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Thermal conductivity improvement of stearic acid using expanded graphite and carbon fiber for energy storage applications
ARTICLE IN PRESS
Renewable Energy 32 (2007) 2201–2210 www.elsevier.com/locate/renene
Thermal conductivity improvement of stearic acid using expanded graphite and carbon fiber for energy storage applications Ali Karaipeklia, Ahmet Sarıa,, Kamil Kaygusuzb Department of Chemistry, Gaziosmanpas- a University, 60240 Tokat, Turkey Department of Chemistry, Karadeniz Technical University, 61080 Trabzon, Turkey a
b
Received 28 February 2006; accepted 21 November 2006 Available online 26 January 2007
Abstract The influence of expanded graphite (EG) and carbon fiber (CF) as heat diffusion promoters on thermal conductivity improvement of stearic acid (SA), as a phase change material (PCM), was evaluated. EG and CF in different mass fractions (2%, 4%, 7%, and 10%) were added to SA, and thermal conductivities of SA/EG and SA/CF composites were measured by using hot-wire method. An almost linear relationship between mass fractions of EG and CF additives, and thermal conductivity of SA was found. Thermal conductivity of SA (0.30 W/mK) increased by 266.6% (206.6%) by adding 10% mass fraction EG (CF). The improvement in thermal conductivity of SA was also experimentally tested by comparing melting time of the pure SA with that of SA/EG and SA/CF composites. The results indicated that the melting times of composite PCMs were reduced significantly with respect to that of pure SA. Furthermore, the latent heat capacities of the SA/EG and SA/CF (90/10 wt%) composite PCMs were determined by differential scanning calorimetry (DSC) technique and compared with that of pure SA. On the basis of all results, it was concluded that the use of EG and CF can be considered an effective method to improve thermal conductivity of SA without reducing much its latent heat storage capacity. r 2006 Elsevier Ltd. All rights reserved. Keywords: Stearic acid; Expanded graphite; Carbon fiber; PCM; Thermal conductivity
Corresponding author. Tel.: +90 356 2521582; fax: +90 356 2521585.
E-mail address: [email protected] (A. Sarı). 0960-1481/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2006.11.011
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1. Introduction Rapid economic and social development has led to huge demand on energy. In an attempt to conserve energy and reduce dependency on fossil fuels and also to reduce the greenhouse gas emission, it is essential to seek effective means of reducing peaks in power consumption and to shift portions of the load from periods of maximum demand. Storage of thermal energy, hence, becomes an important aspect in engineering application, especially in energy conservation in buildings. There are various thermal energy storage methods, but latent heat storage is the most attractive one due to high storage density and small temperature variation from storage to retrieval. In a latent heat storage system, energy is stored during melting and recovered during freezing of a phase change material (PCM) [1–3]. In recent times, several candidate inorganic and organic PCMs and their mixtures have been studied as PCMs for latent heat thermal energy storage (LHTES) applications [3–5]. Among the investigated PCMs, stearic acid (SA), which is a fatty acid, possesses, over other PCMs, some superior properties such as proper melting temperature range, high latent heat capacity, congruently melting, little or no supercooling during the phase transition, low vapor pressure, nontoxicity, noncorrosivity against metal containers, good chemical and thermal stability [6–10]. In spite of these desirable properties of SA, the low thermal conductivity (0.2–0.3 W/mK) is its major drawback decreasing heat storing and releasing rates during melting and crystallization processes, which in turn limits their utility areas [6]. To overcome the low thermal conductivity problem of SA as PCM, several studies have been carried out with purpose of developing LHTES systems using unfinned and finned configurations, dispersing high-conductivity particles and inserting a metal matrix into PCM [11–14]. However, such type heat transfer promoters considerably increase the weight and the volume of LHTES systems. Moreover, carbon fiber (CF) and CF brushes with high thermal conductivity (190–220 W/mK) have been used to enhance the heat transfer in LHTES systems [15–17]. On the other hand, porous graphite matrix (4–100 W/mK) and expanded graphite (EG) were suggested as promising promoters to increase heat diffusion in the convenient PCMs [18–20] and form stable PCMs [21,22]. Among the studied heat transfer promoters, in particular, EG and CF have been considered as excellent promoters because of their advantageous properties: being inert to chemical reaction, uniformly dispersing into PCMs, being compatible with the PCMs, having lower density than that of metals, and thus making the LHTES system of less weight compared to same-volume LHTES system with metal promoters [16–22]. In this paper, thermal conductivity improvement of SA as a PCM using EG and CF as heat diffusion promoters was investigated. The effect of the thermal conductivity enhancement of SA on melting times of SA was also estimated experimentally. In addition, the latent heat capacities of the prepared SA/EG and SA/CF composite PCMs were measured by DSC analysis technique and the results were evaluated by comparing with that of pure SA. 2. Experimental 2.1. Materials SA with melting temperature range of 67–70 1C was obtained from Merck Company. Graphite with the particle size of 35–75 mm was supplied from Astas- Company
