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Energy efficient Bio-based PCM with silica fume composites to apply in concrete for energy saving in buildings
Solar Energy Materials & Solar Cells 143 (2015) 430–434
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Energy efficient Bio-based PCM with silica fume composites to apply in concrete for energy saving in buildings Yujin Kang, Su-Gwang Jeong, Seunghwan Wi, Sumin Kim n Building Environment & Materials Lab, School of Architecture, Soongsil University, Seoul 156-743, Republic of Korea
art ic l e i nf o
a b s t r a c t
Article history: Received 2 May 2015 Received in revised form 6 July 2015 Accepted 17 July 2015
Bio-based PCMs are made from underused feedstock, such as soybean oil, and are significantly less flammable than paraffins. In this study, Bio-based PCMs were prepared with silica fume through a vacuum impregnation process. Silica fume is a mineral admixture product with an impermeable pore structure, and it is one of the most feasible candidates for use as economical, light-weight thermal energy storage materials for buildings [15,16]. We also used exfoliated graphite nanoplatelets (xGnP) to improve the thermal conductivity because PCMs have a low thermal conductivity. The vacuum impregnation process that was used in this study guarantees that the Bio-based PCM composites can store heat after incorporation. Scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), Differential scanning calorimetry (DSC), Thermo gravimetric analyses (TGA), and TCi analysis were then carried out to determine the characteristics of the Bio-based PCM and Bio-based PCM composites. & 2015 Elsevier B.V. All rights reserved.
Keywords: Bio-based PCM Silica fume Exfoliated graphite nanoplatelets (xGnP) Thermal conductivity
1. Introduction Final global energy consumption increased by 23% between 1990 and 2005, and space heating accounts for 53% of the energy consumed by households [1]. Thus, latent heat thermal energy storage (LHTES) through the use of phase change materials (PCMs) to store and release thermal energy is one of the most efficient and reliable ways to reduce energy consumption. PCMs have a high heat storage capacity and nearly isothermal phase change behavior [2,3] and are available as three types: organic, inorganic, and eutectic. Inorganic PCMs include salts, salt hydrates, metals, and alloys that have high latent heat per unit volume, a good thermal conductivity, low cost, and are also non-flammable. However, these have a limited usefulness because most metals that go through phase decomposition are corrosive and exhibit supercooling effects [4,5]. Organic PCMs are classified into paraffinic or non-paraffinic. Paraffinic PCMs have been extensively used because they possess a high heat latent, varied phase change temperatures, little supercooling, low vapor pressure in a melt, and good chemical stability [6]. However, most paraffins have a relatively low ignition resistance relative to non-paraffinic PCMs [7], and for this reason, we have chosen non-paraffinic alternatives, such as Bio-based PCM, that are significantly less flammable. Bio-based PCMs are made from underused and renewable n
Corresponding author. Tel.: þ 82 2 820 0665; fax: þ 82 2 816 3354. E-mail address: [email protected] (S. Kim).
http://dx.doi.org/10.1016/j.solmat.2015.07.026 0927-0248/& 2015 Elsevier B.V. All rights reserved.
feedstock, such as palm oil, coconut oil and soybean oil. These oils are composed of fatty acids, such as α- linolenic acid, which is an organic compound in many common vegetable oils that has a molecular formula C18H30O2. Bio-based PCMs are fully hydrogenated and so are expected to exhibit a good thermal stability throughout many phase change cycles without any risk of oxidation. In addition, they can absorb, store, and release large amounts of latent heat in a manner similar to conventional paraffin, and they are equally amenable to microencapsulation. They can be manufactured for the melting point to vary between 22.7 °C and 78.33 °C, and they are available from various fields in various climatic zones [8–10]. Bio-based PCMs can be loaded with silica fume, which is a secondary product from electric furnaces used in the manufacture of silicon metal or silicon alloy [11–14]. Silica fume is a mineral admixture that has an impermeable pore structure in concrete, and it is a highly effective material that can be used to develop high strength concrete with good performance characteristics [15,16]. Therefore, silica fume is a feasible candidate for use as economical and light-weight thermal energy storage materials in buildings. In this study, exfoliated graphite nanoplatelets (xGnP) were also used to improve the thermal conductivity because PCMs have a low thermal conductivity and doing so has a great effect on the heat storage efficiency of the materials. These particles consist of several layers of graphene sheets that have a very high aspect ratio [17,18]. This study is comparable to research that was previously conducted by our research team [2,19]. In one such study, the changes
Y. Kang et al. / Solar Energy Materials & Solar Cells 143 (2015) 430–434
in the thermal performance of silica fume were evaluated in response to incorporating three types of organic PCM for use as an aggregate for concrete. Another focused using carbon nanomaterials to improve the thermal conductivity of the Bio-based PCM. However, the first study did not attempt to improve the thermal conductivity of the samples that were prepared, and the second did not seek to form Bio-based PCMs with stable properties. Thus, in this study, we prepared Bio-based PCM/silica fume/xGnP composites and evaluated their thermal properties both to improve their thermal conductivity and to stabilize their shapes. Finally, we verified that the prepared composites have a high thermal conductivity and a stable shape. The Bio-based PCMs were impregnated with silica fume, and a mixture of silica fume 5 wt% xGnP was prepared using a vacuum impregnation process that was discuss in previous papers. The vacuum impregnation method guarantees that the PCMs store heat after they have been incorporated as a result of the capillary phenomenon and surface tension forces that are experienced during the process. In this paper, SEM, FT-IR, DSC, TGA, and TCi analyses were carried out to analyze the microstructure, chemical bonding, heat capacity, thermal resistance, and thermal conductivity of the Bio-based PCM composites.
