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Impregnation of porous material with phase change material for thermal energy storage
Materials Chemistry and Physics 115 (2009) 846–850
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Impregnation of porous material with phase change material for thermal energy storage Takahiro Nomura ∗ , Noriyuki Okinaka, Tomohiro Akiyama Center for Advanced Research of Energy Conversion Materials, Hokkaido University, Kita 13 Nishi 8, Kita-ku, Sapporo 060-8628, Japan
a r t i c l e
i n f o
Article history: Received 18 August 2008 Received in revised form 13 January 2009 Accepted 19 February 2009 Keywords: Phase change material Latent heat Composite Porous material
a b s t r a c t In order to efficiently recover waste heat in the form of latent heat, we studied the impregnation of a porous material with a phase change material (PCM); erythritol was selected as the PCM and expanded perlite (EP), diatom earth (DE), and gamma-alumina (GA) were selected as porous materials. Effects of vacuum in impregnation, pore size of porous materials, holding time and cyclic test on thermal properties of composites; latent heat, melting temperature, were mainly examined by using DSC. The following results were obtained. (1) The pores of EP were completely filled with liquid erythritol by the vacuum impregnation treatment, and the latent heat of the EP/erythritol composite reached 83% of the theoretical latent heat of pure erythritol (294.4 J g−1 ). (2) Porous materials with small pore sizes showed a low melting temperature for phase change composites. (3) The pores of EP were completely filled with erythritol at 1.8 ks immersion in the vacuum impregnation treatment. (4) EP/erythritol composite retained 75% of the impregnated PCM, even in a cyclic process of heating and cooling. In conclusion, the impregnation of porous material with erythritol is a promising method for conserving latent heat with high thermal storage density. © 2009 Elsevier B.V. All rights reserved.
1. Introduction New and renewable energy sources are being investigated all over the world. The development of energy storage devices is as important as developing new energy sources. Thermal energy storage systems have the potential to help in conserving energy, which in turn reduces the environmental impact. In fact, these systems balance the energy supply and demand [1]. Latent heat storage by using phase change material (PCM) for recovering waste heat from industries is quite an attractive option from the following four viewpoints [2,3]. Heat source of constant temperature: First, the PCM stores heat in the form of latent heat of fusion and then releases thermal energy at a fixed melting point of the PCM during solidification. High storage density: Generally, the latent heat of the PCM is 50–100 times more than the sensible heat. Heat recovery with small temperature drop: The PCM can regenerate thermal energy at high temperatures at a melting point slightly lower than the temperature of waste heat. Repeatable utilization: The melting and solidifying processes of the PCM can be repeated for a long time. The three typical heat exchangers for latent heat storage are direct contact, shell and tube, and packed beds [4]. In particular, a packed-bed heat exchanger using PCM capsules has advantages
∗ Corresponding author. Tel.: +81 11 706 6842; fax: +81 11 706 6849. E-mail address: [email protected] (T. Nomura). 0254-0584/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2009.02.045
such as quick thermal response and large heat storage density due to a large area of heat transfer. Therefore, many researches have focused on the mass production of PCM capsules [5,6]. For example, Maruoka and Akiyama proposed an electroplating method for PCM encapsulation [7]; in this method a spherical metal chosen as the PCM was uniformly coated with nickel and was successfully used at low temperatures. However, a PCM encapsulation process for high-temperature (∼100 ◦ C) applications has never been established because of problems such as PCM leakage during melting and the difference in the expansion ratio between the PCM and the capsule that sometimes causes cracks in the latter. For this reason, the thickness of the capsule layer must be increased, decreasing heat storage density of capsule beds. In addition, another problem is the short operating life of the PCM beds. This is because the metallic capsule is corroded by the molten metallic PCM. In contrast, a granular phase change composite is attractive since it need not be encapsulated, provides no corrosion, enables quick heat transfer, and offers a larger heat storage density if porous media with high porosity are selected. For preparing the composite, a very simple impregnation method is used. Impregnation methods for the granular phase change composite have already been studied [8–14]; however, to the best of our knowledge, reports on such composites are limited to low-temperature applications. The purpose of this paper is to study granular phase change composite with a large heat storage density and long operating life for high-temperature application, by impregnation methods. In the experiments, we examined
