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Heat transfer enhancement of neopentyl glycol using compressed expanded natural graphite for thermal energy storage
Renewable Energy 51 (2013) 241e246
Contents lists available at SciVerse ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Heat transfer enhancement of neopentyl glycol using compressed expanded natural graphite for thermal energy storage Xianglei Wang a, b, Quangui Guo a, *, Yajuan Zhong c, Xinghai Wei a, Lang Liu a a
Key Laboratory of Carbon Materials, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China Graduate University of Chinese Academy of Sciences, Beijing 100049, China c CAS Key Laboratory of Materials for Energy Conversion, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 April 2012 Accepted 19 September 2012 Available online 24 October 2012
Neopentyl glycol (NPG) was saturated into the compressed expanded natural graphite (CENG) matrices with different densities in an attempt to increase the thermal performance of NPG for latent heat thermal energy storage (LHTES) application. NPG uniformly disperses in the porous network of the expanded graphite. Measured results indicated that thermal conductivities of the composites can be enhanced 11 e88 times as compared with that of the pure NPG. The latent heat of the NPG/CENG composites increased with the increasing mass ratio of the NPG in the composites. Compared with the pure NPG, the deformation of the composites due to phase change has been greatly reduced. After phase transition, thermal conductivity of the composites decreased slightly. The trends concluded from the finite element simulation are coincident with the trends from the thermal imager. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Phase change Expanded graphite Neopentyl glycol Latent heat Finite element simulation
1. Introduction The utilization of phase change materials (PCMs) for thermal energy storage is of increasing concern during the past 30 years [1e 5]. PCMs have been widely used in many fields, such as waste heat recovering, storage and reutilization, solar power utilization, active and passive cooling of the electronic devices and building applications [4e7]. Thermal energy can be stored as a change in internal energy of a material as sensible heat, latent heat and thermochemical heat or combination of these [2]. Latent heat storage is based on heat absorption or release when a storage material undergoes a phase change. And it has been proved to be an effective thermal management application due to the high storage density and a moderate temperature variation [8]. As a kind of non-paraffin organic latent heat storage PCM, polyalcohols as the solidesolid phase change materials have the advantages of smaller volume changes during the phase change processes, no leakage problems, smaller erosion to the devices and longer service life among others, unfortunately, most of them have the drawback of higher phase change temperatures [9]. Therefore it is necessary to search for a lower phase change temperature
* Corresponding author. Tel.: þ86 351 4084106; fax: þ86 351 4083952. E-mail address: [email protected] (Q. Guo). 0960-1481/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.renene.2012.09.029
material, and the neopentyl glycol (NPG) is an appropriate choice (its solidesolid phase change temperature is 44.5 C). Nevertheless, NPG is characterized by low thermal conductivity that reduces heat exchange rates during the phase change process. Recently, expanded natural graphite with high thermal conductivity was reported to enhance the heat transfer in latent heat thermal energy storage (LHTES) systems due to their low density, simplicity involved in forming the matrix, being compatible with PCMs and against high temperature, oxidation resistance, corrosion resistance, radiation resistance [8,10e17]. Up to now, few papers reported how to improve the thermal performance of NPG for LHTES application. The solidesolid phase transition, and the parameters that affect the performance of such composites in thermal management applications did not receive extensive research. In the present work, NPG was saturated into the compressed expanded natural CENG matrices with different densities in an attempt to increase the thermal performance of NPG for LHTES application. Thermal conductivity and latent heat capacity of the composites have been measured. Morphology of the composites was characterized. Anisotropy of the heat transfer with phase change in the composites has been investigated experimentally and theoretically. The influences of structure and thermal properties of the CENG matrices on the thermal behavior of the composites have been discussed in detail.
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2. Experimental 2.1. Sample preparation In this experiment, chemical pure NPG supplied by sinopharm chemical reagent Co. Ltd. which has a solidesolid phase change temperature of 318 K and a latent heat of 116 J/g with a thermal conductivity of 0.43 W/(m K) was used as phase change material. Expanded graphite worms were prepared by intercalation with natural flake graphite as raw materials, concentrated nitric acid as oxidizing agent, perchloric acid as inserting reagent and then obtained by heating the reactant to 900 C for 20 s [18]. The achieved expanded graphite worms were molded to obtain the CENG matrices with different bulk densities ranging from 0.08 to 0.23 g/cm3 according to various pressures. The CENG matrices were impregnated in NPG at 418 K (above the melting point 397 K of NPG) in a vacuum oven for 1 h. 2.2. Sample characterization S4800 scanning electron microscopy (FE-SEM) was used to observe the morphology of the matrices and the composites. The thermal diffusivities for all samples were measured at 298 K and 323 K by LFA 447 nanoflash apparatus. The specific heat of all samples during the phase change process was tested using differential scanning calorimeter (DSC) technique (NETZSCH DSC 200F3). The heating rate was 5 K/min under the protection of nitrogen atmosphere. The formula for thermal conductivity multiplied together density, thermal diffusivity and specific heat. As we know, thermal conductivity of the CENG matrix depends on the orientation of graphite crystallite sheets and porosity. So thermal conductivity of NPG/CENG composites along perpendicular to the compression force direction and parallel to the compression force direction were measured. To measure the time evaluation of temperature distribution function, two cubic samples whose plane orientations were along perpendicular to the compression force direction and parallel to the compression force direction respectively were placed on a hot copper plate in oil bath with a constant temperature 353 K (above the phase change point of the NPG 318 K). Pictures were taken every 10 s interval by a thermal imager (Fluke Ti32 Thermal Imager) to record the temperature distribution. The surface emissivities of copper and composites which were measured by thermal imager are 0.19 and 0.79 respectively. The usual method of testing the emissivity is to place a standard black insulation tape with the emissivity 0.93 in contact with the surface of the object. If the temperature reading is different, the emissivity level on the imager can be adjusted until the object reads the same temperature as the standard black insulation tape. The latent heat
of the samples with a heating speed of 0.5 K/min under the protection argon atmospheres was measured by DSC technique (NETZSCH STA 409 PC). 3. Numerical simulation and modeling 3.1. Governing equations Three physical processes have been simulated in order to study the entire energy storage processes happened inside the entire system: heat transfer by conduction, radiation and phase change heat transfer. The modeling was according to the surroundings that the thermal image experiment had been exposed to. This section presents a summary of the equations needed to account for all three processes. 1. Heat transfer: conduction Heat transfer from the bottom of PCMs to the top of PCMs happens by conduction. Here, the PCMs are the NPG/CENG composites. Since the volumes of PCMs were roughly kept unchanged, it can be assumed that the effect of convection in the phase changed PCMs are negligible. In that case, the heat conduction equation as to be solved:
rCp
vT V,ðkVTÞ ¼ Q vt
It has these material properties: density r, heat capacity Cp, thermal conductivity k, and Q, which is the heat source. 2. Heat transfer: radiation Heat transfer from the boundaries of the copper and PCMs to air happens by radiation.