2. Experiment 2.1. Materials In this study, Bio-based PCM with a melting temperature of 29.24 °C and a latent heat capacity of 147.2 J/g were used directly as PCM. These materials were obtained from the Korea C&S Corporation in South Korea. The silica fume was supplied from Taiwang, a leading international trading company in South Korea [19]. Exfoliated graphite nanoplatelets (xGnP) were prepared from sulfuric acid-intercalated expandable graphite (3772) (Asbury Graphite Mills, NJ) by applying the cost- and time-effective exfoliation process that was initially proposed by Drzal's group [17]. 2.2. Preparation The Bio-based PCM composites were prepared using vacuum impregnation process that conformed to the procedure described in a previous study. The silica fume and the xGnP were dried at 105 °C before the impregnation process. The 5 wt% xGnP was mixed in silica fume according to the mass percentage of the silica fume. The silica fume and the mixture were placed inside of a filtering flask connected to a water tromp apparatus to evacuate the air from the porous structure. The valve between the flask and the liquid PCM container was turned open to allow the flow into the flask, and the vacuum process continued for 90 min. Then, air was allowed to enter the flask again to force the liquid PCM to penetrate into the porous structure of the silica fume and the xGnP. After the penetration, a filtering process was needed to remove the excess PCM that remained in the flask. Two types of Bio-based PCM composite in a colloidal state were filtered with 1 mm filter paper until a granular sample appeared, and the sample was then dried in a vacuum drier at 80 °C for 24 h.
431
transform infrared spectroscopy (FT-IR, 300E Jasco) were then used to confirm the changes in the chemical groups of the composites at room temperature by analyzing the FT-IR spectra of the materials and the Bio-based PCM composites. Differential scanning calorimetry (DSC Q 1000, TA instrument, USA) was conducted to measure the thermal properties of the Bio-based PCM and PCM composites, such as the melting and freezing temperature and latent heat capacity. The DSC measurements were conducted at a 5 °C/min heating and cooling rate with a temperature ranging from 0 to 80 °C. The thermal durability of the Bio-based PCM and PCM composites was assessed via thermo gravimetric analysis (TGA, TA instrument, TGA Q 5000) on approximately 11–14 mg of the samples within the range from 20 to 600 °C and at a heating rate of 10 °C/min. TGA was measured in an atmosphere with 99.5% of nitrogen and 0.5% oxygen to prevent oxidation. A TCi thermal conductivity analyzer was used to measure the thermal conductivity of the Bio-based PCM and PCM composites at room temperature. The TCi developed by C-Therm Technologies Ltd. is a device that uses the Modified Transient Plane Source (MTPS) method to measure the thermal conductivity of a small sample [20].
3. Results and discussion 3.1. Microstructure of the Bio-based PCM composites The Bio-based PCM and Bio-based PCM composites exhibit morphologies with similar surfaces, as shown in Fig. 1. This means that the silica fume and mixture are uniformly dispersed into the microstructure of the Bio-based PCM. Fig. 2 shows SEM images of the Bio-based PCM, silica fume, xGnP, Bio-based PCM/silica fume, and Bio-based PCM/silica fume þxGnP 5 wt%. As shown in Fig. 2 (a), the Bio-based PCM has a smooth surface with particles of various sizes because these contains fatty acids, such as palm oils, soybean oils, and coconut oils [21]. Fig. 2(b) and (c) shows the porous structure of the silica fume and xGnP, respectively. Fig. 2 (d) and (e) confirmed that the Bio-based PCM was incorporated well into the silica fume and the xGnP mixture. Also, Fig. 2 (e) shows form of the xGnP flakes on the surface of Fig. 2(d). 3.2. FT-IR analysis of Bio-based PCM composites FT-IR spectroscopy was used to examine the chemical compatibility among the components of the composites. The FT-IR absorption spectra of the xGnP, silica fume, Bio-based PCM, and Bio-based PCM composites are shown in Fig. 3. The Bio-based PCM were made from fatty acids of vegetable oils, soybean oils, etc., with a molecular formula of C18H30O2. The Bio-based PCM has –CH3 bonding and –CH2 bonding because the PCMs are composed of oily components. The spectra of Bio-based PCM exhibit absorption bands in the 2916 cm 1 and 2845–2849 cm 1 region as a result of the stretching vibration of functional groups of –CH3 and –CH2. The peaks at 1739 cm 1 and 1171 cm 1 represent the C¼ O and C–O groups, respectively. Silica fume peaks were shown at 12001000 cm 1 and at 805 cm 1 due to the Si–O–Si bonding. These FT-IR spectra peak are shown in
2.3. Characterization techniques Scanning electron microscopy (SEM, JEOL JSM-6360A) was carried out at room temperature to assess the microstructure of the materials and the Bio-based PCM composites. SEM images were obtained with an accelerating voltage of 12 kV and a working distance of 12 mm. The samples were then coated with gold at a few nanometers in thickness to increase the electrical conductivity. Fourier
Fig. 1. Sample images of the Bio-based PCM, Bio-based PCM/silica fume, and Biobased PCM/silica fumeþ xGnP 5 wt%.
