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8/11/2019 Cu Paraffin

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1894r 2010 American Chemical Society pubs.acs.org/EF

Energy Fuels2010,24, 18941898: DOI:10.1021/ef9013967

Published on Web 02/19/2010

Preparation and Melting/Freezing Characteristics of Cu/Paraffin Nanofluidas 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 ofnanoparticles in melting paraffin. Cu, Al, and C/Cu nanoparticles were selected to add to the meltingparaffin to enhance the heat-transfer rate of paraffin. Cu nanoparticles have the best performance for heattransfer. Five dispersants were chosen to make Cu nanoparticles stably suspended in melting paraffin. Thenanofluids with Cu nanoparticles show good stability in melting paraffin under the action of Hitenol BC-10, 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 differentialscanning 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 thesame as pure paraffin. The latent heats and phase-change temperatures change very little after 100thermalcycles. Furthermore, the heating andcooling rates of PCMs were significantly improved upon theadditionof Cu nanoparticles. For composites with 1 wt % Cu nanoparticle, the heating and cooling times can bereduced by 30.3 and 28.2%, respectively.

1. Introduction

Since the outbreak of the energy crisis in 1973, thermalenergy storage technologies are receiving more and moreattention. There are various thermal energy storage methods,but latent heat storage is the most attractive one because of theadvantages of its high storage density and isothermal charac-

teristics.1

It has broad application prospects2,3

in solar energyuse, electricity from peak to off peak, waste heat recovery, etc.

Paraffin is one of the most commonly used phase-changematerials (PCMs) in storing thermal energy. It is regarded asthe most promising PCM for large latent heat, low cost,stability, nontoxicity, and no corrosion.4 However, its inher-ent low thermal conductivity limits its utility applications.Many methods have been proposed to enhance the thermalconducitvity of pureparaffin, suchas placing a metal structurein PCM,5 impregnating porous material,6,7 and dispersinghigh thermal conductivity particles in PCM.8,9

Recently, with the development of nanotechnology, resear-chers have started to study the thermal conductivity perfor-mance of adding nanoparticles to various fluids, so-callednanofluids,10 which can result in the thermal conductivityenhancementbeing significantly higherthan the predictions ofthe classical solid-liquid models.11

-13 Ho et al.14 enhancedthe thermal conductivity of paraffin (n-octadecane) by adding

Al2O3 nanoparticles. Xie et al.15 dispersed multi-walled car-bon nanotubes (MWNTs) into paraffin (melting point,Tm=52-54 C) by the ball-milling method.For the composite witha mass fraction of 2.0 wt %, the thermal conductivity enhan-cement ratios reach 35.0 and 40.0% in solid and liquid states,respectively. Zeng et al. studied the thermal conductivityenhancement of Ag nanowires/1-tetradecanol16 and carbonnanotubes (CNTs)/palmitic acid17 as PCMs for the formationof the network in the PCMs. Elgafy et al.18 prepared acomposite with carbon nanofibers filled in with paraffin,and the results showed that the thermal conductivity of thecomposite enhanced significantly, which increased the coolingrate in the solidification process. Wu et al.19 investigated theAl

2O

3/H

2O nanofluids as a PCM for cool storage, and the

*To whom correspondence should be addressed. E-mail: [email protected].

(1) Shukla, A.; Buddhi, D.; Sawhney, R. L. Renewable SustainableEnergy 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. Energy2009,83, 323.(7) Sar, 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 EngineeringCongress and Exposition, San Francisco, CA, Nov 12-17, 1995; Vol.231, pp 99-105.

(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.NanoLett.2009,9, 4128.

(13) Wang, X. J.; Li, X. F.; Yang, S.Energy Fuels2009,23, 2684.(14) Ho, C. J.; Gao, J. Y.Int. Commun. Heat Mass Transfer2009,36,

467.(15) Xie, H. Q.; Wang, J. F.; Xin, Z.Thermochim. Acta2009,488, 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.Carbon2005,43, 3067.(19) Wu, S. Y.; Zhu, D. S.; Li, X. F.; Li, H.; Lei, J. X. Thermochim.

Acta2009,483, 73.


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Energy Fuels2010,24, 18941898: DOI:10.1021/ef9013967 Wu et al.

results stated that the total freezing time of Al2O3/H2Onanofluids reduced by 20.5% with the addition of 0.2 wt %nanoparticles. Liu et al.20 evaluated the cool storage capacityof the BaCl2aqueous solution by suspending TiO2nanopar-ticles, and the cool storage/supply rate and the cool storage/supply capacity both increased higher than those of BaCl2aqueous solution without nanoparticles. Khodadadi et al.21

numerically simulated the solidification of Cu/water nano-fluid in a vertical square enclosure. A higher heat extraction

rate of the freezing process was found as a result of theaddition of Cu nanoparticles. In other words, the additionof nanoparticles could improve the thermal conductivity andthe heating-cooling rate. However, there are only a fewexperimental studies focused on the phase-change propertiesof nanofluids as a latent heat storage material. It is too limitedcompared to the massive study on the viscosity, thermal con-ductivities, mechanisms, and models of nanofluids.22-24

In this paper, a new kind of nanofluid PCM was preparedby adding nanoparticles to paraffin. Different nanoparticleswere chosen and added to melting paraffin to obtain the bestheat-transfer enhancement. Furthermore, different disper-sants wereadded to meltingCu/paraffin to ensure the stabilityof PCMs. At last, the thermal reliability, latent heats, phase-

change temperatures, and heating-

cooling rate of Cu/paraf-fin were investigated experimentally.