4 q ¼ εs Tamb T4 where ε is the surface emissivity, s is the StefaneBoltzmann constant, and Tamb is the ambient temperature. 3. Phase change heat transfer In order to account for the phase change process happening when the PCM is phase changing, the following equation should be solved at the transition interface:
k
/
s1V
Ts1 k
/
s2V
Ts2 ¼ rDH
dX dt
Fig. 1. (a) SEM image of the pore structure of the expanded natural graphite; (b) SEM image of the NPG/CENG composite (CENG 0.085 g/cm3).
X. Wang et al. / Renewable Energy 51 (2013) 241e246
243
Table 1 Parameters of the samples. Bulk density of the CENG (g/cm3)
Mass ratio of NPG in the composites (%)
0.085 0.117 0.156 0.193 0.233
90 87 82 76 71
Thermal conductivity of the composites(W/m K) 25 C X
Y
X
Y
7.3 14.2 25.0 29.0 37.6
4.6 7.1 10.3 11.2 9.0
6.1 13.2 20.1 25.6 36.3
3.9 6.6 8.8 9.8 9.5
where the subscripts s1 and s2 stand for the phases of between before phase change and after phase change, DH is the enthalpy of phase transition, and X is the position of the phase transition interface.
3.2. Model and boundary conditions The model was then created using the following initial and boundary conditions: I. Initial temperatures of the hot copper and two composites are 340 K and 296 K respectively, and the hot copper is keeping in constant temperature 340 K. II. The surface emissivities of copper and composites are 0.19 and 0.79 respectively. III. The thermal expansion is not considered. 3.3. Numerical resolution One major problem has to be dealt with: taking into account the phase change process, i.e., accounting for the phase change interface and the large amount of energy needed to phase transition. This problem is dealt with by introducing equivalent heat capacity method [19]. Using a NPG/CENG composite that has an enthalpy of phase transition of 96 kJ/kg (DH) and changes over a 3.0 K temperature range (from 313.5 K to 316.5 K), the specific heat of the NPG is modified in the following way:
106 100 93 82 75
8 < 2:4 KJ=ðKg*KÞ; T < 313:5 K Cp ¼ ð2:4 þ d*DHÞKJ=ðKg*KÞ; 313:5 K T 316:5 K : 2:4 KJ=ðKg*KÞ; T>316:5 K exp ðT Tm Þ2 =ðDTÞ2 pffiffiffiffi d¼ DT p Here Tm is the phase change temperature and DT denotes the half width of the curve, in this case set to1.5 K, representing half the transition temperature span.
4. Results and discussion 4.1. Structure of the samples The characteristic of the microstructure of a material is determined by its nature. Multi-pores structure is observed from high magnification (3000) of expanded graphite shown in Fig. 1a. There are many interconnected micro-honeycomb pores in the expanded graphite, which can easily be filled with melted NPG. Capillary forces and high surface activity of the graphite microcrystalline goads CENG to be saturated in plenty of melted NPG. As shown in Fig. 1b, the NPG in the composites was embedded and uniformly dispersed in the porous network of expanded graphite. The CENG matrix was compatible with NPG and graphite thanks to the high wetting ability of the NPG. Calculated results indicated that NPG filled at least 80% of the interspaces in the CENG matrices. Evidently, NPG as a solidesolid phase change material will transfer from a low symmetry crystal structure to a high symmetry one
kbefore=-9.29+203.42x R=0.98
40
kafter=-10.59+196.71x R=0.99 35 30 25
DSC, mW/mg
Thermal Conductivity of the composites,W/m.K
Latent heat of the composites (J/g)
50 C
20 15
100% 90% 87% 82% 76% 71%
10 5 0.08
0.10
0.12
0.14
0.16
0.18
0.20
0.22
Bulk density of the CENG matrices, g/cm
0.24
3
Fig. 2. Variation of the thermal conductivity of the composites with the bulk density of the CENG matrices.
