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Enhance heat dissipation for projection lamps by MWCNTs nano-coating
Applied Thermal Engineering 51 (2013) 1098e1106
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Enhance heat dissipation for projection lamps by MWCNTs nano-coating Tun-Ping Teng a, Tun-Chien Teng b, * a b
Department of Industrial Education, National Taiwan Normal University, No. 162, Sec. 1, He-ping E. Rd., Da-an District, Taipei City 10610, Taiwan, ROC Department of Mechatronic Technology, National Taiwan Normal University, No. 162, Sec. 1, He-ping E. Rd., Da-an District, Taipei City 10610, Taiwan, ROC
h i g h l i g h t s < We prepared MWCNTs nano-coating and applied to heat dissipation for projection lamp. < The temperature, power consuming and illumination of the fixture were evaluated. < The internal temperature of the fixture adopting halogen lamps reduces 5.4% at most. < The internal temperature of the fixture adopting LED lamps reduces 3.2% at most. < The power consumption reduces 12.9% at most for LED lamps with MWCNTs nano-coating.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 March 2012 Accepted 7 November 2012 Available online 15 November 2012
In this study, we used two-step synthesis method to prepare nano-coating containing multi-walled carbon nanotubes (MWCNTs) of different concentrations and applied the MWCNTs to heat dissipation for the projection lamp fixtures. We quantitatively coated the nano-coating on the inner surface of the housing of the fixture and evaluated its effects on the internal temperature, power consumption, and luminous output of the fixture for different conditions. The results show that both concentration of the MWCNT nano-coating and the projecting angle of the fixture affect the internal temperature and power consumption. With the MWCNT nano-coating of concentration at 1.328 wt.% and the projecting angle of 0 , the internal temperature of the fixture adopting the halogen lamp and that adopting the LED lamp reduce 5.4% 2.1% and 3.2% 2.1% at most, respectively; the power consumption reduces 12.9% 2.1% at most for the LED lamp; the luminous output has no significant difference for either the halogen or LED lamp. This study demonstrates MWCNT Nano-coating has great feasibility for heat dissipation of the lighting fixture. Further, this concept can also be applied to the cases requiring non-contact heat dissipation. Therefore, this study can widely contributes to energy saving and carbon emission reduction. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Heat dissipation Illumination Multi-walled carbon nanotubes Nano-coating Power consumption
1. Introduction Recently, the carbon nanotubes (CNTs) have been being concerned by many researchers in the nano-material fields. Since the CNTs have excellent thermal conductivity and mechanical properties [1], adding the CNTs into the traditional working fluid (CNT nanofluid) [2e7], ceramic materials [8e11], polymers [12], paraffin [13], and thermal paste [14,15] to improve both thermal performance and mechanical properties of the base material demonstrates many excellent results. The coating or paint added by CNTs * Corresponding author. Department of Mechatronic Technology, National Taiwan Normal University, No. 162, Sec. 1, He-ping E. Rd., Da-an District, Taipei City 10610, Taiwan, ROC. Tel.: þ886 2 7734 3527; fax: þ886 2 2358 3074. E-mail addresses: [email protected], [email protected] (T.-C. Teng). 1359-4311/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.applthermaleng.2012.11.006
can be regarded as a CNT nanofluid with higher viscosity. Although the concept of adding CNTs into the working fluid to improve the resultant thermal conductivity is well known, but the related experimental results of the resultant thermal conductivity have not been identical. The resultant thermal conductivity might be due to some factors such as aspect ratios, material, structure of CNTs, suspension performance, dispersants, and the synthesis method for preparing the experimental sample of nanofluids. Choi et al. [2] dispersed the multi-walled carbon nanotubes (MWCNTs) with a mean diameter of 25 nm and a length of 50 mm into oil. The experimental results demonstrated that the thermal conductivity of the MWCNT nanofluid at 1.0 vol.% enhanced more than 160% as compared with the base fluid at room temperature. Xie et al. [3] conducted the thermal conductivity experiments that dispersed the CNTs in distilled water, ethylene glycol and decene.
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Nomenclature $
q A h Int k L lx pc R Rs T
heat flux, W area, m2 heat transfer coefficient, W/m2 C intensity, counts thermal conductivity, W/m C thickness, m illumination, lx power consumption, W thermal resistance, C/W or ratio reflection, % temperature, C
Experimental results indicated that at least up to 20% of the thermal conductivity as compared with the base fluid. Garg et al. [4] reported the enhancement of thermal conductivity about 3%e5% at 25 C for the MWCNT nanofluid of 1.0 wt.% that was ultrasonicated for 20e 80 min. Chen et al. [5] prepared the MWCNT/ethylene glycol nanofluid of 1.0 vol.% by a wetemechanical reaction without using surfactant or dispersant. The thermal conductivity of the MWCNT/ ethylene glycol nanofluid was 17.5% higher than the base fluid. Chen and Xie [6] adopted the wetemechanochemical reaction method to produce the single, double-walled carbon nanotubes (SWCNTs and DWCNTs) and MWCNTs. The enhancement of thermal conductivity was 15.6%, 14.2%, and 12.1% for the SWCNTs, DWCNTs, and MWCNTs at 0.2 vol.%, respectively. Phuoc et al. [7] adopted the cationic chitosan as a dispersant to stabilize the MWCNT/water nanofluid. The thermal conductivity was enhanced from 2.3% to 13% for the nanofluid containing the MWCNTs at 0.5 wt.% to 3 wt.%. Although most research has demonstrated that the nanofluid containing CNTs has enhancement of the thermal conductivity as compared with the base fluid, the effect of CNTs on coating needs to be further studied. When the coating material containing nanoparticles (called ‘nano-coating’ hereafter) is dried to become the solid state, some mechanisms that enhance the thermal conductivity through the particle motion will disappear. Some studies demonstrated that adding CNTs in solid materials could enhance the thermal conductivity of the base material [8,10,12,15], in which Biercuk et al. [12] pointed that adding 1 vol.% SWCNTs to epoxy could improve its thermal conductivity up to 125%. In contrast, some studies showed that adding CNTs into the solid materials did not enhance the thermal conductivity of the base materials [11,16,17]. The main reasons are due to some factors such as interface thermal resistance between the base materials and CNTs, increased porosity, inhomogeneous distribution, agglomeration of CNTs, and incomplete consolidation processing [11]. Therefore, the nanoparticles must be uniformly dispersed into coating liquid, and the coating film after drying must be closely integrated with the coating substrate. Generally, the ultrasonic vibrator, electromagnetic stirrer, homogenizer, and other physical dispersion are most commonly used to mix nano-materials into base fluid for better suspension [4,18,19]. However, it is difficult for these physical dispersion methods to resolve agglomeration problem resulted from the van der Waals interaction forces, non-reactive surface properties, very large specific surface areas, and aspect ratios of CNTs [7,20e22]. Therefore, many researchers add the surfactant or dispersant in order to improve dispersion in nanofluids. The commonly used dispersant contains nitric/sulfuric acid mixture, potassium hydroxide group [5,23], and various surfactants such as sodium dodecylbenzene sulphonate (SDBS), sodium dodecyl sulfate (SDS), and gum Arabic (GA) [18,24]. Although adding dispersants can improve dispersion of nano-materials in the base fluid to maintain
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Subscripts a ambient bs back surface c convection ex external i inner in internal o outer r radiation T total w wall
a good suspension performance for a long period, the dispersants that generally have low thermal conductivity wrap the nanoparticles to lead increase of the contact resistance on the solide liquid interface. Therefore, overdosing surfactant or dispersant will decrease the thermal conductivity of nanofluids. From the above literature we can realize the CNTs can improve heat transfer in either the fluid or solid state if it is well dispersed. In this study we tried to apply MWCNTs to heat dissipation for another important practical case–lighting fixtures because of ecoconsciousness. Recently, energy saving and carbon emission reduction is a very important topic. So-called ‘energy saving and carbon emission reduction’ focuses on how to efficiently utilize energy, use less fossil energy, and reduce carbon emission. In addition, energy saving and carbon emission reduction should also include extending the product lifetime to reduce the wasted material. The lighting industry is one of the world’s great industrial sectors. It sells annually about 20 billion lamps of the three major types: incandescent lamp, fluorescent lamp, and high intensity discharge lamp. Operating those lamps is estimated currently to consume over 2700 TW-h (nearly 20% of total global electricity production, or approximately 3% of total energy consumption) and thus to produce about 2000 Megatons of CO2 emission [25]. Therefore, how to improve luminous efficiency to reduce lighting energy consumption is a very important task. Regarding the above concern, the light emitting diode (LED) lighting is expected as a potential replacement for the traditional lamp (especially for the incandescent lamp) due to its advantages such as long lifetime, energy efficiency, wide range of colors, lowvoltage power supply, and environmental compatibility [26,27]. Particularly, the development of LED technology has driven efficiency and luminous output of LED to rise exponentially. For example, a laboratory prototype LED achieving 208 lumens per watt at room temperature was also announced in 2010 [28,29]. Therefore, LED has prospect of providing the majority of light sources by 2035 [30]. Despite the above advantages of LED, LED still has the serious problem that efficiency and lifetime falls sharply with heat rising to be resolved [31,32]. So, how to manage heat dissipation for LED is a study-worthy topic; consequently many related researches have been proposed [33e37]. In this study we selected a projection lamp fixture as the experiment subject, and this fixture can adopt either the LED lamp or halogen lamp. We coated the inner housing wall of the fixture with the MWCNT nano-coating to improve its heat dissipation and then investigated the effect on both the internal temperature and power consumption of the fixture that adopted the LED lamp. In addition, since the traditional light sources still play a role in the immediate future through developments in the design and control of lighting installations to provide substantial energy saving opportunities [38], the projection lamp fixture adopting halogen lamps was also tested as comparison group in this experiment.
