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Study on high performance ice slurry formed by cooling emulsion in ice storage (discussion on adaptability of emulsion to thermal storage material)
International Journal of Refrigeration 29 (2006) 1010e1019 www.elsevier.com/locate/ijrefrig
Study on high performance ice slurry formed by cooling emulsion in ice storage (discussion on adaptability of emulsion to thermal storage material) Koji Matsumotoa,*, Ken Oikawab, Masashi Okadac,1, Yoshikazu Teraokad, Tetsuo Kawagoee,2 a
Department of Precision Mechanics, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan b Sumitomo Heavy Industries, Ltd, 5-9-11 Kitashinagawa, Shinagawa-ku, Tokyo 141-8686, Japan c Department of Mechanical Engineering, Aoyama Gakuin University, 5-10-1 Fuchinobe, Sagamihara-shi, Kanagawa 237-8559, Japan d Department of Mechanical Engineering, Aoyama Gakuin University, 5-10-1 Fuchinobe, Sagamihara-shi, Kanagawa 229-8558, Japan e 2-28-3 Katakurachou, Kanagawa-ku, Yokohama 221-0865, Japan Received 12 September 2005; received in revised form 22 December 2005; accepted 22 December 2005 Available online 5 June 2006
Abstract This study focuses on an emulsion as a new thermal storage material for ice storage. Two types of emulsions were formed using an oilewater mixture with a small amount of additive. A silicone, light and lump oils were used. The water contents of the emulsions were 70, 80 and 90%. The additive was an amino group modified silicone oil. No depression of freezing point was observed for the emulsions because of their hydrophobic properties. In order to determine the structure of the emulsions, their electrical resistances were measured. Moreover, components of the liquids separating from the emulsions were analyzed. The results indicated that one emulsion was a W/O type emulsion, while the other was an O/W type. Finally, adaptability of the two emulsions to ice storage was discussed, it was concluded that a high performance ice slurry could be formed by the W/O type emulsion. Ó 2006 Elsevier Ltd and IIR. Keywords: Ice slurry; Thermal storage; Experiment; Binary mixture; Water; Oil; Performance
* Corresponding author. Tel.: þ81 3 3817 1837; fax: þ81 3 3817 1820. E-mail addresses: [email protected] (K. Matsumoto), [email protected] (M. Okada), [email protected]. ac.jp (Y. Teraoka). 1 Tel.: þ81 3 5384 3175; fax þ81 3 5384 6300. 2 Tel./fax: þ81 45 491 7973. 0140-7007/$35.00 Ó 2006 Elsevier Ltd and IIR. doi:10.1016/j.ijrefrig.2005.12.013
K. Matsumoto et al. / International Journal of Refrigeration 29 (2006) 1010e1019
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Etude sur la performance e´leve´e d’un syste`me de production de coulis de glace a` base de refroidissement d’e´mulsion et d’accumulation thermique de glace: discussion sur l’utilisation de l’e´mulsion en tant que mate´riau utilise´ dans l’accumulation thermique Mots cle´s : Coulis de glace ; Accumulation thermique ; Expe´rimentation ; Me´lange binaire ; Eau ; Huile ; Performance
1. Introduction In Japan, a peak cut and peak shift of the demand for electric power can be achieved by spreading out the usage of power for ice storage systems by employing nighttime electric power, instead of the much more costly daytime electric power. In addition, the release of CO2 gas can be much reduced by the spread because of the characteristics of nighttime electric power. Thus, the spread of ice storage systems can reduce the environmental load. An ice storage equipment can be smaller because the amount of thermal storage per unit volume is larger than that required for other thermal storage systems. Especially, in a dynamic ice storage system, the ice slurry used as the thermal storage material has good fluidity. Thus, a large amount of cold energy can be transported with less pumping work. The dynamic system can also respond quickly to changes in heat load because the ice particles have a large surface area. Many researchers have studied the formation methods of ice slurries [1e3], however, the IPF (Ice Packing Factor) of formed ice is low at present. In our study [4], we reported that a high-IPF ice slurry could be formed without adhesion of ice to the cooling wall by cooling and stirring a functional fluid of 10 vol% silicone oil and 90 vol% water with a small amount of additive (silane-coupler) in a resin beaker. Moreover, the ice particles in the slurry remained granular and well-dispersed even after the slurry was preserved for a long time in a frozen state. In our previous study [5], we reported that at a very small depression of the freezing point, all the water in the functional fluid could be frozen because the additive combined with ice by hydrogen bonding. However, the latent heat of fusion of an ice slurry made from the functional fluid dropped according to the depression of freezing point, though the depression of freezing point was very small. Moreover, in the case of using a metal beaker, ice adhesion to the cooling beaker wall often occurred. In order to solve the above problems, a new thermal storage material was developed. As additive, an amino group modified silicone oil was used, rather than the silanecoupler. This silicone oil is hydrophobic, hence, it seems that the latent heat of fusion does not drop. Two types of
emulsions were formed using an oilewater mixture with a small amount of the amino group modified silicone oil. The present study investigates the structure and characteristics of the two emulsions. Moreover, their performance as thermal storage material is assessed from the viewpoints of IPF, fluidity, and ice adhesion to cooling wall. The optimal emulsion is selected for ice storage. 2. Experiment 2.1. Experimental apparatus The experimental apparatus and procedure are the same as in our previous study [5]. Thus, only a summary is provided in the present study. Fig. 1 schematizes the experimental apparatus. PMP (polymethylpentene) and stainless steel beakers were used. Their height and inner diameter were about 190 mm and 130 mm, respectively. The temperatures of the emulsion and ice slurry were measured by a platinum resistance thermometer. The temperature measurement points were 600 mm above the beaker bottom and 15 mm and 65 mm radially from the center of the beaker. The stirring velocity was 250 rpm. The emulsion was formed
Stirrer Beaker
Constant temperature bath Emulsion Brine Fig. 1. Experimental apparatus.
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at room temperature; it was cooled by cold brine at 5.4 C. The electrical resistance of the emulsion was measured without stirring in order to reveal its structure. For these measurements, two 135 35 mm stainless steel plates were immersed in the beaker; the distance between the two plates was 120 mm.
CH H2N
R’
Si
O
Si
O
R
2.2. Preparation method for emulsions
CH The emulsion compositions are shown in Table 1. Each emulsion will hereafter be expressed by the oil type followed by the ratio of water to oil (by volume), for example, ‘‘silicone oil (9:1)’’. In this study, the oil used was a silicone oil (kinetic viscosity: 1.0 105 m2/s at 25 C[6]). In addition, light (kinetic viscosity: 3.5 106 m2/s at 20 C [6]) and lump (kinetic viscosity: 1.75 106 m2/s at 20 C [6]) oils were used because of oil cost considerations. An amino group modified silicone oil with hydrophobic property was used as additive. The amino group modified silicone oil will hereafter be referred to simply as additive. The prescribed amount of water was gradually added, with stirring, into a beaker containing the oil with 0.9 vol% additive. The mixture slowly turned into a cloudy emulsion. This emulsion will be called ‘‘type 1’’. Meanwhile, a mixture of water and oil with additive was ultrasonicated at 20 kHz for 3 min. The mixture, labeled ‘‘type 2’’, also changed into a cloudy emulsion. The constitutional formula of the additive used is shown in Fig. 2. The additive acts as a surfactant, reducing the surface tension of oil at the oilewater interface. Hence, the surface tension of water becomes larger than that of oil. Thus, a W/O type (water in oil) emulsion could easily be formed. The force to keep structure of the emulsion becomes stronger with increasing additive concentration, simultaneously, viscosity of the emulsion increases. An O/W type emulsion (oil in water) could be formed by employing a special method of stirring, such as ultrasonication. 2.3. Characterization of emulsion structure In order to determine the structure of the two types of emulsions, components of liquids separating partially from those emulsions were analyzed, and the electrical resistance of those emulsions was measured. When an emulsion is stirred at high speed, part of the emulsion separates out. Since the emulsion consists of continuous and dispersed
Table 1 Emulsion compositions Tap water (ml) Oil (ml) Additive (ml) Ratio of water to oil
990 110 10 9:1
Additive: amino group modified silicone oil.