30
35
40
45
50
55
60
Temperature,οc Fig. 3. DSC curves of the NPG/CENG composites with different NPG contents.
65
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X. Wang et al. / Renewable Energy 51 (2013) 241e246
higher than that of in perpendicular to the compression force and 11e26 times higher than that of in parallel to the compression force directions for NPG respectively. After phase transition, thermal conductivity of the composites had decreased slightly. The network of the CENG matrix remains very well in the composites, which supplies a path for heat transfer. And the higher the bulk density of CENG matrix, the denser the network. Heat can quickly transfer through the well-developed network of the CENG matrix. As is expected, thermal conductivity of the NPG/CENG composite increased obviously with increasing bulk density of CENG matrix. A relationship between thermal conductivity in perpendicular to the compression force direction and bulk density of the CENG matrix can be easily drawn out from Fig. 2 as follows:
110
Calculated value Experimental value
105
Latent heat, J/g
100
H=1.16w R=1
95
90
H=-42.42+1.64w R=0.99
85
80
kbefore ¼ 9:29 þ 203:42x; kafter ¼ 10:59 þ 196:71x
75 70
75
80
85
90
Mass ratio of NPG in CENG matrix, % Fig. 4. Variation of the latent heat of the composites with the mass ratio of NPG in them.
during the phase transformation [9]. Its integral structure will be damaged and even broken into pieces without the container. Due to the introduction of the CENG, the composites will not leak and the most important its integrity can be ensured. 4.2. Thermal conductivity enhancement Thermal conductivity of NPG at the room temperature was measured as 0.42 W/(m K). As shown in Table 1, thermal conductivity of the NPG/CENG composites improved evidently compared to that of the pure NPG. As following: it was roughly 17e88 times
where x represents the bulk density of the CENG matrix (g/cm3). kbefore and kafter stand for the thermal conductivity (W/(m K)) along perpendicular to the compression force direction of the composite before phase and after phase change respectively. Fig. 2 shows that the linear correlation coefficient R is 0.98, 0.99 respectively for before and after phase change, which reveals a good linear relationship between thermal conductivity and bulk density of the CENG matrix.
4.3. Latent heat capacity DSC analysis was conducted to investigate the latent heat capacity of the NPG/CENG composites as a thermal energy storage system. Fig. 3 shows the DSC curves of the phase change composites of different NPG contents. It is obviously that every composite exhibits a more sharp-angled phase change peak than the pure NPG. The phase-change temperature of the NPG/CENG composites is slightly less than that of the pure NPG. Compared with the pure
Fig. 5. Thermal images of the NPG/CENG composites at different time: heat transferred perpendicular to the compression force in the left one and along the direction of the compression force in the right one.
X. Wang et al. / Renewable Energy 51 (2013) 241e246
245
Fig. 6. Simulations of the NPG/CENG composites at different time: heat transferred perpendicular to the compression force in the left one and along the direction of the compression force in the right one.
Here, H is the latent heat (J/g), w% is the mass ratio of the NPG in the CENG matrices. Experimental values listed in Table 1 can be drawn out an almost linear relationship from Fig. 4 as follows:
H ¼ 42:42 þ 1:64w The high coefficient of determination (R ¼ 0.99) indicates a strong relationship between latent heat and mass ratio of the NPG in the composite. As shown in Fig. 4, the experimental values of latent heat accord well with the calculated values with differences less than 3%. Such small differences in latent heat capacity of the NPG are within the margin of error.
As we know, CENG is a kind of anisotropic material due to the stress differences between parallel and perpendicular to the compression force direction. In compression process, the direction of every graphite piece layer trends to be perpendicular to the compression force direction. From Table 1, anisotropy of thermal conductivity can be seen obviously even the CENG has saturated
70
Parallel to compression force Perpendicular to compression force
60
o
H ¼ 1:16w
4.4. Anisotropy of heat transfer
Temperature, C
NPG, the phase change speed of the composites accelerated because of the existence of graphite microcrystallines. The latent heat values were obtained by means of the integral form of the DSC curve. As reported in Table 1, with the increase of the bulk density of CENG, latent heat of the composite made of it decreases. Graphite occupies space of the NPG in the composites so that the energy storage density of the system is lower than that of the pure NPG. The latent heat data of the NPG/CENG composites are compared with the calculated data according to the mass ratio of the NPG in the composite shown in Fig. 4. Latent heat of the NPG/CENG composite increases with the increase of the mass ratio of the NPG in the composite. And calculated value of the latent heat is 82, 88, 95, 100, and 104 J/g respectively. The calculated latent heat and mass ratio of the NPG in the composites can be drawn a linear plot in Fig. 4:
50
40
30
20 0
200
400
600
800
Time, s Fig. 7. Temperature response in different heat transfer directions of thermal imager experiment.
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X. Wang et al. / Renewable Energy 51 (2013) 241e246
5. Conclusions
70 Perpendicular to compression force Parallel to compression force
CENG matrices with different densities were used to enhance the thermal performance of the NPG for LHTES application. The thermal conductivity and latent heat of the composite depend on the bulk density of the CENG. Thermal conductivity of the NPG/CENG composites can be 11e88 higher times than that of the pure NPG. Compared with the pure NPG, the deformation of composites due to phase change has been greatly reduced. The thermal conductivities of composites have gone down after phase transformation.
o
Temperature, C
60
50
40
Acknowledgments 30
This project was supported by National Basic Research Program of China under the contact No. 2011CB605802, and Shanxi Province Nature Science Foundation of China (No. 2011021022-1).