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2. Theories for heat transfer Fig. 1 shows the heat transfer process from space A to B based on $ the concept of steady-state heat transfer, the heat flux (ðqT Þ) can be expressed as: $
qT ¼ hT AðT1 T2 Þ ¼ ðT1 T2 Þ=RT ;
(1)
where hT is total heat transfer coefficient; A is area of heat transfer; T is temperature; RT is total thermal resistance; the subscripts 1 and 2 refer to space A and B, respectively. In the process of heat transfer, the relationship of heat transfer coefficient (h) and the thermal resistance (R) in the conduction, convection, and radiation are sequentially expressed as:
Rw ¼ ð1=hw AÞ ¼ ðL=kAÞ;
(2)
Rc ¼ ð1=hc AÞ;
(3)
Rr ¼ ð1=hr AÞ;
(4)
where the subscripts w, c, and r refer to conduction, convection, and radiation, respectively. The convection and radiation occur simultaneously at a surface, and both the thermal resistance of convection and radiation can be combined into a single thermal resistance. The total thermal resistance based on the concept of electrical series and parallel resistance can be expressed as follows:
RT ¼ R1;c ==R1;r þ Rw þ R2;c ==R2;r :
(5)
The heat flux is constant when the heat transfer from space A to B under steady state conditions, and then the Eq. (1) can be rewritten as follows:
$ qT ¼ ðT1 Ts1 Þ= R1;c ==R1;r ¼ ½ðTs1 Ts2 Þ=Rw ¼ ðTs2 T2 Þ= R2;c ==R2;r :
(6)
In the Eq. (6), we can find that the smaller temperature difference represents the lower thermal resistance of the interface when the heat flux is constant. In contrast, when T1 keeps at a constant
Fig. 1. The diagram of heat transfer process.
value, T2 closer to T1 represents the lower total thermal resistance between space A and B. According to the above-mentioned concept of heat transfer, reducing the thermal resistance can increase the heat transfer performance. The thermal resistance of radiation is mainly determined by both the shape factor and surface optical properties of materials; the thermal resistance of convection depends mainly on both the airflow rate and surface properties; thermal conductivity is determined by the thermal conductivity of the material itself. Therefore, using the higher absorption coating or anti-reflective structure on the wall can reduce the thermal resistance of radiation; increasing the airflow vents can reduce the convective thermal resistance; using the material with high thermal conductivity to make the wall can effectively reduce the thermal resistance of heat conduction. Similarly, coating a film containing the MWCNTs that has both high thermal conductivity and high optical absorption properties on the either inner or outer surface of a closed or semi-enclosed lampshade can also both enhance the heat dissipation performance of the lighting fixture and extend lifetime of the lamp. 3. Preparation of MWCNT nano-coating and experimental design 3.1. Preparation of test samples Fig. 2 and Fig. 3 are the photographs of Multi-walled carbon nanotubes (MWCNTs, Cheap Tubes Inc.) through the highresolution field emission scanning electron microscope (HRFESEM, S4800, Hitachi) and transmission electron microscope (TEM, H-7100, Hitachi), respectively. According to the specification provided by the manufacturer, the MWCNTs have an outside diameter of 20e30 nm, inside diameter of 5e10 nm, length of 10e 30 mm, average density of 2.1 g/cm3, and thermal conductivity of about 300e400 W/mK. As we can see from those figures, the MWCNTs aggregate, and their primary outside diameter is approximately in line with the specification. We used the MWCNTs/water nanofluid produced by two-step synthesis method as the dilute liquid. First, we added 0.2 wt.% of cationic dispersant (water-soluble chitosan) into distilled water as the base liquid in order to obtain good dispersion performance for the MWCNTs/water nanofluid. Chitosan is widely used in medicine, agriculture, chemical, and food processing areas without causing
Fig. 2. HR-FESEM images of MWCNTs.
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(80 80 0.3 mm) to form the sample 2, 3, and 4 under the spray pressure at 2.0 kg/cm2. After each spraying process was finished, the samples were dried in an oven for 3 h at 50 C. We measured the thickness of the coating films on the samples by thickness meter (LZ-370, KETT) at accuracy of 2%, and the thickness measured 70 5 mm. We also measured reflection spectra of each sample by VIS-NIR spectrometer (BRC112E, B&W) at a resolution of 2 nm. 3.2. Design and process of heat dissipation experiment
Fig. 3. TEM images of MWCNTs.
environmental pollution due to its non-toxic and biodegradable features, so this study used the water-soluble chitosan as a dispersant for the MWCNTs/water nanofluid [7,19]. Second, we gradually added the MWCNTs into the base liquid step by step and up to the needed weight through a precision electronic balance (XS125A, Precisa) at a precision of 0.1 mg. Thirdly, in order to keep the nanoparticle with stable suspension in the base liquid, we alternately used an electromagnetic agitation (PC-420D, Corning), homogenizer (T25 digital, IKA), and ultrasonic vibrator (D400H, TOHAMA) for about 3 h to disperse the MWCNTs. The prepared MWCNTs/water nanofluid had three weight fractions (1.0, 2.0, 4.0 wt.%). Next, the MWCNT nano-coating was produced. First, we used the trial and error method to repeatedly test the optimal amount of solvent (MWCNTs/water nanofluid) to dilute the bulk material for easy spray, and thus obtained the optimal dilution of volume ratio at 40 vol.%. The ingredients of the different kinds of the dilution solvent are listed in Table 1. The bulk material is water-based paint (National Ltd. Co., Taiwan). The density of the bulk material is 1340 kg/m3, and its main ingredient is an acrylic polymer. The MWCNT nano-coating was dispersed several times by a homogenizer, electromagnetic stirrer, and ultrasonic vibrator in order to make it easy to be sprayed. We sprayed the MWCNT nano-coating of each different concentration twice times on each galvanized steel plate
In order to test the heat transfer performance of the galvanized steel plate coated with the MWCNTs, the test configuration was setup as shown in Fig. 4. In this test, the coated surface (front surface) of the experimental sample facing the hot plate (PC-420D, Corning) was heated at a fixed distance (250 mm) away from the hot plate at the different heating temperatures (60, 80, 100, 120, and 140 C), and two thermocouples were attached on the back surface of the sample to measure the temperature. Thus, the temperature of back surface of the test samples can determine the heat transfer performance. The accuracy of the K-type thermocouple is 0.1 C. In order to investigate the performance of the MWCNT nanocoating applied to heat dissipation for the projection lamp fixture, we implemented the heat dissipation performance experiment on the projection lamp fixture. Fig. 5 shows the structure of the projection lamp fixture in this study. This projection lamp fixture contains a cylindrical housing with vents thereon, vents (the lower and upper), a lamp, and a holder that can pose the cylindrical housing at different projecting angle. The projecting angle affects the heat dissipation of the vents due to direction change of the airflow in the vents. In order to investigate the effects of the MWCNT nano-coating and projecting angle (0 or 90 ) on heat dissipation of the fixture, we attached the K-type thermocouple with accuracy of 0.1 C on both the lamp socket and outer surface of the housing by high thermal conductivity adhesive (plastic steel epoxy 3344, PowerBon), and connected the thermocouple with a data logger (TRM-20, TOHO) to measure the temperature when the lamp was turned on. Before the experiment was implemented, we carefully selected three fixtures, three halogen lamps, and three LED lamps for prior test to make sure that the difference in illumination, power consumption, and internal temperature between the three fixtures with the lamp of the same kind is within 2.0%.
Table 1 Preparation parameters of MWCNTs nano-coating. Sample no.
Sample 1
Density of water-based paint (kg/m3) MWCNTs/water nanofluid (wt.%) Dilution of volume ratio (vol.%) MWCNTs concentration in coating (wt.%)
Non-coating
Sample 2
Sample 3
Sample 4
1340 1.0
2.0
4.0
40 0.332
0.664
1.328 Fig. 4. Setup to test heat transfer performance for MWCNTs coating.
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housing was coated by the paint containing the MWCNTs according to the procedure same as the above-mentioned background experiment. Finally, we compared the difference in the temperature, illumination, and power consumption of the projection lamp fixtures between those with and without the MWCNT nano-coating in order to investigate the effects of the MWCNT nano-coating on heat dissipation and lighting performance. Furthermore, In order to reduce experimental deviation, the environmental temperature was controlled at 25 0.5 C during the experiments. 3.3. Uncertainty and data analysis The uncertainty of the results of this experiment were determined by measuring the deviation in the parameters, including the illumination, power consumption, and temperature. Because we mainly evaluated the difference between the same fixture without coating and with coating, the deviation between the fixtures had little impact on evaluation. In the heat dissipation experiment for the projection lamp; the illumination (lx) was determined from readings of the illuminometer; the power consumption (pc) was measured by the digital power meter; the internal (Tin) and external (Tex) temperature was measured by the data logger; the ambient temperature (Ta) was controlled by the temperature controller of DC inverter air conditioner. The uncertainty of illumination, power consumption, and temperature in the heat dissipation experiment for the projection lamp experiment can be expressed as Eqs. (7)e(9), respectively.
Fig. 5. The structure of projection lamp fixture at different projecting angle. (a) 90 , (b) 0 .