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(Sivas, Turkey). Carbon fiber with filament diameter of 6 mm was obtained from the Teknoyapı Company (I˙stanbul, Turkey), and it was cut in to pieces of length 5 mm before use in the experiments. EG was prepared by rapid expansion and exfoliation of expandable graphite (particle size: 35–75 mm) in a furnace over 900 1C for 60 s [22]. Thermo-physical properties of SA, EG and CF are given in Table 1. 2.2. Preparation and calorimetric analysis of the composites SA/EG and SA/CF composites were prepared separately by adding EG and CF to melted SA. In order to determine the variation in thermal conductivity improvement of SA, mass fractions of EG and CF in the composites were selected as 2%, 4%, 7%, and 10%. The mixtures were shaken to obtain homogeneous distribution and kept for 120 h at a temperature (75 1C) over the melting point of SA to be sure of the sustainability of homogeneity of the composites. To determine the effects of EG and CF additives on latent heat capacity of SA, DSC analysis (DuPont 2000 model DSC) was conducted on the pure SA, SA/EG and SA/CF composite samples at a heating rate of 5 1C/min and in a nitrogen atmosphere. DSC measurements were repeated three times for each sample, and the standard deviation in latent heat values was found to be 71.02 J/g. 2.3. Thermal conductivity measurement and estimation of melting time Fig. 1 shows the apparatus used to measure thermal conductivity by transient hot-wire method. The platinum wire in the apparatus was chosen 105 mm in length and 0.18 mm in diameter to obtain the optimum ratio between length and diameter (575), justifying the validity of the method [23]. The composites with different mass fractions (2%, 4%, 7%, and 10%) were placed into sample container (Fig. 1). An almost constant voltage (170.02 V) was applied for 10 s and the temperature raise by circulating the current along the platinum wire was recorded by a thermocouple (Pt-RhPt) with an accuracy of 70.1 1C. Measurements were repeated three times for each sample in order to minimize the error. Thermal conductivities (K) of the prepared samples were calculated based on transient hot-wire method [17] using K¼
ðVIÞ=ð4pLÞ , DT=Dðln tÞ
(1)
Table 1 Thermo-physical properties of pure SA, EG and CF Density (g/cm3)
Thermal conductivity (W/mK)
Specific heat (J/g 1C)
Melting point (1C)
Latent heat capacity (J/g)
Stearic acid (SA)
0.94
198.8
1.3 1.8
2.83 at 40 1C 2.38 at 80 1C — —
68.8
Expanded graphite (EG) Carbon fiber (CF)
0.29a 0.30b 4–100 190
— —
— —
a
Literature value [24]. Measured value at present study.
b
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Fig. 1. Experimental apparatus used for thermal conductivity measurements.
where V is the applied voltage, I is the current circulating to along the platinum wire, L is the length of the platinum wire, DT is the temperature change, and D(ln t) is logarithmic time interval for the associated time t2 and t1. Fig. 2 illustrates the temperature rise as a function of time for consecutive three measurements performed for pure SA. The slopes of the linear plots (1.410, 1.408, and 1.406) indicated that reproducibility of measurements was reliable. The accuracy of the hot-wire apparatus was also checked by measuring the thermal conductivity of solid SA (0.30 W/mK) with a standard error of 70.01 W/mK. This value was very close to literature value (0.29 W/mK) [24]. Furthermore, thermal conductivity improvement of SA was tested by comparing the melting times of the composite PCMs with that of pure SA. Melting times were measured with the experimental setup and procedure used in our former study [25]. 3. Result and discussion 3.1. Compatibility of EG and CF with SA The SA/EG and SA/CF composite PCMs were prepared by adding EG and CF in mass fractions 2%, 4%, 7%, and 10%. The composites were maintained for 120 h at the temperature (75 1C) over the melting point of the SA. It was observed that the additives did not settle out and the mixtures still kept their homogenous state. In fact, when density values of SA, EG and CF were considered (Table 1), it was expected that EG and CF should separate from composite by deposition at the bottom of vessel because they have a density higher than that of SA. However, after keeping EG and CF in melted SA for a long time (120 h), the composites remaining homogenous was due to high viscosity of PCM at liquid state and, thus, not allowing suspension of EG and CF. Based on the observations,
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Fig. 2. Temperature variation vs. ln(t) recorded for thermal conductivity measurements of pure SA for repeated three times.
