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expanded graphite composite as phase change material
Applied Thermal Engineering 27 (2007) 1271–1277 www.elsevier.com/locate/apthermeng
Thermal conductivity and latent heat thermal energy storage characteristics of paraffin/expanded graphite composite as phase change material Ahmet Sarı *, Ali Karaipekli Department of Chemistry, Gaziosmanpasßa University, 60240 Tokat, Turkey Received 12 December 2005; accepted 8 November 2006 Available online 3 January 2007
Abstract This study aimed determination of proper amount of paraffin (n-docosane) absorbed into expanded graphite (EG) to obtain formstable composite as phase change material (PCM), examination of the influence of EG addition on the thermal conductivity using transient hot-wire method and investigation of latent heat thermal energy storage (LHTES) characteristics of paraffin such as melting time, melting temperature and latent heat capacity using differential scanning calorimetry (DSC) technique. The paraffin/EG composites with the mass fraction of 2%, 4%, 7%, and 10% EG were prepared by absorbing liquid paraffin into the EG. The composite PCM with mass fraction of 10% EG was considered as form-stable allowing no leakage of melted paraffin during the solid–liquid phase change due to capillary and surface tension forces of EG. Thermal conductivity of the pure paraffin and the composite PCMs including 2, 4, 7 and 10 wt% EG were measured as 0.22, 0.40, 0.52, 0.68 and 0.82 W/m K, respectively. Melting time test showed that the increasing thermal conductivity of paraffin noticeably decreased its melting time. Furthermore, DSC analysis indicated that changes in the melting temperatures of the composite PCMs were not considerable, and their latent heat capacities were approximately equivalent to the values calculated based on the mass ratios of the paraffin in the composites. It was concluded that the composite PCM with the mass fraction of 10% EG was the most promising one for LHTES applications due to its form-stable property, direct usability without a need of extra storage container, high thermal conductivity, good melting temperature and satisfying latent heat storage capacity. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Paraffin; Expanded graphite; Form-stable composite PCM; Thermal conductivity; Latent heat thermal energy storage
1. Introduction Energy storage is critical in enhancing the applicability, performance, and reliability of a wide range of energy systems as the discrepancy between energy supply and its demand can be eliminated by use of proper thermal energy storage (TES) methods [1–7]. Of various TES methods, latent heat thermal energy storage (LHTES) which was phase change material (PCM) is one of the most preferred
*
Corresponding author. Tel.: +90 356 252 16 16; fax: +90 356 252 15
85. E-mail address: [email protected] (A. Sarı). 1359-4311/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2006.11.004
forms because of its high storage density and small temperature variation from storage to retrieval [8–10]. Several inorganic and organic PCMs and their mixtures have been investigated as LHTES materials [8–13]. Among the investigated PCMs, paraffins have been widely used for LHTES applications due to their large latent heat and proper thermal characteristics such as little or no super cooling, low vapor pressure, good thermal and chemical stability, and self-nucleating behavior [8,12–15]. In spite of these desirable properties of paraffins, the low thermal conductivity (0.21–0.24 W/m K) is its major drawback decreasing the rates of heat stored and released during melting and crystallization processes which in turn limits their utility areas [8,12–14]. To overcome the low thermal
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conductivity problem of paraffins as PCMs, studies have been carried out with the purpose of developing LHTES systems with unfinned and finned configurations [16–19], dispersing high conductivity particles [20] and inserting a metal matrix into paraffin wax [21]. However, uses of such type heat transfer promoters considerably increase the weight and volume of LHTES systems. In addition, carbon fibers and carbon fiber brushes have been used as heat diffusion promoters in paraffin and other PCMs due to their high thermal conductivities and low weight [22,23]. In recent years, porous graphite matrices have been used to improve thermal conductivity of paraffins. Py et al. was prepared the composite of paraffin (m.p.: 73–80 °C)/compressed expanded natural graphite (CENG) as a high and power thermal storage material and they determined the relationship between the thermal conductivity of the composite and the bulk density of CENG [24]. Mills et al. investigated the thermal conductivity enhancement of paraffin (m.p.: 35–55 °C) using a porous graphite matrix and established the performance of a passive thermal management system compacted with the PCM-composite system [25]. On the other hand, paraffins have been encapsulated in polymeric network structures to obtain shape-stabilized or form-stable composite PCM [26–28]. In such a composite PCM, polymeric construct prevents the leakage of liquid PCM when it is changed from solid to liquid, and thus they can be directly used in LHTES systems without a need of extra encapsulation. However, polymer wall of the composite PCM with low thermal conductivity considerably decreases heat transfer from PCM to the energy storage medium. To solve this problem, the expanded graphite (EG) with mass fraction of 3% was added to improve the thermal conductivity of the composite PCM [26,27]. Zhang et al. investigated the influence of some additives on thermal conductivity of paraffin/styrene butadiene styrene composite prepared as shape-stabilized PCM, and they recorded an increase as much as 221% in the thermal conductivity of the composite PCM by addition of the graphite with mass fraction of 20% [29]. In addition, Zhang and Fang studied the effect of the EG addition on the thermal properties of the paraffin (m.p.: 48–50 °C)/EG composite prepared as form-stable PCM, and they reported that the latent heat capacity of the PCM decreased with increase of the mass fraction of the graphite [30]. This study aimed to prepare the composites of paraffin (n-docosane, m.p.: 42–44 °C)/expanded graphite (EG) with varying mass fraction of EG to obtain a form-stable composite PCM and to investigate the effect of EG addition on thermal conductivity and melting time, melting temperature, and latent heat capacity of the paraffin. Although similar papers were published, there is no such a comprehensive study in literature that presents detailed experimental data on thermal conductivities and LHTES characteristics of paraffin (n-docosane; m.p.: 42–44 °C)/ EG composite PCMs and in particular as form-stable composite PCM.
