Energy Fuels 2010, 24, 1894–1898 Published on Web 02/19/2010
: DOI:10.1021/ef9013967
Preparation and Melting/Freezing Characteristics of Cu/Paraffin Nanofluid as Phase-Change Material (PCM) Shuying Wu,† Dongsheng Zhu,*,† Xiurong Zhang,† and Jin Huang‡ †
Key Lab of Enhanced Heat Transfer and Energy Conservation, the Ministry of Education, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China, and ‡College of Material and Energy, Guangdong University of Technology, Guangzhou 510006, China Received November 17, 2009. Revised Manuscript Received February 1, 2010
A new sort of nanofluid phase-change material (PCM) is developed by suspending a small amount of nanoparticles in melting paraffin. Cu, Al, and C/Cu nanoparticles were selected to add to the melting paraffin to enhance the heat-transfer rate of paraffin. Cu nanoparticles have the best performance for heat transfer. Five dispersants were chosen to make Cu nanoparticles stably suspended in melting paraffin. The nanofluids with Cu nanoparticles show good stability in melting paraffin under the action of Hitenol BC10, even suspending for 12 h in a constant temperature trough. The Fourier transform infrared (FTIR) spentrum shows that it is a physical interaction among Cu, paraffin, and Hitenol BC-10. The differential scanning calorimetric (DSC) results reveal that the latent heats of Cu/paraffin shift to lower values compared to those of pure paraffin; however, the melting and freezing temperatures are kept almost the same as pure paraffin. The latent heats and phase-change temperatures change very little after 100 thermal cycles. Furthermore, the heating and cooling rates of PCMs were significantly improved upon the addition of Cu nanoparticles. For composites with 1 wt % Cu nanoparticle, the heating and cooling times can be reduced by 30.3 and 28.2%, respectively.
Recently, with the development of nanotechnology, researchers have started to study the thermal conductivity performance of adding nanoparticles to various fluids, so-called “nanofluids”,10 which can result in the thermal conductivity enhancement being significantly higher than the predictions of the classical solid-liquid models.11-13 Ho et al.14 enhanced the thermal conductivity of paraffin (n-octadecane) by adding Al2O3 nanoparticles. Xie et al.15 dispersed multi-walled carbon nanotubes (MWNTs) into paraffin (melting point, Tm = 52-54 °C) by the ball-milling method. For the composite with a mass fraction of 2.0 wt %, the thermal conductivity enhancement ratios reach 35.0 and 40.0% in solid and liquid states, respectively. Zeng et al. studied the thermal conductivity enhancement of Ag nanowires/1-tetradecanol16 and carbon nanotubes (CNTs)/palmitic acid17 as PCMs for the formation of the network in the PCMs. Elgafy et al.18 prepared a composite with carbon nanofibers filled in with paraffin, and the results showed that the thermal conductivity of the composite enhanced significantly, which increased the cooling rate in the solidification process. Wu et al.19 investigated the Al2O3/H2O nanofluids as a PCM for cool storage, and the
1. Introduction Since the outbreak of the energy crisis in 1973, thermal energy storage technologies are receiving more and more attention. There are various thermal energy storage methods, but latent heat storage is the most attractive one because of the advantages of its high storage density and isothermal characteristics.1 It has broad application prospects2,3 in solar energy use, electricity from peak to off peak, waste heat recovery, etc. Paraffin is one of the most commonly used phase-change materials (PCMs) in storing thermal energy. It is regarded as the most promising PCM for large latent heat, low cost, stability, nontoxicity, and no corrosion.4 However, its inherent low thermal conductivity limits its utility applications. Many methods have been proposed to enhance the thermal conducitvity of pure paraffin, such as placing a metal structure in PCM,5 impregnating porous material,6,7 and dispersing high thermal conductivity particles in PCM.8,9 *To whom correspondence should be addressed. E-mail: cedshzhu@ scut.edu.cn. (1) Shukla, A.; Buddhi, D.; Sawhney, R. L. Renewable Sustainable Energy Rev. 2009, 13, 2119. (2) Sharma, S. D.; Sagara, K. Int. J. Green Energy 2005, 2, 1. (3) Liu, C.; Li, F.; Ma, L. P.; Cheng, H. M. Adv. Mater. 2010, 22, E1. (4) Mondal, S. Appl. Therm. Eng. 2008, 28, 1536. (5) Shatikian, V.; Ziskind, G.; Letan, R. Int. J. Heat Mass Transfer 2005, 48, 3689. (6) Karaipekli, A.; Sari, A. Sol. Energy 2009, 83, 323. (7) Sarı, A.; Karaipekli, A. Appl. Therm. Eng. 2007, 27, 1271. (8) Karaipekli, A.; Sari, A.; Kaygusuz, K. Renewable Energy 2007, 32, 2201. (9) Frusteri, F.; Leonardi, V.; Vasta, S.; Restuccia, G. Appl. Therm. Eng. 2005, 25, 1623. (10) Choi, S. U.S. Proceedings of the 1995 American Society of Mechanical Engineers (ASME) International Mechanical Engineering Congress and Exposition, San Francisco, CA, Nov 12-17, 1995; Vol. 231, pp 99-105. r 2010 American Chemical Society