880 220 10 8:2
770 330 10 7:3
n
3
Fig. 2. Constitutional formula of amino group modified silicone oil.
phases, it is thought that the liquid separating partially from the emulsion is the dispersed phase. Moreover, it is thought that the electrical resistance of the emulsion strongly depends on the continuous phase. 2.4. Characteristics of the emulsions In order to discuss the characteristics of the two types of emulsions, temperature variations of the emulsions during the cooling process and their viscosities were measured. Viscosity was measured by a rotational viscometer; a shearing velocity of 5 rpm was used. 2.5. Formation of ice slurry The beaker with the emulsion (freezing point: 0 C) was soaked directly in a thermostatic bath filled with brine at 5.4 C. The emulsion was cooled with stirring and supercooling of the emulsion was dissolved by throwing a 0.1-ml ice nucleus into the emulsion when the emulsion temperature reached about 3.6 C. Thus, the emulsion changed into an ice slurry. In order to assess the performance of the emulsion as a thermal storage material, the adhesion of ice to the cooling wall was monitored, and its IPF and viscosity were measured. IPF was measured as follows: the amount of ice formed (mice) was determined by the volume change (DV) that accompanied the freezing of water. To determine the volume change, the change in height of the emulsion in the beaker was measured by a laser displacement meter. The volume change of the oil contained in the emulsion was negligible over the temperature range of the present measurements. The value of mice was calculated by Eq. (1). The values of rice and rw in Eq. (1) were obtained from the literature [7]. The value of IPF was calculated using mice: IPF ¼ (mice)/(total mass of ice slurry) 100 (%). In order to check the IPF results obtained by this method, the amount of ice formed was also calculated by measuring the heat supplied to melt all the ice in the ice slurry with a heater. In the calculation, the latent heat of fusion of ice was measured to be about 334 kJ/kg.
K. Matsumoto et al. / International Journal of Refrigeration 29 (2006) 1010e1019
mice ¼ DV
rice rw ðrw rice Þ
ð1Þ
where rice is the density of ice at 273 K, kg/m3; rw density of water at 273.16 K, kg/m3. 2.6. Repetitive formation of ice slurry from emulsion When all the ice in an ice slurry having a certain IPF melts, the slurry changes into a mixture of water, oil and emulsion. When using the emulsions discussed in this paper as thermal storage material, it is necessary that a new emulsion is remade from the mixture of the water, oil and emulsion, and an ice slurry be remade from the new emulsion. Thus, the following experiments were carried out. An ice slurry with a certain IPF was formed by cooling an emulsion with stirring. After all the ice in this slurry was melted by maintaining the slurry at room temperature without stirring, the slurry changed into a mixture of water, oil and emulsion. Then, we tried to re-form an emulsion by stirring this mixture, and to re-form the ice slurry by cooling the new emulsion with stirring. 3. Experimental results and discussion 3.1. Determination of emulsion structure When an oil color, which dyes red only oil, was added to a liquid separating partially from a type 1 emulsion, the liquid was not dyed red, suggesting that type 1 was a W/O emulsion. By contrast, since the liquid separating from a type 2 emulsion was dyed red, it was thought that type 2 was an O/W emulsion. The results of electrical resistance measurements for the type 2 emulsion are shown in Fig. 3. The type 2 emulsions selected were silicone oil (8:2) and (7:3). The electrical resistances measured were on the order of several hundred kU, which is 100 times the electrical resistance of water. ( × 100K Ω )
Meanwhile, for all kinds of type 1 emulsions containing silicone oil, the electrical resistance values were beyond the limits of our device as was that for oil alone; the values were much higher than those of type 2 emulsions. Similar results were obtained in the case of using other kinds of oils (light and lump oils). Thus, it was concluded that type 1 and type 2 were W/O and O/W type emulsions, respectively. Type 1 and type 2 will hereafter be expressed as W/O type and O/W type, respectively. Photographs of silicone oil (9:1) for W/O and O/W types are shown in Figs. 4 and 5, respectively. Figs. 4 and 5 show that the W/O type was cloudy with high-viscosity, while the O/W type was also cloudy with low-viscosity. Enlarged photographs corresponding to Figs. 4 and 5 are shown in Figs. 6 and 7, respectively. Figs. 6 and 7 reveal that the W/O type had a dispersed phase consisting of many spherical water droplets with a mean diameter of several tens of mm, each coated with a thin oil layer, by contrast, the O/W type had a dispersed phase consisting of many spherical oil droplets with a mean diameter of several tens of mm. 3.2. Emulsion characteristics 3.2.1. Temperature variations of emulsions during dissolution of supercooling Temperature variations of the two types of emulsions are shown in Fig. 8. In the case of the O/W type with over 70 vol% water content, the temperature reached 0 C (freezing point) immediately after dissolution of supercooling because its continuous phase was water. The behavior of dissolution of supercooling was similar to that of water alone. On the other hand, in the case of the W/O type with over 70 vol% water content, it took longer to reach 0 C because oil prevented propagation of dissolution among the many dispersed water droplets. 3.2.2. Apparent viscosity of emulsion The viscosity was measured in order to know the fluidity of the emulsion. Measurement results are shown in Table 2. Since preliminary experiments revealed that all
8
(Ω)
6
4
Emulsion Type2 (8:2) Emulsion Type2 (7:3)
2
Water 0 0
1000
2000
3000
Time(s) Fig. 3. Comparison of electrical resistance of water with type 2 emulsions.
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Fig. 4. W/O type (silicone oil (9:1)).
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Water(Continuous phase)
Oil(Dispersed phase)
Fig. 5. O/W type (silicone oil (9:1)).
W/O types in this study were non-Newtonian fluids, the viscosity was expressed as an apparent viscosity. We could not measure the viscosity of O/W types with our device because their viscosity was too low. When the composition ratio of water to oil for the emulsions used was the same, the apparent viscosity of the W/O type emulsion increased with increasing oil viscosity except for silicone oil (7:3) and light oil (7:3). Moreover, if the same oil was used, the apparent viscosity increased with increasing water content. In particular, the apparent viscosity of silicone oil (9:1) was much higher than those of silicone oil (8:2) and (7:3). The relative viscosity of an emulsion increases in proportion to the volumetric fraction of the emulsion [8], where relative viscosity is defined as (viscosity of emulsion)/ (viscosity of continuous phase). This was in agreement with the results shown in Table 2. However, it is known that the
40 µm
Fig. 7. Enlarged photo of O/W type (silicone oil (9:1)).
apparent viscosity depends on not only the volumetric fraction of the dispersed phase, but also its size. In the case using silicone oil, the finding that the apparent viscosity of silicone oil (9:1) with its much smaller dispersed phase size was much larger than those of others, can be explained on the basis of a laser microscopic observation result that the size of the dispersed phase decreased with increase of water content. In the case of using other kinds of oils (light and lump oils), similar results were obtained. 3.2.3. Destruction of emulsion Emulsions are unstable. It is difficult to maintain the structure of an emulsion if it is left to stand for a long time. This is called destruction of the emulsion. When the emulsions formed were left to stand without stirring, no destruction was observed in the O/W types, but it was observed
Oil(Continuous phase) W/O emulsion
Water(Dispersed phase)
40 µm
Fig. 6. Enlarged photo of W/O type (silicone oil (9:1)).
Temperature (°C)
5
O/W emulsion
0
3000
4000
5000
6000
Time(s) Fig. 8. Comparison of dissolution process of supercooled emulsions.