2. Experimental 2.1. Materials Paraffin (n-docosane) with melting temperature of 42– 44 °C was obtained from Merck company. The thermophysical properties of the paraffin are given in Table 1. Graphite powder (average particle size: 270 lm, bulk density: 300 kg/m3, thermal conductivity: 2–90 W/m K) was supplied by Astasß Company (Turkey). 2.2. Preparation of expanded graphite (EG) Expanded graphite (EG) was prepared from graphite to maximize mass fraction of paraffin to be adsorbed into it porous structure. The graphite sample was first converted to intercalated or expandable graphite through chemical oxidation in the presence of a mixture of sulfuric and nitric acid and then dried in a vacuum oven at 65 °C for 24 h. EG was then obtained by rapid expansion and exfoliation of expandable graphite in a furnace over 900 °C for 60 s. The surface area of the EG was measured as 46 m2/g by gas adsorption technique (BET). 2.3. Preparation of paraffin/EG composite PCMs To establish the relationship between thermal conductivity of the composite PCM and the mass fraction of EG and determine the minimum mass fraction of EG that is adequate to obtain paraffin/EG composite as form-stable PCM, the composite PCMs were prepared by impregnation of liquid paraffin into the EG with mass fraction of 2%, 4%, 7%, and 10%. Preliminary experiments indicated that a time as 60 min was enough to reach maximum saturation level of EG with liquid paraffin, and therefore this time was selected as absorption equilibrium time in the preparation of the composite samples. The samples were then filtered and dried. Finally, they were tested in terms of form-stability by heating of them over the melting temperature (42– 44 °C) of paraffin. The composite PCM allowing no liquid paraffin leakage was considered as form-stable composite PCM. In addition, the densities of the paraffin/EG composite PCMs were determined using the pycnometer method with an accuracy of ±3%.
Table 1 Thermo-physical properties of the paraffin (n-docosane) Thermo-physical properties
Paraffin (n-docosane)
Solid–solid phase transition temperature (°C) Melting temperature (°C) Total latent heat of fusion (kJ/kg) Specific heat (kJ/kg °C) Density (kg/m3) Thermal conductivity (W/m K)
28.8 41.6 194.6 1.93(s); 2.38(l) 785 0.22
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of the prepared samples was calculated using Eq. (1) based on transient hot-wire method [22,28] k¼
Fig. 1. The apparatus used for thermal conductivity measurement by transient hot-wire method.
2.4. Thermal conductivity measurement Fig. 1 shows the apparatus used to measure thermal conductivity by transient hot-wire method. The platinum wire in the apparatus was chosen as 105 mm in length and 0.18 mm in diameter to obtain the optimum ratio between the length and the diameter (575), justifying the validity of the method [22]. The composites with different mass fraction (2%, 4%, 7%, and 10%) were placed into sample container (Fig. 1). A constant voltage (1 V) was applied for 10 s and the temperature raise by circulating the current (0.8 A) along the platinum wire was recorded by a thermocouple (Pt-RhPt) with an accuracy of ±0.1 °C. The measurements were repeated three times for each sample in order to minimize the error. The thermal conductivity (k)
q lnðt2 =t1 Þ 4pDT
ð1Þ
where, q is the power input per unit length of the wire, DT is the temperature difference 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 paraffin. The slopes of the plots (0.0702, 0.0691 and 0.0694) indicated that the reproducibility of measurements was reliable. The accuracy of the hot-wire apparatus was also checked by measuring the thermal conductivity of solid stearic acid and solid paraffin and it was found as 0.30 W/m K and 0.22 W/m K, respectively with a standard error of ±0.01. These results were in agreement with the published data; 0.29 W/m K for solid stearic acid [31] and 0.21–0.24 W/m K for solid paraffin [8,12–14]. 2.5. Establishment of melting time Increase in thermal conductivity of paraffin with EG additive in varying mass fractions was tested by comparison of melting times of the composite PCMs with that of pure paraffin. Melting times were measured with the experimental set-up and procedure used in our former study [11]. 2.6. Determination of thermal properties Thermal properties such as melting temperature and latent heat capacity of pure paraffin and paraffin/EG composite PCMs were measured using a DSC instrument (SETARAM DSC-131). Indium was used as a reference material for the calibration of the instrument. DSC measurements were performed at 5 °C/min heating rate and temperature range of 20–80 °C. The solid–solid phase transition temperature and melting temperature were measured by drawing a line at the point of maximum slope of the leading edge of the peak and extrapolating to base line. The latent heats of the PCMs were determined as total by numerical integration of area under the peaks that represent solid–solid and solid–liquid phase transition. 3. Results and discussion 3.1. Compatibility of EG with paraffin
Fig. 2. Temperature rise versus time for three measurements used in the calculation of thermal conductivity of pure paraffin.