First, we turned on the three fixtures with a halogen lamp (Decostar 51/50W/12V/38deg., OSRAM) for 70 min as background experiment, and then replaced the lamp with a LED lamp (MR-16/3.5W/ 12V/50deg., China Electric mfg. Corporation) for the same background experiment. The temperature data were averages of the data measured in the last 10 min of the background experiment. When the illuminating time reached 60 min, we started to measure both the illumination and power consumption of each fixture by an illuminometer (TES-1336A, TES) with accuracy of 3.0% and a digital power meter (WT230, Yokogawa) with accuracy of 0.35% for different lighting sources (halogen lamps and LED lamps) and projecting angles (0 and 90 ) in order to obtain the differences in power consumption and illumination on the surface at a fixed distance below the fixture. The data of illumination and power consumption were averages of the ten measurements with an interval of 1 min during the last 10 min in the experiment. Next, the inner surfaces of the cylindrical housing of the three fixtures were coated with the paint containing the MWCNTs of three different concentrations at 0.332, 0.664, and 1.328 wt.%, respectively. In order to obtain the uniform coating on the inner surface and reduce the thickness differences between the samples, we fixed the cylindrical housing horizontally and concentrically at a motor shaft to let the housing rotate at 600 rpm for 20 min to make the coated paint dry and uniform after the paint was quantitatively brush (1.0 ml) by quantitative dropper and rubber spatula. Then, the samples were dried in the oven at 50 C. Next, we conducted the heat dissipation experiment on the projection lamp fixtures whose
um;lx ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðDlx=lxÞ2 þðDTa =Ta Þ2
(7)
um;pc ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðDpc=pcÞ2 þðDTa =Ta Þ2
(8)
um;T ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðDT=TÞ2 þðDTa =Ta Þ2
(9)
The accuracy of the illuminometer is within 3.0%. The accuracy of the digital power meter is within 0.35%. The accuracy of the thermocouple for internal and external temperature of the projection lamp is within 0.1 C. The accuracy of the temperature controller of DC inverter air conditioner for ambient temperature is within 0.5 C. Therefore, the uncertainty of the illumination, power consumption, and temperature experiment is within 3.6%, 2.1%, and 2.1%, respectively. 3.4. Data analysis The measurement data of heat dissipation performance, illumination, and power consumption for the lamp fixture with uncoated housing are referred as baseline values (Db), and the measurement data for the lamp fixture with housing coated by paint containing the MWCNTs are marked as Dc. For explicit investigation, the experimental data Dc were compared with Db. The differences between Dc and Db are presented as proportions (R) and are expressed as follows:
R ¼ ðDc =Db Þ 100%:
(10)
4. Results and discussion Fig. 6 shows the reflection spectra of the samples. The sample 1 is bare galvanized steel plate without coating, sample 2e4 are coated by the paint containing the MWCNTs of 0.332, 0.664, and 1.328 wt.%, respectively. In the Fig. 6, we can find that adding
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Fig. 6. Reflection spectra of the test samples.
Fig. 8. Internal temperature trends of halogen lamp at different projecting angles.
the MWCNTs can reduce the reflectivity within UVeNIR band (350e950 nm), and the reflectivity decreases with increase in concentration of the MWCNTs. This phenomenon is mainly because the MWCNTs have excellent optical absorption within UVeNIR band. However, when concentration of the MWCNTs exceed above 0.664 wt.%, there is no significant difference in reflectivity because absorption reaches saturation. Fig. 7 shows the heat transfer performance of the samples. In Fig. 7, we can find that adding the MWCNTs can increase the temperature of the back surface of the sample, and the temperature increases with increase in concentration of the MWCNTs. For the same heating temperature of the hot plate, the higher back surface temperature means that there is greater heat flux through the sample, and the sample has lower thermal resistance; consequently providing better heat dissipation. This phenomenon is mainly
because the MWCNTs have both excellent optical absorption and high thermal conductivity (about 300e400 W/mK) to reduce thermal resistance in both radiation and conduction. As seen from Fig. 6, MWCNT nano-coating has high optical absorption such that the thermal resistance in radiation can be reduced. Further, because the overall thermal conductivity of the mixture depends on the thermal conductivity of each ingredient, adding the material with high thermal conductivity into the bulk material can increase the overall thermal conductivity [39,40]. Therefore, the total thermal resistance also reduces when the thermal resistance decreases in both radiation and heat conduction. Fig. 8 and Fig. 10 show the temperature of the lamp socket (called ‘internal temperature’ hereafter) of the lamp fixtures at different projecting angle in the background test, and the related data can be referred to Table 2. Figs. 9 and 11 show the temperature
Fig. 7. Heat transfer performance of the samples.
Fig. 9. External temperature trends of halogen lamp at different projecting angles.
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Fig. 10. Internal temperature trends of LED lamp at different projecting angles.
of the outer housing surface (called ‘external temperature’ hereafter) of the lamp fixtures at different projecting angle in the background test. As we can see from those figures, both the internal temperature and external temperature at projecting angle of 90 is higher than that at the projecting angle of 0 . This phenomenon can be explained as follows (please also refer to Fig. 5): at the projecting angle of 90 , the cold air enters into the bottom vents and the heated air discharges from the top vents according to the principle of heat convection. Thus, the heated air flows by the lamp socket and heats it; major portion of the heat generated by the lamp is transferred by convection, and minor portion is dissipated through the housing wall. In contrast, at the projecting angle of 0 , most of the heated air does not flow by the lamp socket, so the internal temperature is lower. Furthermore, at the projecting angle of 0 , the housing wall provides greater effective heat transfer area, and the heated air easily discharge from the vents on the housing wall, so the external temperature is also lower. From Figs. 8 and 9, we can find that the difference of internal and external temperature between 90 and 0 at 60 min after the halogen lamp was turned on is about 28 C and 10 C, respectively. From Figs. 10 and 11, we can find that the difference of internal and external temperature between 90 and 0 at 60 min after the LED lamp was turned on is about 3 C and 1.5 C, respectively. In order to investigate the effects of the MWCNT nano-coating on the internal temperature of the fixtures, the temperature of each fixture with coated housing is compared with that of the same fixture before its housing is coated. The three fixtures with inner housing wall coated by paint containing the MWCNTs of
concentration at 0.332, 0.664, and 1.328 wt.%, respectively are called ‘fixture 1’, ‘fixture 2’, and ‘fixture 3’, respectively. Fig. 12 shows the ratio of internal temperature of the fixture with coated housing to that of the corresponding fixture with uncoated housing. In Fig. 12, we can find that there is more significant effect for the fixtures at a projecting angle of 0 . For the halogen lamp, little effect is observed at a projecting angle of 90 . However, for the fixtures at a projecting angle of 0 , the internal temperature of the fixtures with inner housing wall coated by paint containing the MWCNTs of concentration at 0.332, 0.664 wt.% (fixture 1 and fixture 2) increases instead. It is because the bulk material of the paint has higher thermal resistance such that the total thermal resistance of the paint is still higher when the added the MWCNTs are fewer. When concentration of the added MWCNTs reaches 1.328 wt.%, the internal temperature is reduced 5.4% 2.1% as compared with the uncoated case. For the LED lamp, similarly, little effect is observed at projecting angle of 90 . For the fixtures at a projecting angle of 0 , the internal temperature is reduced 3.2% 2.1% for the fixture with inner housing wall coated by paint containing the MWCNTs of concentration at 1.328 wt.%. For both the halogen and LED lamps at a projecting angle of 0 , the internal temperature decreases with increase in concentration of the MWCNTs. Similarly, in order to investigate the effects of the MWCNT nanocoating on power consumption of the fixtures, the power consumption of each fixture with coated housing is compared with that of the same fixture before its housing is coated. Fig. 13 shows the ratio of power consumption of the fixture with coated housing to that of the corresponding fixture with uncoated housing. In Fig. 13, we can find that there is no effect of the MWCNT nanocoating on power consumption of the fixtures for the halogen lamp. In fact, the power consumption of halogen lamp is affected by temperature of the filament. In order to stabilize the power consumption of the lamp, the resistance of the filament is designed to increase with the increase in temperature. For such design, when the temperature of the filament increases due to increase in dissipation power, the resistance of the filament also increases such that the power consumption is suppressed. In contrast, when the temperature of the filament decreases due to decrease in dissipation power, the resistance of the filament decreases such that power consumption increases. It should be noted that the luminous efficiency decreases as the temperature of the filament decreases according to the blackbody irradiation principle. Therefore, although the lower temperature benefits the lifetime of the filament, the temperature is still set at the optimal regarding the luminous efficiency [41e43]. For the above reasons, we just cool down the local temperature of the lamp socket instead of the filament in this study. For the halogen lamp, just little portion of the heat generated by the filament dissipates through the lamp socket, so cooling down the local temperature of the lamp socket has very little impact on the filament temperature. It can be proved by checking if the output spectra of the lamp changes. In this study, the output spectra were monitored and had very little variation
Table 2 Experimental results of initial condition for baseline. Test parameters
Deg.
Halogen lamp
Illumination (lx)
90 0 90 0 90 0
9980 10,060 173.15 146.78 39.60 40.08
Internal temperature ( C) Power consumption (W)
9980 10,090 174.91 151.41 40.08 40.56
Note: Relative standard deviation (RSD) ¼ (standard deviation/mean) 100%.
9760 9860 170.19 153.18 38.64 39.12
RSD
LED lamp
1.05% 1.02% 1.13% 1.79% 1.52% 1.50%
556 554 51.81 48.69 3.88 3.92
RSD 554 535 52.89 49.48 3.88 3.90
545 533 50.96 48.03 3.88 3.90
0.87% 1.75% 1.52% 1.22% 0.00% 0.24%
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Fig. 11. External temperature trends of LED lamp at different projecting angles.
during the experiment. Since the filament temperature was not affected in the experiment, the power consumption does not change. In contrast, for the LED lamp, Fig. 13 shows the MWCNT nano-coating has some effect on the power consumption. Especially for the higher concentration of the MWCNT nano-coating, the power consumption reduces 12.9% 2.1% at most. It is mainly due to both the characteristic of LED chip and rectifier electronics integrated in the LED lamp. The junction temperature quite impacts the luminous efficiency and lifetime of the LED chip; the luminous efficiency and lifetime exponentially decreases as the junction temperature increases. Because LED is a ‘cold light’ source, the most heat generated from the LED chip is conducted out through the substrate. Thus, cooling down the temperature of the lamp socket
can also reduce the junction temperature of LED chip. Further, since LED needs DC electricity power, the related rectifier electronics are integrated on the bottom of the LED lamp. Thus, lower temperature of the lamp socket also can reduce the power consumption and benefit the lifetime of the electronics. Based on the above reasons, the MWCNT nano-coating has more obvious effect on power consumption of the LED lamps. Similarly, in order to investigate the effects of the MWCNT nanocoating on the illumination, the illumination of each fixture with coated housing is compared with that of the same fixture before its
Fig. 12. Internal temperature ratio of fixtures at different projecting angles.