K. Matsumoto et al. / International Journal of Refrigeration 29 (2006) 1010e1019 Table 2 Apparent viscosity of W/O type (shear velocity: 5 rpm at 0 C) Composition ratio of water to oil
Kind of oil
Apparent viscosity (Pa s)
7:3 8:2 9:1 7:3 8:2 9:1 7:3 8:2 9:1
Silicone oil Silicone oil Silicone oil Light oil Light oil Light oil Lump oil Lump oil Lump oil
10.1 23.0 72.1 12.1 16.9 46.1 0.2 6.9 18.9
that, over time, W/O type emulsions separated gradually into water and oil. In the typical destruction process, the dispersed phase separates from the emulsion due to the density difference between the continuous and dispersed phases (this is called ‘‘creaming’’), and the dispersed phase that separates out condenses (‘‘flocculation’’), and the condensing dispersed phase forms a lump (‘‘coalescence’’). The state of silicone oil (9:1) and lump oil (7:3) at 2 h after stirring is stopped is shown in Fig. 9. Fig. 9 reveals that the separation rate of the emulsion depends on the emulsion composition. On the basis of Table 2, it can be said that the separation rate of the emulsion had a tendency to increase with decreasing apparent viscosity because creaming and flocculation occurred more easily at lower apparent viscosities. Moreover, since the emulsion becomes more stable with decreasing dispersed phase size, it is thought that the dispersed phase size decreases according to the decrease in the separation rate. 3.3. Formation of ice slurry 3.3.1. Ice adhesion to cooling wall In the case of the PMP beaker, ice adhesion to the cooling wall was observed for all O/W type emulsions used in this
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study. Adhesion occurred within 30 min after dissolution of supercooling. State of ice adhesion for silicone oil (8:2) is shown in Fig. 10. The figure reveals that the ice adhering to the cooling wall was colorless and transparent. Thus, it is thought that the ice was composed of water from the continuous phase frozen at the cooling wall. The adhesive force of the ice was very strong, and hence, it was difficult to remove ice from the cooling wall. Similar results were obtained when other kinds of O/W types were used. Thus, O/W type emulsions were deemed unsuitable as thermal storage material. Ice adhesion was also investigated in W/O type emulsions. In the case of silicone oil, for the composition of (9:1), ice adhesion occurred within 30 min after dissolution of supercooling, however, for (8:2), the ice slurry could be formed continuously for about 2 h without ice adhesion, and for (7:3), the ice slurry could be formed continuously for over 7 h without ice adhesion. In the case using light oil, for (9:1) ice adhesion occurred within 30 min after dissolution of supercooling, however, for (8:2) ice slurry could be formed continuously for about 5 h without ice adhesion, and for (7:3) ice slurry could be also formed continuously for over 7 h without ice adhesion. In the case of lump oil, for (9:1), the ice slurry could be formed continuously for over 7 h without ice adhesion, and for (8:2), the ice slurry could be formed continuously for about 4 h without ice adhesion. However, in the case of lump oil (7:3), ice adhesion occurred at about 2 h after dissolution of supercooling. The state of ice adhesion for silicone oil (8:2) is shown in Fig. 11. From the figure, which reveals that the ice adhering to the cooling wall was milk white, indicating that the ice contained oil and additive. Adhesive force of ice to the cooling wall was very weak because of the effect of oil and additive, and hence, it was very easy to remove ice from the cooling wall. Thus, it was easy to dissolve ice adhesion. Similar results were obtained for all W/O type emulsions except for silicone oil (9:1), light oil (9:1) and lump oil (7:3). For these three emulsions, the adhesive force of ice was very strong,
Wall Silicone oil (9:1)
Lump oil (7:3) Oil
Emulsion
Emulsion Water
Fig. 9. Separation of emulsion (W/O type).
Ice Fig. 10. Adhesion of ice to cooling wall (O/W type, silicone oil (8:2)).
K. Matsumoto et al. / International Journal of Refrigeration 29 (2006) 1010e1019
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Wall Ice
Fig. 11. Adhesion of ice to cooling wall (W/O type, silicone oil (8:2)).
and thus, it was very difficult to remove ice from the cooling wall. The state of ice adhesion for lump oil (7:3) is shown in Fig. 12. Since the state shown in Fig. 12 very much resembled that for the O/W type with silicone oil (8:2) shown in Fig. 10, it is thought that only water separating from the W/O type emulsion with lump oil (7:3) froze at the cooling wall. Thus, all W/O type emulsions except for silicone oil (9:1), light oil (9:1) and lump oil (7:3) were deemed suitable as thermal storage material. The above discussion reveals that the possibility of ice adhesion was much lower for the W/O type than for the O/W type. This is because in the latter case, the water originating from the continuous phase contacts the cooling wall directly, while in the case of the W/O type, water from the dispersed phase hardly contacts the cooling wall because of the presence of oil. For the W/O type, possibility of ice adhesion depended on the emulsion composition. It is generally believed that the possibility of ice adhesion decreases with increasing oil content because water hardly comes in direct contact with the cooling wall in the case of higher oil contents. However, the case of lump oil was an exception. Thus, the relationship between possibility of ice adhesion and apparent
viscosity of the emulsion was very important. As shown in Table 2, in the cases of silicone oil (9:1) and light oil (9:1) with their high apparent viscosities, the effect of stirring in the vicinity of the cooling wall was not enough, and the formed ice stagnated at the cooling wall, leading to ice adhesion. By contrast, in the case of lump oil (7:3) with its lower apparent viscosity, it was concluded, on the basis of the above discussion on the relationship between emulsion stability and apparent viscosity, that the force maintaining the W/O emulsion structure was weak. Thus, it is thought that the water separating partially from the emulsion froze at the cooling wall. In the present system, the possibility of ice adhesion was lower in the apparent viscosity range of 10e23 Pa s (shearing velocity: 5 rpm, 0 C), and the ice adhesion was easy to dissolve if it occurred. However, in cases of silicone oil (9:1) and light oil (9:1), which far exceed this viscosity range, ice adhesion to the cooling wall occurred within 30 min after dissolution of supercooling and the adhesive force of the ice was very strong. By contrast, in the case of lump oil (7:3), which was far below the abovementioned viscosity range, ice adhesion occurred at about 2 h, and the adhesion force of the ice was strong. However, it is possible to change the apparent viscosity of the W/O type by varying the stirring conditions and the additive concentration. Thus, for silicone oil (9:1), light oil (9:1), and lump oil (7:3), it is thought that good ice formation can be realized by optimizing their apparent viscosities. 3.3.2. Ice particles Since best ice formation in this study was achieved for W/O type emulsions of silicone oil (7:3), light oil (7:3), and lump oil (9:1), the following discussion will focus on these three cases. For silicone oil (7:3), a photograph of the ice slurry formed immediately after dissolution of supercooling is shown in Fig. 13, and a photograph of ice particles at 7 h after dissolution of supercooling is shown in Fig. 14. For light oil (7:3), a photograph of ice
Ice
Wall Fig. 12. Adhesion of ice to cooling wall (W/O type, lump oil (7:3)).
Fig. 13. Ice particles immediately after dissolution of supercooling (W/O type, silicone oil (7:3)).
K. Matsumoto et al. / International Journal of Refrigeration 29 (2006) 1010e1019
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100
Ice particle
IPF measurement with volume change IPF measurement with heater
IPF( )
80
60
40
20
0
0
2
4
6
Time(h) Fig. 14. Ice particles at 7 h after dissolution of supercooling (W/O type, silicone oil (7:3)).