T. Nomura et al. / Materials Chemistry and Physics 115 (2009) 846–850
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Table 1 Specifications of erythritol used in this study. Melting point (◦ C)
Latent heat (kJ kg−1 )
Density (kg m−3 )
Specific heat (kJ(kg K)−1 )
Liquid
Solid
Liquid
Solid
118
354.7
1.28
1.45
2.66
1.68
Viscosity (mPa s)
16.0
Table 2 Specifications of porous materials used in this study. Material
Main components
True density (kg m−3 )
Bulk density (kg m−3 )
Porosity (–)
Average pore size (nm)
Average particle size (mm)
Shape
Expanded perlite (EP) Diatom earth (DE) Gamma-alumina (GA)
SiO2 , Al2 O3 SiO2 Al2 O3
1366 2445 3300
234 638 780
0.83 0.74 0.55
110 47 8
2.0 ± 1.0 1.0 ± 0.5 1.0 ± 0.5
Grain Grain Grain
the effect of the impregnation conditions on the latent heat and melting point of the composite. This study provides information on how to easily produce a thermal storage medium with a large heat storage density, thereby conserving a significant amount of energy. 2. Experimental 2.1. Materials Table 1 provides the specifications of erythritol. In the experiments, erythritol (C4 H8 O4 , Tmp : 118 ◦ C, latent heat: 354.7 kJ kg−1 ) was selected as the PCM. Erythritol has a high latent heat of 354.7 kJ kg−1 and an appropriate melting point for storing industrial waste heat. In fact, erythritol is used for the recovery of waste heat. In addition, it is a kind of sugar alcohol and completely safe. Expanded perlite (EP), diatom earth (DE), and gamma-alumina (GA) were selected as the porous materials. Table 2 shows the specifications of these three porous materials used in the experiments, and Fig. 1 shows their pore size distribution. EP is a volcanic amorphous porous material that is prepared by expanding perlite to 7–16 times its original volume; the expansion occurs when perlite is heated at a temperature range of 700–1200 ◦ C and water trapped in the structure of the material vaporizes and escapes. DE is also a natural porous material and consists of fossilized
remains of diatoms, a type of hard-shelled algae. These two porous materials were selected as supporting media from very large porosity. 2.2. Impregnation experiments Fig. 2 shows the procedure of impregnation treatment with/without vacuuming. Firstly, solid PCM and porous materials were placed in an electric furnace with a vacuum pump; the vacuum pump was used for evacuating the air in the furnace. Secondly, the furnace was maintained at 150 ◦ C in order to melt erythritol. Thirdly, porous materials have been physically placed in liquid PCM for 1.8 ks. When using EP, the holding time in liquid PCM was changed to 0.6 ks, 1.2 ks, 1.8 ks, and 3.6 ks to study the effect of time on impregnation. Fourth, the vacuum pump was turned off to allow air to reenter the furnace; the porous materials filled with the PCM were taken out from liquid PCM with a stainless mesh and then for removing liquid PCM captured by the surface of composites or not supported in pore, they were kept in the furnace which was keeping at 150 ◦ C. Finally, the products were taken out from the furnace and dried. Subsequently, differential scanning calorimetry (DSC) was used to evaluate the latent heat and melting point of the products, and a scanning electron microscope (SEM) aided in clarifying the internal structure of the product. In addition, cyclic tests of heating and cooling were performed on the EP/erythritol composite to evaluate the usable heat as latent heat.
3. Results and discussions 3.1. Macro- and microstructure Fig. 3 shows the photos and SEM images of EP before and after the vacuum impregnation treatment. The macroscopic view of EP appears the same before and after the impregnation treatment, as shown in Fig. 3(a) and (b), respectively; however, the microstructure of the sample had changed; the dark-colored area, that is, the pores was completely filled with erythritol after the impregnation treatment (see Fig. 3(c) and (b)). Similar microscopic images were also observed in other samples. This fact demonstrated that the PCM was well impregnated into the porous material under the vacuum conditions, as expected. 3.2. Thermophysical properties
Fig. 1. Pore size distribution of porous materials as measured by mercury porosimetry and BET.