2. Experimental Section

2.1. Materials. Paraffin (Tm=58-60 C)wasused asPCMinthe preparation of the composite PCMs. It was purchased fromthe Shanghai Specimen andModel Factory (China). Cu, Al,andC/Cu nanopowders (Shenzhen Junye Nano Material Ltd.,China) with all contents >99.9% were used. The averageparticle size of those particles was 25 nm. Five kinds of dis-persants were used here. Their structures are shown in Figure 1.GA, Span-80, cetyl trimethyl ammonium bromide (CTAB), andsodium dodecylbenzenesulfonate (SDBS) were supplied by theGuangzhou Chemical Reagent Factory (China). Hitenol BC-10

was purchased from Montello, Inc., Japan. All chemicals usedwere without any further purification.2.2. Preparation of PCMs.A two-step method was selected to

prepare the PCMs. First, to confirm which nanoparticle shouldbe used, different nanoparticles were dispersed into meltingparaffin. To observe the stability of the composite, the PCMsconsisting of paraffin, Cu nanoparticles, and different disper-sants were prepared. All of the preparation processes wereperformed using an ultrasonic vibrator for 2 h. A longer timeof high-energy sonication would introduce defects.25 The ultra-sonic temperature was above 58 C to ensure that the sampleswere kept sufficiently above the melting point of the paraffin.

2.3. Heating-Cooling Rate Test.A thermal performance testwas conductedto verify the improvement of heat-transfer rate inthe presence of Cu particles. The experimental setup is shown inFigure 2. The water in the constant temperature trough wasmaintained at 70 C for the heating process and 30 C for thecooling process. Forthe cooling process, themelting PCMs were

transferred to thetest tubes. Then, the tubes were placed into theconstant temperature trough for freezing. After this process, thePCMs were immediately subjected to the heating process in thesame way at a constant temperature of 70 C. The transienttemperature response at the center of the tubes was recorded bythe 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 aheating rate of 5 C/min in a purified argon atmosphere. Liquidnitrogen was used as the cooling medium during the freezingprocess. The melting and freezing temperatures were estimatedby the tangent at the point of greatest slope on the face portionof the peak of the DSC curve. The latent heats of phase changewere determined by numerical integration of the area under thepeaks.

Infrared spectra of solid PCMs were obtained using a Fouriertransform 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 usedto determine thermal reliability of PCMin terms of thechangeinphase-change temperatures and latent heats with respect to thethermal cycling number. A thermal cycling test consisted of a

Figure 1.Chemical structure of dispersants.

Figure 2.Experimental setup for the heating-cooling rate test.

(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 MassTransfer2007,34, 534.

(22) Krishnamurthy,S.; Bhattacharya, P.; PhelanR, P. E.; Prasher, S.Nano Lett.2006,6, 419.

(23) Lee, D. Langmuir2007,23, 6011.(24) Zhu, H. T.; Zhang, C. Y.; Tang, Y. M.; Wang, J. X. J. Phys.

Chem. C2007,111, 1646.(25) OConnell, 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.Science2002,297, 593.


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between the experimental data and the predicted values. Wethink that the heat-transfer performance of nanofluid is

different from the conventional solid-

liquid mixture.32 Sur-face and size effects of the nanoparticle have effects on thethermal characteristics.33 Therefore, the equation for thesimple solid-liquid mixture does not suit the calculation ofthe latent heat of nanofluid. A new model is needed fornanofluid. Kenisarin et al.34 reported that fatty acid compo-sitions with latent heat capacity of about 120 kJ/kg can be

used for solar energy storage. The latent heats of all of thePCM composites in our experiments are about 180 kJ/kg,which indicates that these PCM composites are suitable forlatent heat thermal energy storage applications.

3.5. Heating-Cooling Rate Evaluation.The improvementof the heat-transfer rate was verified by comparing the

heating-

cooling rate process of pure paraffin and Cu/para-ffin composite. The typical temperature curves of the PCMsare shown in Figures 9 and 10, respectively. It shows thatboth heating and cooling processes are influenced by theaddition of Cu particles.