(11) Maxwell, C. J. Electricity and Magnetism; Clarendon Press: Oxford, U.K., 1873. (12) Gao, J. W.; Zheng, R. T.; Ohtani, H.; Zhu, D. S.; Chen, G. Nano Lett. 2009, 9, 4128. (13) Wang, X. J.; Li, X. F.; Yang, S. Energy Fuels 2009, 23, 2684. (14) Ho, C. J.; Gao, J. Y. Int. Commun. Heat Mass Transfer 2009, 36, 467. (15) Xie, H. Q.; Wang, J. F.; Xin, Z. Thermochim. Acta 2009, 488, 39. (16) Zeng, J. L.; Cao, Z.; Yang, D. W.; Sun, L. X.; Zhang L. J. Therm. Anal. Calorim. 2010, doi: 10.1007/s10973-009-0472-y. (17) Zeng, J. L.; Cao, Z.; Yang, D. W.; Xu, F.; Sun, L. X.; Zhang, X. F.; Zhang, L. J. Therm. Anal. Calorim. 2009, 95, 507. (18) Elgafy, A.; Lafdi, K. Carbon 2005, 43, 3067. (19) Wu, S. Y.; Zhu, D. S.; Li, X. F.; Li, H.; Lei, J. X. Thermochim. Acta 2009, 483, 73.
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results stated that the total freezing time of Al2O3/H2O nanofluids reduced by 20.5% with the addition of 0.2 wt % nanoparticles. Liu et al.20 evaluated the cool storage capacity of the BaCl2 aqueous solution by suspending TiO2 nanoparticles, and the cool storage/supply rate and the cool storage/ supply capacity both increased higher than those of BaCl2 aqueous solution without nanoparticles. Khodadadi et al.21 numerically simulated the solidification of Cu/water nanofluid in a vertical square enclosure. A higher heat extraction rate of the freezing process was found as a result of the addition of Cu nanoparticles. In other words, the addition of nanoparticles could improve the thermal conductivity and the heating-cooling rate. However, there are only a few experimental studies focused on the phase-change properties of nanofluids as a latent heat storage material. It is too limited compared to the massive study on the viscosity, thermal conductivities, mechanisms, and models of nanofluids.22-24 In this paper, a new kind of nanofluid PCM was prepared by adding nanoparticles to paraffin. Different nanoparticles were chosen and added to melting paraffin to obtain the best heat-transfer enhancement. Furthermore, different dispersants were added to melting Cu/paraffin to ensure the stability of PCMs. At last, the thermal reliability, latent heats, phasechange temperatures, and heating-cooling rate of Cu/paraffin were investigated experimentally. 2. Experimental Section 2.1. Materials. Paraffin (Tm =58-60 °C) was used as PCM in the preparation of the composite PCMs. It was purchased from the Shanghai Specimen and Model Factory (China). Cu, Al, and C/Cu nanopowders (Shenzhen Junye Nano Material Ltd., China) with all contents >99.9% were used. The average particle size of those particles was 25 nm. Five kinds of dispersants were used here. Their structures are shown in Figure 1. GA, Span-80, cetyl trimethyl ammonium bromide (CTAB), and sodium dodecylbenzenesulfonate (SDBS) were supplied by the Guangzhou Chemical Reagent Factory (China). Hitenol BC-10 was purchased from Montello, Inc., Japan. All chemicals used were without any further purification. 2.2. Preparation of PCMs. A two-step method was selected to prepare the PCMs. First, to confirm which nanoparticle should be used, different nanoparticles were dispersed into melting paraffin. To observe the stability of the composite, the PCMs consisting of paraffin, Cu nanoparticles, and different dispersants were prepared. All of the preparation processes were performed using an ultrasonic vibrator for 2 h. A longer time of high-energy sonication would introduce defects.25 The ultrasonic temperature was above 58 °C to ensure that the samples were kept sufficiently above the melting point of the paraffin. 2.3. Heating-Cooling Rate Test. A thermal performance test was conducted to verify the improvement of heat-transfer rate in the presence of Cu particles. The experimental setup is shown in Figure 2. The water in the constant temperature trough was maintained at 70 °C for the heating process and 30 °C for the cooling process. For the cooling process, the melting PCMs were
Figure 1. Chemical structure of dispersants.