Fig. 16. Comparison of IPF values (W/O type, silicone oil (7:3)).
particles at 7 h after dissolution of supercooling is shown in Fig. 15. Several white spots about 1 mm in size can be seen in Fig. 13; these are ice particles that formed immediately after dissolution. Though Fig. 13 cannot be simply compared with Fig. 6 because of the difference in composition, on the basis of the enlarged photograph of the W/O type in Fig. 6, it was thought that 10e20 dispersed water droplets formed one lump, and this lump transformed into the ice particle shown in Fig. 13 upon cooling with stirring. Fig. 14 reveals that the ice particles that formed at 7 h after dissolution of supercooling were much larger than those immediately after dissolution. The sizes of the former were about 2e3 mm. The one ice particle that formed immediately after dissolution contacted the surrounding dispersed water droplets with time, the ice particle and dispersed water droplets coalesced, and the ice particle grew upon cooling, and it became spherical with stirring. Fig. 15 shows that, in the case of light oil (7:3), the mean size of the ice particles formed was 1e2 mm, which was smaller than that for silicone oil
(7:3). The difference in mean ice particle size is attributable to differences in heat transfer and degree of coalescence of the dispersed phase caused by difference in apparent viscosity. 3.3.3. IPF of ice slurry Fig. 16 shows the IPF measurement results for the W/O type emulsion of silicone oil (7:3), obtained by the two different methods mentioned above. The difference between the results of the two methods was very small, suggesting that the IPF measurements were reliable. IPF results for W/O type silicone oil (7:3) and lump oil (9:1) are shown in Fig. 17. The PMP beaker was used. For silicone oil (7:3), the IPF became 70% at 7 h after dissolution of supercooling, and simultaneously, all the water in the emulsion froze. For lump oil (9:1), the IPF became 90% at 6 h after dissolution of supercooling, and simultaneously, all the water in the emulsion froze. That is, the ice formation rate of lump oil (9:1) was faster than that of silicone oil (7:3). This is because the thermal conductivity 100
IPF( )
80
60
40
20
Ice particle
0
Lump oil(9:1) Silicone oil(7:3) 0
2
4
6
Time(h) Fig. 15. Ice particles at 7 h after dissolution of supercooling (W/O type, light oil (7:3)).
Fig. 17. Time dependence of IPF (W/O type, silicone oil (7:3) and lump oil (9:1)).
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Fig. 18. Ice slurry (W/O type, lump oil (9:1), IPF ¼ 10.7%).
Fig. 20. Ice slurry (W/O type, lump oil (9:1), IPF ¼ 90.0%).
of water is larger than those of silicone oil and lump oil, and the water content of lump oil (9:1) is larger than that of silicone oil (7:3). For lump oil (9:1), the states of the ice slurry that formed at 1, 2 and 6 h after dissolution are shown in Figs. 18e20, respectively. Figs. 18 and 19 reveal that the ice slurry with 10e20% IPF had good fluidity. Fig. 20 shows that granular ice particles were formed when IPF became 90%, at which point all the water in the emulsion froze. To discuss transport of the ice slurries, apparent viscosities of ice slurries made from silicone oil (7:3) and lump oil (9:1) are compared in Fig. 21. Those apparent viscosities were measured in the process of increasing IPF. The figure reveals that the difference between the two slurries was small up to an IPF of 10%, but increased with increasing IPF thereafter. It is thought that the difference was larger because the apparent viscosity depends on the emulsion composition, the ice particle size and the dispersed phase size. For silicone oil (7:3), apparent viscosity was smaller for IPF values below about 30%, and thus, the ice slurry made from silicone oil (7:3) had good fluidity.
3.3.4. Ice formation in stainless beaker The ice formation rate in PMP beaker is slow. It is essential to increase the ice formation rate for practical applications. Thus, a stainless steel beaker, whose thermal conductivity was about 50 times that of the PMP beaker, was used. The IPF of the ice slurry made from silicone oil (7:3) is shown in Fig. 22. For comparison, the IPF obtained in the case of the PMP beaker is shown as well. Fig. 22 shows that in the case of the stainless steel beaker, the IPF reached 70% at about 2 h after dissolution of supercooling without ice adhesion to the cooling wall, at which time, all the water in the emulsion froze. The ice formation rate was much higher in the case of the stainless steel beaker than in the case of the PMP beaker. Similar results were obtained for light oil (7:3) and lump oil (9:1), which exhibited the best ice formation in this study. The above discussion indicates that water hardly contacts the cooling wall directly because of the structural property of the W/O type emulsion, i.e., it is thought that the inherent structure of the W/O type
Apparent viscosity(Pa . s)
800 Lump oil(9:1) Silicone oil(7:3) 600
400
200
0 0
10
20
30
40
50
IPF( ) Fig. 19. Ice slurry (W/O type, lump oil (9:1), IPF ¼ 23.8%).
Fig. 21. Relationship between IPF and apparent viscosity of ice slurry (W/O type, shear velocity: 5 rpm, 0 C).
K. Matsumoto et al. / International Journal of Refrigeration 29 (2006) 1010e1019
4. Conclusions
100 Stainless-steel vessel PMP vessel
IPF( )
80 60 40 20 0
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0
100
200
300
400
Time(m)
(1) O/W and W/O type emulsions were developed as thermal storage material for ice storage. (2) The two types of emulsions were characterized and their adaptability to ice storage was investigated. (3) The W/O type emulsion was demonstrated to be suitable for ice storage. (4) It was found that the inherent structure of the W/O type emulsion prevented ice adhesion, even when a stainless steel beaker was used. (5) It was found that the W/O emulsions silicone oil (7:3), light oil (7:3) and lump oil (9:1) exhibited the best ice formation.
Fig. 22. Time dependence of IPF (W/O type, silicone oil (7:3)).
Acknowledgements emulsion prevents ice adhesion regardless of the beaker material. 3.3.5. Repetitive formation of ice slurry from emulsion In the case of the W/O type with silicone oil (7:3), once an ice slurry with an IPF below 40% made from the W/O type emulsion changed into a mixture of water, oil and W/O type emulsion due to melting at room temperature without stirring, a new W/O type emulsion could be made from the mixture with stirring. If the IPF was above 40%, however, a new emulsion could not be formed. For W/O type with light oil (7:3), re-formation of an emulsion was possible below 70% IPF. For W/O type with lump oil (9:1), re-formation of an emulsion was possible below 50% IPF. New ice slurries could be made from the reformed W/O type emulsions by cooling with stirring without ice adhesion to the cooling wall. During transport, the IPF of an ice slurry is about 20e30%. Thus, W/O type emulsions made from silicone oil (7:3), light oil (7:3), and lump oil (9:1) are suitable for repetitive formation of emulsion and ice slurry. Moreover, in the cases of these three W/O type emulsions, after an ice slurry with the highest IPF was formed by freezing all the water in the emulsion with stirring, all the ice in the ice slurry was melted by maintaining the ice slurry at room temperature with 50 rpm stirring, which was lower than the ordinary stirring speed (250 rpm), and then a new W/O type emulsion could be re-formed from the ice slurry melted by ordinary stirring. A new ice slurry could be made from each re-formed W/O type emulsion by cooling with stirring without ice adhesion.
This study was financially supported by Chuo University as one of the 2005 Research Projects for Promotion of Advanced Research at Graduate School. The authors wish to thank Mr. Nishiyama, Mr. Sekine, Mr. Doi, Mr. Sakae and Mr. Mitamura, who are graduates of Chuo University, for their collaboration.