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Y. Kang et al. / Solar Energy Materials & Solar Cells 143 (2015) 430–434
Fig. 2. Scanning electron microphotographs ( 1000) of (a) Bio-based PCM, (b) silica fume, (c) xGnP, (d) Bio-based PCM/silica fume, and (e) Bio-based PCM/silica fumeþ xGnP 5 wt%.
Table 1, and as a result, the spectra of the Bio-based PCM composites is the same as that for the Bio-based PCM and silica fume. This means that these composites are composed of PCM and silica fume through a physical interaction without a chemical reaction. As a consequence, the heat storage characteristics of the PCM were maintained in the silica fume structure, and its latent heat storage performance can be revealed after applying them to mortar or concrete. 3.3. Thermal properties analysis A DSC analysis was carried out to determine the thermal energy storage properties of the Bio-based PCM and Bio-based PCM composites, such as the melting temperature, freezing temperature, and latent heat. The DSC graph of the Bio-based PCM and
Bio-based PCM composites is shown in Fig. 4. Table 2 presents the DSC data, including the melting temperature, freezing temperature, latent heat, and incorporated rate. In the DSC data, the melting and freezing temperature was determined to be 29.24 °C and 16.43 °C for the Bio-based PCM, 30.87 °C and 14.13 °C for the Bio-based PCM/silica fume, 29.87 °C and 18.10 °C for the Bio-based PCM/silica fume þxGnP 5 wt%, respectively. This graph confirms that the melting and freezing temperature range for the Bio-based PCM composites was lower than that of the Bio-based PCM because the Bio-based PCM was influenced by supercooling. The latent heat capacity of the Bio-based PCM was 118.5 J/g and 119.0 J/g during melting and freezing, and the results for the Biobased PCM composites showed latent heat capacities of 103.0 J/g and 110.8 J/g, respectively, for melting and latent heat capacities of
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100.2 J/g and 109.9 J/g, respectively, for freezing. The values of the latent heat capacity of the Bio-based PCM composites were 86.91% and 93.50% that of the Bio-based PCM. This means that the xGnP was more the porous than the silica fume. Therefore, the xGnP of Bio-based PCM composites helps these achieve a greater latent heat capacity, and as a result, we expect that the Bio-based PCM composites have important potential for heating and cooling applications in buildings.
3.4. Thermal stability of the Bio-based PCM composites
Fig. 3. FT-IR spectra of the xGnP, silica fume, Bio-based PCM, and Bio-based PCM composites.
Table 1 FT-IR spectra of the Bio-based PCM and silica fume. Vibration
Wave number range (cm 1)
–CH3 –CH2 C ¼O C–O Si–O–Si
2916 2845–2849 1739 1171 1200–1000, –805
The thermal stability of the Bio-based PCM composites was evaluated via thermo gravimetric analysis (TGA). Fig. 5 shows TGA results for the Bio-based PCM and Bio-based PCM composites, and the temperature increased from 20 to 600 °C. All samples can be seen in the graph to have two curves for thermal oxidation degradation. In the case of the Bio-based PCM, the first degradation peak started at 80.56 °C, and the second degradation peak started at 254.76 °C. However, the degradation curve for the Biobased PCM composite with silica fume exhibits first peak at 105.81 °C and a second peak at 247.48 °C. The peak temperatures of the Bio-based PCM composite with silica fume and xGnP 5 wt% is showing 92.38 °C and 249.30 °C, respectively. These graphs indicate that the Bio-based PCM composites had a higher peak than the Bio-based PCM due to the higher flame retardant properties. After about 300 °C, the oxidation rates of the Bio-based PCM composites were shown to be 72.99% and 76.17%, respectively. The Bio-based PCM/silica fume þxGnP 5 wt% was higher than the Biobased PCM/silica fume. Furthermore, the graph of the Bio-based PCM/silica fume þxGnP 5 wt% showed that it started at a lower temperature of thermal decomposition relative to the Bio-based PCM/silica fume. This seems to indicate that the xGnP has a high thermal conductivity.
Fig. 4. DSC curves of the Bio-based PCM, Bio-based PCM/silica fume, and Bio-based PCM/silica fume þxGnP 5 wt%.
Fig. 5. TGA curves of the Bio-based PCM and Bio-based PCM composites.