Fig. 14. Illumination ratio of fixtures at different projecting angles.
Fig. 13. Power consumption ratio of fixtures at different projecting angles.
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housing is coated. Fig. 14 shows the ratio of illumination of the fixture with coated housing to that of the corresponding fixture with uncoated housing. In Fig. 14, we can find that there is little of the MWCNT nano-coating on illumination of the fixtures for either the halogen lamp or LED lamp. The difference in the experiment data is within the deviation of the illumination experiment (3.6%). In this study, the above results demonstrate that the MWCNT nano-coating can indeed reduce the internal temperature of the fixtures and reduce the power consumption (for LED lamps) while maintains the same luminous output. Therefore, this study will contribute to lifetime of the components, energy saving and carbon emission reduction. 5. Conclusions In this study, we quantitatively painted the inner housing wall of the projecting lamp fixtures with the MWCNT nano-coating of different concentration in order to reduce the internal temperature of the fixtures for the purpose of both extending the lifetime of the components and saving energy. The experimental results show that the internal temperature reduces 5.4% 2.1% (for the halogen lamp) and 3.2% 2.1% (for the LED lamp) at most for the fixture with housing coated by the MWCNT nano-coating of 1.328 wt.% at the projecting angle of 0 . The power consumption reduces 12.9% 2.1% (for the LED lamp) at most for the fixture with housing coated by the MWCNT nano-coating of 1.328 wt.% at the projecting angle of 0 . The MWCNT nano-coating has no effect on luminous output for either the halogen lamp or LED lamp. This study demonstrates that the MWCNT nano-coating has high feasibility in heat dissipation for lighting fixtures, which benefits the lifetime of the related components. Further, the MWCNTs can also extend to be applied to the cases requiring non-contact heat dissipation. Therefore, this study can widely contribute to energy saving and carbon emission reduction. Acknowledgements The authors would like to thank National Science Council of the Republic of China, Taiwan for their financial support to this research under contract no.: NSC-100-2221-E-003-029 and NSC-100-2221E-003-015, respectively. References [1] S.M.S. Murshed, K.C. Leong, C. Yang, Thermophysical and electrokinetic properties of nanofluids e a critical review, Appl. Therm. Eng. 28 (2008) 2109e2125. [2] S.U.S. Choi, Z.G. Zang, W. Yu, F.E. Lookwood, E.A. Grulke, Anomalous thermal conductivity enhancement in nanutube suspension, Appl. Phys. Lett. 79 (2001) 2252e2254. [3] H. Xie, H. Lee, W. Youn, M. Choi, Nanofluids containing multiwalled carbon nanotubes and their enhanced thermal conductivities, J. Appl. Phys. 94 (2003) 4967e4971. [4] P. Garg, J.L. Alvarado, C. Marsh, T.A. Carlson, D.A. Kessler, K. Annamalai, An experimental study on the effect of ultrasonication on viscosity and heat transfer performance of multi-wall carbon nanotube-based aqueous nanofluids, Int. J. Heat Mass Transf. 52 (2009) 5090e5101. [5] L. Chen, H. Xie, Y. Li, W. Yu, Nanofluids containing carbon nanotubes treated by mechanochemical reaction, Thermochim. Acta 477 (2008) 21e24. [6] L. Chen, H. Xie, Surfactant-free nanofluids containing double- and singlewalled carbon nanotubes functionalized by a wet-mechanochemical reaction, Thermochim. Acta 497 (2010) 67e71. [7] T.X. Phuoc, M. Massoudi, R.H. Chen, Viscosity and thermal conductivity of nanofluids containing multi-walled carbon nanotubes stabilized by chitosan, Int. J. Therm. Sci. 50 (2011) 12e18. [8] C.H. Liu, H. Huang, Y. Wu, S.S. Fan, Thermal conductivity improvement of silicone elastomer with carbon nanotube loading, Appl. Phys. Lett. 84 (2004) 4248e4250. [9] G.D. Zhan, J.D. Kuntz, J. Wan, A.K. Mukherjee, Single-wall carbon nanotubes as attractive toughening agents in alumina-based nanocomposites, Nat. Mater. 2 (2003) 38e42.
[10] R. Sivakumar, S.Q. Guo, T. Nishimura, Y. Kagawa, Thermal conductivity in multi-wall carbon nanotube/silica-based nanocomposites, Scr. Mater. 56 (2007) 265e268. [11] K. Chu, Q. Wu, C. Jia, X. Liang, J. Nie, W. Tian, G. Gai, H. Guo, Fabrication and effective thermal conductivity of multi-walled carbon nanotubes reinforced Cu matrix composites for heat sink applications, Compos. Sci. Technol. 70 (2010) 298e304. [12] M.J. Biercuk, M.C. Llaguno, M. Radosavljevic, J.K. Hyun, A.T. Johnson, Carbon nanotube composites for thermal management, Appl. Phys. Lett. 80 (2002) 2767e2769. [13] T.P. Teng, B.G. Lin, Y.Y. Yeh, Characterization of heat storage by nanocomposite-enhanced phase change materials, Adv. Mater. Res. 287e290 (2011) 1448e1455. [14] D.D.L. Chung, Materials for thermal conduction, Appl. Therm. Eng. 21 (2001) 1593e1605. [15] Y. Xu, C.K. Leong, D.D.L. Chung, Carbon nanotube thermal pastes for improving thermal contacts, J. Electron. Mater. 36 (2007) 1181e1187. [16] C. Kim, B. Lim, B. Kim, U. Shim, S. Oh, B. Sung, J. Choi, J. Ki, S. Baik, Strengthening of copper matrix composites by nickel-coated singlewalled carbon nanotube reinforcements, Synth. Met. 159 (2009) 424e429. [17] C.W. Nan, R. Birringer, Determining the Kapitza resistance and the thermal conductivity of polycrystals: a simple model, Phys. Rev. B 57 (1998) 8264e8268. [18] Y. Ding, H. Alias, D. Wen, R.A. Williams, Heat transfer of aqueous suspensions of carbon nanotubes (CNT nanofluids), Int. J. Heat Mass. Transf. 49 (2006) 240e250. [19] Y.H. Hung, T.P. Teng, T.C. Teng, J.H. Chen, Assessment of heat dissipation performance for nanofluid, Appl. Therm. Eng. 32 (2012) 132e140. [20] L.A. Girifalco, M. Hodak, R.S. Lee, Carbon nanotubes, buckyballs, ropes, and a universal graphic potential, Phys. Rev. B 62 (2000) 13104e13110. [21] T. Lin, V. Bajpai, T. Ji, L. Dai, Chemistry of carbon nanotubes, Aust. J. Chem. 56 (2003) 635e651. [22] C. Park, Z. Ounaies, K.A. Watson, R.E. Crooks, J.J. Smith, S.E. Lowther, Dispersion of single wall carbon nanotubes by in situ polymerization under sonication, Chem. Phys. Lett. 364 (2002) 303e308. [23] G.H. Ko, K. Heo, K. Lee, D.S. Kim, C. Kim, Y. Sohn, M. Choi, An experimental study on the pressure drop of nanofluids containing carbon nanotubes in a horizontal tube, Int. J. Heat Mass. Transf. 50 (2007) 4749e4753. [24] L. Jiang, L. Gao, J. Sun, Production of aqueous colloidal dispersions of carbon nanotubes, J. Colloid Interface Sci. 260 (2003) 89e94. [25] OECDJIEA, Light’s Labour’s Lost: Policies for Energy-efficient Lighting. Paris Cedex, France (2006). [26] R. Devonshire, The competitive technology environment for LED lighting, J. Light Vis. Environ. 32 (2008) 275e287. [27] N. Zheludev, The life and times of the LED- a 100-year history, Nat. Photonics 1 (2007) 189e192. [28] Nature Photonics Editorial, Haitz’s law, Nat. Photonics 1 (2007) 23. [29] CREE, Cree breaks 200 lumen per watt efficacy barrier (Press release), CREE, Durham, N.C, 2010. [30] M. Richards, D. Carter, Good lighting with less energy: where next? Lighting Res. Technol. 41 (2009) 285e286. [31] A.A. Efremov, N.I. Bochkareva, R.I. Gorbunov, D.A. Lavrinovich, Y.T. Rebane, D.V. Tarkhin, Y.G. Shreter, Effect of the joule heating on the quantum efficiency and choice of thermal conditions for high-power blue InGaN/GaN LEDs, Semiconductors 40 (2006) 605e610. [32] S.L. Chuang, A. Ishibashi, S. Kijima, N. Nakayama, M. Ukita, S. Taniguchi, Kinetic model for degradation of light-emitting diodes, IEEE J. Quantum Electron. 33 (1997) 970e979. [33] Philips and Lumileds Co. Ltd., Understanding Power LED Lifetime Analysis, in: Technology White Paper, Philips and Lumileds Co. Ltd., 2007. [34] E. Hong, N. Narendran, A method for projecting useful life of LED lighting systems, 3rd International Conference on Solid State Lighting, Proc. SPIE 5187 (2004) 93e99. [35] J. Hu, L. Yang, M.W. Shin, Mechanism and thermal effect of delamination in light-emitting diode packages, Microelectron. J. 38 (2007) 157e163. [36] M. Bürmen, F. Pernus, B. Likar, Prediction of intensity and color degradation of LEDs, Proc. SPIE 6486 (2007) 64860M1e10. [37] J.H. Choi, M.W. Shin, Thermal investigation of LED lighting module, Microelectron. Reliab. (2011). http://dx.doi.org/10.1016/j.microrel.2011.04.009. [38] D. Loe, Energy efficiency in lighting e considerations and possibilities, Lighting Res. Technol. 41 (2009) 209e218. [39] J.C. Maxwell, A Treatise on Electricity and Magnetism, third ed., vol. 1, Clarendon, Oxford, 1904. [40] R.L. Hamilton, O.K. Crosser, Thermal conductivity of heterogeneous two component systems, I&EC Fundam. 1 (1962) 182e191. [41] S. Garbe, S. Hanloh, Growth of potassium-filled bubbles in doped tungsten and its relation to hot spot development and intergranular fracture, Philips J. Res. 38 (1983) 248e262. [42] Publication Number TP-110Incandescent Lamps, General Electric Company, Nela Park, Cleveland, OH, 1964. [43] R. Kane, Heinz Sell Revolution in Lamps: A Chronicle of 50 Years of Progress, second ed., The Fairmont Press, Inc, 2001.