Fig. 4(a) shows the DSC curves of pure erythritol and the EP and DE/erythritol composites prepared by vacuum impregnation (impregnation time; 1.8 ks). The EP and DE/erythritol composites have slightly different DSC curves in comparison to pure erythritol. The melting point of the samples can be evaluated from the slope of the curves. Note that the melting point of the EP/erythritol composite was the same as that of pure erythritol, but the melting point of the DE/erythritol composite was slightly lower than that of pure erythritol. Fig. 4(b) shows the DSC curve of the GA/erythritol composite. The GA/erythritol composite, together with pure erythritol, is shown in Fig. 4(b). Interestingly, the GA/erythritol composite had a significantly different DSC curve from the others; that is, it had two small endothermic peaks, with one peak being observed around the melting temperature of pure erythritol (118.0 ◦ C) and the other at
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T. Nomura et al. / Materials Chemistry and Physics 115 (2009) 846–850
Fig. 2. Image of vacuum impregnation treatment for preparing phase change composite (VP: vacuum pump).
a much lower and wide temperature zone. The results suggested that erythritol in the nano-sized pores of porous materials had significantly low melting temperature. Moreover, for the EP/erythritol composites, the latent heat calculated by multiplying the mass ratio of erythritol in the composite and its latent heat value [15,16]; 332.2 kJ kg−1 , is approximately consistent with the value which were measured by DSC. On the contrary, for DE/erythritol composite, the measured value was slightly less than the calculated value; 248.8 kJ kg−1 , moreover for GA/erythritol composite, the measured value was much less than the calculated value; 134.8 kJ kg−1 . These phenomena were probably caused by the abnormal interaction between the PCM and the inner surface of the nano-sized pore [17–19]. Especially, for the GA/erythritol composite, the effect is quite preeminent and it has a small latent heat at low melting points. Therefore it is of significant academic interest; however, owing to this property, it is not suitable for practical heat storage medium. In conclusion, EP and DE/erythritol composites showing only one endothermic peak around the melting point of pure erythritol (118.0 ◦ C) were well explained by the mass fraction between
erythritol and porous material and these composites are promising candidates for heat storage media. 3.3. Latent heat in composites and effect of vacuuming Fig. 5 shows the measured latent heat of the samples prepared by impregnation with and without vacuuming, together with the theoretical latent heat value. The theoretical value was calculated according to Eq. (1) under the assumption that the pores were completely filled with erythritol. HTheoretical =
εerythritol HErythritol porous (1 − ε) + εerythritol
(1)
Here, H, ε, and represent the latent heat of pure erythritol, porosity, and density, respectively. The latent heat of the sample prepared with vacuuming was always the largest among the three samples. For EP and DE, the measured value was larger than the theoretical one because the PCM was captured by the surface and not by the pores. Note that the vacuum impregnation treatment increased
Fig. 3. Photos and SEM images of EP before and after vacuum impregnation treatment.
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Fig. 6. Histories of latent heat of EP/erythritol composites prepared using different impregnation treatments.
the impregnation. In the impregnation experiments, the pore geometry of DE was sufficiently simple for replacing the air within the pores by the PCM; the pores were completely filled with the PCM by the impregnation treatment at normal pressure without vacuuming. In contrast, the pore geometry of EP was too complicated to evacuate the air within the pores before the impregnation treatment. Furthermore, the latent heat of the EP and DE/erythritol composites was greater than the theoretical values because the PCM was attached on the surface, in addition to being impregnated in all the pores. As result, the EP/erythritol composite prepared by the vacuum impregnation treatment had the largest latent heat, which was 83% of that of pure erythritol. Fig. 4. (a) DSC curves of pure erythritol and EP and DE/erythritol composites by vacuum impregnation treatment (impregnation time: 1.8 ks). (b) DSC curves of pure erythritol and GA/erythritol composite by vacuum impregnation treatment (impregnation time: 1.8 ks).