Table 2 DSC analysis of the Bio-based PCM and Bio-based PCM composites. Samples
Bio-based PCM Bio-based PCM/silica fume Bio-based PCM/silica fume þxGnP 5 wt%
Melting
Freezing
Incorporated rate (%)
Temperature (°C)
Latent heat (J/g)
Temperature (°C)
Latent heat (J/g)
29.24 30.87 29.87
118.5 103.0 110.8
16.43 14.13 18.10
119.0 100.2 109.9
– 86.91 93.50
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Y. Kang et al. / Solar Energy Materials & Solar Cells 143 (2015) 430–434
useful to include in building materials, such as mortar or concrete, because they improve the thermal properties of the material, and, as a result, such materials should be considered for use in energysaving buildings.
Acknowledgments This work was supported by the National Research Foundation of Korea(NRF) grant funded by the Korea government(MSIP) (No. 2014R1A2A1A11053829).
References
Fig. 6. Thermal conductivity of the Bio-based PCM and Bio-based PCM composites loaded with silica fume and xGnP.
3.5. Thermal conductivity analysis Fig. 6 shows the thermal conductivity analysis of the Bio-based PCM and Bio-based PCM composites. It shows that the thermal conductivities of the Bio-based PCM composites remarkably improved relative to that of the Bio-based PCM. This means that the addition of silica fume and xGnP led to an improvement in the thermal conductivity of the Bio-based PCM. Fig. 6 shows that the average thermal conductivity of the Bio-based PCM, Bio-based PCM/silica fume, and Bio-based PCM/silica fume þxGnP 5 wt% is 0.2034 W/mK, 0.3179 W/mK, and 0.4936 W/mK, respectively. The Bio-based PCM/silica fume increased by 156% relative to the PCM, but the rate of increase for the composite with xGnP was of 243%. Consequently, we confirmed that loading xGnP led to an improvement in the thermal conductivity over loading with silica fume.
4. Conclusion Bio-based PCM composites with silica fume and xGnP were prepared using a vacuum impregnation process. The SEM analysis indicated that the Bio-based PCM and Bio-based PCM composites have similar surfaces, which means that there is no chemical reaction between the Bio-based PCM and the silica fume. The heat storage characteristics of the Bio-based PCM were maintained in the structure of the silica fume, even after applying these to mortar or concrete. The DSC analysis showed that the latent heat capacity of the Bio-based PCM composites was of 86.91% and 93.50%, respectively. This means that the xGnP helps obtain a greater latent heat capacity. A TGA analysis was carried out to evaluate the thermal stability of the Bio-based PCM composites. In the graphs, the oxidation rates of the Bio-based PCM composites were shown to be 72.99% and 76.17%, respectively, after approximately 300 °C, and the thermal decomposition of the Bio-based PCM composites with xGnP started at a lower temperature than for the Bio-based PCM/silica fume. A TCi thermal conductivity analyzer was used to assess the thermal conductivity of the Biobased PCM and Bio-based PCM composites. The thermal conductivity of the Bio-based PCM composites increased by 156% and 243% relative to that of the Bio-based PCM, and as a result, the silica fume and xGnP led to an improvement in the thermal conductivity of the Bio-based PCM. In conclusion, we confirmed that the Bio-based PCM composites with silica fume and xGnP could be
[1] IEA, Worldwide Trends in Energy Use and Efficiency: Key Insights From IEA Indicator Analysis, France: OECD/IEA, 2008. [2] S. Yu, S. Jeong, O. Chung, S. Kim, Bio-based PCM/carbon nanomaterials composites with enhanced thermal conductivity, Sol. Energy Mater. Sol. Cells 120 (2014) 549–554. [3] S.M. Hasnain, Review on sustainable thermal energy storage technologies. Part I: heat storage materials and techniques, Energy Convers. Manag. 39 (1998) 1127–1138. [4] A. Sharma, V.V. Tyagi, C.R. Chen, D. Buddhi, Review on thermal energy storage with phase change materials and applications, Renew. Sustain. Energy Rev. 13 (2009) 318–345. [5] C.Y. Zhao, G.H. Zhang, Review on microencapsulated phase change materials (MEPCMs): fabrication, characterization and applications, Renew. Sustain. Energy Rev. 15 (2011) 3813–3832. [6] M.K. Rathod, J. Banerjee, Thermal stability of phase change materials used in latent heat energy storage systems: a review, Renew. Sustain. Energy Rev. 18 (2013) 246–258. [7] J. Kosny, E. Kossecka, A. Brzezinski, A. Tleoubaev, D. Yarbrough, Dynamic thermal performance analysis of fiber insulations containing bio-based phase change materials (PCMs), Energy Build. 52 (2012) 122–131. [8] S. Jeong, O. Chung, S. Yu, S. Kim, S. Kim, Improvement of the thermal properties of Bio-based PCM using exfoliated graphite nanoplatelets, Sol. Energy Mater. Sol. Cells 117 (2013) 87–92. [9] O. Chung, S. Jeong, S. Yu, S. Kim, Thermal performance of organic PCMs/ micronized silica composite for latent heat thermal energy storage, Energy Build. 70 (2014) 180–185. [10] S. Jeong, J. Lee, J. Seo, S. Kim, Thermal performance evaluation of Bio-based shape stabilized PCM with boron nitride for energy saving, Int. J. Heat Mass Transf. 