Contents lists available at SciVerse ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Enhance heat dissipation for projection lamps by MWCNTs nano-coating Tun-Ping Teng a, Tun-Chien Teng b, * a b
Department of Industrial Education, National Taiwan Normal University, No. 162, Sec. 1, He-ping E. Rd., Da-an District, Taipei City 10610, Taiwan, ROC Department of Mechatronic Technology, National Taiwan Normal University, No. 162, Sec. 1, He-ping E. Rd., Da-an District, Taipei City 10610, Taiwan, ROC
h i g h l i g h t s < We prepared MWCNTs nano-coating and applied to heat dissipation for projection lamp. < The temperature, power consuming and illumination of the fixture were evaluated. < The internal temperature of the fixture adopting halogen lamps reduces 5.4% at most. < The internal temperature of the fixture adopting LED lamps reduces 3.2% at most. < The power consumption reduces 12.9% at most for LED lamps with MWCNTs nano-coating.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 March 2012 Accepted 7 November 2012 Available online 15 November 2012
In this study, we used two-step synthesis method to prepare nano-coating containing multi-walled carbon nanotubes (MWCNTs) of different concentrations and applied the MWCNTs to heat dissipation for the projection lamp fixtures. We quantitatively coated the nano-coating on the inner surface of the housing of the fixture and evaluated its effects on the internal temperature, power consumption, and luminous output of the fixture for different conditions. The results show that both concentration of the MWCNT nano-coating and the projecting angle of the fixture affect the internal temperature and power consumption. With the MWCNT nano-coating of concentration at 1.328 wt.% and the projecting angle of 0 , the internal temperature of the fixture adopting the halogen lamp and that adopting the LED lamp reduce 5.4% 2.1% and 3.2% 2.1% at most, respectively; the power consumption reduces 12.9% 2.1% at most for the LED lamp; the luminous output has no significant difference for either the halogen or LED lamp. This study demonstrates MWCNT Nano-coating has great feasibility for heat dissipation of the lighting fixture. Further, this concept can also be applied to the cases requiring non-contact heat dissipation. Therefore, this study can widely contributes to energy saving and carbon emission reduction. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Heat dissipation Illumination Multi-walled carbon nanotubes Nano-coating Power consumption
1. Introduction Recently, the carbon nanotubes (CNTs) have been being concerned by many researchers in the nano-material fields. Since the CNTs have excellent thermal conductivity and mechanical properties [1], adding the CNTs into the traditional working fluid (CNT nanofluid) [2e7], ceramic materials [8e11], polymers [12], paraffin [13], and thermal paste [14,15] to improve both thermal performance and mechanical properties of the base material demonstrates many excellent results. The coating or paint added by CNTs * Corresponding author. Department of Mechatronic Technology, National Taiwan Normal University, No. 162, Sec. 1, He-ping E. Rd., Da-an District, Taipei City 10610, Taiwan, ROC. Tel.: þ886 2 7734 3527; fax: þ886 2 2358 3074. E-mail addresses: [email protected], [email protected] (T.-C. Teng). 1359-4311/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.applthermaleng.2012.11.006
can be regarded as a CNT nanofluid with higher viscosity. Although the concept of adding CNTs into the working fluid to improve the resultant thermal conductivity is well known, but the related experimental results of the resultant thermal conductivity have not been identical. The resultant thermal conductivity might be due to some factors such as aspect ratios, material, structure of CNTs, suspension performance, dispersants, and the synthesis method for preparing the experimental sample of nanofluids. Choi et al. [2] dispersed the multi-walled carbon nanotubes (MWCNTs) with a mean diameter of 25 nm and a length of 50 mm into oil. The experimental results demonstrated that the thermal conductivity of the MWCNT nanofluid at 1.0 vol.% enhanced more than 160% as compared with the base fluid at room temperature. Xie et al. [3] conducted the thermal conductivity experiments that dispersed the CNTs in distilled water, ethylene glycol and decene.
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Nomenclature $
q A h Int k L lx pc R Rs T
heat flux, W area, m2 heat transfer coefficient, W/m2 C intensity, counts thermal conductivity, W/m C thickness, m illumination, lx power consumption, W thermal resistance, C/W or ratio reflection, % temperature, C
Experimental results indicated that at least up to 20% of the thermal conductivity as compared with the base fluid. Garg et al. [4] reported the enhancement of thermal conductivity about 3%e5% at 25 C for the MWCNT nanofluid of 1.0 wt.% that was ultrasonicated for 20e 80 min. Chen et al. [5] prepared the MWCNT/ethylene glycol nanofluid of 1.0 vol.% by a wetemechanical reaction without using surfactant or dispersant. The thermal conductivity of the MWCNT/ ethylene glycol nanofluid was 17.5% higher than the base fluid. Chen and Xie [6] adopted the wetemechanochemical reaction method to produce the single, double-walled carbon nanotubes (SWCNTs and DWCNTs) and MWCNTs. The enhancement of thermal conductivity was 15.6%, 14.2%, and 12.1% for the SWCNTs, DWCNTs, and MWCNTs at 0.2 vol.%, respectively. Phuoc et al. [7] adopted the cationic chitosan as a dispersant to stabilize the MWCNT/water nanofluid. The thermal conductivity was enhanced from 2.3% to 13% for the nanofluid containing the MWCNTs at 0.5 wt.% to 3 wt.%. Although most research has demonstrated that the nanofluid containing CNTs has enhancement of the thermal conductivity as compared with the base fluid, the effect of CNTs on coating needs to be further studied. When the coating material containing nanoparticles (called ‘nano-coating’ hereafter) is dried to become the solid state, some mechanisms that enhance the thermal conductivity through the particle motion will disappear. Some studies demonstrated that adding CNTs in solid materials could enhance the thermal conductivity of the base material [8,10,12,15], in which Biercuk et al. [12] pointed that adding 1 vol.% SWCNTs to epoxy could improve its thermal conductivity up to 125%. In contrast, some studies showed that adding CNTs into the solid materials did not enhance the thermal conductivity of the base materials [11,16,17]. The main reasons are due to some factors such as interface thermal resistance between the base materials and CNTs, increased porosity, inhomogeneous distribution, agglomeration of CNTs, and incomplete consolidation processing [11]. Therefore, the nanoparticles must be uniformly dispersed into coating liquid, and the coating film after drying must be closely integrated with the coating substrate. Generally, the ultrasonic vibrator, electromagnetic stirrer, homogenizer, and other physical dispersion are most commonly used to mix nano-materials into base fluid for better suspension [4,18,19]. However, it is difficult for these physical dispersion methods to resolve agglomeration problem resulted from the van der Waals interaction forces, non-reactive surface properties, very large specific surface areas, and aspect ratios of CNTs [7,20e22]. Therefore, many researchers add the surfactant or dispersant in order to improve dispersion in nanofluids. The commonly used dispersant contains nitric/sulfuric acid mixture, potassium hydroxide group [5,23], and various surfactants such as sodium dodecylbenzene sulphonate (SDBS), sodium dodecyl sulfate (SDS), and gum Arabic (GA) [18,24]. Although adding dispersants can improve dispersion of nano-materials in the base fluid to maintain
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Subscripts a ambient bs back surface c convection ex external i inner in internal o outer r radiation T total w wall
a good suspension performance for a long period, the dispersants that generally have low thermal conductivity wrap the nanoparticles to lead increase of the contact resistance on the solide liquid interface. Therefore, overdosing surfactant or dispersant will decrease the thermal conductivity of nanofluids. From the above literature we can realize the CNTs can improve heat transfer in either the fluid or solid state if it is well dispersed. In this study we tried to apply MWCNTs to heat dissipation for another important practical case–lighting fixtures because of ecoconsciousness. Recently, energy saving and carbon emission reduction is a very important topic. So-called ‘energy saving and carbon emission reduction’ focuses on how to efficiently utilize energy, use less fossil energy, and reduce carbon emission. In addition, energy saving and carbon emission reduction should also include extending the product lifetime to reduce the wasted material. The lighting industry is one of the world’s great industrial sectors. It sells annually about 20 billion lamps of the three major types: incandescent lamp, fluorescent lamp, and high intensity discharge lamp. Operating those lamps is estimated currently to consume over 2700 TW-h (nearly 20% of total global electricity production, or approximately 3% of total energy consumption) and thus to produce about 2000 Megatons of CO2 emission [25]. Therefore, how to improve luminous efficiency to reduce lighting energy consumption is a very important task. Regarding the above concern, the light emitting diode (LED) lighting is expected as a potential replacement for the traditional lamp (especially for the incandescent lamp) due to its advantages such as long lifetime, energy efficiency, wide range of colors, lowvoltage power supply, and environmental compatibility [26,27]. Particularly, the development of LED technology has driven efficiency and luminous output of LED to rise exponentially. For example, a laboratory prototype LED achieving 208 lumens per watt at room temperature was also announced in 2010 [28,29]. Therefore, LED has prospect of providing the majority of light sources by 2035 [30]. Despite the above advantages of LED, LED still has the serious problem that efficiency and lifetime falls sharply with heat rising to be resolved [31,32]. So, how to manage heat dissipation for LED is a study-worthy topic; consequently many related researches have been proposed [33e37]. In this study we selected a projection lamp fixture as the experiment subject, and this fixture can adopt either the LED lamp or halogen lamp. We coated the inner housing wall of the fixture with the MWCNT nano-coating to improve its heat dissipation and then investigated the effect on both the internal temperature and power consumption of the fixture that adopted the LED lamp. In addition, since the traditional light sources still play a role in the immediate future through developments in the design and control of lighting installations to provide substantial energy saving opportunities [38], the projection lamp fixture adopting halogen lamps was also tested as comparison group in this experiment.