Among the composite PCMs, the one with the mass fraction of 10% EG was proved to be as form-stable composite PCM as it could keep the same form in solid state even when the temperature of the PCM was over than the melting temperature of paraffin. It was due to that paraffin was hold by the capillary force and the surface tension force of the porous EG. This result also signified that the EG in mass fraction of 10% absorbed the liquid paraffin as nine times as itself without to be leakage of the melted
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Fig. 3. The photographs of (a) pure paraffin as PCM and (b) paraffin/ 10 wt% EG composite as form-stable PCM.
paraffin during the solid–liquid phase change process. This result is in agreement with literature. For instance, Py et al. reported that the paraffin with melting temperatures of 73– 80 °C was loaded to the compressed expanded natural graphite (CENG) from 65 to 95 wt% depending on the bulk density of CENG [24]. Mills et al. found that the paraffin with melting temperatures of 35–55 °C was impregnated as 85–90 wt% into porous graphite matrix with density of 61 g/l [25]. In addition, the mass fraction of paraffin (m.p.: 58–60 °C) absorbed into the EG for form-stable composite PCM has been reported as 86.6% by Zhang and Fang [30]. The paraffin distributed uniformly in EG due to its good structural compatibility (Fig. 3). This achieved that paraffin/10 wt% EG composite as form-stable PCM. The density of the form-stable composite PCM was measured as 721 kg/m3 at 25 °C by pycnometer method. It was less than the density of the paraffin (785 kg/m3), suggesting that a LHTES system using the form-stable paraffin/EG composite PCM to be lighter than a LHTES system using paraffin provided that of the systems have the same volume. In addition, the form-stable composite PCMs had a good mechanical robustness.
Fig. 4. Thermal conductivities of paraffin/EG composite PCMs with varying mass fraction of EG.
Thermal conductivities of the composite PCMs with mass fraction of 2%, 4%, 7%, and 10% EG indicated that the thermal conductivity of paraffin (0.22 W/m K) increased as 81.2%, 136.3%, 209.1%, and 272.7%, respectively. This was attributed to high thermal conductivity of the EG [24,25,29]. The results also showed that the thermal conductivity of paraffin could be achieved further by addition of EG more than 10 wt%. However, this mass fraction is adequate to obtain form-stable composite PCM, and further increase in EG over 10 wt% will results in decrease in latent heat capacity of the composite due to decrease in the mass fraction of paraffin (below 90 wt%) in the composite. Therefore the mass fraction of EG in the composite was limited with 10%. The high thermal conductivity of form-stable composite PCM (about four times of that of pure paraffin) makes it much more attractive than pure paraffin for passive LHTES applications. 3.3. Comparison of melting time of the composite PCMs with that of paraffin
3.2. Thermal conductivity improvement Fig. 4 shows thermal conductivity of the composite PCMs with different mass fraction of EG, indicating that the thermal conductivity increased with increasing mass fraction of EG. A relationship between the thermal conductivity and mass fraction of EG can be easily drawn out from Fig. 4 as follows: ^y ¼ 0:0524x þ 0:3038 ðr2 ¼ 0:9974Þ
ð2Þ
where, ^y and x represent thermal conductivity (k) and mass fraction of EG (%), respectively. The high coefficient of determination (r2 = 0.9974) indicated a strong relationship between the thermal conductivity and mass fraction of EG.
The improvement in thermal conductivity of paraffin was tested by comparing melting times of the composite PCMs with that of paraffin. Fig. 5 shows the melting temperature curves of the pure paraffin as PCM, paraffin/ 4 wt% EG mixture as composite PCM and paraffin/ 10 wt% EG as form-stable composite PCM. The melting time was estimated from the temperature curves as a time elapsed until the temperatures of the PCMs to reach from the initial temperature (25.2 °C) to over melting point of the PCMs (41.6 °C). The melting times of pure paraffin, the composite PCM with 4 wt% EG and form-stable composite PCM were determined as 91, 76 and 62 min, respectively, showing that the melting times of composite PCMs
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Fig. 5. Temperature curves of pure paraffin and paraffin/EG composite PCMs.
decreased by 16% and 32%, respectively when compared to melting time of pure paraffin. This is due to increased heat transfer by high thermal conductivity. The reduction in melting time of the composite PCMs confirmed the improvement in thermal conductivity of the paraffin. It can be also noteworthy that the form-stable composite PCM can more rapidly absorb heat supplied from a heat source compared with pure paraffin as PCM.
Table 2 Melting temperatures and total latent heat capacity of pure paraffin and paraffin/EG composite PCMs Material
Melting temperature (°C)
Total latent heat capacity (kJ/kg)
Pure paraffin 98 wt%paraffin/2 wt% EG 96 wt%paraffin/4 wt% EG 93 wt%paraffin/7 wt% EG 90 wt%paraffin/10 wt% EG
41.6 41.1 41.0 40.7 40.2
194.6 192.6 188.0 181.9 178.3
3.4. Thermal properties of paraffin/EG composite PCMs DSC analysis was conducted to investigate the influence of EG addition on thermal properties such as melting temperature and the latent heat storage capacity of paraffin and the composite PCMs. Thermal properties of paraffin and the composite PCMs depending on the amount of paraffin are given in Table 2. The DSC curve of paraffin in Fig. 6 was taken as reference to evaluate the change in thermal properties of composite PCMs. In DSC thermograms, the main peak represents solid–liquid phase change (melting) of paraffin, and the minor peak corresponds to solid–solid phase transition of paraffin. Table 2 shows that the melting temperatures of paraffin/EG composite PCMs saturated with paraffin in mass ratios of 98%, 96%, 93%, and 90% were lower than that of pure paraffin as 0.5, 0.6, 0.9 and 1.4 °C, respectively. On the other hand, total latent heat (for both of solid–solid and solid–liquid phase change) capacity of the composite PCMs composed of 98%, 96%, 93%, and 90% paraffin were lower 1.3%, 3.4%, 6.5% and 8.4%, respectively, compared to that of pure paraffin. This indicated that the total latent heats of the com-
Fig. 6. The DSC thermograms of pure paraffin and paraffin/10 wt% EG composite as form-stable PCM.