71 (2014) 245–250. [11] M.S. Amin, F.S. Hashem, Hydration characteristics of hydrothermal treated cement kiln dust–sludge–silica fume pastes, Constr. Build. Mater. 25 (2011) 1870–1876. [12] W. Wongkeo, A. Chaipanich, Compressive strength, microstructure and thermal analysis of autoclaved and air cured structural lightweight concrete made with coal bottom ash and silica fume, Mater. Sci. Eng.: A 527 (2010) 3676–3684. [13] M. Barati, S. Sarder, A. McLean, R. Roy, Recovery of silicon from silica fume, J. Non Cryst. Solids 357 (2011) 18–23. [14] G. İnan Sezer, Compressive strength and sulfate resistance of limestone and/or silica fume mortars, Constr. Build. Mater. 26 (2012) 613–618. [15] H. Song, S. Pack, S. Nam, J. Jang, V. Saraswathy, Estimation of the permeability of silica fume cement concrete, Constr. Build. Mater. 24 (2010) 315–321. [16] R. Siddique, Utilization of silica fume in concrete: review of hardened properties, Resour. Conserv. Recycl. 55 (2011) 923–932. [17] S. Kim, L.T. Drzal, High latent heat storage and high thermal conductive phase change materials using exfoliated graphite nanoplatelets, Sol. Energy Mater. Sol. Cells 93 (2009) 136–142. [18] J. Zeng, S. Zheng, S. Yu, F. Zhu, J. Gan, L. Zhu, Z. Xiao, X. Zhu, Z. Zhu, L. Sun, Z. Cao, Preparation and thermal properties of palmitic acid/polyaniline/exfoliated graphite nanoplatelets form-stable phase change materials, Appl. Energy 115 (2014) 603–609. [19] S. Jeong, J. Jeon, J. Cha, J. Kim, S. Kim, Preparation and evaluation of thermal enhanced silica fume by incorporating organic PCM, for application to concrete, Energy Build. 62 (2013) 190–195. [20] J. Cha, J. Seo, S. Kim, Building materials thermal conductivity measurement and correlation with heat flow meter, laser flash analysis and TCi, J. Therm. Anal. Calorim. 109 (2012) 295–300. [21] A. Sarı, A. Karaipekli, Fatty acid esters-based composite phase change materials for thermal energy storage in buildings, Appl. Therm. Eng. 37 (2012) 208–216.
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Energy efficient Bio-based PCM with silica fume composites to apply in concrete for energy saving in buildings Yujin Kang, Su-Gwang Jeong, Seunghwan Wi, Sumin Kim n Building Environment & Materials Lab, School of Architecture, Soongsil University, Seoul 156-743, Republic of Korea
art ic l e i nf o
a b s t r a c t
Article history: Received 2 May 2015 Received in revised form 6 July 2015 Accepted 17 July 2015
Bio-based PCMs are made from underused feedstock, such as soybean oil, and are significantly less flammable than paraffins. In this study, Bio-based PCMs were prepared with silica fume through a vacuum impregnation process. Silica fume is a mineral admixture product with an impermeable pore structure, and it is one of the most feasible candidates for use as economical, light-weight thermal energy storage materials for buildings [15,16]. We also used exfoliated graphite nanoplatelets (xGnP) to improve the thermal conductivity because PCMs have a low thermal conductivity. The vacuum impregnation process that was used in this study guarantees that the Bio-based PCM composites can store heat after incorporation. Scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), Differential scanning calorimetry (DSC), Thermo gravimetric analyses (TGA), and TCi analysis were then carried out to determine the characteristics of the Bio-based PCM and Bio-based PCM composites. & 2015 Elsevier B.V. All rights reserved.
Keywords: Bio-based PCM Silica fume Exfoliated graphite nanoplatelets (xGnP) Thermal conductivity
1. Introduction Final global energy consumption increased by 23% between 1990 and 2005, and space heating accounts for 53% of the energy consumed by households [1]. Thus, latent heat thermal energy storage (LHTES) through the use of phase change materials (PCMs) to store and release thermal energy is one of the most efficient and reliable ways to reduce energy consumption. PCMs have a high heat storage capacity and nearly isothermal phase change behavior [2,3] and are available as three types: organic, inorganic, and eutectic. Inorganic PCMs include salts, salt hydrates, metals, and alloys that have high latent heat per unit volume, a good thermal conductivity, low cost, and are also non-flammable. However, these have a limited usefulness because most metals that go through phase decomposition are corrosive and exhibit supercooling effects [4,5]. Organic PCMs are classified into paraffinic or non-paraffinic. Paraffinic PCMs have been extensively used because they possess a high heat latent, varied phase change temperatures, little supercooling, low vapor pressure in a melt, and good chemical stability [6]. However, most paraffins have a relatively low ignition resistance relative to non-paraffinic PCMs [7], and for this reason, we have chosen non-paraffinic alternatives, such as Bio-based PCM, that are significantly less flammable. Bio-based PCMs are made from underused and renewable n
Corresponding author. Tel.: þ 82 2 820 0665; fax: þ 82 2 816 3354. E-mail address: [email protected] (S. Kim).
