunctional fluids, and that research amounts to nothing more than selecting preparation techniques which have empirically been
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Nomenclature
Abbreviations TD tetradecane HD hexadecane OD octadecane Roman letter symbols n empirical shape factor T temperature, C Greek letters b sphericity h viscosity, Pa s f volume fraction l thermal conductivity, W m1 K1 j mass fraction Subscripts e emulsion in inlet out outlet p dispersed particle in emulsion w water
successful in many cases. There, in this research, an emulsion was prepared using the D-phase emulsification method, which is easy and requires little energy compared to the phase inversion emulsification method which has previously been the most common technique for emulsion preparation [17e19]. Then the stability of that emulsion was evaluated, and its thermal properties were measured. Heat storage and radiation characteristics were also experimentally examined.
2. Phase change emulsion Generally speaking, emulsion preparation methods can be divided into two types: mechanical techniques and surface chemistry techniques. With mechanical techniques, the dispersed particles are reduced to microparticles using a homogenizer with high shearing force or high pressure, and this technique is suitable for large-scale mass production. With surface chemistry techniques, on the other hand, the HLB value (hydrophilicelipophilic balance) of the emulsifier (surfactant) is adjusted, and emulsification is achieved by obtaining the D-phase, which is a surfactant associate, through adjustment of the temperature or adding an additive. This method does not require large-scale equipment, and energy consumption for producing the emulsion can be reduced. In this research, the emulsion was prepared using a surface chemistry technique called the D-phase emulsification method, and the particle size distribution and characteristics of the obtained emulsion were examined. Also, the emulsion which was the subject of this research was the O/W type in which the continuous phase is water. The D-phase emulsification method is a technique developed by Sagitani et al. [20]. With this method, an emulsion is produced by adding water-soluble polyalcohol to a nonionic surfactant, oil and water system, adjusting the HLB value, and obtaining a D-phase and O/D gel. The details of the preparation procedure are given in the references [10], but since energy consumed in emulsion production can be reduced because the method requires no heating or cooling
as with the PIT method, and no great mechanical force for agitation, it was thought to be suitable for the application which was the subject of this research. Furthermore, the method can use a watersoluble surfactant with a comparatively wide range of HLB values, and thus the impact on the environment is low and the amount which can be prepared at one time is larger than with the PIT method. Fig. 1 shows an overview of the method of emulsion production. For the detailed procedure of D-phase emulsification and information on characteristics of the produced emulsion, please refer our previous study [16]. The D-phase emulsification needs the following steps: preparing surfactant-polyalcohol water solution with surfactant; formation of clear gel emulsion; formation of oilin-water emulsion. For process of making this O/W emulsion, the addition of polyalcohol water to the usual emulsion components (oil, water, and surfactant) is necessary since the addition of polyalcohol water changes the liquid crystalline phase to the surfactant isotropic solution. Fig. 2 shows the phase diagram of emulsification process of the D-phase emulsification method. The mixture rates are indicated by taking each vertex as 100mass%, and the opposing side as 0mass%. For example, a solution at point A in the diagram indicates a blend of a mixed liquid of polyalcohol water (50mass%) and surfactant (50mass%). Moving along the arrow from point (a) to point (b) means that oil is added until it accounts for approximately 80mass% of the mixed liquid. In the D-phase emulsification method, the first step is formation of an O/D gel emulsion by dispersing an oil phase (photo (b) in Fig. 3) into a solution containing water, polyalcohol and surfactant (point A). This is then made into an O/W solution by diluting the gel emulsion with water (point C). Photographs of the appearance at points (a), (b) and (c) in Fig. 2 are shown, respectively, in Fig. 3 (a), (b) and (c). For preparing an emulsion using the D-phase emulsification method in this research, polyoxyethylene sorbitan monooleate (HLB value 15.0) was selected as the nonionic surfactant. This surfactant is highly hydrophilic, and thus the cloud point which indicates hydrophobicity as the temperature rises is not near the ordinary use temperature and it is thought that the emulsion can be kept in a stable condition even through repeated cycles of heating and cooling. Regarding the substances used as emulsion materials in this research, purified water was used as the continuous phase. Polyoxyethylene sorbitan monooleate (C64H124O26) was used as the surfactant, and 1,3-Butane diol (C4H10O2) and ethylene glycol (C2H6O2) were adopted as the polyalcohol. In addition, tetradecane (C14H30, melting point 5.9 C), hexadecane (C16H34, melting point 18.2 C), and octadecane (C18H38, melting point 27.0 C) as the dispersed phase which are the phase change materials. In this experiment, an emulsion blended
Polyol Water
+
O/D emulsion Clear gel
Non-ionic surfactant
Isotropic solution
Oil phase (with stirring, dropwise)
Water phase (with stirring) O/W emulsion Fig. 1. Process for production of emulsion by D-phase emulsification method.