References [1] T. Hirata, K. Nagasaka, M. Ishikawa, Crystal ice formation of solution and its removal phenomena at cooled horizontal solid surface (part I: ice removal phenomena), Int. J. Heat Mass Transf. 43 (3) (2000) 333e339. [2] T. Hirata, M. Kato, K. Nagasaka, M. Ishikawa, Crystal ice formation of solution and its removal phenomena at cooled horizontal solid surface (part II: ice removal condition), Int. J. Heat Mass Transf. 43 (5) (2000) 757e765. [3] H. Kawabe, S. Fukusako, M. Yamada, K. Yanagita, Continuous production characteristics of slush ice by use of a horizontal oscillating cooled wall, Trans. JSRAE 15 (3) (1998) 193e201 (in Japanese). [4] K. Matsumoto, M. Okada, T. Kawagoe, C. Kan, Ice storage system with watereoil mixture (formation of suspension with high IPF), Int. J. Refrigeration 23 (5) (2000) 336e344. [5] K. Matsumoto, Y. Shiokawa, M. Okada, T. Kawagoe, C. Kan, Ice storage system with watereoil mixture (discussion about influence of additive on ice formation process), Int. J. Refrigeration (2002) 11e18. [6] JSME Data Book: Thermophysical Properties of Fluids, JSME, 1997, 461 pp. (in Japanese). [7] Handbook of Heat Transfer, fourth ed., JSME, 1986 (in Japanese). [8] K. Yosizawa, H. Kitahara, ‘‘Bunsan Nyuka no Kagaku’’, Kougaku Tosho, 1979, pp. 25e257 (in Japanese).
Study on high performance ice slurry formed by cooling emulsion in ice storage (discussion on adaptability of emulsion to thermal storage material) Koji Matsumotoa,*, Ken Oikawab, Masashi Okadac,1, Yoshikazu Teraokad, Tetsuo Kawagoee,2 a
Department of Precision Mechanics, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan b Sumitomo Heavy Industries, Ltd, 5-9-11 Kitashinagawa, Shinagawa-ku, Tokyo 141-8686, Japan c Department of Mechanical Engineering, Aoyama Gakuin University, 5-10-1 Fuchinobe, Sagamihara-shi, Kanagawa 237-8559, Japan d Department of Mechanical Engineering, Aoyama Gakuin University, 5-10-1 Fuchinobe, Sagamihara-shi, Kanagawa 229-8558, Japan e 2-28-3 Katakurachou, Kanagawa-ku, Yokohama 221-0865, Japan Received 12 September 2005; received in revised form 22 December 2005; accepted 22 December 2005 Available online 5 June 2006
Abstract This study focuses on an emulsion as a new thermal storage material for ice storage. Two types of emulsions were formed using an oilewater mixture with a small amount of additive. A silicone, light and lump oils were used. The water contents of the emulsions were 70, 80 and 90%. The additive was an amino group modified silicone oil. No depression of freezing point was observed for the emulsions because of their hydrophobic properties. In order to determine the structure of the emulsions, their electrical resistances were measured. Moreover, components of the liquids separating from the emulsions were analyzed. The results indicated that one emulsion was a W/O type emulsion, while the other was an O/W type. Finally, adaptability of the two emulsions to ice storage was discussed, it was concluded that a high performance ice slurry could be formed by the W/O type emulsion. Ó 2006 Elsevier Ltd and IIR. Keywords: Ice slurry; Thermal storage; Experiment; Binary mixture; Water; Oil; Performance
* Corresponding author. Tel.: þ81 3 3817 1837; fax: þ81 3 3817 1820. E-mail addresses: [email protected] (K. Matsumoto), [email protected] (M. Okada), [email protected]. ac.jp (Y. Teraoka). 1 Tel.: þ81 3 5384 3175; fax þ81 3 5384 6300. 2 Tel./fax: þ81 45 491 7973. 0140-7007/$35.00 Ó 2006 Elsevier Ltd and IIR. doi:10.1016/j.ijrefrig.2005.12.013
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Etude sur la performance e´leve´e d’un syste`me de production de coulis de glace a` base de refroidissement d’e´mulsion et d’accumulation thermique de glace: discussion sur l’utilisation de l’e´mulsion en tant que mate´riau utilise´ dans l’accumulation thermique Mots cle´s : Coulis de glace ; Accumulation thermique ; Expe´rimentation ; Me´lange binaire ; Eau ; Huile ; Performance
1. Introduction In Japan, a peak cut and peak shift of the demand for electric power can be achieved by spreading out the usage of power for ice storage systems by employing nighttime electric power, instead of the much more costly daytime electric power. In addition, the release of CO2 gas can be much reduced by the spread because of the characteristics of nighttime electric power. Thus, the spread of ice storage systems can reduce the environmental load. An ice storage equipment can be smaller because the amount of thermal storage per unit volume is larger than that required for other thermal storage systems. Especially, in a dynamic ice storage system, the ice slurry used as the thermal storage material has good fluidity. Thus, a large amount of cold energy can be transported with less pumping work. The dynamic system can also respond quickly to changes in heat load because the ice particles have a large surface area. Many researchers have studied the formation methods of ice slurries [1e3], however, the IPF (Ice Packing Factor) of formed ice is low at present. In our study [4], we reported that a high-IPF ice slurry could be formed without adhesion of ice to the cooling wall by cooling and stirring a functional fluid of 10 vol% silicone oil and 90 vol% water with a small amount of additive (silane-coupler) in a resin beaker. Moreover, the ice particles in the slurry remained granular and well-dispersed even after the slurry was preserved for a long time in a frozen state. In our previous study [5], we reported that at a very small depression of the freezing point, all the water in the functional fluid could be frozen because the additive combined with ice by hydrogen bonding. However, the latent heat of fusion of an ice slurry made from the functional fluid dropped according to the depression of freezing point, though the depression of freezing point was very small. Moreover, in the case of using a metal beaker, ice adhesion to the cooling beaker wall often occurred. In order to solve the above problems, a new thermal storage material was developed. As additive, an amino group modified silicone oil was used, rather than the silanecoupler. This silicone oil is hydrophobic, hence, it seems that the latent heat of fusion does not drop. Two types of
emulsions were formed using an oilewater mixture with a small amount of the amino group modified silicone oil. The present study investigates the structure and characteristics of the two emulsions. Moreover, their performance as thermal storage material is assessed from the viewpoints of IPF, fluidity, and ice adhesion to cooling wall. The optimal emulsion is selected for ice storage. 2. Experiment 2.1. Experimental apparatus The experimental apparatus and procedure are the same as in our previous study [5]. Thus, only a summary is provided in the present study. Fig. 1 schematizes the experimental apparatus. PMP (polymethylpentene) and stainless steel beakers were used. Their height and inner diameter were about 190 mm and 130 mm, respectively. The temperatures of the emulsion and ice slurry were measured by a platinum resistance thermometer. The temperature measurement points were 600 mm above the beaker bottom and 15 mm and 65 mm radially from the center of the beaker. The stirring velocity was 250 rpm. The emulsion was formed
Stirrer Beaker
Constant temperature bath Emulsion Brine Fig. 1. Experimental apparatus.
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at room temperature; it was cooled by cold brine at 5.4 C. The electrical resistance of the emulsion was measured without stirring in order to reveal its structure. For these measurements, two 135 35 mm stainless steel plates were immersed in the beaker; the distance between the two plates was 120 mm.
CH H2N
R’
Si
O
Si
O
R
2.2. Preparation method for emulsions
CH The emulsion compositions are shown in Table 1. Each emulsion will hereafter be expressed by the oil type followed by the ratio of water to oil (by volume), for example, ‘‘silicone oil (9:1)’’. In this study, the oil used was a silicone oil (kinetic viscosity: 1.0 105 m2/s at 25 C[6]). In addition, light (kinetic viscosity: 3.5 106 m2/s at 20 C [6]) and lump (kinetic viscosity: 1.75 106 m2/s at 20 C [6]) oils were used because of oil cost considerations. An amino group modified silicone oil with hydrophobic property was used as additive. The amino group modified silicone oil will hereafter be referred to simply as additive. The prescribed amount of water was gradually added, with stirring, into a beaker containing the oil with 0.9 vol% additive. The mixture slowly turned into a cloudy emulsion. This emulsion will be called ‘‘type 1’’. Meanwhile, a mixture of water and oil with additive was ultrasonicated at 20 kHz for 3 min. The mixture, labeled ‘‘type 2’’, also changed into a cloudy emulsion. The constitutional formula of the additive used is shown in Fig. 2. The additive acts as a surfactant, reducing the surface tension of oil at the oilewater interface. Hence, the surface tension of water becomes larger than that of oil. Thus, a W/O type (water in oil) emulsion could easily be formed. The force to keep structure of the emulsion becomes stronger with increasing additive concentration, simultaneously, viscosity of the emulsion increases. An O/W type emulsion (oil in water) could be formed by employing a special method of stirring, such as ultrasonication. 2.3. Characterization of emulsion structure In order to determine the structure of the two types of emulsions, components of liquids separating partially from those emulsions were analyzed, and the electrical resistance of those emulsions was measured. When an emulsion is stirred at high speed, part of the emulsion separates out. Since the emulsion consists of continuous and dispersed
Table 1 Emulsion compositions Tap water (ml) Oil (ml) Additive (ml) Ratio of water to oil
990 110 10 9:1
Additive: amino group modified silicone oil.