it can also be noted that EG is more homogenously distributed in the SA compared to the CF because of its being more compatible in terms of physical and chemical. 3.2. Thermal conductivity improvement Fig. 3 shows the variation of thermal conductivity of SA/EG and the SA/CF composite PCMs with different mass fractions of EG and CF additives. As clearly seen from this figure, thermal conductivity of SA increased with increasing mass fraction of EG and CF additives. An almost linear relationship between thermal conductivity and mass fraction of EG and CF can be easily drawn out from Fig. 3 as follows, respectively: y ¼ 0:0841x þ 0:2194
ðR2 ¼ 0:97 for the linear plot represents EG additionÞ,
(2)
y ¼ 0:0659x þ 0:2831
ðR2 ¼ 0:98 for the linear plot represents CF additionÞ,
(3)
where y and x represent the thermal conductivity (K) of the composite and mass fraction of EG or CF (%), respectively. The coefficients of determination (R2 ¼ 0.97 for linear plot represent EG addition and R2 ¼ 0.98 for linear plot represent CF addition) indicated the existence of an almost linear relationship between thermal conductivity and mass fraction of EG and CF. It can also clear from Fig. 3 that thermal conductivity of the SA increased as 27.6%, 58.6%, 179.3%, and 279.3%, for addition EG in mass fraction 2%, 4%, 7%, and 10%, respectively and it increased as 24.1%, 106.9%, 162.1%, and 217.2% for addition of CF in mass fractions 2%, 4%, 7%, and 10%, respectively. These results were attributed to high thermal conductivity of the EG (4–100 W/mK) and CF (190 W/mK) [15,17,20–22].
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Fig. 3. Variation of thermal conductivity of the SA with the mass fraction of the EG and the CF addives.
Moreover, it was clearly noticed that thermal conductivity of SA was better enhanced using the EG although EG had lower conductivity than CF. It was due to high homogeneity grade of SA/EG composite and good physical compatibility of EG with SA, and thus providing large heat diffusion in the SA. On the other hand, Fig. 4 shows the effective thermal conductivity of the composite, KC/KPCM, which is the ratio of the thermal conductivity of the composite to the thermal conductivity of pure SA. Based on coefficients of determination obtained from linear plots (R2 ¼ 0.97 for linear plots obtained for both EG and CF additives), one can be notice an almost linear relationship between effective thermal conductivity and volume fraction of EG (XEG) and CF (XCF). The effective thermal conductivity for SA/EG composite was increased by a factor of 1.23, 2.07, 2.47, and 3.13 for XEG ¼ 0.010, 0.021, 0.038, and 0.055, respectively, and it increased by a factor of 1.20, 1.58, 2.81, and 3.67 for XCF ¼ 0.011, 0.022, 0.040, and 0.058, respectively. From these results, it is plausible to think that efficiency of heat diffusion should be a function of homogeneity grade and physical compatibility of the composite. 3.3. Comparison of melting times The improvement in thermal conductivity of the SA caused by adding EG and CF was tested by comparing melting times of composite PCMs with that of pure SA. Figs. 5 and 6 show the melting temperature curves of the pure SA as PCM, SA/EG (98/2 wt%), SA/EG (96/4 wt%), and SA/CF (98/2 wt%), SA/CF (93/7 wt%) composite PCMs. The melting time was estimated from the temperature curves as a time elapsed until the temperatures of the PCMs reaches the same initial temperature (33 1C) to over melting point of the PCMs
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Fig. 4. Variation of effective thermal conductivity with volume fractions of the EG and the CF additives.
Fig. 5. Melting temperature curves of the pure SA, SA/EG (98/2 wt%) and SA/EG (96/4 wt%) composite.
(70 1C). The melting time was determined as 95, 75, 62, 78, and 60 min for the pure SA, SA/EG (98/2 wt%), SA/EG (96/4 wt%), SA/CF (98/2 wt%), and SA/CF (93/7 wt%) composite PCMs, respectively. These results indicate that the melting times of composite
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Fig. 6. Melting temperature curves of the pure SA, SA/CF (98/2 wt%) and SA/CF (93/7 wt%) composite.