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Water Surfactant
50
0
Table 2 Composition of testing emulsions.
(c)
100
D (a)
3
50
Emulsions
Mass fraction of PCM alkanes, j ()
TD10 e EG: Tetradecane base TD40 e EG: Tetradecane base HD10 e 1e3,BD: Hexadecane base OD10 e EG: Tetradecane base
10 40 10 0.1e20
D+O (b)
100 0
100
50
Diol water
0
Liquid paraffin
Gel
Fig. 2. Phase diagram for production of emulsion by D-phase emulsification method.
This section describes the results of those measurements. A laser diffraction particle size analyzer (Shimadzu, SALD-3000) was used to measure particle size. Fig. 4 shows the particle size distribution of the hexadecanebased emulsion immediately after preparation. The horizontal
Fig. 3. Photos of emulsion process: (a) polyalcohol water; (b) O/D phase emulsion; (c) diluted with water.
produced by in the ratios indicates the and Table 2
3. Physical properties of emulsion Thermophysical properties of slurry systems and microcapsule fluids have been studied widely [21e23]. In this section, important properties of nanoemulsions with PCM as thermofunctional fluids are evaluated. 3.1. Stability There is a correlation between emulsion particle size and emulsion stability, and in general the smaller the particle size, the higher the stability of the emulsion. Therefore, it is crucial to ascertain the particle size of the prepared emulsion. In order to determine the size of the hexadecane particles dispersed in the emulsion, and their changes over time, the particle size and particle size distribution of the prepared emulsion were measured from immediately after the emulsion was finished until 65 days later. Table 1 Composition of polyalcohol water with surfactant (mass ratio of surfactant: polyalcohol: water). Surfactants
Ethylene glycol
1,3-Butane diol
POE(20) sorbitan monostearate [10] POE(10) oleyl ether [10] POE(20) sorbitan monooleate
4:3:1
4:2:2
4:2.8:1.2 4:3:1
4:2.4:1.6 4:2:2
axis of the graph indicates particle size, and the vertical axis indicates the number of particles of a certain size as a percentage of the total number of particles. Immediately after preparing the emulsion, the minimum particle size was 0.333 mm, maximum particle size 0.604 mm, and median diameter 0.422 mm. On the other hand, Fig. 5 shows changes in the most common particle size over the 65-day interval. It can be confirmed from the figure that a highly stable emulsion was prepared, with almost no evident changes in particle diameter even after 65 days had elapsed. 3.2. Viscosity In this research, a rotating viscometer (Brookfield, DV-II Pro) was used to measure the viscosity of the prepared hexadecanebased emulsion with the uncertainty of ±1.02%. In the
30 Repetition rate (%)
with the liquid PCM (phase change material) was mixing purified water, surfactant and polyalcohol given in the table to yield a total of 100 g. Table 1 composition of polyalcohol water with surfactant, shows the composition of testing emulsions.
25 20 15 10 5 0 0.1
1 Diameter [ m]
Fig. 4. Repetition rate of diameter of dispersed phase in emulsion.