posite PCMs are large and almost equivalent to values calculated by multiplying the latent heat value of pure
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paraffin (194.6 kJ/kg) with its mass fractions (98, 96, 93, and 90 wt%). In addition, the latent heat of form-stable composite PCM saturated with 90 wt% paraffin had a satisfying latent heat capacity (178.3 kJ/kg) for LHTES applications. 4. Conclusions The following conclusions were reached based on the experimental results: (1) The paraffin (n-docosane)/EG composite PCMs can be easily prepared by impregnation of liquid paraffin into the porous structure of EG. The paraffin/EG composite PCM with mass fraction of 10% EG was accepted as form-stable composite PCM as it allowed no leakage of melted paraffin from the pores of EG when subjected to a solid–liquid phase change process. In particular, the use of form-stable composite PCM can reduce the weight of the LHTES system due to its low density besides its direct usability in energy storage systems without a requirement of an extra storage container. (2) Increasing the mass fraction of EG from 2% to 10% gradually increased the thermal conductivity of paraffin/EG composite PCM. A very high correlation (r = 0.9986) occurred between thermal conductivity and mass fraction of EG, indicating a strong association between the two attributes. (3) The reduction of the melting times of the composite PCMs was in accordance with the increase in the thermal conductivity. In overall, it may be suggested that a composite with a mass fraction of 10% EG can be considered as the most promising PCM for thermal energy storage using LHTES method due to its form-stable property, direct usability without a need of extra storage container, high thermal conductivity, proper melting temperature and satisfying latent heat capacity. Acknowledgement The authors thank Dr. Orhan UZUN for DSC analyses made in Department of Physics of Gaziosmanpasßa University. References [1] G.A. Lane, Solar Heat Storage: Latent Heat Materials, Technology, vol. II, CRC Press Inc., Florida, 1986. [2] H.P. Garg, S.C. Mullick, A.K. Bhargava, Solar Thermal Energy Storage, D. Reidel Publishing Company, Dordrecht, Holland, 1985. [3] I. Dinc¸er, Thermal energy storage systems as a key technology in energy conservation, Int. J. Energy Res. 26 (2002) 567–588. [4] H.G. Lorsch, K.W. Kauffman, J.C. Denton, Thermal energy storage for heating and air conditioning, Energy Conver. Manag. 15 (1975) 69–81.
[5] S.M. Hasnain, Review on sustainable thermal energy storage, Technologies, Part 1: Heat storage materials and techniques, Energy Conver. Manag. 39 (1998) 1127–1138. [6] R. Domanski, G. Fellah, Thermoeconomic analysis of sensible heat, thermal energy storage, Appl. Therm. Eng. 18 (1998) 693– 704. [7] S.M. Vakilaltojjar, W. Saman, Analysis and modelling of a phase change storage system for air conditioning applications, Appl. Therm. Eng. 21 (2001) 249–263. [8] A. Abhat, Low temperature latent thermal energy storage system: heat storage materials, Sol. Energy 30 (1983) 313–332. [9] I. Dinc¸er, M.A. Rosen, Thermal Energy Storage, Systems and Applications, Wiley, England, 2002. [10] K. Kaygusuz, The viability of thermal energy storage, Energy Sour. 21 (1999) 745–756. [11] A. Sarı, Thermal characteristics of a eutectic mixture of myristic and palmitic acids as phase change material for heating applications, Appl. Therm. Eng. 23 (2003) 1005–1017. [12] S.D. Sharma, K. Sagara, Latent heat storage materials and systems: a review, Int. J. Green Energy 2 (2005) 1–56. [13] B. Zalba, J.M. Marin, L.F. Cabeza, H. Mehling, Review on thermal energy storage with phase change: materials, heat transfer analysis and applications, Appl. Therm. Eng. 23 (2003) 251–283. [14] S. Himran, A. Suwono, G.A. Mansoori, Characterization of alkanes and paraffin waxes for application as phase change energy storage medium, Energy Sour. 16 (1994) 117–128. [15] X. Liu, H. Liu, S. Wang, L. Zhang, H. Cheng, Preparation and thermal properties of form stable paraffin phase change material encapsulation, Energy Conver. Manag. 47 (2006) 2515–2522. [16] K.A.R. Ismail, C.L.F. Alves, M.S. Modesto, Numerical and experimental study on the solidification of pcm around a vertical axially finned isothermal cylinder, Appl. Therm. Eng. 21 (2001) 53– 77. [17] R. Velraj, R.V. Seeniraj, B. Hafner, C. Faber, K. Schwarzer, Heat transfer enhancement in a latent heat storage system, Sol. Energy 65 (1999) 171–180. [18] Z. Liu, X. Sun, C. Ma, Experimental investigations on the characteristics of melting processes of stearic acid in an annulus and its thermal conductivity enhancement by fins, Energy Conver. Manag. 46 (2005) 959–969. [19] Z. Liu, X. Sun, C. Ma, Experimental study of the characteristics of solidification of stearic acid in an annulus and its thermal conductivity enhancement, Energy Conver. Manag. 46 (2005) 971–984. [20] R. Sigel, Solidification of low conductivity material containing dispersed high conductivity particles, Int. J. Heat Mass Transf. 20 (1977) 1087–1089. [21] X. Tong, J.A. Khan, M.R. Amin, Enhancement of heat transfer by inserting a metal matrix into a phase change material, Num. Heat Trans., Part A 30 (1996) 125–141. [22] F. Frusteri, V. Leonardi, S. Vasta, G. Restuccia, Thermal conductivity measurement of a pcm based storage system containing carbon fibers, Appl. Therm. Eng. 25 (2005) 1623–1633. [23] J. Fukai, M. Makoto, Y. Kodama, O. Miyatake, Thermal conductivity enhancement of energy storage media using carbon fibers, Energy Conver. Manag. 