20
0
200
400
600
800
Time, s Fig. 8. Temperature response in different heat transfer directions of the finite element simulation.
with the NPG. The higher the density of CENG is, the bigger anisotropic ratio of thermal conductivity of both before and after phase change is. Thermal imager experiment and the finite element simulation were used to further prove the point. Heat transfers along perpendicular to the compression force direction and parallel to the compression force direction were measured. Temperature distribution images of both thermal imager and the finite element simulation at 100, 200, 300, 400, 600 and 800 s are shown in Figs. 5 and 6 respectively for composite with a bulk density of CENG 0.156 g/cm3 and a mass ratio of NPG of 82%. In the two different heat transfer directions, various heat transfer speed is obvious. Both thermal imager experiment and the finite element simulation have proved the point. The right one of heat transfer speed is obviously quicker than the left one. For the right one, heat can quickly spread out before and after its phase change. So its temperature distribution range is very little. Its phase change roughly occurs from 100 to 200 s, in this time period, temperature distribution range is relatively larger. For the left one, temperature distribution range has been larger until after 800 s. In order to observe this phenomenon directly, two curves of the temperature versus time at the same height of the samples are drawn in Figs. 7 and 8 for thermal imager experiment and the finite element simulation respectively. It is indicated that along the two different directions the samples have the different temperature response during the whole process. But the trend is roughly the same for every cure. Specifically as follows: They undergo a relatively long phase change process after their temperatures rapidly get to about 40 C. And then their temperatures first-quick-then-slow rise until they get up to a constant value. What is different is that the degrees of quickness and slowness for every curve are different. The trends that were concluded from the finite element simulation are coincident with the trends from the thermal imager as are shown in Figs. 5e8.
References [1] Zalba B, Marín JM, Cabeza LF, Mehling H. Review on thermal energy storage with phase change: materials, heat transfer analysis and applications. Appl Therm Eng 2003;23:251e83. [2] Sharma A, Tyagi VV, Chen CR, Buddhi D. Review on thermal energy storage with phase change materials and applications. Renew Sustain Energ Rev 2009;13:318e45. [3] Abhat A. Low temperature latent heat thermal energy storage: heat storage materials. Sol Energ 1983;30:313e32. [4] Raj VAA, Velarj R. Review on free cooling of buildings using phase change materials. Renew Sustain Energ Rev 2010;14:2819e29. [5] Darkwa K, OCallaghan PW, Tetlow D. Phase-change drywalls in a passive-solar building. Appl Energ 2006;83:425e35. [6] Yagi J, Akiyama T. Storage of thermal energy for effective use of waste heat from industries. J Mater Process Technol 1995;48:793e804. [7] Fok SC, Shen W, Tan FL. Cooling of portable hand-held electronic devices using phase change materials in finned heat sinks. Int J Therm Sci 2010;49:109e17. [8] Zhong YJ, Li SZ, Wei XH, Liu ZJ, Guo QG. Heat transfer enhancement of paraffin wax using compressed expanded natural graphite for thermal energy storage. Carbon 2010;48:300e4. [9] Wang XW, Lu ER, Lin WX, Wang CZ. Micromechanism of heat storage in a binary system of two kinds of polyalcohols as a solid-solid phase change material. Energ Convers Manag 2000;41:135e44. [10] Pincemin S, Olives R, Py X, Christ M. Highly conductive composites made of phase change materials and graphite for thermal storage. Sol Energ Mater Sol Cell 2008;92:603e13. [11] Acem Z, Lopez J, Barrio EP. KNO3/NaNO3-graphite materials for thermal energy storage at high temperature: part I. e elaboration methods and thermal properties. Appl Therm Eng 2010;30:1580e5. [12] Lopez J, Acem Z, Barrio EP. KNO3/NaNO3-graphite materials for thermal energy storage at high temperature: part II. e phase transition properties. Appl Therm Eng 2010;30:1586e93. [13] Karaipekli A, Sari A, Kaygusuz K. Thermal conductivity improvement of stearic acid using expanded graphite and carbon fiber for energy storage applications. Renew Energ 2007;32:2201e10. [14] Kim S, Drzal LT. High latent heat storage and high thermal conductive phase change materials using exfoliated graphite nanoplatelets. Sol Energ Mater Sol Cell 2009;93:136e42. [15] Zhao CY, Wu ZG. Heat transfer enhancement of high temperature thermal energy storage using metal foams and expanded graphite. Sol Energ Mater Sol Cell 2011;95:636e43. [16] Fang GY, Li H, Chen Z, Liu X. Preparation and characterization of stearic acid/ expanded graphite composites as thermal energy storage materials. Energy 2010;35:4622e6. [17] Zhao JG, Guo Y, Feng F, Tong QH, Qv WS, Wang HQ. Microstructure and thermal properties of a paraffin/expanded graphite phase-change composite for thermal storage. Renew Energ 2011;36:1339e42. [18] Wei XH, Liu L, Zhang JX, Shi JL, Guo QG. Synthesis of HClO4-GICs and the performance of flexible graphite produced from them. New Carbon Mater 2007;22:342e8. [19] Voller VR, Prakash C. A fixed grid numerical modeling methodology for convection-diffusion mushy region phase change problems. Int J Heat Mass Transf 1987;30:1709e19.