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0.45 0.4
(-)
1
0.9
e/ w
Diameter ( m)
0.5
0.8
H-C model by Eq.(1)
0.35 0.7
0.3
0
10
20
30
40
50
60
0
70
Elapsed days (days)
5 10 15 20 Concentration of octadecane (mass%)
Fig. 7. Relationship between thermal conductivity and concentration of emulsion. Fig. 5. Stability of emulsion.
experiment, temperature control was carried out by connecting the rotating viscometer with a thermostatic chamber, and the emulsion viscosity was measured from 25 C to 0 C. In addition, the rotation speed of the viscometer was set to 200 rpm. Fig. 6 shows the relationship between temperature and viscosity h. From the figure, it was found that viscosity of the prepared emulsion is 2.2 times that of water at 10 C, and 2.1 times that of water at 20 C.
3.3. Thermal conductivity Thermal conductivity tester (Decagon, KD2 Pro) was used to measure thermal conductivity. During measurement, the sample temperature was kept constant using a thermostatic bath. Measurement was repeated 5 times at the same temperature, and then the average value was found. The uncertainty of each measurement is estimated as ±5.0%. Fig. 7 shows the relationship between the dispersed particle concentration and thermal conductivity ratio for the octadecanebased emulsions. The test liquid temperature was set at 20 C. The thermal conductivity ratio is the value obtained by dividing the thermal conductivity of the sample emulsion le by the thermal conductivity of the base liquid of water lw. The solid line indicates the values obtained using the HamiltonCrosser model [24].
le lp þ ðn 1Þlw ðn 1Þf lw lp ¼ lw lp þ ðn 1Þlw þ f lw lp
(1)
n¼
3
(2)
b
Here, f indicates the volume fraction. The measured value was used as the thermal conductivity of water lw and the literature value [25] was used as the thermal conductivity of the dispersed particles lp. b and n are the sphericity and the empirical shape factor, respectively. In this research, b ¼ 1 based on the assumption that the dispersed particles have a perfectly spherical shape in the emulsion. From Fig. 7 that the thermal conductivity ratio of the emulsion decreases monotonically as the dispersed particle concentration rises. This is attributable to the fact that thermal conductivity of octadecane (the dispersed particle) is lower than thermal conductivity of water (the dispersion medium). In addition, if the measurements of the emulsion are compared against theoretical values, they are found to match over the entire range within ±4%. Fig. 8 shows the relationship between temperature and thermal conductivity of the octadecane-based emulsion. The sample concentration is 10mass% and 20mass%. Measurement was conducted while continuously raising the sample temperature from 10 C to 40 C. The solid line in the figure indicates theoretical values obtained from the HeC model. The graph shows that thermal conductivity of the emulsion increases with rising temperature. On the other hand, a large drop in thermal conductivity was seen near 27 C regardless of the emulsion concentration. This factor appears because, as the temperature rises, the dispersed nano-particle (octadecane) changes from a solid phase with high thermal conductivity to a liquid phase with low thermal conductivity. Also, when the measurements and theoretical values were compared, with a 10mass% emulsion a large difference was seen at and above
5
0.75 (W m-1 K-1)
4 3 2
e
(mPa s)
0.8
Emulsion Water (reference value)
1
0.7 0.65 0.6
10mass% Measurement H-C model by Eq.(1) 20mass% Measurement H-C model by Eq.(1)
Melting point of octadecane
0.55 0.5 0.45
0
0
5
10
15
Temperature Fig. 6. Viscosity of emulsion.
20
25
0.4
10
15
20
25
30
35
40
Temperature Fig. 8. Temperature dependence of thermal conductivity.