the latent heat of the EP/erythritol composite by as much as 30% in comparison to the normal pressure treatment. On the other hand, there was no remarkable difference in the DE/erythritol composite. The influence of the treatment probably depended on the pore geometry of the sample used. Under the impregnation treatment, the actual relationship is expressed by the following equation: PPCM D2 + 4D cos > Pair D2
(2)
Here, P, D, , and represent the pressure, pore diameter, surface tension of the PCM, and contact angle, respectively. Liquid PCM was impregnated through capillary forces in a porous material, but the air pressure within the pores prevented
3.4. Effect of impregnation time on latent heat in composites Fig. 6 shows the changes in the latent heat of the EP/erythritol composites prepared by different treatments with/without vacuuming. The latent heat of both samples changed with time, and during the change, the liquid PCM replaced the air within the pores. The vacuum-impregnated sample increased the latent heat very rapidly from the beginning to 0.6 ks, then increased slightly from 0.6 ks to 1.8 ks, and finally reached a constant value at 1.8 ks. The value was 10% larger than that prepared at 0.6 ks; this was influenced from the fact that the pores in EP were completely filled with erythritol. In contrast, in the treatment without vacuuming, the impregnated sample without vacuum did not increase the latent heat in the duration between 0.6 ks and 1.8 ks. This is the reason why the capillary force and the air pressure within the pores were balanced, and it can be expressed by the following equation. PPCM D2 + 4D cos = Pair D2
(3)
In this case, the sample had scattered air within the pores even after cooling and the pores were also filled with the PCM. The sample that is not fully impregnated cannot be used as a PCM composite in practice because the scattered air within the pores expands significantly to cause the ejection of the PCM during the actual operation at elevated temperatures. 3.5. Cyclic test
Fig. 5. Comparison between vacuum impregnation treatment and impregnation treatment for each porous material.
Fig. 7 shows changes in the latent heat of the EP/erythritol composite in the cyclic test of heating and cooling. The latent heat decreased up to the forth repetition of the cyclic test, and subsequently, it was constant at 75 mass% of the initial value. This was probably caused by the leakage of the PCM attached on the surface and within the pores due to the thermal expansion during the melting. The pore diameter of EP had a wide distribution (see Fig. 1).
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(1) The latent heat of the EP/erythritol composite prepared by vacuum-impregnation was as much as 83 mass% of pure erythritol. (2) The sample with small pore sizes had a low melting temperature due to the nano-size effect. (3) Vacuum impregnation at 1.8 ks was quite effective for completely filling the pores of EP with erythritol. (4) In the cyclic test of heating and cooling, the EP/erythritol composite maintained 75 mass% of the initial latent heat after the test was repeated four times. References Fig. 7. Latent heat of EP/erythritol composites (impregnation time: 1.8 ks) in cyclic heating and cooling process.
Relatively large pores had a very small capillary force to retain the liquid PCM at the melting point. From the experiments and Fig. 1, a pore diameter of 10 m can be considered as the threshold for enabling EP to retain liquid erythritol. 4. Conclusions Granular phase change composites were prepared by the impregnation method in which the pores of three different porous materials were filled with liquid erythritol (as PCM) for the different vacuum conditions, impregnation times, and cycles of heating and cooling. The following conclusions were obtained.