Figure 2. Experimental setup for the heating-cooling rate test.
transferred to the test tubes. Then, the tubes were placed into the constant temperature trough for freezing. After this process, the PCMs were immediately subjected to the heating process in the same way at a constant temperature of 70 °C. The transient temperature response at the center of the tubes was recorded by the temperature datalogger at a time interval of 10 s. 2.4. Analysis Methods. A TA differential scanning calorimeter (DSC) was used to determine the latent heats and phase-change temperatures of melting and freezing. It was performed at a heating rate of 5 °C/min in a purified argon atmosphere. Liquid nitrogen was used as the cooling medium during the freezing process. The melting and freezing temperatures were estimated by the tangent at the point of greatest slope on the face portion of the peak of the DSC curve. The latent heats of phase change were determined by numerical integration of the area under the peaks. Infrared spectra of solid PCMs were obtained using a Fourier transform infrared spectrometer (FTIR, Bruker TENSON 27) with KBr pellets in the range of 4000-400 cm-1. 2.5. Thermal Cycling Test. The thermal cycling test was used to determine thermal reliability of PCM in terms of the change in phase-change temperatures and latent heats with respect to the thermal cycling number. A thermal cycling test consisted of a
(20) Liu, Y. D.; Zhou, Y. G.; Tong, M. W.; Zhou, X. S. Microfluid. Nanofluid. 2009, 7, 579. (21) Khodadadi, J. M.; Hosseinizadeh, S. F. Int. Commun. Heat Mass Transfer 2007, 34, 534. (22) Krishnamurthy, S.; Bhattacharya, P.; PhelanR, P. E.; Prasher, S. Nano Lett. 2006, 6, 419. (23) Lee, D. Langmuir 2007, 23, 6011. (24) Zhu, H. T.; Zhang, C. Y.; Tang, Y. M.; Wang, J. X. J. Phys. Chem. C 2007, 111, 1646. (25) O’Connell, M. J.; Bachilo, S. M.; Huffman, C. B.; Moore, V. C.; Strano, M. S.; Haroz, E. H.; Rialon, K. L.; Boul, P. J.; Noon, W. H.; Kittrell, C.; Ma, J.; Hauge, R. H.; Weisman, R. B.; Smalley, R. E. Science 2002, 297, 593.
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Figure 3. Heating curves of the PCMs with different nanoparticles.
Figure 4. Cooling curves of the PCMs with different nanoparticles.
melting and freezing process of PCM. DSC analysis was performed to determine the thermal stability of the composite PCM after thermal cycles.