880 220 10 8:2
770 330 10 7:3
n
3
Fig. 2. Constitutional formula of amino group modified silicone oil.
phases, it is thought that the liquid separating partially from the emulsion is the dispersed phase. Moreover, it is thought that the electrical resistance of the emulsion strongly depends on the continuous phase. 2.4. Characteristics of the emulsions In order to discuss the characteristics of the two types of emulsions, temperature variations of the emulsions during the cooling process and their viscosities were measured. Viscosity was measured by a rotational viscometer; a shearing velocity of 5 rpm was used. 2.5. Formation of ice slurry The beaker with the emulsion (freezing point: 0 C) was soaked directly in a thermostatic bath filled with brine at 5.4 C. The emulsion was cooled with stirring and supercooling of the emulsion was dissolved by throwing a 0.1-ml ice nucleus into the emulsion when the emulsion temperature reached about 3.6 C. Thus, the emulsion changed into an ice slurry. In order to assess the performance of the emulsion as a thermal storage material, the adhesion of ice to the cooling wall was monitored, and its IPF and viscosity were measured. IPF was measured as follows: the amount of ice formed (mice) was determined by the volume change (DV) that accompanied the freezing of water. To determine the volume change, the change in height of the emulsion in the beaker was measured by a laser displacement meter. The volume change of the oil contained in the emulsion was negligible over the temperature range of the present measurements. The value of mice was calculated by Eq. (1). The values of rice and rw in Eq. (1) were obtained from the literature [7]. The value of IPF was calculated using mice: IPF ¼ (mice)/(total mass of ice slurry) 100 (%). In order to check the IPF results obtained by this method, the amount of ice formed was also calculated by measuring the heat supplied to melt all the ice in the ice slurry with a heater. In the calculation, the latent heat of fusion of ice was measured to be about 334 kJ/kg.
K. Matsumoto et al. / International Journal of Refrigeration 29 (2006) 1010e1019
mice ¼ DV
rice rw ðrw rice Þ
ð1Þ
where rice is the density of ice at 273 K, kg/m3; rw density of water at 273.16 K, kg/m3. 2.6. Repetitive formation of ice slurry from emulsion When all the ice in an ice slurry having a certain IPF melts, the slurry changes into a mixture of water, oil and emulsion. When using the emulsions discussed in this paper as thermal storage material, it is necessary that a new emulsion is remade from the mixture of the water, oil and emulsion, and an ice slurry be remade from the new emulsion. Thus, the following experiments were carried out. An ice slurry with a certain IPF was formed by cooling an emulsion with stirring. After all the ice in this slurry was melted by maintaining the slurry at room temperature without stirring, the slurry changed into a mixture of water, oil and emulsion. Then, we tried to re-form an emulsion by stirring this mixture, and to re-form the ice slurry by cooling the new emulsion with stirring. 3. Experimental results and discussion 3.1. Determination of emulsion structure When an oil color, which dyes red only oil, was added to a liquid separating partially from a type 1 emulsion, the liquid was not dyed red, suggesting that type 1 was a W/O emulsion. By contrast, since the liquid separating from a type 2 emulsion was dyed red, it was thought that type 2 was an O/W emulsion. The results of electrical resistance measurements for the type 2 emulsion are shown in Fig. 3. The type 2 emulsions selected were silicone oil (8:2) and (7:3). The electrical resistances measured were on the order of several hundred kU, which is 100 times the electrical resistance of water. ( × 100K Ω )
Meanwhile, for all kinds of type 1 emulsions containing silicone oil, the electrical resistance values were beyond the limits of our device as was that for oil alone; the values were much higher than those of type 2 emulsions. Similar results were obtained in the case of using other kinds of oils (light and lump oils). Thus, it was concluded that type 1 and type 2 were W/O and O/W type emulsions, respectively. Type 1 and type 2 will hereafter be expressed as W/O type and O/W type, respectively. Photographs of silicone oil (9:1) for W/O and O/W types are shown in Figs. 4 and 5, respectively. Figs. 4 and 5 show that the W/O type was cloudy with high-viscosity, while the O/W type was also cloudy with low-viscosity. Enlarged photographs corresponding to Figs. 4 and 5 are shown in Figs. 6 and 7, respectively. Figs. 6 and 7 reveal that the W/O type had a dispersed phase consisting of many spherical water droplets with a mean diameter of several tens of mm, each coated with a thin oil layer, by contrast, the O/W type had a dispersed phase consisting of many spherical oil droplets with a mean diameter of several tens of mm. 3.2. Emulsion characteristics 3.2.1. Temperature variations of emulsions during dissolution of supercooling Temperature variations of the two types of emulsions are shown in Fig. 8. In the case of the O/W type with over 70 vol% water content, the temperature reached 0 C (freezing point) immediately after dissolution of supercooling because its continuous phase was water. The behavior of dissolution of supercooling was similar to that of water alone. On the other hand, in the case of the W/O type with over 70 vol% water content, it took longer to reach 0 C because oil prevented propagation of dissolution among the many dispersed water droplets. 3.2.2. Apparent viscosity of emulsion The viscosity was measured in order to know the fluidity of the emulsion. Measurement results are shown in Table 2. Since preliminary experiments revealed that all
8
(Ω)
6
4
Emulsion Type2 (8:2) Emulsion Type2 (7:3)
2
Water 0 0
1000
2000
3000
Time(s) Fig. 3. Comparison of electrical resistance of water with type 2 emulsions.
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Fig. 4. W/O type (silicone oil (9:1)).
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Water(Continuous phase)
Oil(Dispersed phase)
Fig. 5. O/W type (silicone oil (9:1)).
W/O types in this study were non-Newtonian fluids, the viscosity was expressed as an apparent viscosity. We could not measure the viscosity of O/W types with our device because their viscosity was too low. When the composition ratio of water to oil for the emulsions used was the same, the apparent viscosity of the W/O type emulsion increased with increasing oil viscosity except for silicone oil (7:3) and light oil (7:3). Moreover, if the same oil was used, the apparent viscosity increased with increasing water content. In particular, the apparent viscosity of silicone oil (9:1) was much higher than those of silicone oil (8:2) and (7:3). The relative viscosity of an emulsion increases in proportion to the volumetric fraction of the emulsion [8], where relative viscosity is defined as (viscosity of emulsion)/ (viscosity of continuous phase). This was in agreement with the results shown in Table 2. However, it is known that the
40 µm
Fig. 7. Enlarged photo of O/W type (silicone oil (9:1)).
apparent viscosity depends on not only the volumetric fraction of the dispersed phase, but also its size. In the case using silicone oil, the finding that the apparent viscosity of silicone oil (9:1) with its much smaller dispersed phase size was much larger than those of others, can be explained on the basis of a laser microscopic observation result that the size of the dispersed phase decreased with increase of water content. In the case of using other kinds of oils (light and lump oils), similar results were obtained. 3.2.3. Destruction of emulsion Emulsions are unstable. It is difficult to maintain the structure of an emulsion if it is left to stand for a long time. This is called destruction of the emulsion. When the emulsions formed were left to stand without stirring, no destruction was observed in the O/W types, but it was observed
Oil(Continuous phase) W/O emulsion
Water(Dispersed phase)
40 µm
Fig. 6. Enlarged photo of W/O type (silicone oil (9:1)).