41 (2000) 1543–1556. [24] X. Py, S. Olives, S. Mauran, Paraffin/porous graphite matrix composite as a high and constant power thermal storage material, Int. J. Heat Mass Transf. 44 (2001) 2727–2737. [25] A. Mills, M. Farid, J.R. Selman, S. Al-Hallaj, Thermal conductivity enhancement of phase change materials using a graphite matrix, Appl. Therm. Eng. 26 (2006) 1652–1661. [26] A. Sarı, Form-stable paraffin/high density polyethylene composites as solid–liquid phase change material for thermal energy storage: preparation and thermal properties, Energy Conver. Manag. 45 (2004) 2033–2042. [27] M. Xiao, B. Feng, K. Gong, Preparation and performance of shapestabilized phase change thermal storage materials with high thermal conductivity, Energy Conver. Manag. 43 (2002) 103–108.
A. Sarı, A. Karaipekli / Applied Thermal Engineering 27 (2007) 1271–1277 [28] H. Inaba, P. Tu, Evaluation of thermo-physical characteristics on shape-stabilized paraffin as a solid–liquid phase change material, Heat and Mass Transf. 32 (1997) 307–312. [29] Y. Zhang, J. Ding, X. Wang, R. Yang, K. Lin, Influence of additives on thermal conductivity of shape-stabilized phase change material, Sol. Energy Mater. Sol. Cells 90 (2006) 1692–1702.
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Thermal conductivity and latent heat thermal energy storage characteristics of paraffin/expanded graphite composite as phase change material Ahmet Sarı *, Ali Karaipekli Department of Chemistry, Gaziosmanpasßa University, 60240 Tokat, Turkey Received 12 December 2005; accepted 8 November 2006 Available online 3 January 2007
Abstract This study aimed determination of proper amount of paraffin (n-docosane) absorbed into expanded graphite (EG) to obtain formstable composite as phase change material (PCM), examination of the influence of EG addition on the thermal conductivity using transient hot-wire method and investigation of latent heat thermal energy storage (LHTES) characteristics of paraffin such as melting time, melting temperature and latent heat capacity using differential scanning calorimetry (DSC) technique. The paraffin/EG composites with the mass fraction of 2%, 4%, 7%, and 10% EG were prepared by absorbing liquid paraffin into the EG. The composite PCM with mass fraction of 10% EG was considered as form-stable allowing no leakage of melted paraffin during the solid–liquid phase change due to capillary and surface tension forces of EG. Thermal conductivity of the pure paraffin and the composite PCMs including 2, 4, 7 and 10 wt% EG were measured as 0.22, 0.40, 0.52, 0.68 and 0.82 W/m K, respectively. Melting time test showed that the increasing thermal conductivity of paraffin noticeably decreased its melting time. Furthermore, DSC analysis indicated that changes in the melting temperatures of the composite PCMs were not considerable, and their latent heat capacities were approximately equivalent to the values calculated based on the mass ratios of the paraffin in the composites. It was concluded that the composite PCM with the mass fraction of 10% EG was the most promising one for LHTES applications due to its form-stable property, direct usability without a need of extra storage container, high thermal conductivity, good melting temperature and satisfying latent heat storage capacity. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Paraffin; Expanded graphite; Form-stable composite PCM; Thermal conductivity; Latent heat thermal energy storage
1. Introduction Energy storage is critical in enhancing the applicability, performance, and reliability of a wide range of energy systems as the discrepancy between energy supply and its demand can be eliminated by use of proper thermal energy storage (TES) methods [1–7]. Of various TES methods, latent heat thermal energy storage (LHTES) which was phase change material (PCM) is one of the most preferred
*
Corresponding author. Tel.: +90 356 252 16 16; fax: +90 356 252 15
85. E-mail address: [email protected] (A. Sarı). 1359-4311/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2006.11.004
forms because of its high storage density and small temperature variation from storage to retrieval [8–10]. Several inorganic and organic PCMs and their mixtures have been investigated as LHTES materials [8–13]. Among the investigated PCMs, paraffins have been widely used for LHTES applications due to their large latent heat and proper thermal characteristics such as little or no super cooling, low vapor pressure, good thermal and chemical stability, and self-nucleating behavior [8,12–15]. In spite of these desirable properties of paraffins, the low thermal conductivity (0.21–0.24 W/m K) is its major drawback decreasing the rates of heat stored and released during melting and crystallization processes which in turn limits their utility areas [8,12–14]. To overcome the low thermal