Contents lists available at SciVerse ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Heat transfer enhancement of neopentyl glycol using compressed expanded natural graphite for thermal energy storage Xianglei Wang a, b, Quangui Guo a, *, Yajuan Zhong c, Xinghai Wei a, Lang Liu a a
Key Laboratory of Carbon Materials, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China Graduate University of Chinese Academy of Sciences, Beijing 100049, China c CAS Key Laboratory of Materials for Energy Conversion, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 April 2012 Accepted 19 September 2012 Available online 24 October 2012
Neopentyl glycol (NPG) was saturated into the compressed expanded natural graphite (CENG) matrices with different densities in an attempt to increase the thermal performance of NPG for latent heat thermal energy storage (LHTES) application. NPG uniformly disperses in the porous network of the expanded graphite. Measured results indicated that thermal conductivities of the composites can be enhanced 11 e88 times as compared with that of the pure NPG. The latent heat of the NPG/CENG composites increased with the increasing mass ratio of the NPG in the composites. Compared with the pure NPG, the deformation of the composites due to phase change has been greatly reduced. After phase transition, thermal conductivity of the composites decreased slightly. The trends concluded from the finite element simulation are coincident with the trends from the thermal imager. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Phase change Expanded graphite Neopentyl glycol Latent heat Finite element simulation
1. Introduction The utilization of phase change materials (PCMs) for thermal energy storage is of increasing concern during the past 30 years [1e 5]. PCMs have been widely used in many fields, such as waste heat recovering, storage and reutilization, solar power utilization, active and passive cooling of the electronic devices and building applications [4e7]. Thermal energy can be stored as a change in internal energy of a material as sensible heat, latent heat and thermochemical heat or combination of these [2]. Latent heat storage is based on heat absorption or release when a storage material undergoes a phase change. And it has been proved to be an effective thermal management application due to the high storage density and a moderate temperature variation [8]. As a kind of non-paraffin organic latent heat storage PCM, polyalcohols as the solidesolid phase change materials have the advantages of smaller volume changes during the phase change processes, no leakage problems, smaller erosion to the devices and longer service life among others, unfortunately, most of them have the drawback of higher phase change temperatures [9]. Therefore it is necessary to search for a lower phase change temperature
* Corresponding author. Tel.: þ86 351 4084106; fax: þ86 351 4083952. E-mail address: [email protected] (Q. Guo). 0960-1481/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.renene.2012.09.029
material, and the neopentyl glycol (NPG) is an appropriate choice (its solidesolid phase change temperature is 44.5 C). Nevertheless, NPG is characterized by low thermal conductivity that reduces heat exchange rates during the phase change process. Recently, expanded natural graphite with high thermal conductivity was reported to enhance the heat transfer in latent heat thermal energy storage (LHTES) systems due to their low density, simplicity involved in forming the matrix, being compatible with PCMs and against high temperature, oxidation resistance, corrosion resistance, radiation resistance [8,10e17]. Up to now, few papers reported how to improve the thermal performance of NPG for LHTES application. The solidesolid phase transition, and the parameters that affect the performance of such composites in thermal management applications did not receive extensive research. In the present work, NPG was saturated into the compressed expanded natural CENG matrices with different densities in an attempt to increase the thermal performance of NPG for LHTES application. Thermal conductivity and latent heat capacity of the composites have been measured. Morphology of the composites was characterized. Anisotropy of the heat transfer with phase change in the composites has been investigated experimentally and theoretically. The influences of structure and thermal properties of the CENG matrices on the thermal behavior of the composites have been discussed in detail.
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X. Wang et al. / Renewable Energy 51 (2013) 241e246
2. Experimental 2.1. Sample preparation In this experiment, chemical pure NPG supplied by sinopharm chemical reagent Co. Ltd. which has a solidesolid phase change temperature of 318 K and a latent heat of 116 J/g with a thermal conductivity of 0.43 W/(m K) was used as phase change material. Expanded graphite worms were prepared by intercalation with natural flake graphite as raw materials, concentrated nitric acid as oxidizing agent, perchloric acid as inserting reagent and then obtained by heating the reactant to 900 C for 20 s [18]. The achieved expanded graphite worms were molded to obtain the CENG matrices with different bulk densities ranging from 0.08 to 0.23 g/cm3 according to various pressures. The CENG matrices were impregnated in NPG at 418 K (above the melting point 397 K of NPG) in a vacuum oven for 1 h. 2.2. Sample characterization S4800 scanning electron microscopy (FE-SEM) was used to observe the morphology of the matrices and the composites. The thermal diffusivities for all samples were measured at 298 K and 323 K by LFA 447 nanoflash apparatus. The specific heat of all samples during the phase change process was tested using differential scanning calorimeter (DSC) technique (NETZSCH DSC 200F3). The heating rate was 5 K/min under the protection of nitrogen atmosphere. The formula for thermal conductivity multiplied together density, thermal diffusivity and specific heat. As we know, thermal