http://dx.doi.org/10.1016/j.solmat.2015.07.026 0927-0248/& 2015 Elsevier B.V. All rights reserved.
feedstock, such as palm oil, coconut oil and soybean oil. These oils are composed of fatty acids, such as α- linolenic acid, which is an organic compound in many common vegetable oils that has a molecular formula C18H30O2. Bio-based PCMs are fully hydrogenated and so are expected to exhibit a good thermal stability throughout many phase change cycles without any risk of oxidation. In addition, they can absorb, store, and release large amounts of latent heat in a manner similar to conventional paraffin, and they are equally amenable to microencapsulation. They can be manufactured for the melting point to vary between 22.7 °C and 78.33 °C, and they are available from various fields in various climatic zones [8–10]. Bio-based PCMs can be loaded with silica fume, which is a secondary product from electric furnaces used in the manufacture of silicon metal or silicon alloy [11–14]. Silica fume is a mineral admixture that has an impermeable pore structure in concrete, and it is a highly effective material that can be used to develop high strength concrete with good performance characteristics [15,16]. Therefore, silica fume is a feasible candidate for use as economical and light-weight thermal energy storage materials in buildings. In this study, exfoliated graphite nanoplatelets (xGnP) were also used to improve the thermal conductivity because PCMs have a low thermal conductivity and doing so has a great effect on the heat storage efficiency of the materials. These particles consist of several layers of graphene sheets that have a very high aspect ratio [17,18]. This study is comparable to research that was previously conducted by our research team [2,19]. In one such study, the changes
Y. Kang et al. / Solar Energy Materials & Solar Cells 143 (2015) 430–434
in the thermal performance of silica fume were evaluated in response to incorporating three types of organic PCM for use as an aggregate for concrete. Another focused using carbon nanomaterials to improve the thermal conductivity of the Bio-based PCM. However, the first study did not attempt to improve the thermal conductivity of the samples that were prepared, and the second did not seek to form Bio-based PCMs with stable properties. Thus, in this study, we prepared Bio-based PCM/silica fume/xGnP composites and evaluated their thermal properties both to improve their thermal conductivity and to stabilize their shapes. Finally, we verified that the prepared composites have a high thermal conductivity and a stable shape. The Bio-based PCMs were impregnated with silica fume, and a mixture of silica fume 5 wt% xGnP was prepared using a vacuum impregnation process that was discuss in previous papers. The vacuum impregnation method guarantees that the PCMs store heat after they have been incorporated as a result of the capillary phenomenon and surface tension forces that are experienced during the process. In this paper, SEM, FT-IR, DSC, TGA, and TCi analyses were carried out to analyze the microstructure, chemical bonding, heat capacity, thermal resistance, and thermal conductivity of the Bio-based PCM composites.
2. Experiment 2.1. Materials In this study, Bio-based PCM with a melting temperature of 29.24 °C and a latent heat capacity of 147.2 J/g were used directly as PCM. These materials were obtained from the Korea C&S Corporation in South Korea. The silica fume was supplied from Taiwang, a leading international trading company in South Korea [19]. Exfoliated graphite nanoplatelets (xGnP) were prepared from sulfuric acid-intercalated expandable graphite (3772) (Asbury Graphite Mills, NJ) by applying the cost- and time-effective exfoliation process that was initially proposed by Drzal's group [17]. 2.2. Preparation The Bio-based PCM composites were prepared using vacuum impregnation process that conformed to the procedure described in a previous study. The silica fume and the xGnP were dried at 105 °C before the impregnation process. The 5 wt% xGnP was mixed in silica fume according to the mass percentage of the silica fume. The silica fume and the mixture were placed inside of a filtering flask connected to a water tromp apparatus to evacuate the air from the porous structure. The valve between the flask and the liquid PCM container was turned open to allow the flow into the flask, and the vacuum process continued for 90 min. Then, air was allowed to enter the flask again to force the liquid PCM to penetrate into the porous structure of the silica fume and the xGnP. After the penetration, a filtering process was needed to remove the excess PCM that remained in the flask. Two types of Bio-based PCM composite in a colloidal state were filtered with 1 mm filter paper until a granular sample appeared, and the sample was then dried in a vacuum drier at 80 °C for 24 h.
431
transform infrared spectroscopy (FT-IR, 300E Jasco) were then used to confirm the changes in the chemical groups of the composites at room temperature by analyzing the FT-IR spectra of the materials and the Bio-based PCM composites. Differential scanning calorimetry (DSC Q 1000, TA instrument, USA) was conducted to measure the thermal properties of the Bio-based PCM and PCM composites, such as the melting and freezing temperature and latent heat capacity. The DSC measurements were conducted at a 5 °C/min heating and cooling rate with a temperature ranging from 0 to 80 °C. The thermal durability of the Bio-based PCM and PCM composites was assessed via thermo gravimetric analysis (TGA, TA instrument, TGA Q 5000) on approximately 11–14 mg of the samples within the range from 20 to 600 °C and at a heating rate of 10 °C/min. TGA was measured in an atmosphere with 99.5% of nitrogen and 0.5% oxygen to prevent oxidation. A TCi thermal conductivity analyzer was used to measure the thermal conductivity of the Bio-based PCM and PCM composites at room temperature. The TCi developed by C-Therm Technologies Ltd. is a device that uses the Modified Transient Plane Source (MTPS) method to measure the thermal conductivity of a small sample [20].