PCMs reduced by 21%, 38%, 18%, and 37%, respectively, with respect to melting time of pure SA. This was due to accelerated conduction heat diffusion at the initial stage of the heat charging process and increased convection heat diffusion during phase change of the composite PCM by high thermal conductivity. The reduction in melting time of composite PCMs confirmed the thermal conductivity improvement of SA. It is also noteworthy that the prepared composite PCMs can more rapidly absorb heat supplied from a heat source compared with pure SA as PCM. Similar results were in agreement with the data obtained for the effect of enhanced thermal conductivity by using different heat transfer promoters on melting time of octadecane and Li2CO3 as PCM by Shiina and Inagaki [26]. 3.4. Latent heat capacity of composites DSC measurements have been conducted to evaluate the influence of EG and CF additive on loss of latent heat capacity of SA as PCM. As reported in Table 1, pure SA has a latent heat capacity of 198.8 J/g. Calorimetric measurements of composite PCMs indicate that latent heat capacity of SA decreased to 183.1 and 184.6 J/g when adding EG and CF in mass fraction of 10%. The latent heats of SA/EG and SA/CF (90/10 wt%) composite PCMs were 8% and 7% less than that of the pure SA, respectively. Such small decreases in latent heat capacity of SA in case of composite are insignificant for LHTES applications. Moreover, the prepared SA/EG and SA/CF (90/10 wt%) composite PCMs have satisfying latent heat storage capacity for a LHTES system as well as high thermal conductivity. 4. Conclusions Thermal conductivity of the SA as PCM was efficiently improved using EG and CF additives as heat diffusion promoter. The results clearly indicated that an almost linear
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relationship between thermal conductivity and mass fraction of EG and CF additives. The variation in thermal conductivity improvement of SA caused by using the same mass fraction of EG and CF additives was due to differences in homogeneity grade of the composite (PCM/promoter) and physical compatibility of promoter with PCM. Therefore, it was concluded that the higher as the effects of both factors the higher as the improvement of thermal conductivity. Thermal conductivity improvement of pure SA was also tested by examining melting times of pure SA as PCM, SA/EG and SA/CF (90/10 wt%) composite PCMs. The reduction of melting times of composite PCMs with respect to melting time of pure SA confirmed to the thermal conductivity improvement of SA. This result also proved that the heat transfer rate of composite PCMs was obviously higher than that of pure SA owing to combination with EG and CF that have high thermal conductivity. Furthermore, the effect of EG and CF additives on latent heat capacity of pure SA was investigated by DSC analysis. DSC results indicated that the decrease in latent heat capacity of SA after addition of EG and CF was little, and SA/EG and SA/CF (90/10 wt%) composite PCMs had satisfactory latent heat storage capacity for LHTES applications. As a result, EG and CF can be considered as an effective heat diffusion promoter to improve thermal conductivity of SA without much reducing its latent heat storage capacity.
Acknowledgment The authors wish to thanks Sevim Ulupınar in Middle East Technical University for carrying out the DSC analysis.
References [1] Abhat A. Low temperature latent thermal energy storage system: Heat storage materials. Sol Energy 1983; 30:313–32. [2] Dinc- er I, Rosen MA. Thermal energy storage systems and applications. Chichester, UK: Wiley; 2002. [3] Zalba B, Marin JM, Cabeza LF, Mehling H. Review on thermal energy storage with phase change: materials, heat transfer analysis and applications. Appl Therm Eng 2003;23:251–83. [4] Sharma SD, Sagara K. Latent heat storage materials and systems: a review. Int J Green Energy 2005;2:1–56. [5] Kaygusuz K. The viability of thermal energy storage. Energy Sour 2001;21:745–56. [6] Rozanna D, Chuah TG, Salmiah A, Thomas Choong SY, Sa’ari M. Fatty acids as phase change materials (PCMs) for thermal energy storage: a Review. Int J Green Energy 2004;1:495–513. [7] Feldman D, Shapiro MM, Banu D, Fuks CJ. Fatty acids and their mixtures as phase change materials for thermal energy storage. Sol Energy Mater 1989;18:201–16. [8] Hasan A, Sayigh AAM. Some fatty acids as phase change thermal energy storage materials. Renew Energy 1994;4:69–76. [9] Sarı A, Kaygusuz K. Thermal energy storage system using stearic acid as a phase change material. Sol Energy 2001;71:365–76. [10] Sarı A, Kaygusuz K. Some fatty acids used for latent heat storage: thermal stability and corrosion of metals with respect to thermal cycling. Renew Energy 2003;28:339–48. [11] Eftekhar J, Sheikh AH, Lou DYS. Heat transfer enhancement in a paraffin wax thermal storage system. J Sol Energy Eng 1984;106:299–306. [12] Lacroix M. Study of the heat transfer behavior of a latent heat thermal energy storage unit with a finned tube. Int J Heat Mass Transfer 1993;36:2083–92. [13] Velraj V, Seenıraj RV, Hafner B, Schwarzer K. Heat transfer enhancement in a latent heat storage system. Sol Energy 1999;65:171–80.