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
A. Pasupathy, R. Velraj, R.V. Seeniraj, Renew. Sust. Energ. Rev. 12 (2008) 39. N. Maruoka, K. Sato, J. Yagi, T. Akiyama, ISIJ Int. 42 (2002) 215. N. Maruoka, T. Akiyama, ISIJ Int. 42 (2002) 1189. T. Akiyama, Y. Ashizawa, J. Yagi, Heat transf Jpn Res 21 (1992) 199. T. Akiyama, J. Yagi, High Temp. Mater. Proc. 19 (2000) 3. G.A.A. Asselman, Energy Conv. 16 (1976) 35. N. Maruoka, T. Akiyama, J. Chem. Eng. Jpn. 36 (2003) 794. Z. Zhang, X. Fang, Energy Convers. Manage. 47 (2006) 303. X. Pay, R. Olives, S. Mauran, Heat Mass Transf. 44 (2001) 2727. A. Sari, A. Karaipekli, Appl. Therm. Eng. 27 (2007) 1271. D. Zang, S. Tian, D. Xiao, Sol. Energy 81 (2007) 653. A.D. Fedden, M.E. Franke. AIAA 2006. D. Zhang, J. Zhou, K. Wu, Z. Li, Sol. Energy 78 (2005) 471. A. Sari, A. Karaipeki, Mater. Chem. Phys. 109 (2008) 459. A. Karaipeki, A. Sari, Renew. Energy 33 (2008) 2599. A. Sari, A. Karaipeki, K. Kaygusuz, Energy Source A 30 (2008) 464. K. Ishikiriyama, A. Sakamoto, M. Todoroki, T. Tayama, K. Tanaka, T. Kobayashi, J. Colloid Interf. Sci. 173 (1995) 419. [18] K. Ishikiriyama, A. Sakamoto, M. Todoroki, T. Tayama, K. Tanaka, T. Kobayashi, Thermochim. Acta 267 (1995) 169. [19] R. Radhakrishnan, K.E. Gubbins, M.S. Bartkowiak, Thermochim. Acta 89 (2002) 7.
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Impregnation of porous material with phase change material for thermal energy storage Takahiro Nomura ∗ , Noriyuki Okinaka, Tomohiro Akiyama Center for Advanced Research of Energy Conversion Materials, Hokkaido University, Kita 13 Nishi 8, Kita-ku, Sapporo 060-8628, Japan
a r t i c l e
i n f o
Article history: Received 18 August 2008 Received in revised form 13 January 2009 Accepted 19 February 2009 Keywords: Phase change material Latent heat Composite Porous material
a b s t r a c t In order to efficiently recover waste heat in the form of latent heat, we studied the impregnation of a porous material with a phase change material (PCM); erythritol was selected as the PCM and expanded perlite (EP), diatom earth (DE), and gamma-alumina (GA) were selected as porous materials. Effects of vacuum in impregnation, pore size of porous materials, holding time and cyclic test on thermal properties of composites; latent heat, melting temperature, were mainly examined by using DSC. The following results were obtained. (1) The pores of EP were completely filled with liquid erythritol by the vacuum impregnation treatment, and the latent heat of the EP/erythritol composite reached 83% of the theoretical latent heat of pure erythritol (294.4 J g−1 ). (2) Porous materials with small pore sizes showed a low melting temperature for phase change composites. (3) The pores of EP were completely filled with erythritol at 1.8 ks immersion in the vacuum impregnation treatment. (4) EP/erythritol composite retained 75% of the impregnated PCM, even in a cyclic process of heating and cooling. In conclusion, the impregnation of porous material with erythritol is a promising method for conserving latent heat with high thermal storage density. © 2009 Elsevier B.V. All rights reserved.
1. Introduction New and renewable energy sources are being investigated all over the world. The development of energy storage devices is as important as developing new energy sources. Thermal energy storage systems have the potential to help in conserving energy, which in turn reduces the environmental impact. In fact, these systems balance the energy supply and demand [1]. Latent heat storage by using phase change material (PCM) for recovering waste heat from industries is quite an attractive option from the following four viewpoints [2,3]. Heat source of constant temperature: First, the PCM stores heat in the form of latent heat of fusion and then releases thermal energy at a fixed melting point of the PCM during solidification. High storage density: Generally, the latent heat of the PCM is 50–100 times more than the sensible heat. Heat recovery with small temperature drop: The PCM can regenerate thermal energy at high temperatures at a melting point slightly lower than the temperature of waste heat. Repeatable utilization: The melting and solidifying processes of the PCM can be repeated for a long time. The three typical heat exchangers for latent heat storage are direct contact, shell and tube, and packed beds [4]. In particular, a packed-bed heat exchanger using PCM capsules has advantages
∗ Corresponding author. Tel.: +81 11 706 6842; fax: +81 11 706 6849. E-mail address: [email protected] (T. Nomura). 0254-0584/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2009.02.045