3. Results and Discussion 3.1. Selection of Nanoparticles. To select an effective nanoparticle, a heating-cooling rate test was conducted using the experimental setup shown in Figure 2. Cu, Al, and C/Cu nanoparticles were chosen. The mass fractions of Cu/paraffin, Al/paraffin, and (C/Cu)/paraffin all remain 0.1%. The heating and cooling curves of the PCMs are shown in Figures 3 and 4. The results show that different nanoparticles have different effects on the heat-transfer rate. It took 1278, 1311, 1356, and 1416 s for Cu/paraffin, Al/ paraffin, (C/Cu)/paraffin, and pure paraffin to reach 67 °C, respectively. It took 1419, 1518, 1587, and 1512 s for Cu/ paraffin, Al/paraffin, (C/Cu)/paraffin, and pure paraffin to drop to 30 °C. The heating time was reduced by 9.7, 7.4, and 4.2% for Cu/paraffin, Al/paraffin, and (C/Cu)/paraffin, respectively. The cooling time was reduced by 6.7, -0.4, and -5% for them, respectively. Thus, the Cu particle has the best performance for heat transfer. However, without the dispersant, the melting Cu/paraffin will not be stable.26 In the next study, the work was focused on the thermal properties of Cu/paraffin in the presence of dispersants. 3.2. Preparation of Cu/Paraffin. Preparation of PCMs is the first key step in applying nanophase particles to enhance the heat-transfer performance of PCM. To improve the dispersion behavior of nanoparticles in liquid fluids, the methods, such as using ultrasonic vibration and adding dispersants, were widely used in preparing nano-suspensions. In this study, ultrasonication and different dispersants were used at the same time. The mass ratio for Cu nanoparticles and dispersants was controlled at 1:3. Figure 5 shows the sediment photographs after 12 h. It clearly indicates that the suspension with the surfactant of Hitenol BC-10 is the most stable compared to others. The mechanism of Hitesol BC-10 on the copper suspension should be steric hindrance.27 In the structure of Hitesol BC-10, there is a long carbon chain and a benzene ring, which make the steric hindrance very large. The Cu nanoparticles in the suspension are hard to be close to each other and could disperse stably. 3.3. Characterization of PCMs. FTIR is an effective method to investigate a specific interaction in the composite
Figure 5. Sediment photographs with different dispersants depositing for 12 h.
PCMs. Figure 6 shows the FTIR absorption spectrum of the PCMs. No significant new peaks were observed. The FTIR spectrum proved that there are only physical interactions between the components of the composite PCM and no new chemical-bond generation.28 3.4. Thermal Properties of PCMs. DSC analysis was conducted to investigate the influence of Cu addition on the thermal properties of composite PCMs. The phase-change latent heat and temperature of pure paraffin and Cu/paraffin composites are shown in Figures 7 and 8. As can be seen from the data, the changes in the melting and freezing temperatures are very little for different contents of Cu nanoparticles. The maximum change is -0.6% for melting temperatures and 2% for freezing temperatures, which could be negligible. As can been seen in Figure 8, the maximum reduction is 11.1% for melting latent heats and 11.7% for freezing latent heats, respectively. According to the theory of mixtures, the latent heat of Cu/paraffin composite is equal to the values calculated by multiplying the latent heat value of pure paraffin with its mass fraction in the composite PCM. The calculated values are shown in Figure 8. It shows that the latent heat of each composite is lower than the calculated latent heat. With the increase of the mass fractions of Cu, the deviation becomes larger. Similar results were also found by Sarı and Karaipekli,29-31 Ho and Gao,14 and Xie et al.15 However, only a vague reason was given for the discrepancy (28) Wang, W. L.; Yang, X. X.; Fang, Y. T.; Ding, J. Appl. Energy 2009, 86, 170. (29) Sarı, A.; Karaipekli, A. Mater. Chem. Phys. 2008, 109, 459. (30) Karaipekli, A.; Sarı, A. Renewable Energy 2008, 33, 2599. (31) Karaipekli, A.; Sarı, A.; Alkan, C. Chem. Eng. J. 2009, 155, 899.
(26) Li, X. F.; Zhu, D. S.; Wang, X. J. J. Colloid Interface Sci. 2007, 310, 456. (27) Hwang, Y.; Lee, J. K.; Lee, J. K.; Jeong, Y. M.; Cheong, S. I.; Ahnb, Y. C.; Kim, S. H. Powder Technol. 2008, 186, 145.