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[14] Hafner B, Schwarzer K. Improvement of the heat transfer in a phase-change-material storage. In: Proceedings of the IEA ECES IA annex 10. Fourth workshop. Benedikbauern, Germany: October 1999. p. 28–29. [15] Fukai J, Kanou M, Kodama Y, Miyatake O. Thermal conductivity enhancement of energy storage media using carbon fibers. Energy Convers Management 2000;41:1543–56. [16] Fukai J, Hamada Y, Morozumi Y, Miyatake O. Improvement of thermal characteristics of latent heat thermal energy storage units using carbon-fiber brushes: experiments and modeling. Int J Heat Mass Transfer 2002;46:4513–25. [17] Frusteri F, Leonardi V, Vasta S, Restuccia G. Thermal conductivity measurement of a PCM based storage system containing carbon fibers. Appl Therm Eng 2005;25:1623–33. [18] Mehling H, Hiebler S, Ziegler F. Latent heat storage using a PCM-graphite composite material: advantages and potential applications. In: Proceedings of the Fourth workshop of IEA ECES IA annex 10. Bendiktbeuern, Germany, October 1999. p. 28–29. [19] Py X, Olives S, Mauran S. Paraffin/porous graphite matrix composite as a high and constant power thermal storage material. Int J Heat Mass Transfer 2001;44:2727–37. [20] Marin JM, Zalba B, Cabeza LF, Mehling H. Improvement of a thermal energy storage using plates with paraffin-graphite composite. Int J Heat Mass Transfer 2005;48:2561–70. [21] Sarı A. Form-stable paraffin/high density polyethylene composites as solid-liquid phase change material for thermal energy storage: preparation and thermal properties. Energy Convers Manage 2004;45:2033–42. [22] Xiao M, Feng B, Gong K. Preparation and performance of shape stabilized phase change thermal storage materials with high thermal conductivity. Energy Convers Manage 2002;43:103–8. [23] Carslaw HS, Jaeger JC. Conduction of heat in solids. Oxford: Cladendon Press; 1959. [24] Bansal NK, Budhi D. An analytical study of latent heat storage system in acylinder. Energy Convers Manage 1992;33:235–42. [25] Sarı A, Kaygusuz K. Performance of myristic acid as a phase change material for energy storage application. Renew Energy 2001;24:303–17. [26] Shiina Y, Inagaki T. Study on the efficiency of effective thermal conductivities on melting characteristics of latent heat storage capsules. Int J Heat Mass Transfer 2005;48:373–83.
Renewable Energy 32 (2007) 2201–2210 www.elsevier.com/locate/renene
Thermal conductivity improvement of stearic acid using expanded graphite and carbon fiber for energy storage applications Ali Karaipeklia, Ahmet Sarıa,, Kamil Kaygusuzb Department of Chemistry, Gaziosmanpas- a University, 60240 Tokat, Turkey Department of Chemistry, Karadeniz Technical University, 61080 Trabzon, Turkey a
b
Received 28 February 2006; accepted 21 November 2006 Available online 26 January 2007
Abstract The influence of expanded graphite (EG) and carbon fiber (CF) as heat diffusion promoters on thermal conductivity improvement of stearic acid (SA), as a phase change material (PCM), was evaluated. EG and CF in different mass fractions (2%, 4%, 7%, and 10%) were added to SA, and thermal conductivities of SA/EG and SA/CF composites were measured by using hot-wire method. An almost linear relationship between mass fractions of EG and CF additives, and thermal conductivity of SA was found. Thermal conductivity of SA (0.30 W/mK) increased by 266.6% (206.6%) by adding 10% mass fraction EG (CF). The improvement in thermal conductivity of SA was also experimentally tested by comparing melting time of the pure SA with that of SA/EG and SA/CF composites. The results indicated that the melting times of composite PCMs were reduced significantly with respect to that of pure SA. Furthermore, the latent heat capacities of the SA/EG and SA/CF (90/10 wt%) composite PCMs were determined by differential scanning calorimetry (DSC) technique and compared with that of pure SA. On the basis of all results, it was concluded that the use of EG and CF can be considered an effective method to improve thermal conductivity of SA without reducing much its latent heat storage capacity. r 2006 Elsevier Ltd. All rights reserved. Keywords: Stearic acid; Expanded graphite; Carbon fiber; PCM; Thermal conductivity
Corresponding author. Tel.: +90 356 2521582; fax: +90 356 2521585.