3. Results and discussion 3.1. Microstructure of the Bio-based PCM composites The Bio-based PCM and Bio-based PCM composites exhibit morphologies with similar surfaces, as shown in Fig. 1. This means that the silica fume and mixture are uniformly dispersed into the microstructure of the Bio-based PCM. Fig. 2 shows SEM images of the Bio-based PCM, silica fume, xGnP, Bio-based PCM/silica fume, and Bio-based PCM/silica fume þxGnP 5 wt%. As shown in Fig. 2 (a), the Bio-based PCM has a smooth surface with particles of various sizes because these contains fatty acids, such as palm oils, soybean oils, and coconut oils [21]. Fig. 2(b) and (c) shows the porous structure of the silica fume and xGnP, respectively. Fig. 2 (d) and (e) confirmed that the Bio-based PCM was incorporated well into the silica fume and the xGnP mixture. Also, Fig. 2 (e) shows form of the xGnP flakes on the surface of Fig. 2(d). 3.2. FT-IR analysis of Bio-based PCM composites FT-IR spectroscopy was used to examine the chemical compatibility among the components of the composites. The FT-IR absorption spectra of the xGnP, silica fume, Bio-based PCM, and Bio-based PCM composites are shown in Fig. 3. The Bio-based PCM were made from fatty acids of vegetable oils, soybean oils, etc., with a molecular formula of C18H30O2. The Bio-based PCM has –CH3 bonding and –CH2 bonding because the PCMs are composed of oily components. The spectra of Bio-based PCM exhibit absorption bands in the 2916 cm 1 and 2845–2849 cm 1 region as a result of the stretching vibration of functional groups of –CH3 and –CH2. The peaks at 1739 cm 1 and 1171 cm 1 represent the C¼ O and C–O groups, respectively. Silica fume peaks were shown at 12001000 cm 1 and at 805 cm 1 due to the Si–O–Si bonding. These FT-IR spectra peak are shown in
2.3. Characterization techniques Scanning electron microscopy (SEM, JEOL JSM-6360A) was carried out at room temperature to assess the microstructure of the materials and the Bio-based PCM composites. SEM images were obtained with an accelerating voltage of 12 kV and a working distance of 12 mm. The samples were then coated with gold at a few nanometers in thickness to increase the electrical conductivity. Fourier
Fig. 1. Sample images of the Bio-based PCM, Bio-based PCM/silica fume, and Biobased PCM/silica fumeþ xGnP 5 wt%.
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Fig. 2. Scanning electron microphotographs ( 1000) of (a) Bio-based PCM, (b) silica fume, (c) xGnP, (d) Bio-based PCM/silica fume, and (e) Bio-based PCM/silica fumeþ xGnP 5 wt%.
Table 1, and as a result, the spectra of the Bio-based PCM composites is the same as that for the Bio-based PCM and silica fume. This means that these composites are composed of PCM and silica fume through a physical interaction without a chemical reaction. As a consequence, the heat storage characteristics of the PCM were maintained in the silica fume structure, and its latent heat storage performance can be revealed after applying them to mortar or concrete. 3.3. Thermal properties analysis A DSC analysis was carried out to determine the thermal energy storage properties of the Bio-based PCM and Bio-based PCM composites, such as the melting temperature, freezing temperature, and latent heat. The DSC graph of the Bio-based PCM and
Bio-based PCM composites is shown in Fig. 4. Table 2 presents the DSC data, including the melting temperature, freezing temperature, latent heat, and incorporated rate. In the DSC data, the melting and freezing temperature was determined to be 29.24 °C and 16.43 °C for the Bio-based PCM, 30.87 °C and 14.13 °C for the Bio-based PCM/silica fume, 29.87 °C and 18.10 °C for the Bio-based PCM/silica fume þxGnP 5 wt%, respectively. This graph confirms that the melting and freezing temperature range for the Bio-based PCM composites was lower than that of the Bio-based PCM because the Bio-based PCM was influenced by supercooling. The latent heat capacity of the Bio-based PCM was 118.5 J/g and 119.0 J/g during melting and freezing, and the results for the Biobased PCM composites showed latent heat capacities of 103.0 J/g and 110.8 J/g, respectively, for melting and latent heat capacities of
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100.2 J/g and 109.9 J/g, respectively, for freezing. The values of the latent heat capacity of the Bio-based PCM composites were 86.91% and 93.50% that of the Bio-based PCM. This means that the xGnP was more the porous than the silica fume. Therefore, the xGnP of Bio-based PCM composites helps these achieve a greater latent heat capacity, and as a result, we expect that the Bio-based PCM composites have important potential for heating and cooling applications in buildings.