conductivity of the CENG matrix depends on the orientation of graphite crystallite sheets and porosity. So thermal conductivity of NPG/CENG composites along perpendicular to the compression force direction and parallel to the compression force direction were measured. To measure the time evaluation of temperature distribution function, two cubic samples whose plane orientations were along perpendicular to the compression force direction and parallel to the compression force direction respectively were placed on a hot copper plate in oil bath with a constant temperature 353 K (above the phase change point of the NPG 318 K). Pictures were taken every 10 s interval by a thermal imager (Fluke Ti32 Thermal Imager) to record the temperature distribution. The surface emissivities of copper and composites which were measured by thermal imager are 0.19 and 0.79 respectively. The usual method of testing the emissivity is to place a standard black insulation tape with the emissivity 0.93 in contact with the surface of the object. If the temperature reading is different, the emissivity level on the imager can be adjusted until the object reads the same temperature as the standard black insulation tape. The latent heat
of the samples with a heating speed of 0.5 K/min under the protection argon atmospheres was measured by DSC technique (NETZSCH STA 409 PC). 3. Numerical simulation and modeling 3.1. Governing equations Three physical processes have been simulated in order to study the entire energy storage processes happened inside the entire system: heat transfer by conduction, radiation and phase change heat transfer. The modeling was according to the surroundings that the thermal image experiment had been exposed to. This section presents a summary of the equations needed to account for all three processes. 1. Heat transfer: conduction Heat transfer from the bottom of PCMs to the top of PCMs happens by conduction. Here, the PCMs are the NPG/CENG composites. Since the volumes of PCMs were roughly kept unchanged, it can be assumed that the effect of convection in the phase changed PCMs are negligible. In that case, the heat conduction equation as to be solved:
rCp
vT V,ðkVTÞ ¼ Q vt
It has these material properties: density r, heat capacity Cp, thermal conductivity k, and Q, which is the heat source. 2. Heat transfer: radiation Heat transfer from the boundaries of the copper and PCMs to air happens by radiation.
4 q ¼ εs Tamb T4 where ε is the surface emissivity, s is the StefaneBoltzmann constant, and Tamb is the ambient temperature. 3. Phase change heat transfer In order to account for the phase change process happening when the PCM is phase changing, the following equation should be solved at the transition interface:
k
/
s1V
Ts1 k
/
s2V
Ts2 ¼ rDH
dX dt
Fig. 1. (a) SEM image of the pore structure of the expanded natural graphite; (b) SEM image of the NPG/CENG composite (CENG 0.085 g/cm3).
X. Wang et al. / Renewable Energy 51 (2013) 241e246
243
Table 1 Parameters of the samples. Bulk density of the CENG (g/cm3)
Mass ratio of NPG in the composites (%)
0.085 0.117 0.156 0.193 0.233
90 87 82 76 71
Thermal conductivity of the composites(W/m K) 25 C X
Y
X
Y
7.3 14.2 25.0 29.0 37.6
4.6 7.1 10.3 11.2 9.0
6.1 13.2 20.1 25.6 36.3
3.9 6.6 8.8 9.8 9.5
where the subscripts s1 and s2 stand for the phases of between before phase change and after phase change, DH is the enthalpy of phase transition, and X is the position of the phase transition interface.
3.2. Model and boundary conditions The model was then created using the following initial and boundary conditions: I. Initial temperatures of the hot copper and two composites are 340 K and 296 K respectively, and the hot copper is keeping in constant temperature 340 K. II. The surface emissivities of copper and composites are 0.19 and 0.79 respectively. III. The thermal expansion is not considered. 3.3. Numerical resolution One major problem has to be dealt with: taking into account the phase change process, i.e., accounting for the phase change interface and the large amount of energy needed to phase transition. This problem is dealt with by introducing equivalent heat capacity method [19]. Using a NPG/CENG composite that has an enthalpy of phase transition of 96 kJ/kg (DH) and changes over a 3.0 K temperature range (from 313.5 K to 316.5 K), the specific heat of the NPG is modified in the following way:
106 100 93 82 75
8 < 2:4 KJ=ðKg*KÞ; T < 313:5 K Cp ¼ ð2:4 þ d*DHÞKJ=ðKg*KÞ; 313:5 K T 316:5 K : 2:4 KJ=ðKg*KÞ; T>316:5 K exp ðT Tm Þ2 =ðDTÞ2 pffiffiffiffi d¼ DT p Here Tm is the phase change temperature and DT denotes the half width of the curve, in this case set to1.5 K, representing half the transition temperature span.
4. Results and discussion 4.1. Structure of the samples The characteristic of the microstructure of a material is determined by its nature. Multi-pores structure is observed from high magnification (3000) of expanded graphite shown in Fig. 1a. There are many interconnected micro-honeycomb pores in the expanded graphite, which can easily be filled with melted NPG. Capillary forces and high surface activity of the graphite microcrystalline goads CENG to be saturated in plenty of melted NPG. As shown in Fig. 1b, the NPG in the composites was embedded and uniformly dispersed in the porous network of expanded graphite. The CENG matrix was compatible with NPG and graphite thanks to the high wetting ability of the NPG. Calculated results indicated that NPG filled at least 80% of the interspaces in the CENG matrices. Evidently, NPG as a solidesolid phase change material will transfer from a low symmetry crystal structure to a high symmetry one
kbefore=-9.29+203.42x R=0.98
40
kafter=-10.59+196.71x R=0.99 35 30 25
DSC, mW/mg
Thermal Conductivity of the composites,W/m.K
Latent heat of the composites (J/g)
50 C
20 15
100% 90% 87% 82% 76% 71%
10 5 0.08
0.10
0.12
0.14
0.16
0.18
0.20
0.22
Bulk density of the CENG matrices, g/cm
0.24
3
Fig. 2. Variation of the thermal conductivity of the composites with the bulk density of the CENG matrices.