E-mail address: [email protected] (A. Sarı). 0960-1481/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2006.11.011
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1. Introduction Rapid economic and social development has led to huge demand on energy. In an attempt to conserve energy and reduce dependency on fossil fuels and also to reduce the greenhouse gas emission, it is essential to seek effective means of reducing peaks in power consumption and to shift portions of the load from periods of maximum demand. Storage of thermal energy, hence, becomes an important aspect in engineering application, especially in energy conservation in buildings. There are various thermal energy storage methods, but latent heat storage is the most attractive one due to high storage density and small temperature variation from storage to retrieval. In a latent heat storage system, energy is stored during melting and recovered during freezing of a phase change material (PCM) [1–3]. In recent times, several candidate inorganic and organic PCMs and their mixtures have been studied as PCMs for latent heat thermal energy storage (LHTES) applications [3–5]. Among the investigated PCMs, stearic acid (SA), which is a fatty acid, possesses, over other PCMs, some superior properties such as proper melting temperature range, high latent heat capacity, congruently melting, little or no supercooling during the phase transition, low vapor pressure, nontoxicity, noncorrosivity against metal containers, good chemical and thermal stability [6–10]. In spite of these desirable properties of SA, the low thermal conductivity (0.2–0.3 W/mK) is its major drawback decreasing heat storing and releasing rates during melting and crystallization processes, which in turn limits their utility areas [6]. To overcome the low thermal conductivity problem of SA as PCM, several studies have been carried out with purpose of developing LHTES systems using unfinned and finned configurations, dispersing high-conductivity particles and inserting a metal matrix into PCM [11–14]. However, such type heat transfer promoters considerably increase the weight and the volume of LHTES systems. Moreover, carbon fiber (CF) and CF brushes with high thermal conductivity (190–220 W/mK) have been used to enhance the heat transfer in LHTES systems [15–17]. On the other hand, porous graphite matrix (4–100 W/mK) and expanded graphite (EG) were suggested as promising promoters to increase heat diffusion in the convenient PCMs [18–20] and form stable PCMs [21,22]. Among the studied heat transfer promoters, in particular, EG and CF have been considered as excellent promoters because of their advantageous properties: being inert to chemical reaction, uniformly dispersing into PCMs, being compatible with the PCMs, having lower density than that of metals, and thus making the LHTES system of less weight compared to same-volume LHTES system with metal promoters [16–22]. In this paper, thermal conductivity improvement of SA as a PCM using EG and CF as heat diffusion promoters was investigated. The effect of the thermal conductivity enhancement of SA on melting times of SA was also estimated experimentally. In addition, the latent heat capacities of the prepared SA/EG and SA/CF composite PCMs were measured by DSC analysis technique and the results were evaluated by comparing with that of pure SA. 2. Experimental 2.1. Materials SA with melting temperature range of 67–70 1C was obtained from Merck Company. Graphite with the particle size of 35–75 mm was supplied from Astas- Company
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(Sivas, Turkey). Carbon fiber with filament diameter of 6 mm was obtained from the Teknoyapı Company (I˙stanbul, Turkey), and it was cut in to pieces of length 5 mm before use in the experiments. EG was prepared by rapid expansion and exfoliation of expandable graphite (particle size: 35–75 mm) in a furnace over 900 1C for 60 s [22]. Thermo-physical properties of SA, EG and CF are given in Table 1. 2.2. Preparation and calorimetric analysis of the composites SA/EG and SA/CF composites were prepared separately by adding EG and CF to melted SA. In order to determine the variation in thermal conductivity improvement of SA, mass fractions of EG and CF in the composites were selected as 2%, 4%, 7%, and 10%. The mixtures were shaken to obtain homogeneous distribution and kept for 120 h at a temperature (75 1C) over the melting point of SA to be sure of the sustainability of homogeneity of the composites. To determine the effects of EG and CF additives on latent heat capacity of SA, DSC analysis (DuPont 2000 model DSC) was conducted on the pure SA, SA/EG and SA/CF composite samples at a heating rate of 5 1C/min and in a nitrogen atmosphere. DSC measurements were repeated three times for each sample, and the standard deviation in latent heat values was found to be 71.02 J/g. 2.3. Thermal conductivity measurement and estimation of melting time Fig. 1 shows the apparatus used to measure thermal conductivity by transient hot-wire method. The platinum wire in the apparatus was chosen 105 mm in length and 0.18 mm in diameter to obtain the optimum ratio between length and diameter (575), justifying the validity of the method [23]. The composites with different mass fractions (2%, 4%, 7%, and 10%) were placed into sample container (Fig. 1). An almost constant voltage (170.02 V) was applied for 10 s and the temperature raise by circulating the current along the platinum wire was recorded by a thermocouple (Pt-RhPt) with an accuracy of 70.1 1C. Measurements were repeated three times for each sample in order to minimize the error. Thermal conductivities (K) of the prepared samples were calculated based on transient hot-wire method [17] using K¼
ðVIÞ=ð4pLÞ , DT=Dðln tÞ
(1)
Table 1 Thermo-physical properties of pure SA, EG and CF Density (g/cm3)
Thermal conductivity (W/mK)
Specific heat (J/g 1C)
Melting point (1C)
Latent heat capacity (J/g)
Stearic acid (SA)
0.94
198.8
1.3 1.8
2.83 at 40 1C 2.38 at 80 1C — —
68.8
Expanded graphite (EG) Carbon fiber (CF)
0.29a 0.30b 4–100 190
— —
— —
a
Literature value [24]. Measured value at present study.