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conductivity problem of paraffins as PCMs, studies have been carried out with the purpose of developing LHTES systems with unfinned and finned configurations [16–19], dispersing high conductivity particles [20] and inserting a metal matrix into paraffin wax [21]. However, uses of such type heat transfer promoters considerably increase the weight and volume of LHTES systems. In addition, carbon fibers and carbon fiber brushes have been used as heat diffusion promoters in paraffin and other PCMs due to their high thermal conductivities and low weight [22,23]. In recent years, porous graphite matrices have been used to improve thermal conductivity of paraffins. Py et al. was prepared the composite of paraffin (m.p.: 73–80 °C)/compressed expanded natural graphite (CENG) as a high and power thermal storage material and they determined the relationship between the thermal conductivity of the composite and the bulk density of CENG [24]. Mills et al. investigated the thermal conductivity enhancement of paraffin (m.p.: 35–55 °C) using a porous graphite matrix and established the performance of a passive thermal management system compacted with the PCM-composite system [25]. On the other hand, paraffins have been encapsulated in polymeric network structures to obtain shape-stabilized or form-stable composite PCM [26–28]. In such a composite PCM, polymeric construct prevents the leakage of liquid PCM when it is changed from solid to liquid, and thus they can be directly used in LHTES systems without a need of extra encapsulation. However, polymer wall of the composite PCM with low thermal conductivity considerably decreases heat transfer from PCM to the energy storage medium. To solve this problem, the expanded graphite (EG) with mass fraction of 3% was added to improve the thermal conductivity of the composite PCM [26,27]. Zhang et al. investigated the influence of some additives on thermal conductivity of paraffin/styrene butadiene styrene composite prepared as shape-stabilized PCM, and they recorded an increase as much as 221% in the thermal conductivity of the composite PCM by addition of the graphite with mass fraction of 20% [29]. In addition, Zhang and Fang studied the effect of the EG addition on the thermal properties of the paraffin (m.p.: 48–50 °C)/EG composite prepared as form-stable PCM, and they reported that the latent heat capacity of the PCM decreased with increase of the mass fraction of the graphite [30]. This study aimed to prepare the composites of paraffin (n-docosane, m.p.: 42–44 °C)/expanded graphite (EG) with varying mass fraction of EG to obtain a form-stable composite PCM and to investigate the effect of EG addition on thermal conductivity and melting time, melting temperature, and latent heat capacity of the paraffin. Although similar papers were published, there is no such a comprehensive study in literature that presents detailed experimental data on thermal conductivities and LHTES characteristics of paraffin (n-docosane; m.p.: 42–44 °C)/ EG composite PCMs and in particular as form-stable composite PCM.
2. Experimental 2.1. Materials Paraffin (n-docosane) with melting temperature of 42– 44 °C was obtained from Merck company. The thermophysical properties of the paraffin are given in Table 1. Graphite powder (average particle size: 270 lm, bulk density: 300 kg/m3, thermal conductivity: 2–90 W/m K) was supplied by Astasß Company (Turkey). 2.2. Preparation of expanded graphite (EG) Expanded graphite (EG) was prepared from graphite to maximize mass fraction of paraffin to be adsorbed into it porous structure. The graphite sample was first converted to intercalated or expandable graphite through chemical oxidation in the presence of a mixture of sulfuric and nitric acid and then dried in a vacuum oven at 65 °C for 24 h. EG was then obtained by rapid expansion and exfoliation of expandable graphite in a furnace over 900 °C for 60 s. The surface area of the EG was measured as 46 m2/g by gas adsorption technique (BET). 2.3. Preparation of paraffin/EG composite PCMs To establish the relationship between thermal conductivity of the composite PCM and the mass fraction of EG and determine the minimum mass fraction of EG that is adequate to obtain paraffin/EG composite as form-stable PCM, the composite PCMs were prepared by impregnation of liquid paraffin into the EG with mass fraction of 2%, 4%, 7%, and 10%. Preliminary experiments indicated that a time as 60 min was enough to reach maximum saturation level of EG with liquid paraffin, and therefore this time was selected as absorption equilibrium time in the preparation of the composite samples. The samples were then filtered and dried. Finally, they were tested in terms of form-stability by heating of them over the melting temperature (42– 44 °C) of paraffin. The composite PCM allowing no liquid paraffin leakage was considered as form-stable composite PCM. In addition, the densities of the paraffin/EG composite PCMs were determined using the pycnometer method with an accuracy of ±3%.
Table 1 Thermo-physical properties of the paraffin (n-docosane) Thermo-physical properties
Paraffin (n-docosane)
Solid–solid phase transition temperature (°C) Melting temperature (°C) Total latent heat of fusion (kJ/kg) Specific heat (kJ/kg °C) Density (kg/m3) Thermal conductivity (W/m K)
28.8 41.6 194.6 1.93(s); 2.38(l) 785 0.22
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of the prepared samples was calculated using Eq. (1) based on transient hot-wire method [22,28] k¼
Fig. 1. The apparatus used for thermal conductivity measurement by transient hot-wire method.