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the melting point of octadecane, compared with a good match below the melting point. Due to the above results from Figs. 6 and 7, the HeC model was shown to be valid for predicting thermal conductivity of the emulsion within the range of dispersed particle concentrations in this research. 3.4. Thermal characteristics The melting point of the hexadecane dispersed in the prepared emulsion is 18.2 C. On the other hand, it is known that liquid phase change materials dispersed in an emulsion exhibit lower crystallization temperature with smaller particles sizes [26e30], and to enable industrial use, it is necessary to know the solidifying point of the hexadecane dispersed in the emulsion. In this research, differential scanning calorimetry (DSC; Rigaku, Thermo plus EVO2) was used to measure the melting point and solidifying point of the prepared emulsion. Fig. 9 shows the DSC measurement results and their relationship with heat flow and temperature. The vertical axis of the figure indicates heat flow, and the horizontal axis temperature. The line at the top of the figure is the DSC curve during cooling process, and the line at the bottom is the DSC curve during heating process. The rate of temperature rise was set to 2 K/min. From the graph it is observed that, in the cooling process, there is a peak accompanying latent heat absorption near 1 C. Since latent heat is released as the sample material solidifies, this peak is thought to be due to the solidification heat of the hexadecane contained in the emulsion. In addition, a peak is evident at approximately 18 C in measurement of the heating process, and it is inferred that release of latent heat occurred here. Due to the above results, it was found that the degree of supercooling of the hexadecane dispersed in the emulsion prepared in this research was approximately 17 K. These results are almost the same as the results of Eric Dickerson et al. [31]. In addition to evaluate the phase change temperature, we consider the amount of latent heat of tetradecane-based emulsion using DSC. The results are indicated in Table 3. It is found that the latent heat is measured to be 19.3 kJ/kg for 10mass emulsion and 73.3 kJ/kg for 40mass% one at 5.9 C. 3.5. Heat transport characteristics In order to ascertain the heat exchange characteristics of the phase change emulsion in this research, a convection heat dissipation experiment was carried out using a parallel flow type double-pipe heat exchanger.
Heat flow (mW)
10 Cooling
5 0 -5 Heating
-10 -15 -10
-5
0
5
10
15
20
25
Temperature Fig. 9. Heat flow measurement of emulsion with DSC.
30
5
Table 3 Amount of latent heat of emulsion. Emulsions
Amount of latent heat (kJ/kg of emulsion)
Pure tetradecane TD10 TD40
229 19.3 (8.4% of tetradecene) 73.3 (32.0% of tetradecene)
Figs. 10 and 11 show a schematic drawing and a setup of the experimental apparatus for the heat exchange test, respectively. The apparatus is comprised of a cooling liquid circulation system and emulsion circulation system. The cooling liquid circulation system is comprised of a cooling water pipe, low-temperature thermostatic chamber, testing section, and temperature measurement system. The emulsion circulation system is comprised of an emulsion storage tank, emulsion transport pipe, pump, flow control valve, test section, and temperature measurement system. The test section as a flow channel is a double-pipe heat exchanger having 2 m length, and it has a structure in which cooling liquid and emulsion flow in the same direction. In addition, thermocouples are provided at the inlet and outlet of each unit, so that inlet and outlet temperature can be measured for each channel. Fig. 12 depicts the cross-sectional view of the flow channel. The outer tube is made of transparent acrylic. Its inner and outer diameter are 15 mm and 25 mm, respectively. On the other hand, a copper tube having 12 mm outer diameter is adopted as the inner flow tube. A thermal insulator is installed along the outer circumference of the outer tube. Table 4 shows the experiment conditions. In this experiment, an aqueous solution of propylene glycol with adjusted concentration (approximately 33 mass%) was used as the cooling liquid. In all experiment conditions, the temperature difference was set to 25 C at the test section inlet for the sample liquid and cooling liquid. This was done in order to set the difference in temperature of each sample fluid between the test section inlet and outlet to approximately 10 C, and ensure an adequate temperature difference between the inlet and outlet. For example, in experiment Condition 1, the emulsion inlet temperature