b
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Fig. 1. Experimental apparatus used for thermal conductivity measurements.
where V is the applied voltage, I is the current circulating to along the platinum wire, L is the length of the platinum wire, DT is the temperature change, and D(ln t) is logarithmic time interval for the associated time t2 and t1. Fig. 2 illustrates the temperature rise as a function of time for consecutive three measurements performed for pure SA. The slopes of the linear plots (1.410, 1.408, and 1.406) indicated that reproducibility of measurements was reliable. The accuracy of the hot-wire apparatus was also checked by measuring the thermal conductivity of solid SA (0.30 W/mK) with a standard error of 70.01 W/mK. This value was very close to literature value (0.29 W/mK) [24]. Furthermore, thermal conductivity improvement of SA was tested by comparing the melting times of the composite PCMs with that of pure SA. Melting times were measured with the experimental setup and procedure used in our former study [25]. 3. Result and discussion 3.1. Compatibility of EG and CF with SA The SA/EG and SA/CF composite PCMs were prepared by adding EG and CF in mass fractions 2%, 4%, 7%, and 10%. The composites were maintained for 120 h at the temperature (75 1C) over the melting point of the SA. It was observed that the additives did not settle out and the mixtures still kept their homogenous state. In fact, when density values of SA, EG and CF were considered (Table 1), it was expected that EG and CF should separate from composite by deposition at the bottom of vessel because they have a density higher than that of SA. However, after keeping EG and CF in melted SA for a long time (120 h), the composites remaining homogenous was due to high viscosity of PCM at liquid state and, thus, not allowing suspension of EG and CF. Based on the observations,
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Fig. 2. Temperature variation vs. ln(t) recorded for thermal conductivity measurements of pure SA for repeated three times.
it can also be noted that EG is more homogenously distributed in the SA compared to the CF because of its being more compatible in terms of physical and chemical. 3.2. Thermal conductivity improvement Fig. 3 shows the variation of thermal conductivity of SA/EG and the SA/CF composite PCMs with different mass fractions of EG and CF additives. As clearly seen from this figure, thermal conductivity of SA increased with increasing mass fraction of EG and CF additives. An almost linear relationship between thermal conductivity and mass fraction of EG and CF can be easily drawn out from Fig. 3 as follows, respectively: y ¼ 0:0841x þ 0:2194
ðR2 ¼ 0:97 for the linear plot represents EG additionÞ,
(2)
y ¼ 0:0659x þ 0:2831
ðR2 ¼ 0:98 for the linear plot represents CF additionÞ,
(3)
where y and x represent the thermal conductivity (K) of the composite and mass fraction of EG or CF (%), respectively. The coefficients of determination (R2 ¼ 0.97 for linear plot represent EG addition and R2 ¼ 0.98 for linear plot represent CF addition) indicated the existence of an almost linear relationship between thermal conductivity and mass fraction of EG and CF. It can also clear from Fig. 3 that thermal conductivity of the SA increased as 27.6%, 58.6%, 179.3%, and 279.3%, for addition EG in mass fraction 2%, 4%, 7%, and 10%, respectively and it increased as 24.1%, 106.9%, 162.1%, and 217.2% for addition of CF in mass fractions 2%, 4%, 7%, and 10%, respectively. These results were attributed to high thermal conductivity of the EG (4–100 W/mK) and CF (190 W/mK) [15,17,20–22].
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Fig. 3. Variation of thermal conductivity of the SA with the mass fraction of the EG and the CF addives.