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2. Theories for heat transfer Fig. 1 shows the heat transfer process from space A to B based on $ the concept of steady-state heat transfer, the heat flux (ðqT Þ) can be expressed as: $
qT ¼ hT AðT1 T2 Þ ¼ ðT1 T2 Þ=RT ;
(1)
where hT is total heat transfer coefficient; A is area of heat transfer; T is temperature; RT is total thermal resistance; the subscripts 1 and 2 refer to space A and B, respectively. In the process of heat transfer, the relationship of heat transfer coefficient (h) and the thermal resistance (R) in the conduction, convection, and radiation are sequentially expressed as:
Rw ¼ ð1=hw AÞ ¼ ðL=kAÞ;
(2)
Rc ¼ ð1=hc AÞ;
(3)
Rr ¼ ð1=hr AÞ;
(4)
where the subscripts w, c, and r refer to conduction, convection, and radiation, respectively. The convection and radiation occur simultaneously at a surface, and both the thermal resistance of convection and radiation can be combined into a single thermal resistance. The total thermal resistance based on the concept of electrical series and parallel resistance can be expressed as follows:
RT ¼ R1;c ==R1;r þ Rw þ R2;c ==R2;r :
(5)
The heat flux is constant when the heat transfer from space A to B under steady state conditions, and then the Eq. (1) can be rewritten as follows:
$ qT ¼ ðT1 Ts1 Þ= R1;c ==R1;r ¼ ½ðTs1 Ts2 Þ=Rw ¼ ðTs2 T2 Þ= R2;c ==R2;r :
(6)
In the Eq. (6), we can find that the smaller temperature difference represents the lower thermal resistance of the interface when the heat flux is constant. In contrast, when T1 keeps at a constant
Fig. 1. The diagram of heat transfer process.
value, T2 closer to T1 represents the lower total thermal resistance between space A and B. According to the above-mentioned concept of heat transfer, reducing the thermal resistance can increase the heat transfer performance. The thermal resistance of radiation is mainly determined by both the shape factor and surface optical properties of materials; the thermal resistance of convection depends mainly on both the airflow rate and surface properties; thermal conductivity is determined by the thermal conductivity of the material itself. Therefore, using the higher absorption coating or anti-reflective structure on the wall can reduce the thermal resistance of radiation; increasing the airflow vents can reduce the convective thermal resistance; using the material with high thermal conductivity to make the wall can effectively reduce the thermal resistance of heat conduction. Similarly, coating a film containing the MWCNTs that has both high thermal conductivity and high optical absorption properties on the either inner or outer surface of a closed or semi-enclosed lampshade can also both enhance the heat dissipation performance of the lighting fixture and extend lifetime of the lamp. 3. Preparation of MWCNT nano-coating and experimental design 3.1. Preparation of test samples Fig. 2 and Fig. 3 are the photographs of Multi-walled carbon nanotubes (MWCNTs, Cheap Tubes Inc.) through the highresolution field emission scanning electron microscope (HRFESEM, S4800, Hitachi) and transmission electron microscope (TEM, H-7100, Hitachi), respectively. According to the specification provided by the manufacturer, the MWCNTs have an outside diameter of 20e30 nm, inside diameter of 5e10 nm, length of 10e 30 mm, average density of 2.1 g/cm3, and thermal conductivity of about 300e400 W/mK. As we can see from those figures, the MWCNTs aggregate, and their primary outside diameter is approximately in line with the specification. We used the MWCNTs/water nanofluid produced by two-step synthesis method as the dilute liquid. First, we added 0.2 wt.% of cationic dispersant (water-soluble chitosan) into distilled water as the base liquid in order to obtain good dispersion performance for the MWCNTs/water nanofluid. Chitosan is widely used in medicine, agriculture, chemical, and food processing areas without causing
Fig. 2. HR-FESEM images of MWCNTs.
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(80 80 0.3 mm) to form the sample 2, 3, and 4 under the spray pressure at 2.0 kg/cm2. After each spraying process was finished, the samples were dried in an oven for 3 h at 50 C. We measured the thickness of the coating films on the samples by thickness meter (LZ-370, KETT) at accuracy of 2%, and the thickness measured 70 5 mm. We also measured reflection spectra of each sample by VIS-NIR spectrometer (BRC112E, B&W) at a resolution of 2 nm. 3.2. Design and process of heat dissipation experiment
Fig. 3. TEM images of MWCNTs.
environmental pollution due to its non-toxic and biodegradable features, so this study used the water-soluble chitosan as a dispersant for the MWCNTs/water nanofluid [7,19]. Second, we gradually added the MWCNTs into the base liquid step by step and up to the needed weight through a precision electronic balance (XS125A, Precisa) at a precision of 0.1 mg. Thirdly, in order to keep the nanoparticle with stable suspension in the base liquid, we alternately used an electromagnetic agitation (PC-420D, Corning), homogenizer (T25 digital, IKA), and ultrasonic vibrator (D400H, TOHAMA) for about 3 h to disperse the MWCNTs. The prepared MWCNTs/water nanofluid had three weight fractions (1.0, 2.0, 4.0 wt.%). Next, the MWCNT nano-coating was produced. First, we used the trial and error method to repeatedly test the optimal amount of solvent (MWCNTs/water nanofluid) to dilute the bulk material for easy spray, and thus obtained the optimal dilution of volume ratio at 40 vol.%. The ingredients of the different kinds of the dilution solvent are listed in Table 1. The bulk material is water-based paint (National Ltd. Co., Taiwan). The density of the bulk material is 1340 kg/m3, and its main ingredient is an acrylic polymer. The MWCNT nano-coating was dispersed several times by a homogenizer, electromagnetic stirrer, and ultrasonic vibrator in order to make it easy to be sprayed. We sprayed the MWCNT nano-coating of each different concentration twice times on each galvanized steel plate
In order to test the heat transfer performance of the galvanized steel plate coated with the MWCNTs, the test configuration was setup as shown in Fig. 4. In this test, the coated surface (front surface) of the experimental sample facing the hot plate (PC-420D, Corning) was heated at a fixed distance (250 mm) away from the hot plate at the different heating temperatures (60, 80, 100, 120, and 140 C), and two thermocouples were attached on the back surface of the sample to measure the temperature. Thus, the temperature of back surface of the test samples can determine the heat transfer performance. The accuracy of the K-type thermocouple is 0.1 C. In order to investigate the performance of the MWCNT nanocoating applied to heat dissipation for the projection lamp fixture, we implemented the heat dissipation performance experiment on the projection lamp fixture. Fig. 5 shows the structure of the projection lamp fixture in this study. This projection lamp fixture contains a cylindrical housing with vents thereon, vents (the lower and upper), a lamp, and a holder that can pose the cylindrical housing at different projecting angle. The projecting angle affects the heat dissipation of the vents due to direction change of the airflow in the vents. In order to investigate the effects of the MWCNT nano-coating and projecting angle (0 or 90 ) on heat dissipation of the fixture, we attached the K-type thermocouple with accuracy of 0.1 C on both the lamp socket and outer surface of the housing by high thermal conductivity adhesive (plastic steel epoxy 3344, PowerBon), and connected the thermocouple with a data logger (TRM-20, TOHO) to measure the temperature when the lamp was turned on. Before the experiment was implemented, we carefully selected three fixtures, three halogen lamps, and three LED lamps for prior test to make sure that the difference in illumination, power consumption, and internal temperature between the three fixtures with the lamp of the same kind is within 2.0%.
Table 1 Preparation parameters of MWCNTs nano-coating. Sample no.
Sample 1
Density of water-based paint (kg/m3) MWCNTs/water nanofluid (wt.%) Dilution of volume ratio (vol.%) MWCNTs concentration in coating (wt.%)
Non-coating
Sample 2
Sample 3
Sample 4
1340 1.0
2.0
4.0
40 0.332
0.664
1.328 Fig. 4. Setup to test heat transfer performance for MWCNTs coating.
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housing was coated by the paint containing the MWCNTs according to the procedure same as the above-mentioned background experiment. Finally, we compared the difference in the temperature, illumination, and power consumption of the projection lamp fixtures between those with and without the MWCNT nano-coating in order to investigate the effects of the MWCNT nano-coating on heat dissipation and lighting performance. Furthermore, In order to reduce experimental deviation, the environmental temperature was controlled at 25 0.5 C during the experiments. 3.3. Uncertainty and data analysis The uncertainty of the results of this experiment were determined by measuring the deviation in the parameters, including the illumination, power consumption, and temperature. Because we mainly evaluated the difference between the same fixture without coating and with coating, the deviation between the fixtures had little impact on evaluation. In the heat dissipation experiment for the projection lamp; the illumination (lx) was determined from readings of the illuminometer; the power consumption (pc) was measured by the digital power meter; the internal (Tin) and external (Tex) temperature was measured by the data logger; the ambient temperature (Ta) was controlled by the temperature controller of DC inverter air conditioner. The uncertainty of illumination, power consumption, and temperature in the heat dissipation experiment for the projection lamp experiment can be expressed as Eqs. (7)e(9), respectively.