Temperature (°C)
5
O/W emulsion
0
3000
4000
5000
6000
Time(s) Fig. 8. Comparison of dissolution process of supercooled emulsions.
K. Matsumoto et al. / International Journal of Refrigeration 29 (2006) 1010e1019 Table 2 Apparent viscosity of W/O type (shear velocity: 5 rpm at 0 C) Composition ratio of water to oil
Kind of oil
Apparent viscosity (Pa s)
7:3 8:2 9:1 7:3 8:2 9:1 7:3 8:2 9:1
Silicone oil Silicone oil Silicone oil Light oil Light oil Light oil Lump oil Lump oil Lump oil
10.1 23.0 72.1 12.1 16.9 46.1 0.2 6.9 18.9
that, over time, W/O type emulsions separated gradually into water and oil. In the typical destruction process, the dispersed phase separates from the emulsion due to the density difference between the continuous and dispersed phases (this is called ‘‘creaming’’), and the dispersed phase that separates out condenses (‘‘flocculation’’), and the condensing dispersed phase forms a lump (‘‘coalescence’’). The state of silicone oil (9:1) and lump oil (7:3) at 2 h after stirring is stopped is shown in Fig. 9. Fig. 9 reveals that the separation rate of the emulsion depends on the emulsion composition. On the basis of Table 2, it can be said that the separation rate of the emulsion had a tendency to increase with decreasing apparent viscosity because creaming and flocculation occurred more easily at lower apparent viscosities. Moreover, since the emulsion becomes more stable with decreasing dispersed phase size, it is thought that the dispersed phase size decreases according to the decrease in the separation rate. 3.3. Formation of ice slurry 3.3.1. Ice adhesion to cooling wall In the case of the PMP beaker, ice adhesion to the cooling wall was observed for all O/W type emulsions used in this
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study. Adhesion occurred within 30 min after dissolution of supercooling. State of ice adhesion for silicone oil (8:2) is shown in Fig. 10. The figure reveals that the ice adhering to the cooling wall was colorless and transparent. Thus, it is thought that the ice was composed of water from the continuous phase frozen at the cooling wall. The adhesive force of the ice was very strong, and hence, it was difficult to remove ice from the cooling wall. Similar results were obtained when other kinds of O/W types were used. Thus, O/W type emulsions were deemed unsuitable as thermal storage material. Ice adhesion was also investigated in W/O type emulsions. In the case of silicone oil, for the composition of (9:1), ice adhesion occurred within 30 min after dissolution of supercooling, however, for (8:2), the ice slurry could be formed continuously for about 2 h without ice adhesion, and for (7:3), the ice slurry could be formed continuously for over 7 h without ice adhesion. In the case using light oil, for (9:1) ice adhesion occurred within 30 min after dissolution of supercooling, however, for (8:2) ice slurry could be formed continuously for about 5 h without ice adhesion, and for (7:3) ice slurry could be also formed continuously for over 7 h without ice adhesion. In the case of lump oil, for (9:1), the ice slurry could be formed continuously for over 7 h without ice adhesion, and for (8:2), the ice slurry could be formed continuously for about 4 h without ice adhesion. However, in the case of lump oil (7:3), ice adhesion occurred at about 2 h after dissolution of supercooling. The state of ice adhesion for silicone oil (8:2) is shown in Fig. 11. From the figure, which reveals that the ice adhering to the cooling wall was milk white, indicating that the ice contained oil and additive. Adhesive force of ice to the cooling wall was very weak because of the effect of oil and additive, and hence, it was very easy to remove ice from the cooling wall. Thus, it was easy to dissolve ice adhesion. Similar results were obtained for all W/O type emulsions except for silicone oil (9:1), light oil (9:1) and lump oil (7:3). For these three emulsions, the adhesive force of ice was very strong,
Wall Silicone oil (9:1)
Lump oil (7:3) Oil
Emulsion
Emulsion Water
Fig. 9. Separation of emulsion (W/O type).
Ice Fig. 10. Adhesion of ice to cooling wall (O/W type, silicone oil (8:2)).
K. Matsumoto et al. / International Journal of Refrigeration 29 (2006) 1010e1019
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Wall Ice
Fig. 11. Adhesion of ice to cooling wall (W/O type, silicone oil (8:2)).
and thus, it was very difficult to remove ice from the cooling wall. The state of ice adhesion for lump oil (7:3) is shown in Fig. 12. Since the state shown in Fig. 12 very much resembled that for the O/W type with silicone oil (8:2) shown in Fig. 10, it is thought that only water separating from the W/O type emulsion with lump oil (7:3) froze at the cooling wall. Thus, all W/O type emulsions except for silicone oil (9:1), light oil (9:1) and lump oil (7:3) were deemed suitable as thermal storage material. The above discussion reveals that the possibility of ice adhesion was much lower for the W/O type than for the O/W type. This is because in the latter case, the water originating from the continuous phase contacts the cooling wall directly, while in the case of the W/O type, water from the dispersed phase hardly contacts the cooling wall because of the presence of oil. For the W/O type, possibility of ice adhesion depended on the emulsion composition. It is generally believed that the possibility of ice adhesion decreases with increasing oil content because water hardly comes in direct contact with the cooling wall in the case of higher oil contents. However, the case of lump oil was an exception. Thus, the relationship between possibility of ice adhesion and apparent
viscosity of the emulsion was very important. As shown in Table 2, in the cases of silicone oil (9:1) and light oil (9:1) with their high apparent viscosities, the effect of stirring in the vicinity of the cooling wall was not enough, and the formed ice stagnated at the cooling wall, leading to ice adhesion. By contrast, in the case of lump oil (7:3) with its lower apparent viscosity, it was concluded, on the basis of the above discussion on the relationship between emulsion stability and apparent viscosity, that the force maintaining the W/O emulsion structure was weak. Thus, it is thought that the water separating partially from the emulsion froze at the cooling wall. In the present system, the possibility of ice adhesion was lower in the apparent viscosity range of 10e23 Pa s (shearing velocity: 5 rpm, 0 C), and the ice adhesion was easy to dissolve if it occurred. However, in cases of silicone oil (9:1) and light oil (9:1), which far exceed this viscosity range, ice adhesion to the cooling wall occurred within 30 min after dissolution of supercooling and the adhesive force of the ice was very strong. By contrast, in the case of lump oil (7:3), which was far below the abovementioned viscosity range, ice adhesion occurred at about 2 h, and the adhesion force of the ice was strong. However, it is possible to change the apparent viscosity of the W/O type by varying the stirring conditions and the additive concentration. Thus, for silicone oil (9:1), light oil (9:1), and lump oil (7:3), it is thought that good ice formation can be realized by optimizing their apparent viscosities. 3.3.2. Ice particles Since best ice formation in this study was achieved for W/O type emulsions of silicone oil (7:3), light oil (7:3), and lump oil (9:1), the following discussion will focus on these three cases. For silicone oil (7:3), a photograph of the ice slurry formed immediately after dissolution of supercooling is shown in Fig. 13, and a photograph of ice particles at 7 h after dissolution of supercooling is shown in Fig. 14. For light oil (7:3), a photograph of ice
Ice
Wall Fig. 12. Adhesion of ice to cooling wall (W/O type, lump oil (7:3)).
Fig. 13. Ice particles immediately after dissolution of supercooling (W/O type, silicone oil (7:3)).