30
35
40
45
50
55
60
Temperature,οc Fig. 3. DSC curves of the NPG/CENG composites with different NPG contents.
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higher than that of in perpendicular to the compression force and 11e26 times higher than that of in parallel to the compression force directions for NPG respectively. After phase transition, thermal conductivity of the composites had decreased slightly. The network of the CENG matrix remains very well in the composites, which supplies a path for heat transfer. And the higher the bulk density of CENG matrix, the denser the network. Heat can quickly transfer through the well-developed network of the CENG matrix. As is expected, thermal conductivity of the NPG/CENG composite increased obviously with increasing bulk density of CENG matrix. A relationship between thermal conductivity in perpendicular to the compression force direction and bulk density of the CENG matrix can be easily drawn out from Fig. 2 as follows:
110
Calculated value Experimental value
105
Latent heat, J/g
100
H=1.16w R=1
95
90
H=-42.42+1.64w R=0.99
85
80
kbefore ¼ 9:29 þ 203:42x; kafter ¼ 10:59 þ 196:71x
75 70
75
80
85
90
Mass ratio of NPG in CENG matrix, % Fig. 4. Variation of the latent heat of the composites with the mass ratio of NPG in them.
during the phase transformation [9]. Its integral structure will be damaged and even broken into pieces without the container. Due to the introduction of the CENG, the composites will not leak and the most important its integrity can be ensured. 4.2. Thermal conductivity enhancement Thermal conductivity of NPG at the room temperature was measured as 0.42 W/(m K). As shown in Table 1, thermal conductivity of the NPG/CENG composites improved evidently compared to that of the pure NPG. As following: it was roughly 17e88 times
where x represents the bulk density of the CENG matrix (g/cm3). kbefore and kafter stand for the thermal conductivity (W/(m K)) along perpendicular to the compression force direction of the composite before phase and after phase change respectively. Fig. 2 shows that the linear correlation coefficient R is 0.98, 0.99 respectively for before and after phase change, which reveals a good linear relationship between thermal conductivity and bulk density of the CENG matrix.
4.3. Latent heat capacity DSC analysis was conducted to investigate the latent heat capacity of the NPG/CENG composites as a thermal energy storage system. Fig. 3 shows the DSC curves of the phase change composites of different NPG contents. It is obviously that every composite exhibits a more sharp-angled phase change peak than the pure NPG. The phase-change temperature of the NPG/CENG composites is slightly less than that of the pure NPG. Compared with the pure
Fig. 5. Thermal images of the NPG/CENG composites at different time: heat transferred perpendicular to the compression force in the left one and along the direction of the compression force in the right one.
X. Wang et al. / Renewable Energy 51 (2013) 241e246
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Fig. 6. Simulations of the NPG/CENG composites at different time: heat transferred perpendicular to the compression force in the left one and along the direction of the compression force in the right one.
Here, H is the latent heat (J/g), w% is the mass ratio of the NPG in the CENG matrices. Experimental values listed in Table 1 can be drawn out an almost linear relationship from Fig. 4 as follows:
H ¼ 42:42 þ 1:64w The high coefficient of determination (R ¼ 0.99) indicates a strong relationship between latent heat and mass ratio of the NPG in the composite. As shown in Fig. 4, the experimental values of latent heat accord well with the calculated values with differences less than 3%. Such small differences in latent heat capacity of the NPG are within the margin of error.
As we know, CENG is a kind of anisotropic material due to the stress differences between parallel and perpendicular to the compression force direction. In compression process, the direction of every graphite piece layer trends to be perpendicular to the compression force direction. From Table 1, anisotropy of thermal conductivity can be seen obviously even the CENG has saturated
70
Parallel to compression force Perpendicular to compression force
60
o
H ¼ 1:16w
4.4. Anisotropy of heat transfer
Temperature, C
NPG, the phase change speed of the composites accelerated because of the existence of graphite microcrystallines. The latent heat values were obtained by means of the integral form of the DSC curve. As reported in Table 1, with the increase of the bulk density of CENG, latent heat of the composite made of it decreases. Graphite occupies space of the NPG in the composites so that the energy storage density of the system is lower than that of the pure NPG. The latent heat data of the NPG/CENG composites are compared with the calculated data according to the mass ratio of the NPG in the composite shown in Fig. 4. Latent heat of the NPG/CENG composite increases with the increase of the mass ratio of the NPG in the composite. And calculated value of the latent heat is 82, 88, 95, 100, and 104 J/g respectively. The calculated latent heat and mass ratio of the NPG in the composites can be drawn a linear plot in Fig. 4:
50
40
30
20 0
200
400
600
800
Time, s Fig. 7. Temperature response in different heat transfer directions of thermal imager experiment.
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5. Conclusions
70 Perpendicular to compression force Parallel to compression force
CENG matrices with different densities were used to enhance the thermal performance of the NPG for LHTES application. The thermal conductivity and latent heat of the composite depend on the bulk density of the CENG. Thermal conductivity of the NPG/CENG composites can be 11e88 higher times than that of the pure NPG. Compared with the pure NPG, the deformation of composites due to phase change has been greatly reduced. The thermal conductivities of composites have gone down after phase transformation.
o
Temperature, C
60
50
40
Acknowledgments 30
This project was supported by National Basic Research Program of China under the contact No. 2011CB605802, and Shanxi Province Nature Science Foundation of China (No. 2011021022-1).