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Figure 6. FTIR spectra of paraffin and Cu/paraffin. Figure 9. Heating curves of paraffin and Cu/paraffin.
used for solar energy storage. The latent heats of all of the PCM composites in our experiments are about 180 kJ/kg, which indicates that these PCM composites are suitable for latent heat thermal energy storage applications. 3.5. Heating-Cooling Rate Evaluation. The improvement of the heat-transfer rate was verified by comparing the heating-cooling rate process of pure paraffin and Cu/paraffin composite. The typical temperature curves of the PCMs are shown in Figures 9 and 10, respectively. It shows that both heating and cooling processes are influenced by the addition of Cu particles. The figures show a comparison of the heat-transfer rate between pure paraffin and composite PCMs. When the material with a high thermal conductivity is added, the thermal response will become more sensitive. In Figure 9, the temperatures of pure paraffin and the composite PCMs are 30 °C, the same at the beginning of the heating performance test. The temperatures of the paraffin and the composite PCM increased with time elapsing, and the phase change (BC) from solid (AB) to liquid (CD) occurred. The temperature-increasing curves of the composite were steeper than the curve of pure paraffin. The heating times of Cu/ paraffin were typically shortened. For example, it took 1450 s for pure paraffin to increase the temperature from 30 to 68 °C, whereas it took only 1010 s for Cu/paraffin (1 wt %), which was reduced 30.3% compared to that for pure paraffin. It was obvious that the heating rate of the composite PCM was higher than that of pure paraffin. It can also be seen from Figure 10 that the cooling rate of composite PCMs was also higher than that of pure paraffin. It took 1810 s for pure paraffin to drop its temperature from 68 to 30 °C and only 1300 s for Cu/paraffin (1 wt %), indicating that the cooling time for Cu/paraffin (1 wt %) was reduced 28.2% compared to that for pure paraffin. It was concluded from these results that the heat-transfer rate in the composite PCMs was obviously higher than that in pure paraffin because of the addition of Cu particles. There are two possible reasons to explain the behavior of the higher heat-transfer rate. One is the higher thermal conductivity for Cu/paraffin, because the crystal growth mainly depends upon heat transfer. At the process of melting and freezing, a large amount of heat will be discharged. If the heat cannot be released timely, the heating and cooling process will be hindered. The thermal conductivity of pure paraffin is enhanced with the addition of nanoparticles. Therefore, the heating and cooling speeds of PCMs are able to be accelerated. Thermal conductivities of samples were measured by
Figure 7. Phase-change temperature of Cu/paraffin with different mass fractions.
Figure 8. Latent heat of Cu/paraffin with different mass fractions.
between the experimental data and the predicted values. We think that the heat-transfer performance of nanofluid is different from the conventional solid-liquid mixture.32 Surface and size effects of the nanoparticle have effects on the thermal characteristics.33 Therefore, the equation for the simple solid-liquid mixture does not suit the calculation of the latent heat of nanofluid. A new model is needed for nanofluid. Kenisarin et al.34 reported that fatty acid compositions with latent heat capacity of about 120 kJ/kg can be (32) Prasher, R.; Bhattachary, P.; Phelan, P. E. Phys. Rev. Lett. 2005, 94, No. 025901. (33) Wang, B. X.; Zhou, L. P.; Peng, X. F. Int. J. Thermophys. 2006, 27, 139. (34) Kenisarin, M.; Mahkamov, K. Renewable Sustainable Energy Rev. 2007, 11, 1913.
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Figure 11. Effect of thermal cycles on the phase-change temperature. Figure 10. Cooling curves of paraffin and Cu/paraffin.
the transient hot-wire method. The values of 0 and 1 wt % Cu/paraffin are 0.1687 and 0.1878 W m-1 K-1 in liquid and 0.2699 and 0.2908 W m-1 K-1 in solid, respectively. There is a different extent on the increase of thermal conductivity, which has been verified by many other groups.35,36 Therefore, the thermal conductivity improvement of paraffin confirms the reduction of heating and cooling times of Cu/paraffin compared to those of paraffin. The other reason may be the Cu nanoparticles acting as a nucleating agent, and this is also helpful for reducing the heating and cooling times.20 As can be seen from Figures 9 and 10, the melting and freezing temperatures of the Cu/paraffin composite PCM are almost equal to those of pure paraffin. However, the melting and freezing temperatures for all PCMs are lower or higher than those from DSC. The reason is that the measurement principle and the sample weight are different between the heating-cooling rate test and DSC method. The DSC measurement is an express method and differs from the classical thermophysical methods. A similar phenomena were also found by Li et al.37 3.6. Thermal Reliability of PCMs. In this experiment, the composite PCM including 0.5 wt % Cu nanoparticles was used to test its stability on the thermal performance after 0, 20, 50, 70, and 100 times thermal cycling. Figures 11 and 12 show the phase-change temperature and latent heat after thermal cycling, respectively. The maximum change is -1.6% for melting temperature and -1.9% for freezing temperature. It shows little change for the phase-change temperature, which is not significant for applications. It can also be seen that Cu/paraffin has a good thermal reliability in terms of the latent heat values. The greatest change in the melting latent heat is -3.2%, and the greatest change in the freezing latent heat is 2%. These changes are
Figure 12. Effect of thermal cycles on the latent heat.