2.4. Thermal conductivity measurement Fig. 1 shows the apparatus used to measure thermal conductivity by transient hot-wire method. The platinum wire in the apparatus was chosen as 105 mm in length and 0.18 mm in diameter to obtain the optimum ratio between the length and the diameter (575), justifying the validity of the method [22]. The composites with different mass fraction (2%, 4%, 7%, and 10%) were placed into sample container (Fig. 1). A constant voltage (1 V) was applied for 10 s and the temperature raise by circulating the current (0.8 A) along the platinum wire was recorded by a thermocouple (Pt-RhPt) with an accuracy of ±0.1 °C. The measurements were repeated three times for each sample in order to minimize the error. The thermal conductivity (k)
q lnðt2 =t1 Þ 4pDT
ð1Þ
where, q is the power input per unit length of the wire, DT is the temperature difference 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 paraffin. The slopes of the plots (0.0702, 0.0691 and 0.0694) indicated that the reproducibility of measurements was reliable. The accuracy of the hot-wire apparatus was also checked by measuring the thermal conductivity of solid stearic acid and solid paraffin and it was found as 0.30 W/m K and 0.22 W/m K, respectively with a standard error of ±0.01. These results were in agreement with the published data; 0.29 W/m K for solid stearic acid [31] and 0.21–0.24 W/m K for solid paraffin [8,12–14]. 2.5. Establishment of melting time Increase in thermal conductivity of paraffin with EG additive in varying mass fractions was tested by comparison of melting times of the composite PCMs with that of pure paraffin. Melting times were measured with the experimental set-up and procedure used in our former study [11]. 2.6. Determination of thermal properties Thermal properties such as melting temperature and latent heat capacity of pure paraffin and paraffin/EG composite PCMs were measured using a DSC instrument (SETARAM DSC-131). Indium was used as a reference material for the calibration of the instrument. DSC measurements were performed at 5 °C/min heating rate and temperature range of 20–80 °C. The solid–solid phase transition temperature and melting temperature were measured by drawing a line at the point of maximum slope of the leading edge of the peak and extrapolating to base line. The latent heats of the PCMs were determined as total by numerical integration of area under the peaks that represent solid–solid and solid–liquid phase transition. 3. Results and discussion 3.1. Compatibility of EG with paraffin
Fig. 2. Temperature rise versus time for three measurements used in the calculation of thermal conductivity of pure paraffin.
Among the composite PCMs, the one with the mass fraction of 10% EG was proved to be as form-stable composite PCM as it could keep the same form in solid state even when the temperature of the PCM was over than the melting temperature of paraffin. It was due to that paraffin was hold by the capillary force and the surface tension force of the porous EG. This result also signified that the EG in mass fraction of 10% absorbed the liquid paraffin as nine times as itself without to be leakage of the melted
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Fig. 3. The photographs of (a) pure paraffin as PCM and (b) paraffin/ 10 wt% EG composite as form-stable PCM.
paraffin during the solid–liquid phase change process. This result is in agreement with literature. For instance, Py et al. reported that the paraffin with melting temperatures of 73– 80 °C was loaded to the compressed expanded natural graphite (CENG) from 65 to 95 wt% depending on the bulk density of CENG [24]. Mills et al. found that the paraffin with melting temperatures of 35–55 °C was impregnated as 85–90 wt% into porous graphite matrix with density of 61 g/l [25]. In addition, the mass fraction of paraffin (m.p.: 58–60 °C) absorbed into the EG for form-stable composite PCM has been reported as 86.6% by Zhang and Fang [30]. The paraffin distributed uniformly in EG due to its good structural compatibility (Fig. 3). This achieved that paraffin/10 wt% EG composite as form-stable PCM. The density of the form-stable composite PCM was measured as 721 kg/m3 at 25 °C by pycnometer method. It was less than the density of the paraffin (785 kg/m3), suggesting that a LHTES system using the form-stable paraffin/EG composite PCM to be lighter than a LHTES system using paraffin provided that of the systems have the same volume. In addition, the form-stable composite PCMs had a good mechanical robustness.
Fig. 4. Thermal conductivities of paraffin/EG composite PCMs with varying mass fraction of EG.
Thermal conductivities of the composite PCMs with mass fraction of 2%, 4%, 7%, and 10% EG indicated that the thermal conductivity of paraffin (0.22 W/m K) increased as 81.2%, 136.3%, 209.1%, and 272.7%, respectively. This was attributed to high thermal conductivity of the EG [24,25,29]. The results also showed that the thermal conductivity of paraffin could be achieved further by addition of EG more than 10 wt%. However, this mass fraction is adequate to obtain form-stable composite PCM, and further increase in EG over 10 wt% will results in decrease in latent heat capacity of the composite due to decrease in the mass fraction of paraffin (below 90 wt%) in the composite. Therefore the mass fraction of EG in the composite was limited with 10%. The high thermal conductivity of form-stable composite PCM (about four times of that of pure paraffin) makes it much more attractive than pure paraffin for passive LHTES applications. 3.3. Comparison of melting time of the composite PCMs with that of paraffin
3.2. Thermal conductivity improvement Fig. 4 shows thermal conductivity of the composite PCMs with different mass fraction of EG, indicating that the thermal conductivity increased with increasing mass fraction of EG. A relationship between the thermal conductivity and mass fraction of EG can be easily drawn out from Fig. 4 as follows: ^y ¼ 0:0524x þ 0:3038 ðr2 ¼ 0:9974Þ
ð2Þ
where, ^y and x represent thermal conductivity (k) and mass fraction of EG (%), respectively. The high coefficient of determination (r2 = 0.9974) indicated a strong relationship between the thermal conductivity and mass fraction of EG.