Fig. 5. The structure of projection lamp fixture at different projecting angle. (a) 90 , (b) 0 .
First, we turned on the three fixtures with a halogen lamp (Decostar 51/50W/12V/38deg., OSRAM) for 70 min as background experiment, and then replaced the lamp with a LED lamp (MR-16/3.5W/ 12V/50deg., China Electric mfg. Corporation) for the same background experiment. The temperature data were averages of the data measured in the last 10 min of the background experiment. When the illuminating time reached 60 min, we started to measure both the illumination and power consumption of each fixture by an illuminometer (TES-1336A, TES) with accuracy of 3.0% and a digital power meter (WT230, Yokogawa) with accuracy of 0.35% for different lighting sources (halogen lamps and LED lamps) and projecting angles (0 and 90 ) in order to obtain the differences in power consumption and illumination on the surface at a fixed distance below the fixture. The data of illumination and power consumption were averages of the ten measurements with an interval of 1 min during the last 10 min in the experiment. Next, the inner surfaces of the cylindrical housing of the three fixtures were coated with the paint containing the MWCNTs of three different concentrations at 0.332, 0.664, and 1.328 wt.%, respectively. In order to obtain the uniform coating on the inner surface and reduce the thickness differences between the samples, we fixed the cylindrical housing horizontally and concentrically at a motor shaft to let the housing rotate at 600 rpm for 20 min to make the coated paint dry and uniform after the paint was quantitatively brush (1.0 ml) by quantitative dropper and rubber spatula. Then, the samples were dried in the oven at 50 C. Next, we conducted the heat dissipation experiment on the projection lamp fixtures whose
um;lx ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðDlx=lxÞ2 þðDTa =Ta Þ2
(7)
um;pc ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðDpc=pcÞ2 þðDTa =Ta Þ2
(8)
um;T ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðDT=TÞ2 þðDTa =Ta Þ2
(9)
The accuracy of the illuminometer is within 3.0%. The accuracy of the digital power meter is within 0.35%. The accuracy of the thermocouple for internal and external temperature of the projection lamp is within 0.1 C. The accuracy of the temperature controller of DC inverter air conditioner for ambient temperature is within 0.5 C. Therefore, the uncertainty of the illumination, power consumption, and temperature experiment is within 3.6%, 2.1%, and 2.1%, respectively. 3.4. Data analysis The measurement data of heat dissipation performance, illumination, and power consumption for the lamp fixture with uncoated housing are referred as baseline values (Db), and the measurement data for the lamp fixture with housing coated by paint containing the MWCNTs are marked as Dc. For explicit investigation, the experimental data Dc were compared with Db. The differences between Dc and Db are presented as proportions (R) and are expressed as follows:
R ¼ ðDc =Db Þ 100%:
(10)
4. Results and discussion Fig. 6 shows the reflection spectra of the samples. The sample 1 is bare galvanized steel plate without coating, sample 2e4 are coated by the paint containing the MWCNTs of 0.332, 0.664, and 1.328 wt.%, respectively. In the Fig. 6, we can find that adding
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Fig. 6. Reflection spectra of the test samples.
Fig. 8. Internal temperature trends of halogen lamp at different projecting angles.
the MWCNTs can reduce the reflectivity within UVeNIR band (350e950 nm), and the reflectivity decreases with increase in concentration of the MWCNTs. This phenomenon is mainly because the MWCNTs have excellent optical absorption within UVeNIR band. However, when concentration of the MWCNTs exceed above 0.664 wt.%, there is no significant difference in reflectivity because absorption reaches saturation. Fig. 7 shows the heat transfer performance of the samples. In Fig. 7, we can find that adding the MWCNTs can increase the temperature of the back surface of the sample, and the temperature increases with increase in concentration of the MWCNTs. For the same heating temperature of the hot plate, the higher back surface temperature means that there is greater heat flux through the sample, and the sample has lower thermal resistance; consequently providing better heat dissipation. This phenomenon is mainly
because the MWCNTs have both excellent optical absorption and high thermal conductivity (about 300e400 W/mK) to reduce thermal resistance in both radiation and conduction. As seen from Fig. 6, MWCNT nano-coating has high optical absorption such that the thermal resistance in radiation can be reduced. Further, because the overall thermal conductivity of the mixture depends on the thermal conductivity of each ingredient, adding the material with high thermal conductivity into the bulk material can increase the overall thermal conductivity [39,40]. Therefore, the total thermal resistance also reduces when the thermal resistance decreases in both radiation and heat conduction. Fig. 8 and Fig. 10 show the temperature of the lamp socket (called ‘internal temperature’ hereafter) of the lamp fixtures at different projecting angle in the background test, and the related data can be referred to Table 2. Figs. 9 and 11 show the temperature
Fig. 7. Heat transfer performance of the samples.
Fig. 9. External temperature trends of halogen lamp at different projecting angles.
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Fig. 10. Internal temperature trends of LED lamp at different projecting angles.
of the outer housing surface (called ‘external temperature’ hereafter) of the lamp fixtures at different projecting angle in the background test. As we can see from those figures, both the internal temperature and external temperature at projecting angle of 90 is higher than that at the projecting angle of 0 . This phenomenon can be explained as follows (please also refer to Fig. 5): at the projecting angle of 90 , the cold air enters into the bottom vents and the heated air discharges from the top vents according to the principle of heat convection. Thus, the heated air flows by the lamp socket and heats it; major portion of the heat generated by the lamp is transferred by convection, and minor portion is dissipated through the housing wall. In contrast, at the projecting angle of 0 , most of the heated air does not flow by the lamp socket, so the internal temperature is lower. Furthermore, at the projecting angle of 0 , the housing wall provides greater effective heat transfer area, and the heated air easily discharge from the vents on the housing wall, so the external temperature is also lower. From Figs. 8 and 9, we can find that the difference of internal and external temperature between 90 and 0 at 60 min after the halogen lamp was turned on is about 28 C and 10 C, respectively. From Figs. 10 and 11, we can find that the difference of internal and external temperature between 90 and 0 at 60 min after the LED lamp was turned on is about 3 C and 1.5 C, respectively. In order to investigate the effects of the MWCNT nano-coating on the internal temperature of the fixtures, the temperature of each fixture with coated housing is compared with that of the same fixture before its housing is coated. The three fixtures with inner housing wall coated by paint containing the MWCNTs of
concentration at 0.332, 0.664, and 1.328 wt.%, respectively are called ‘fixture 1’, ‘fixture 2’, and ‘fixture 3’, respectively. Fig. 12 shows the ratio of internal temperature of the fixture with coated housing to that of the corresponding fixture with uncoated housing. In Fig. 12, we can find that there is more significant effect for the fixtures at a projecting angle of 0 . For the halogen lamp, little effect is observed at a projecting angle of 90 . However, for the fixtures at a projecting angle of 0 , the internal temperature of the fixtures with inner housing wall coated by paint containing the MWCNTs of concentration at 0.332, 0.664 wt.% (fixture 1 and fixture 2) increases instead. It is because the bulk material of the paint has higher thermal resistance such that the total thermal resistance of the paint is still higher when the added the MWCNTs are fewer. When concentration of the added MWCNTs reaches 1.328 wt.%, the internal temperature is reduced 5.4% 2.1% as compared with the uncoated case. For the LED lamp, similarly, little effect is observed at projecting angle of 90 . For the fixtures at a projecting angle of 0 , the internal temperature is reduced 3.2% 2.1% for the fixture with inner housing wall coated by paint containing the MWCNTs of concentration at 1.328 wt.%. For both the halogen and LED lamps at a projecting angle of 0 , the internal temperature decreases with increase in concentration of the MWCNTs. Similarly, in order to investigate the effects of the MWCNT nanocoating on power consumption of the fixtures, the power consumption of each fixture with coated housing is compared with that of the same fixture before its housing is coated. Fig. 13 shows the ratio of power consumption of the fixture with coated housing to that of the corresponding fixture with uncoated housing. In Fig. 13, we can find that there is no effect of the MWCNT nanocoating on power consumption of the fixtures for the halogen lamp. In fact, the power consumption of halogen lamp is affected by temperature of the filament. In order to stabilize the power consumption of the lamp, the resistance of the filament is designed to increase with the increase in temperature. For such design, when the temperature of the filament increases due to increase in dissipation power, the resistance of the filament also increases such that the power consumption is suppressed. In contrast, when the temperature of the filament decreases due to decrease in dissipation power, the resistance of the filament decreases such that power consumption increases. It should be noted that the luminous efficiency decreases as the temperature of the filament decreases according to the blackbody irradiation principle. Therefore, although the lower temperature benefits the lifetime of the filament, the temperature is still set at the optimal regarding the luminous efficiency [41e43]. For the above reasons, we just cool down the local temperature of the lamp socket instead of the filament in this study. For the halogen lamp, just little portion of the heat generated by the filament dissipates through the lamp socket, so cooling down the local temperature of the lamp socket has very little impact on the filament temperature. It can be proved by checking if the output spectra of the lamp changes. In this study, the output spectra were monitored and had very little variation
Table 2 Experimental results of initial condition for baseline. Test parameters
Deg.
Halogen lamp
Illumination (lx)
90 0 90 0 90 0
9980 10,060 173.15 146.78 39.60 40.08
Internal temperature ( C) Power consumption (W)
9980 10,090 174.91 151.41 40.08 40.56
Note: Relative standard deviation (RSD) ¼ (standard deviation/mean) 100%.