negligible for latent heat thermal energy storage applications. 4. Conclusions In conclusion, the addition of nanoparticles enhanced the heat-transfer rate of PCM significantly. The Cu nanoparticle has better effects than Al and C/Cu nanopowders. Because of large steric hindrance, the Cu/paraffin composite PCM with Hitenol BC-10 shows good dispersed property after 12 h. The FTIR spectroscopy results indicate that there is just a physical interaction among Cu, Hitenol BC-10, and paraffin. When Cu particles are added to paraffin, the changes in the melting and freezing temperatures are in negligible magnitudes. However, the latent heats for melting and freezing are reduced. The maximum reduction is 11.1% for melting latent heats and 11.7% for freezing latent heats, respectively. After 100 heating and cooling cycles, Cu/paraffin still has a good thermal reliability because of the little changes in latent heat and phase-change temperature. The heat-transfer rate of the composite PCM was obviously higher than that of pure paraffin. The heating and cooling rate tests showed that the heating and cooling times were reduced by 30.3 and 28.2% for 1 wt % Cu/ paraffin, respectively. The experimental results reveal that the addition of nanoparticles to paraffin is a good method to enhance the heat-transfer performance of paraffin, because of its good thermal properties, thermal and chemical reliability, and heat-transfer rate.
(35) Buongiorno, J.; Venerus, D. C.; Prabhat, N.; McKrell, T.; Townsend, J.; Christianson, R.; Tolmachev, Y. V.; Keblinski, P.; Hu, L.; Alvarado, J. L.; Bang, I. C.; Bishnoi, S. W.; Bonetti, M.; Botz, F.; Cecere, A.; Chang, Y.; Chen, G.; Chen, H.; Chung, S. J.; Chyu, M. K.; Das, S. K.; Di Paola, R.; Ding, Y.; Dubois, F.; Dzido, G.; Eapen, J.; Escher, W.; Funfschilling, D.; Galand, Q.; Gao, J.; Gharagozloo, P. E.; Goodson, K. E.; Gutierrez, J. G.; Hong, H.; Horton, M.; Hwang, K. S.; Iorio, C. S.; Jang, S. P.; Jarzebski, A. B.; Jiang, Y.; Jin, L.; Kabelac, S.; Kamath, A.; Kedzierski, M. A.; Kieng, L. G.; Kim, C.; Kim, J.-H.; Kim, S.; Lee, S. H.; Leong, K. C.; Manna, I.; Michel, B.; Ni, R.; Patel, H. E.; Philip, J.; Poulikakos, D.; Reynaud, C.; Savino, R.; Singh, P. K.; Song, P.; Sundararajan, T.; Timofeeva, E.; Tritcak, T.; Turanov, A. N.; Van Vaerenbergh, S.; Wen, D.; Witharana, S.; Yang, C.; Yeh, W.-H.; Zhao, X.-Z.; Zhou, S.-Q. J. Appl. Phys. 2009, 106, No. 094312. (36) Yu, W.; Xie, H. Q.; Bao, D. Nanotechnology 2010, 21, No. 055705. (37) Li, J. L.; Xue, P.; Ding, W. Y.; Han, J. M.; Sun, G. L. Sol. Energy Mater. Sol. Cells 2009, 93, 1761.
Acknowledgment. The authors acknowledge the financial support from the Plan Projects for Science and Technology of Guangzhou (Grant 2008Z1-1061) and the Program for New Century Excellent Talents in University (Grant NCET-04-0826). 1898