such as quick thermal response and large heat storage density due to a large area of heat transfer. Therefore, many researches have focused on the mass production of PCM capsules [5,6]. For example, Maruoka and Akiyama proposed an electroplating method for PCM encapsulation [7]; in this method a spherical metal chosen as the PCM was uniformly coated with nickel and was successfully used at low temperatures. However, a PCM encapsulation process for high-temperature (∼100 ◦ C) applications has never been established because of problems such as PCM leakage during melting and the difference in the expansion ratio between the PCM and the capsule that sometimes causes cracks in the latter. For this reason, the thickness of the capsule layer must be increased, decreasing heat storage density of capsule beds. In addition, another problem is the short operating life of the PCM beds. This is because the metallic capsule is corroded by the molten metallic PCM. In contrast, a granular phase change composite is attractive since it need not be encapsulated, provides no corrosion, enables quick heat transfer, and offers a larger heat storage density if porous media with high porosity are selected. For preparing the composite, a very simple impregnation method is used. Impregnation methods for the granular phase change composite have already been studied [8–14]; however, to the best of our knowledge, reports on such composites are limited to low-temperature applications. The purpose of this paper is to study granular phase change composite with a large heat storage density and long operating life for high-temperature application, by impregnation methods. In the experiments, we examined
T. Nomura et al. / Materials Chemistry and Physics 115 (2009) 846–850
847
Table 1 Specifications of erythritol used in this study. Melting point (◦ C)
Latent heat (kJ kg−1 )
Density (kg m−3 )
Specific heat (kJ(kg K)−1 )
Liquid
Solid
Liquid
Solid
118
354.7
1.28
1.45
2.66
1.68
Viscosity (mPa s)
16.0
Table 2 Specifications of porous materials used in this study. Material
Main components
True density (kg m−3 )
Bulk density (kg m−3 )
Porosity (–)
Average pore size (nm)
Average particle size (mm)
Shape
Expanded perlite (EP) Diatom earth (DE) Gamma-alumina (GA)
SiO2 , Al2 O3 SiO2 Al2 O3
1366 2445 3300
234 638 780
0.83 0.74 0.55
110 47 8
2.0 ± 1.0 1.0 ± 0.5 1.0 ± 0.5
Grain Grain Grain
the effect of the impregnation conditions on the latent heat and melting point of the composite. This study provides information on how to easily produce a thermal storage medium with a large heat storage density, thereby conserving a significant amount of energy. 2. Experimental 2.1. Materials Table 1 provides the specifications of erythritol. In the experiments, erythritol (C4 H8 O4 , Tmp : 118 ◦ C, latent heat: 354.7 kJ kg−1 ) was selected as the PCM. Erythritol has a high latent heat of 354.7 kJ kg−1 and an appropriate melting point for storing industrial waste heat. In fact, erythritol is used for the recovery of waste heat. In addition, it is a kind of sugar alcohol and completely safe. Expanded perlite (EP), diatom earth (DE), and gamma-alumina (GA) were selected as the porous materials. Table 2 shows the specifications of these three porous materials used in the experiments, and Fig. 1 shows their pore size distribution. EP is a volcanic amorphous porous material that is prepared by expanding perlite to 7–16 times its original volume; the expansion occurs when perlite is heated at a temperature range of 700–1200 ◦ C and water trapped in the structure of the material vaporizes and escapes. DE is also a natural porous material and consists of fossilized
remains of diatoms, a type of hard-shelled algae. These two porous materials were selected as supporting media from very large porosity. 2.2. Impregnation experiments Fig. 2 shows the procedure of impregnation treatment with/without vacuuming. Firstly, solid PCM and porous materials were placed in an electric furnace with a vacuum pump; the vacuum pump was used for evacuating the air in the furnace. Secondly, the furnace was maintained at 150 ◦ C in order to melt erythritol. Thirdly, porous materials have been physically placed in liquid PCM for 1.8 ks. When using EP, the holding time in liquid PCM was changed to 0.6 ks, 1.2 ks, 1.8 ks, and 3.6 ks to study the effect of time on impregnation. Fourth, the vacuum pump was turned off to allow air to reenter the furnace; the porous materials filled with the PCM were taken out from liquid PCM with a stainless mesh and then for removing liquid PCM captured by the surface of composites or not supported in pore, they were kept in the furnace which was keeping at 150 ◦ C. Finally, the products were taken out from the furnace and dried. Subsequently, differential scanning calorimetry (DSC) was used to evaluate the latent heat and melting point of the products, and a scanning electron microscope (SEM) aided in clarifying the internal structure of the product. In addition, cyclic tests of heating and cooling were performed on the EP/erythritol composite to evaluate the usable heat as latent heat.