20
0
200
400
600
800
Time, s Fig. 8. Temperature response in different heat transfer directions of the finite element simulation.
with the NPG. The higher the density of CENG is, the bigger anisotropic ratio of thermal conductivity of both before and after phase change is. Thermal imager experiment and the finite element simulation were used to further prove the point. Heat transfers along perpendicular to the compression force direction and parallel to the compression force direction were measured. Temperature distribution images of both thermal imager and the finite element simulation at 100, 200, 300, 400, 600 and 800 s are shown in Figs. 5 and 6 respectively for composite with a bulk density of CENG 0.156 g/cm3 and a mass ratio of NPG of 82%. In the two different heat transfer directions, various heat transfer speed is obvious. Both thermal imager experiment and the finite element simulation have proved the point. The right one of heat transfer speed is obviously quicker than the left one. For the right one, heat can quickly spread out before and after its phase change. So its temperature distribution range is very little. Its phase change roughly occurs from 100 to 200 s, in this time period, temperature distribution range is relatively larger. For the left one, temperature distribution range has been larger until after 800 s. In order to observe this phenomenon directly, two curves of the temperature versus time at the same height of the samples are drawn in Figs. 7 and 8 for thermal imager experiment and the finite element simulation respectively. It is indicated that along the two different directions the samples have the different temperature response during the whole process. But the trend is roughly the same for every cure. Specifically as follows: They undergo a relatively long phase change process after their temperatures rapidly get to about 40 C. And then their temperatures first-quick-then-slow rise until they get up to a constant value. What is different is that the degrees of quickness and slowness for every curve are different. The trends that were concluded from the finite element simulation are coincident with the trends from the thermal imager as are shown in Figs. 5e8.
References [1] Zalba B, Marín JM, Cabeza LF, Mehling H. Review on thermal energy storage with phase change: materials, heat transfer analysis and applications. Appl Therm Eng 2003;23:251e83. [2] Sharma A, Tyagi VV, Chen CR, Buddhi D. Review on thermal energy storage with phase change materials and applications. Renew Sustain Energ Rev 2009;13:318e45. [3] Abhat A. Low temperature latent heat thermal energy storage: heat storage materials. Sol Energ 1983;30:313e32. [4] Raj VAA, Velarj R. Review on free cooling of buildings using phase change materials. Renew Sustain Energ Rev 2010;14:2819e29. [5] Darkwa K, OCallaghan PW, Tetlow D. Phase-change drywalls in a passive-solar building. Appl Energ 2006;83:425e35. [6] Yagi J, Akiyama T. Storage of thermal energy for effective use of waste heat from industries. J Mater Process Technol 1995;48:793e804. [7] Fok SC, Shen W, Tan FL. Cooling of portable hand-held electronic devices using phase change materials in finned heat sinks. Int J Therm Sci 2010;49:109e17. [8] Zhong YJ, Li SZ, Wei XH, Liu ZJ, Guo QG. Heat transfer enhancement of paraffin wax using compressed expanded natural graphite for thermal energy storage. Carbon 2010;48:300e4. [9] Wang XW, Lu ER, Lin WX, Wang CZ. Micromechanism of heat storage in a binary system of two kinds of polyalcohols as a solid-solid phase change material. Energ Convers Manag 2000;41:135e44. [10] Pincemin S, Olives R, Py X, Christ M. Highly conductive composites made of phase change materials and graphite for thermal storage. Sol Energ Mater Sol Cell 2008;92:603e13. [11] Acem Z, Lopez J, Barrio EP. KNO3/NaNO3-graphite materials for thermal energy storage at high temperature: part I. e elaboration methods and thermal properties. Appl Therm Eng 2010;30:1580e5. [12] Lopez J, Acem Z, Barrio EP. KNO3/NaNO3-graphite materials for thermal energy storage at high temperature: part II. e phase transition properties. Appl Therm Eng 2010;30:1586e93. [13] Karaipekli A, Sari A, Kaygusuz K. Thermal conductivity improvement of stearic acid using expanded graphite and carbon fiber for energy storage applications. Renew Energ 2007;32:2201e10. [14] Kim S, Drzal LT. High latent heat storage and high thermal conductive phase change materials using exfoliated graphite nanoplatelets. Sol Energ Mater Sol Cell 2009;93:136e42. [15] Zhao CY, Wu ZG. Heat transfer enhancement of high temperature thermal energy storage using metal foams and expanded graphite. Sol Energ Mater Sol Cell 2011;95:636e43. [16] Fang GY, Li H, Chen Z, Liu X. Preparation and characterization of stearic acid/ expanded graphite composites as thermal energy storage materials. Energy 2010;35:4622e6. [17] Zhao JG, Guo Y, Feng F, Tong QH, Qv WS, Wang HQ. Microstructure and thermal properties of a paraffin/expanded graphite phase-change composite for thermal storage. Renew Energ 2011;36:1339e42. [18] Wei XH, Liu L, Zhang JX, Shi JL, Guo QG. Synthesis of HClO4-GICs and the performance of flexible graphite produced from them. New Carbon Mater 2007;22:342e8. [19] Voller VR, Prakash C. A fixed grid numerical modeling methodology for convection-diffusion mushy region phase change problems. Int J Heat Mass Transf 1987;30:1709e19.