The improvement in thermal conductivity of paraffin was tested by comparing melting times of the composite PCMs with that of paraffin. Fig. 5 shows the melting temperature curves of the pure paraffin as PCM, paraffin/ 4 wt% EG mixture as composite PCM and paraffin/ 10 wt% EG as form-stable composite PCM. The melting time was estimated from the temperature curves as a time elapsed until the temperatures of the PCMs to reach from the initial temperature (25.2 °C) to over melting point of the PCMs (41.6 °C). The melting times of pure paraffin, the composite PCM with 4 wt% EG and form-stable composite PCM were determined as 91, 76 and 62 min, respectively, showing that the melting times of composite PCMs
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Fig. 5. Temperature curves of pure paraffin and paraffin/EG composite PCMs.
decreased by 16% and 32%, respectively when compared to melting time of pure paraffin. This is due to increased heat transfer by high thermal conductivity. The reduction in melting time of the composite PCMs confirmed the improvement in thermal conductivity of the paraffin. It can be also noteworthy that the form-stable composite PCM can more rapidly absorb heat supplied from a heat source compared with pure paraffin as PCM.
Table 2 Melting temperatures and total latent heat capacity of pure paraffin and paraffin/EG composite PCMs Material
Melting temperature (°C)
Total latent heat capacity (kJ/kg)
Pure paraffin 98 wt%paraffin/2 wt% EG 96 wt%paraffin/4 wt% EG 93 wt%paraffin/7 wt% EG 90 wt%paraffin/10 wt% EG
41.6 41.1 41.0 40.7 40.2
194.6 192.6 188.0 181.9 178.3
3.4. Thermal properties of paraffin/EG composite PCMs DSC analysis was conducted to investigate the influence of EG addition on thermal properties such as melting temperature and the latent heat storage capacity of paraffin and the composite PCMs. Thermal properties of paraffin and the composite PCMs depending on the amount of paraffin are given in Table 2. The DSC curve of paraffin in Fig. 6 was taken as reference to evaluate the change in thermal properties of composite PCMs. In DSC thermograms, the main peak represents solid–liquid phase change (melting) of paraffin, and the minor peak corresponds to solid–solid phase transition of paraffin. Table 2 shows that the melting temperatures of paraffin/EG composite PCMs saturated with paraffin in mass ratios of 98%, 96%, 93%, and 90% were lower than that of pure paraffin as 0.5, 0.6, 0.9 and 1.4 °C, respectively. On the other hand, total latent heat (for both of solid–solid and solid–liquid phase change) capacity of the composite PCMs composed of 98%, 96%, 93%, and 90% paraffin were lower 1.3%, 3.4%, 6.5% and 8.4%, respectively, compared to that of pure paraffin. This indicated that the total latent heats of the com-
Fig. 6. The DSC thermograms of pure paraffin and paraffin/10 wt% EG composite as form-stable PCM.
posite PCMs are large and almost equivalent to values calculated by multiplying the latent heat value of pure
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paraffin (194.6 kJ/kg) with its mass fractions (98, 96, 93, and 90 wt%). In addition, the latent heat of form-stable composite PCM saturated with 90 wt% paraffin had a satisfying latent heat capacity (178.3 kJ/kg) for LHTES applications. 4. Conclusions The following conclusions were reached based on the experimental results: (1) The paraffin (n-docosane)/EG composite PCMs can be easily prepared by impregnation of liquid paraffin into the porous structure of EG. The paraffin/EG composite PCM with mass fraction of 10% EG was accepted as form-stable composite PCM as it allowed no leakage of melted paraffin from the pores of EG when subjected to a solid–liquid phase change process. In particular, the use of form-stable composite PCM can reduce the weight of the LHTES system due to its low density besides its direct usability in energy storage systems without a requirement of an extra storage container. (2) Increasing the mass fraction of EG from 2% to 10% gradually increased the thermal conductivity of paraffin/EG composite PCM. A very high correlation (r = 0.9986) occurred between thermal conductivity and mass fraction of EG, indicating a strong association between the two attributes. (3) The reduction of the melting times of the composite PCMs was in accordance with the increase in the thermal conductivity. In overall, it may be suggested that a composite with a mass fraction of 10% EG can be considered as the most promising PCM for thermal energy storage using LHTES method due to its form-stable property, direct usability without a need of extra storage container, high thermal conductivity, proper melting temperature and satisfying latent heat capacity. Acknowledgement The authors thank Dr. Orhan UZUN for DSC analyses made in Department of Physics of Gaziosmanpasßa University. References [1] G.A. Lane, Solar Heat Storage: Latent Heat Materials, Technology, vol. II, CRC Press Inc., Florida, 1986. [2] H.P. Garg, S.C. Mullick, A.K. Bhargava, Solar Thermal Energy Storage, D. Reidel Publishing Company, Dordrecht, Holland, 1985. [3] I. Dinc¸er, Thermal energy storage systems as a key technology in energy conservation, Int. J. Energy Res. 26 (2002) 567–588. [4] H.G. Lorsch, K.W. Kauffman, J.C. Denton, Thermal energy storage for heating and air conditioning, Energy Conver. Manag. 15 (1975) 69–81.
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