3. Results and discussions 3.1. Macro- and microstructure Fig. 3 shows the photos and SEM images of EP before and after the vacuum impregnation treatment. The macroscopic view of EP appears the same before and after the impregnation treatment, as shown in Fig. 3(a) and (b), respectively; however, the microstructure of the sample had changed; the dark-colored area, that is, the pores was completely filled with erythritol after the impregnation treatment (see Fig. 3(c) and (b)). Similar microscopic images were also observed in other samples. This fact demonstrated that the PCM was well impregnated into the porous material under the vacuum conditions, as expected. 3.2. Thermophysical properties
Fig. 1. Pore size distribution of porous materials as measured by mercury porosimetry and BET.
Fig. 4(a) shows the DSC curves of pure erythritol and the EP and DE/erythritol composites prepared by vacuum impregnation (impregnation time; 1.8 ks). The EP and DE/erythritol composites have slightly different DSC curves in comparison to pure erythritol. The melting point of the samples can be evaluated from the slope of the curves. Note that the melting point of the EP/erythritol composite was the same as that of pure erythritol, but the melting point of the DE/erythritol composite was slightly lower than that of pure erythritol. Fig. 4(b) shows the DSC curve of the GA/erythritol composite. The GA/erythritol composite, together with pure erythritol, is shown in Fig. 4(b). Interestingly, the GA/erythritol composite had a significantly different DSC curve from the others; that is, it had two small endothermic peaks, with one peak being observed around the melting temperature of pure erythritol (118.0 ◦ C) and the other at
848
T. Nomura et al. / Materials Chemistry and Physics 115 (2009) 846–850
Fig. 2. Image of vacuum impregnation treatment for preparing phase change composite (VP: vacuum pump).
a much lower and wide temperature zone. The results suggested that erythritol in the nano-sized pores of porous materials had significantly low melting temperature. Moreover, for the EP/erythritol composites, the latent heat calculated by multiplying the mass ratio of erythritol in the composite and its latent heat value [15,16]; 332.2 kJ kg−1 , is approximately consistent with the value which were measured by DSC. On the contrary, for DE/erythritol composite, the measured value was slightly less than the calculated value; 248.8 kJ kg−1 , moreover for GA/erythritol composite, the measured value was much less than the calculated value; 134.8 kJ kg−1 . These phenomena were probably caused by the abnormal interaction between the PCM and the inner surface of the nano-sized pore [17–19]. Especially, for the GA/erythritol composite, the effect is quite preeminent and it has a small latent heat at low melting points. Therefore it is of significant academic interest; however, owing to this property, it is not suitable for practical heat storage medium. In conclusion, EP and DE/erythritol composites showing only one endothermic peak around the melting point of pure erythritol (118.0 ◦ C) were well explained by the mass fraction between
erythritol and porous material and these composites are promising candidates for heat storage media. 3.3. Latent heat in composites and effect of vacuuming Fig. 5 shows the measured latent heat of the samples prepared by impregnation with and without vacuuming, together with the theoretical latent heat value. The theoretical value was calculated according to Eq. (1) under the assumption that the pores were completely filled with erythritol. HTheoretical =
εerythritol HErythritol porous (1 − ε) + εerythritol
(1)
Here, H, ε, and represent the latent heat of pure erythritol, porosity, and density, respectively. The latent heat of the sample prepared with vacuuming was always the largest among the three samples. For EP and DE, the measured value was larger than the theoretical one because the PCM was captured by the surface and not by the pores. Note that the vacuum impregnation treatment increased
Fig. 3. Photos and SEM images of EP before and after vacuum impregnation treatment.