K. Matsumoto et al. / International Journal of Refrigeration 29 (2006) 1010e1019
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100
Ice particle
IPF measurement with volume change IPF measurement with heater
IPF( )
80
60
40
20
0
0
2
4
6
Time(h) Fig. 14. Ice particles at 7 h after dissolution of supercooling (W/O type, silicone oil (7:3)).
Fig. 16. Comparison of IPF values (W/O type, silicone oil (7:3)).
particles at 7 h after dissolution of supercooling is shown in Fig. 15. Several white spots about 1 mm in size can be seen in Fig. 13; these are ice particles that formed immediately after dissolution. Though Fig. 13 cannot be simply compared with Fig. 6 because of the difference in composition, on the basis of the enlarged photograph of the W/O type in Fig. 6, it was thought that 10e20 dispersed water droplets formed one lump, and this lump transformed into the ice particle shown in Fig. 13 upon cooling with stirring. Fig. 14 reveals that the ice particles that formed at 7 h after dissolution of supercooling were much larger than those immediately after dissolution. The sizes of the former were about 2e3 mm. The one ice particle that formed immediately after dissolution contacted the surrounding dispersed water droplets with time, the ice particle and dispersed water droplets coalesced, and the ice particle grew upon cooling, and it became spherical with stirring. Fig. 15 shows that, in the case of light oil (7:3), the mean size of the ice particles formed was 1e2 mm, which was smaller than that for silicone oil
(7:3). The difference in mean ice particle size is attributable to differences in heat transfer and degree of coalescence of the dispersed phase caused by difference in apparent viscosity. 3.3.3. IPF of ice slurry Fig. 16 shows the IPF measurement results for the W/O type emulsion of silicone oil (7:3), obtained by the two different methods mentioned above. The difference between the results of the two methods was very small, suggesting that the IPF measurements were reliable. IPF results for W/O type silicone oil (7:3) and lump oil (9:1) are shown in Fig. 17. The PMP beaker was used. For silicone oil (7:3), the IPF became 70% at 7 h after dissolution of supercooling, and simultaneously, all the water in the emulsion froze. For lump oil (9:1), the IPF became 90% at 6 h after dissolution of supercooling, and simultaneously, all the water in the emulsion froze. That is, the ice formation rate of lump oil (9:1) was faster than that of silicone oil (7:3). This is because the thermal conductivity 100
IPF( )
80
60
40
20
Ice particle
0
Lump oil(9:1) Silicone oil(7:3) 0
2
4
6
Time(h) Fig. 15. Ice particles at 7 h after dissolution of supercooling (W/O type, light oil (7:3)).
Fig. 17. Time dependence of IPF (W/O type, silicone oil (7:3) and lump oil (9:1)).
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Fig. 18. Ice slurry (W/O type, lump oil (9:1), IPF ¼ 10.7%).
Fig. 20. Ice slurry (W/O type, lump oil (9:1), IPF ¼ 90.0%).
of water is larger than those of silicone oil and lump oil, and the water content of lump oil (9:1) is larger than that of silicone oil (7:3). For lump oil (9:1), the states of the ice slurry that formed at 1, 2 and 6 h after dissolution are shown in Figs. 18e20, respectively. Figs. 18 and 19 reveal that the ice slurry with 10e20% IPF had good fluidity. Fig. 20 shows that granular ice particles were formed when IPF became 90%, at which point all the water in the emulsion froze. To discuss transport of the ice slurries, apparent viscosities of ice slurries made from silicone oil (7:3) and lump oil (9:1) are compared in Fig. 21. Those apparent viscosities were measured in the process of increasing IPF. The figure reveals that the difference between the two slurries was small up to an IPF of 10%, but increased with increasing IPF thereafter. It is thought that the difference was larger because the apparent viscosity depends on the emulsion composition, the ice particle size and the dispersed phase size. For silicone oil (7:3), apparent viscosity was smaller for IPF values below about 30%, and thus, the ice slurry made from silicone oil (7:3) had good fluidity.
3.3.4. Ice formation in stainless beaker The ice formation rate in PMP beaker is slow. It is essential to increase the ice formation rate for practical applications. Thus, a stainless steel beaker, whose thermal conductivity was about 50 times that of the PMP beaker, was used. The IPF of the ice slurry made from silicone oil (7:3) is shown in Fig. 22. For comparison, the IPF obtained in the case of the PMP beaker is shown as well. Fig. 22 shows that in the case of the stainless steel beaker, the IPF reached 70% at about 2 h after dissolution of supercooling without ice adhesion to the cooling wall, at which time, all the water in the emulsion froze. The ice formation rate was much higher in the case of the stainless steel beaker than in the case of the PMP beaker. Similar results were obtained for light oil (7:3) and lump oil (9:1), which exhibited the best ice formation in this study. The above discussion indicates that water hardly contacts the cooling wall directly because of the structural property of the W/O type emulsion, i.e., it is thought that the inherent structure of the W/O type
Apparent viscosity(Pa . s)
800 Lump oil(9:1) Silicone oil(7:3) 600
400
200
0 0
10
20
30
40
50
IPF( ) Fig. 19. Ice slurry (W/O type, lump oil (9:1), IPF ¼ 23.8%).
Fig. 21. Relationship between IPF and apparent viscosity of ice slurry (W/O type, shear velocity: 5 rpm, 0 C).
K. Matsumoto et al. / International Journal of Refrigeration 29 (2006) 1010e1019
4. Conclusions
100 Stainless-steel vessel PMP vessel
IPF( )
80 60 40 20 0
1019
0
100
200
300
400
Time(m)
(1) O/W and W/O type emulsions were developed as thermal storage material for ice storage. (2) The two types of emulsions were characterized and their adaptability to ice storage was investigated. (3) The W/O type emulsion was demonstrated to be suitable for ice storage. (4) It was found that the inherent structure of the W/O type emulsion prevented ice adhesion, even when a stainless steel beaker was used. (5) It was found that the W/O emulsions silicone oil (7:3), light oil (7:3) and lump oil (9:1) exhibited the best ice formation.
Fig. 22. Time dependence of IPF (W/O type, silicone oil (7:3)).
Acknowledgements emulsion prevents ice adhesion regardless of the beaker material. 3.3.5. Repetitive formation of ice slurry from emulsion In the case of the W/O type with silicone oil (7:3), once an ice slurry with an IPF below 40% made from the W/O type emulsion changed into a mixture of water, oil and W/O type emulsion due to melting at room temperature without stirring, a new W/O type emulsion could be made from the mixture with stirring. If the IPF was above 40%, however, a new emulsion could not be formed. For W/O type with light oil (7:3), re-formation of an emulsion was possible below 70% IPF. For W/O type with lump oil (9:1), re-formation of an emulsion was possible below 50% IPF. New ice slurries could be made from the reformed W/O type emulsions by cooling with stirring without ice adhesion to the cooling wall. During transport, the IPF of an ice slurry is about 20e30%. Thus, W/O type emulsions made from silicone oil (7:3), light oil (7:3), and lump oil (9:1) are suitable for repetitive formation of emulsion and ice slurry. Moreover, in the cases of these three W/O type emulsions, after an ice slurry with the highest IPF was formed by freezing all the water in the emulsion with stirring, all the ice in the ice slurry was melted by maintaining the ice slurry at room temperature with 50 rpm stirring, which was lower than the ordinary stirring speed (250 rpm), and then a new W/O type emulsion could be re-formed from the ice slurry melted by ordinary stirring. A new ice slurry could be made from each re-formed W/O type emulsion by cooling with stirring without ice adhesion.
This study was financially supported by Chuo University as one of the 2005 Research Projects for Promotion of Advanced Research at Graduate School. The authors wish to thank Mr. Nishiyama, Mr. Sekine, Mr. Doi, Mr. Sakae and Mr. Mitamura, who are graduates of Chuo University, for their collaboration.
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