was 30 C, and the outlet temperature was approximately 20 C. In order to ascertain the effects on heat exchange duty due to the phase change of hexadecane dispersed in the emulsion, experiments were conducted under a condition (Condition 1) not straddling the 18.2 C phase change temperature of hexadecane, and under conditions (Condition 2 and 3) straddling the phase change temperature of hexadecane where there is a possibility of a phase change by hexadecane. Also, it was possible to estimate through a preliminary experiment that heat loss when the cooling liquid inlet temperature is 5 C is 5% or less of the quantity of heat obtained by the cooling liquid. Similarly, heat loss at 5 C was estimated to be 8% or less, and at 10 C to be 10% or less. Fig. 13 shows the emulsion temperature at the outlet of the test section. The horizontal axis of the figure indicates the numbers of the experiment conditions given in Table 1, and the vertical axis indicates the water temperature at the test section outlet. It can be seen from the graph that the outlet temperature is approximately 18 C in Condition 1, approximately 10 C in Condition 2, and approximately 5 C in Condition 3. On the other hand, Fig. 14 shows the emulsion temperature differences between the test section inlet and outlet. The horizontal axis in the figure indicates the experiment condition number, and the vertical axis indicates the emulsion temperature difference TinTout between the test section inlet and outlet. It is evident from the figure that the temperature difference is approximately 11 K for Condition 1, approximately 10 K for
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Table 4 Experimental condition. Inlet temperature of emulsion, Tin ( C)
Inlet temperature of coolant ( C)
15
1 2 3
30 20 15
5 5 10
10
Tout
Condition no.
5 Coolant flow
P
0
Thermostatic bath
Tin
Test section
2
3
Condition No. Fig. 13. Outlet temperature of emulsion as a function of experimental condition.
2000mm
15
Emulsion flow
P
1
Tout
Thermocouples
10
Fig. 10. Schematic drawing of experimental apparatus.
Tin - Tout
Emulsion tank
5
0
1
2
3
Condition No. Fig. 14. Temperature difference between Tin and Tout of emulsion.
4. Conclusions
Fig. 11. Photo of experimental setup.
Condition 2, and approximately 8 K for Condition 3. These results show that the temperature difference is smaller to the extent that a condition has a lower set temperature. As is evident from the value for the drop in solidification temperature shown in Fig. 9, it is likely that the actual solidifying point of hexadecane is about 1 C, and this is believed to be due to the fact that the effects of latent heat absorption were seen in the experiment for Condition 3 where that temperature was passed through.
In order to ascertain the thermophysical properties and heat transport characteristics of a phase change emulsion, the particle size distribution, phase change point, viscosity, and thermal conductivity were measured for n-alkanes contained in the phase change emulsion prepared in this experiment. It is found that the particle diameter in the emulsion prepared using the D-phase emulsification method was in the range 0.333e0.604 mm. When the viscosity of emulsion prepared in the research was compared with the viscosity of water, it exhibited a value roughly twice as high. In addition, through an experiment using a DSC to measure the degree of supercooling, it was found that the degree of supercooling of hexadecane-based emulsion was 17 K. The latent heat of tetradecane-based emulsion is indicated to be 19.3 kJ/kg for 10mass emulsion and 73.3 kJ/kg for 40mass% one at 5.9 C in this research. As regard the thermal conductivity of octadecane-based emulsion, a large change in thermal conductivity of emulsion was evident accompanying the phase change near the melting point of the dispersed particles. Furthermore, the heat transport characteristics of the phase change emulsion were experimentally ascertained using a parallel flow double-pipe heat exchanger. From the experiment, it is thought that, in the low-temperature condition, there was a phase change of the hexadecane phase change material contained in the emulsion, and an accompanying release of latent heat. Acknowledgments
Fig. 12. Cross-sectional view of the flow channel.
This work was partially supported by the Strategic Basic Research Programs (Advanced Low Carbon Technology Research
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Please cite this article in press as: Kawanami T, et al., Thermophysical properties and thermal characteristics of phase change emulsion for thermal energy storage media, Energy (2016), http://dx.doi.org/10.1016/j.energy.2016.04.021