The figures show a comparison of the heat-transfer ratebetween pure paraffin and composite PCMs. When thematerial with a high thermal conductivity is added, the ther-mal response will become more sensitive. In Figure 9, thetemperatures of pure paraffin and the composite PCMsare 30 C, the same at the beginning of the heating perfor-mance test. The temperatures of the paraffin and the com-posite PCM increased with time elapsing, and the phasechange (BC) from solid (AB) to liquid (CD) occurred. The

temperature-increasing curves of the composite were steeperthan the curve of pure paraffin. The heating times of Cu/paraffin were typically shortened. For example, it took 1450 sfor pure paraffin to increase the temperature from 30 to68 C, whereas it took only 1010 s for Cu/paraffin (1 wt %),which was reduced 30.3% compared to that for pure paraf-fin. It was obvious that the heating rate of the compositePCM was higher than that of pure paraffin. It can also beseen from Figure 10 that the cooling rate of composite PCMswas also higher than that of pure paraffin. It took 1810 s forpure paraffin to drop its temperature from 68 to 30 C andonly 1300 s for Cu/paraffin (1 wt %), indicating that thecooling time for Cu/paraffin (1 wt %) was reduced 28.2%compared to that for pure paraffin. It was concluded fromthese results that the heat-transfer rate in the composite PCMswas obviously higher than that in pure paraffin because ofthe addition of Cu particles.

There are two possible reasons to explain the behavior ofthe higher heat-transfer rate. One is the higher thermal con-ductivity for Cu/paraffin, because the crystal growth mainlydepends upon heat transfer. At the process of melting andfreezing,a large amount of heat will be discharged. If theheatcannot be released timely, the heating and cooling processwill be hindered. The thermal conductivityof pure paraffin isenhanced with the addition of nanoparticles. Therefore, theheating and cooling speeds of PCMs are able to be accele-rated. Thermal conductivities of samples were measured by

Figure 6.FTIR spectra of paraffin and Cu/paraffin.

Figure 7. Phase-change temperature of Cu/paraffin with differentmass fractions.

Figure 8.Latent heat of Cu/paraffin with different mass fractions.

Figure 9.Heating curves of paraffin and Cu/paraffin.

(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 EnergyRev.2007,11, 1913.


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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 and0.2699 and 0.2908 W m-1 K-1 in solid, respectively. There is adifferent extent on the increase of thermal conductivity, whichhas been verified by many other groups.35,36 Therefore, thethermal conductivity improvement of paraffin confirms thereduction of heating and cooling times of Cu/paraffin com-

pared to those of paraffin. The other reason may be the Cunanoparticles acting as a nucleating agent, and this is alsohelpful for reducing the heating and cooling times.20

As can be seen from Figures 9 and 10, the melting andfreezing temperatures of the Cu/paraffin compositePCM arealmost equal to those of pure paraffin. However, the meltingand freezing temperatures for all PCMs are lower or higherthan those from DSC. The reason is that the measurementprinciple and the sample weight are different between theheating-cooling rate test and DSC method. The DSC mea-surement is an express method and differs from the classicalthermophysical methods. A similar phenomena were alsofound by Li et al.37

3.6. Thermal Reliability of PCMs. In this experiment, thecomposite PCM including 0.5 wt % Cu nanoparticles wasused to test its stability on the thermal performance after 0,20, 50, 70, and 100 times thermal cycling. Figures 11 and 12show the phase-change temperature and latent heat afterthermal cycling, respectively. The maximum change is-1.6% for melting temperature and -1.9% for freezingtemperature. It shows little change for the phase-changetemperature, which is not significant for applications. Itcan also be seen that Cu/paraffin has a good thermalreliability in terms of the latent heat values. The greatestchange in the melting latent heat is -3.2%, and the greatestchange in the freezing latent heat is 2%. These changes are

negligible for latent heat thermal energy storage applica-tions.

4. Conclusions

In conclusion, the addition of nanoparticles enhanced theheat-transfer rate of PCM significantly. The Cu nanoparticlehas better effects than Al and C/Cu nanopowders. Because oflarge steric hindrance, the Cu/paraffin composite PCM withHitenol BC-10 shows good dispersed property after 12 h. TheFTIRspectroscopy results indicate that there is justa physicalinteraction among Cu, Hitenol BC-10,and paraffin. When Cuparticles are added to paraffin, the changes in the melting andfreezing temperatures are in negligible magnitudes. However,the latent heats for melting and freezing are reduced. Themaximum reduction is 11.1% for melting latent heats and11.7% for freezinglatent heats, respectively. After 100heatingand cooling cycles, Cu/paraffin still has a good thermalreliability because of the little changes in latent heat and

phase-change temperature. The heat-transfer rate of the com-posite PCM was obviously higher than that of pure paraffin.The heating and coolingrate tests showed that the heating andcooling times were reduced by 30.3and 28.2% for 1 wt% Cu/paraffin, respectively. The experimental results reveal that theaddition of nanoparticles to paraffin is a good method toenhance the heat-transfer performance of paraffin, because ofits good thermal properties, thermal and chemical reliability,and heat-transfer rate.

Acknowledgment. The authors acknowledge the financialsupport from the Plan Projects for Science and Technology ofGuangzhou (Grant 2008Z1-1061) and the Program for NewCentury Excellent Talents in University (Grant NCET-04-0826).

Figure 10.Cooling curves of paraffin and Cu/paraffin.

Figure 11. Effect of thermal cycles on the phase-change tempera-ture.

Figure 12.Effect of thermal cycles on the latent heat.

(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.

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(37) Li, J.L.; Xue,P.; Ding, W.Y.;Han,J. M.; Sun,G. L. Sol. EnergyMater. Sol. Cells2009,93, 1761.

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