T. Nomura et al. / Materials Chemistry and Physics 115 (2009) 846–850
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Fig. 6. Histories of latent heat of EP/erythritol composites prepared using different impregnation treatments.
the impregnation. In the impregnation experiments, the pore geometry of DE was sufficiently simple for replacing the air within the pores by the PCM; the pores were completely filled with the PCM by the impregnation treatment at normal pressure without vacuuming. In contrast, the pore geometry of EP was too complicated to evacuate the air within the pores before the impregnation treatment. Furthermore, the latent heat of the EP and DE/erythritol composites was greater than the theoretical values because the PCM was attached on the surface, in addition to being impregnated in all the pores. As result, the EP/erythritol composite prepared by the vacuum impregnation treatment had the largest latent heat, which was 83% of that of pure erythritol. Fig. 4. (a) DSC curves of pure erythritol and EP and DE/erythritol composites by vacuum impregnation treatment (impregnation time: 1.8 ks). (b) DSC curves of pure erythritol and GA/erythritol composite by vacuum impregnation treatment (impregnation time: 1.8 ks).
the latent heat of the EP/erythritol composite by as much as 30% in comparison to the normal pressure treatment. On the other hand, there was no remarkable difference in the DE/erythritol composite. The influence of the treatment probably depended on the pore geometry of the sample used. Under the impregnation treatment, the actual relationship is expressed by the following equation: PPCM D2 + 4D cos > Pair D2
(2)
Here, P, D, , and represent the pressure, pore diameter, surface tension of the PCM, and contact angle, respectively. Liquid PCM was impregnated through capillary forces in a porous material, but the air pressure within the pores prevented
3.4. Effect of impregnation time on latent heat in composites Fig. 6 shows the changes in the latent heat of the EP/erythritol composites prepared by different treatments with/without vacuuming. The latent heat of both samples changed with time, and during the change, the liquid PCM replaced the air within the pores. The vacuum-impregnated sample increased the latent heat very rapidly from the beginning to 0.6 ks, then increased slightly from 0.6 ks to 1.8 ks, and finally reached a constant value at 1.8 ks. The value was 10% larger than that prepared at 0.6 ks; this was influenced from the fact that the pores in EP were completely filled with erythritol. In contrast, in the treatment without vacuuming, the impregnated sample without vacuum did not increase the latent heat in the duration between 0.6 ks and 1.8 ks. This is the reason why the capillary force and the air pressure within the pores were balanced, and it can be expressed by the following equation. PPCM D2 + 4D cos = Pair D2
(3)
In this case, the sample had scattered air within the pores even after cooling and the pores were also filled with the PCM. The sample that is not fully impregnated cannot be used as a PCM composite in practice because the scattered air within the pores expands significantly to cause the ejection of the PCM during the actual operation at elevated temperatures. 3.5. Cyclic test
Fig. 5. Comparison between vacuum impregnation treatment and impregnation treatment for each porous material.
Fig. 7 shows changes in the latent heat of the EP/erythritol composite in the cyclic test of heating and cooling. The latent heat decreased up to the forth repetition of the cyclic test, and subsequently, it was constant at 75 mass% of the initial value. This was probably caused by the leakage of the PCM attached on the surface and within the pores due to the thermal expansion during the melting. The pore diameter of EP had a wide distribution (see Fig. 1).
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(1) The latent heat of the EP/erythritol composite prepared by vacuum-impregnation was as much as 83 mass% of pure erythritol. (2) The sample with small pore sizes had a low melting temperature due to the nano-size effect. (3) Vacuum impregnation at 1.8 ks was quite effective for completely filling the pores of EP with erythritol. (4) In the cyclic test of heating and cooling, the EP/erythritol composite maintained 75 mass% of the initial latent heat after the test was repeated four times. References Fig. 7. Latent heat of EP/erythritol composites (impregnation time: 1.8 ks) in cyclic heating and cooling process.
Relatively large pores had a very small capillary force to retain the liquid PCM at the melting point. From the experiments and Fig. 1, a pore diameter of 10 m can be considered as the threshold for enabling EP to retain liquid erythritol. 4. Conclusions Granular phase change composites were prepared by the impregnation method in which the pores of three different porous materials were filled with liquid erythritol (as PCM) for the different vacuum conditions, impregnation times, and cycles of heating and cooling. The following conclusions were obtained.
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