3.4. Thermal stability of the Bio-based PCM composites
Fig. 3. FT-IR spectra of the xGnP, silica fume, Bio-based PCM, and Bio-based PCM composites.
Table 1 FT-IR spectra of the Bio-based PCM and silica fume. Vibration
Wave number range (cm 1)
–CH3 –CH2 C ¼O C–O Si–O–Si
2916 2845–2849 1739 1171 1200–1000, –805
The thermal stability of the Bio-based PCM composites was evaluated via thermo gravimetric analysis (TGA). Fig. 5 shows TGA results for the Bio-based PCM and Bio-based PCM composites, and the temperature increased from 20 to 600 °C. All samples can be seen in the graph to have two curves for thermal oxidation degradation. In the case of the Bio-based PCM, the first degradation peak started at 80.56 °C, and the second degradation peak started at 254.76 °C. However, the degradation curve for the Biobased PCM composite with silica fume exhibits first peak at 105.81 °C and a second peak at 247.48 °C. The peak temperatures of the Bio-based PCM composite with silica fume and xGnP 5 wt% is showing 92.38 °C and 249.30 °C, respectively. These graphs indicate that the Bio-based PCM composites had a higher peak than the Bio-based PCM due to the higher flame retardant properties. After about 300 °C, the oxidation rates of the Bio-based PCM composites were shown to be 72.99% and 76.17%, respectively. The Bio-based PCM/silica fume þxGnP 5 wt% was higher than the Biobased PCM/silica fume. Furthermore, the graph of the Bio-based PCM/silica fume þxGnP 5 wt% showed that it started at a lower temperature of thermal decomposition relative to the Bio-based PCM/silica fume. This seems to indicate that the xGnP has a high thermal conductivity.
Fig. 4. DSC curves of the Bio-based PCM, Bio-based PCM/silica fume, and Bio-based PCM/silica fume þxGnP 5 wt%.
Fig. 5. TGA curves of the Bio-based PCM and Bio-based PCM composites.
Table 2 DSC analysis of the Bio-based PCM and Bio-based PCM composites. Samples
Bio-based PCM Bio-based PCM/silica fume Bio-based PCM/silica fume þxGnP 5 wt%
Melting
Freezing
Incorporated rate (%)
Temperature (°C)
Latent heat (J/g)
Temperature (°C)
Latent heat (J/g)
29.24 30.87 29.87
118.5 103.0 110.8
16.43 14.13 18.10
119.0 100.2 109.9
– 86.91 93.50
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useful to include in building materials, such as mortar or concrete, because they improve the thermal properties of the material, and, as a result, such materials should be considered for use in energysaving buildings.
Acknowledgments This work was supported by the National Research Foundation of Korea(NRF) grant funded by the Korea government(MSIP) (No. 2014R1A2A1A11053829).
References
Fig. 6. Thermal conductivity of the Bio-based PCM and Bio-based PCM composites loaded with silica fume and xGnP.
3.5. Thermal conductivity analysis Fig. 6 shows the thermal conductivity analysis of the Bio-based PCM and Bio-based PCM composites. It shows that the thermal conductivities of the Bio-based PCM composites remarkably improved relative to that of the Bio-based PCM. This means that the addition of silica fume and xGnP led to an improvement in the thermal conductivity of the Bio-based PCM. Fig. 6 shows that the average thermal conductivity of the Bio-based PCM, Bio-based PCM/silica fume, and Bio-based PCM/silica fume þxGnP 5 wt% is 0.2034 W/mK, 0.3179 W/mK, and 0.4936 W/mK, respectively. The Bio-based PCM/silica fume increased by 156% relative to the PCM, but the rate of increase for the composite with xGnP was of 243%. Consequently, we confirmed that loading xGnP led to an improvement in the thermal conductivity over loading with silica fume.
4. Conclusion Bio-based PCM composites with silica fume and xGnP were prepared using a vacuum impregnation process. The SEM analysis indicated that the Bio-based PCM and Bio-based PCM composites have similar surfaces, which means that there is no chemical reaction between the Bio-based PCM and the silica fume. The heat storage characteristics of the Bio-based PCM were maintained in the structure of the silica fume, even after applying these to mortar or concrete. The DSC analysis showed that the latent heat capacity of the Bio-based PCM composites was of 86.91% and 93.50%, respectively. This means that the xGnP helps obtain a greater latent heat capacity. A TGA analysis was carried out to evaluate the thermal stability of the Bio-based PCM composites. In the graphs, the oxidation rates of the Bio-based PCM composites were shown to be 72.99% and 76.17%, respectively, after approximately 300 °C, and the thermal decomposition of the Bio-based PCM composites with xGnP started at a lower temperature than for the Bio-based PCM/silica fume. A TCi thermal conductivity analyzer was used to assess the thermal conductivity of the Biobased PCM and Bio-based PCM composites. The thermal conductivity of the Bio-based PCM composites increased by 156% and 243% relative to that of the Bio-based PCM, and as a result, the silica fume and xGnP led to an improvement in the thermal conductivity of the Bio-based PCM. In conclusion, we confirmed that the Bio-based PCM composites with silica fume and xGnP could be
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