Moreover, it was clearly noticed that thermal conductivity of SA was better enhanced using the EG although EG had lower conductivity than CF. It was due to high homogeneity grade of SA/EG composite and good physical compatibility of EG with SA, and thus providing large heat diffusion in the SA. On the other hand, Fig. 4 shows the effective thermal conductivity of the composite, KC/KPCM, which is the ratio of the thermal conductivity of the composite to the thermal conductivity of pure SA. Based on coefficients of determination obtained from linear plots (R2 ¼ 0.97 for linear plots obtained for both EG and CF additives), one can be notice an almost linear relationship between effective thermal conductivity and volume fraction of EG (XEG) and CF (XCF). The effective thermal conductivity for SA/EG composite was increased by a factor of 1.23, 2.07, 2.47, and 3.13 for XEG ¼ 0.010, 0.021, 0.038, and 0.055, respectively, and it increased by a factor of 1.20, 1.58, 2.81, and 3.67 for XCF ¼ 0.011, 0.022, 0.040, and 0.058, respectively. From these results, it is plausible to think that efficiency of heat diffusion should be a function of homogeneity grade and physical compatibility of the composite. 3.3. Comparison of melting times The improvement in thermal conductivity of the SA caused by adding EG and CF was tested by comparing melting times of composite PCMs with that of pure SA. Figs. 5 and 6 show the melting temperature curves of the pure SA as PCM, SA/EG (98/2 wt%), SA/EG (96/4 wt%), and SA/CF (98/2 wt%), SA/CF (93/7 wt%) composite PCMs. The melting time was estimated from the temperature curves as a time elapsed until the temperatures of the PCMs reaches the same initial temperature (33 1C) to over melting point of the PCMs
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Fig. 4. Variation of effective thermal conductivity with volume fractions of the EG and the CF additives.
Fig. 5. Melting temperature curves of the pure SA, SA/EG (98/2 wt%) and SA/EG (96/4 wt%) composite.
(70 1C). The melting time was determined as 95, 75, 62, 78, and 60 min for the pure SA, SA/EG (98/2 wt%), SA/EG (96/4 wt%), SA/CF (98/2 wt%), and SA/CF (93/7 wt%) composite PCMs, respectively. These results indicate that the melting times of composite
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Fig. 6. Melting temperature curves of the pure SA, SA/CF (98/2 wt%) and SA/CF (93/7 wt%) composite.
PCMs reduced by 21%, 38%, 18%, and 37%, respectively, with respect to melting time of pure SA. This was due to accelerated conduction heat diffusion at the initial stage of the heat charging process and increased convection heat diffusion during phase change of the composite PCM by high thermal conductivity. The reduction in melting time of composite PCMs confirmed the thermal conductivity improvement of SA. It is also noteworthy that the prepared composite PCMs can more rapidly absorb heat supplied from a heat source compared with pure SA as PCM. Similar results were in agreement with the data obtained for the effect of enhanced thermal conductivity by using different heat transfer promoters on melting time of octadecane and Li2CO3 as PCM by Shiina and Inagaki [26]. 3.4. Latent heat capacity of composites DSC measurements have been conducted to evaluate the influence of EG and CF additive on loss of latent heat capacity of SA as PCM. As reported in Table 1, pure SA has a latent heat capacity of 198.8 J/g. Calorimetric measurements of composite PCMs indicate that latent heat capacity of SA decreased to 183.1 and 184.6 J/g when adding EG and CF in mass fraction of 10%. The latent heats of SA/EG and SA/CF (90/10 wt%) composite PCMs were 8% and 7% less than that of the pure SA, respectively. Such small decreases in latent heat capacity of SA in case of composite are insignificant for LHTES applications. Moreover, the prepared SA/EG and SA/CF (90/10 wt%) composite PCMs have satisfying latent heat storage capacity for a LHTES system as well as high thermal conductivity. 4. Conclusions Thermal conductivity of the SA as PCM was efficiently improved using EG and CF additives as heat diffusion promoter. The results clearly indicated that an almost linear
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relationship between thermal conductivity and mass fraction of EG and CF additives. The variation in thermal conductivity improvement of SA caused by using the same mass fraction of EG and CF additives was due to differences in homogeneity grade of the composite (PCM/promoter) and physical compatibility of promoter with PCM. Therefore, it was concluded that the higher as the effects of both factors the higher as the improvement of thermal conductivity. Thermal conductivity improvement of pure SA was also tested by examining melting times of pure SA as PCM, SA/EG and SA/CF (90/10 wt%) composite PCMs. The reduction of melting times of composite PCMs with respect to melting time of pure SA confirmed to the thermal conductivity improvement of SA. This result also proved that the heat transfer rate of composite PCMs was obviously higher than that of pure SA owing to combination with EG and CF that have high thermal conductivity. Furthermore, the effect of EG and CF additives on latent heat capacity of pure SA was investigated by DSC analysis. DSC results indicated that the decrease in latent heat capacity of SA after addition of EG and CF was little, and SA/EG and SA/CF (90/10 wt%) composite PCMs had satisfactory latent heat storage capacity for LHTES applications. As a result, EG and CF can be considered as an effective heat diffusion promoter to improve thermal conductivity of SA without much reducing its latent heat storage capacity.
Acknowledgment The authors wish to thanks Sevim Ulupınar in Middle East Technical University for carrying out the DSC analysis.
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