9760 9860 170.19 153.18 38.64 39.12
RSD
LED lamp
1.05% 1.02% 1.13% 1.79% 1.52% 1.50%
556 554 51.81 48.69 3.88 3.92
RSD 554 535 52.89 49.48 3.88 3.90
545 533 50.96 48.03 3.88 3.90
0.87% 1.75% 1.52% 1.22% 0.00% 0.24%
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Fig. 11. External temperature trends of LED lamp at different projecting angles.
during the experiment. Since the filament temperature was not affected in the experiment, the power consumption does not change. In contrast, for the LED lamp, Fig. 13 shows the MWCNT nano-coating has some effect on the power consumption. Especially for the higher concentration of the MWCNT nano-coating, the power consumption reduces 12.9% 2.1% at most. It is mainly due to both the characteristic of LED chip and rectifier electronics integrated in the LED lamp. The junction temperature quite impacts the luminous efficiency and lifetime of the LED chip; the luminous efficiency and lifetime exponentially decreases as the junction temperature increases. Because LED is a ‘cold light’ source, the most heat generated from the LED chip is conducted out through the substrate. Thus, cooling down the temperature of the lamp socket
can also reduce the junction temperature of LED chip. Further, since LED needs DC electricity power, the related rectifier electronics are integrated on the bottom of the LED lamp. Thus, lower temperature of the lamp socket also can reduce the power consumption and benefit the lifetime of the electronics. Based on the above reasons, the MWCNT nano-coating has more obvious effect on power consumption of the LED lamps. Similarly, in order to investigate the effects of the MWCNT nanocoating on the illumination, the illumination of each fixture with coated housing is compared with that of the same fixture before its
Fig. 12. Internal temperature ratio of fixtures at different projecting angles.
Fig. 14. Illumination ratio of fixtures at different projecting angles.
Fig. 13. Power consumption ratio of fixtures at different projecting angles.
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housing is coated. Fig. 14 shows the ratio of illumination of the fixture with coated housing to that of the corresponding fixture with uncoated housing. In Fig. 14, we can find that there is little of the MWCNT nano-coating on illumination of the fixtures for either the halogen lamp or LED lamp. The difference in the experiment data is within the deviation of the illumination experiment (3.6%). In this study, the above results demonstrate that the MWCNT nano-coating can indeed reduce the internal temperature of the fixtures and reduce the power consumption (for LED lamps) while maintains the same luminous output. Therefore, this study will contribute to lifetime of the components, energy saving and carbon emission reduction. 5. Conclusions In this study, we quantitatively painted the inner housing wall of the projecting lamp fixtures with the MWCNT nano-coating of different concentration in order to reduce the internal temperature of the fixtures for the purpose of both extending the lifetime of the components and saving energy. The experimental results show that the internal temperature reduces 5.4% 2.1% (for the halogen lamp) and 3.2% 2.1% (for the LED lamp) at most for the fixture with housing coated by the MWCNT nano-coating of 1.328 wt.% at the projecting angle of 0 . The power consumption reduces 12.9% 2.1% (for the LED lamp) at most for the fixture with housing coated by the MWCNT nano-coating of 1.328 wt.% at the projecting angle of 0 . The MWCNT nano-coating has no effect on luminous output for either the halogen lamp or LED lamp. This study demonstrates that the MWCNT nano-coating has high feasibility in heat dissipation for lighting fixtures, which benefits the lifetime of the related components. Further, the MWCNTs can also extend to be applied to the cases requiring non-contact heat dissipation. Therefore, this study can widely contribute to energy saving and carbon emission reduction. Acknowledgements The authors would like to thank National Science Council of the Republic of China, Taiwan for their financial support to this research under contract no.: NSC-100-2221-E-003-029 and NSC-100-2221E-003-015, respectively. References [1] S.M.S. Murshed, K.C. Leong, C. Yang, Thermophysical and electrokinetic properties of nanofluids e a critical review, Appl. Therm. Eng. 28 (2008) 2109e2125. [2] S.U.S. Choi, Z.G. Zang, W. Yu, F.E. Lookwood, E.A. Grulke, Anomalous thermal conductivity enhancement in nanutube suspension, Appl. Phys. Lett. 79 (2001) 2252e2254. [3] H. Xie, H. Lee, W. Youn, M. Choi, Nanofluids containing multiwalled carbon nanotubes and their enhanced thermal conductivities, J. Appl. Phys. 94 (2003) 4967e4971. [4] P. Garg, J.L. Alvarado, C. Marsh, T.A. Carlson, D.A. Kessler, K. Annamalai, An experimental study on the effect of ultrasonication on viscosity and heat transfer performance of multi-wall carbon nanotube-based aqueous nanofluids, Int. J. Heat Mass Transf. 52 (2009) 5090e5101. [5] L. Chen, H. Xie, Y. Li, W. Yu, Nanofluids containing carbon nanotubes treated by mechanochemical reaction, Thermochim. Acta 477 (2008) 21e24. [6] L. Chen, H. Xie, Surfactant-free nanofluids containing double- and singlewalled carbon nanotubes functionalized by a wet-mechanochemical reaction, Thermochim. Acta 497 (2010) 67e71. [7] T.X. Phuoc, M. Massoudi, R.H. Chen, Viscosity and thermal conductivity of nanofluids containing multi-walled carbon nanotubes stabilized by chitosan, Int. J. Therm. Sci. 50 (2011) 12e18. [8] C.H. Liu, H. Huang, Y. Wu, S.S. Fan, Thermal conductivity improvement of silicone elastomer with carbon nanotube loading, Appl. Phys. Lett. 84 (2004) 4248e4250. [9] G.D. Zhan, J.D. Kuntz, J. Wan, A.K. Mukherjee, Single-wall carbon nanotubes as attractive toughening agents in alumina-based nanocomposites, Nat. Mater. 2 (2003) 38e42.
[10] R. Sivakumar, S.Q. Guo, T. Nishimura, Y. Kagawa, Thermal conductivity in multi-wall carbon nanotube/silica-based nanocomposites, Scr. Mater. 56 (2007) 265e268. [11] K. Chu, Q. Wu, C. Jia, X. Liang, J. Nie, W. Tian, G. Gai, H. Guo, Fabrication and effective thermal conductivity of multi-walled carbon nanotubes reinforced Cu matrix composites for heat sink applications, Compos. Sci. Technol. 70 (2010) 298e304. [12] M.J. Biercuk, M.C. Llaguno, M. Radosavljevic, J.K. Hyun, A.T. Johnson, Carbon nanotube composites for thermal management, Appl. Phys. Lett. 80 (2002) 2767e2769. [13] T.P. Teng, B.G. Lin, Y.Y. Yeh, Characterization of heat storage by nanocomposite-enhanced phase change materials, Adv. Mater. Res. 287e290 (2011) 1448e1455. [14] D.D.L. Chung, Materials for thermal conduction, Appl. Therm. Eng. 21 (2001) 1593e1605. [15] Y. Xu, C.K. Leong, D.D.L. Chung, Carbon nanotube thermal pastes for improving thermal contacts, J. Electron. Mater. 36 (2007) 1181e1187. [16] C. Kim, B. Lim, B. Kim, U. Shim, S. Oh, B. Sung, J. Choi, J. Ki, S. Baik, Strengthening of copper matrix composites by nickel-coated singlewalled carbon nanotube reinforcements, Synth. Met. 159 (2009) 424e429. [17] C.W. Nan, R. Birringer, Determining the Kapitza resistance and the thermal conductivity of polycrystals: a simple model, Phys. Rev. B 57 (1998) 8264e8268. [18] Y. Ding, H. Alias, D. Wen, R.A. Williams, Heat transfer of aqueous suspensions of carbon nanotubes (CNT nanofluids), Int. J. Heat Mass. Transf. 49 (2006) 240e250. [19] Y.H. Hung, T.P. Teng, T.C. Teng, J.H. Chen, Assessment of heat dissipation performance for nanofluid, Appl. Therm. Eng. 32 (2012) 132e140. [20] L.A. Girifalco, M. Hodak, R.S. Lee, Carbon nanotubes, buckyballs, ropes, and a universal graphic potential, Phys. Rev. B 62 (2000) 13104e13110. [21] T. Lin, V. Bajpai, T. Ji, L. Dai, Chemistry of carbon nanotubes, Aust. J. Chem. 56 (2003) 635e651. [22] C. Park, Z. Ounaies, K.A. Watson, R.E. Crooks, J.J. Smith, S.E. Lowther, Dispersion of single wall carbon nanotubes by in situ polymerization under sonication, Chem. Phys. Lett. 364 (2002) 303e308. [23] G.H. Ko, K. Heo, K. Lee, D.S. Kim, C. Kim, Y. Sohn, M. Choi, An experimental study on the pressure drop of nanofluids containing carbon nanotubes in a horizontal tube, Int. J. Heat Mass. Transf. 50 (2007) 4749e4753. [24] L. Jiang, L. Gao, J. Sun, Production of aqueous colloidal dispersions of carbon nanotubes, J. Colloid Interface Sci. 260 (2003) 89e94. [25] OECDJIEA, Light’s Labour’s Lost: Policies for Energy-efficient Lighting. Paris Cedex, France (2006). [26] R. Devonshire, The competitive technology environment for LED lighting, J. Light Vis. Environ. 32 (2008) 275e287. [27] N. Zheludev, The life and times of the LED- a 100-year history, Nat. Photonics 1 (2007) 189e192. [28] Nature Photonics Editorial, Haitz’s law, Nat. Photonics 1 (2007) 23. [29] CREE, Cree breaks 200 lumen per watt efficacy barrier (Press release), CREE, Durham, N.C, 2010. [30] M. Richards, D. Carter, Good lighting with less energy: where next? Lighting Res. Technol. 41 (2009) 285e286. [31] A.A. Efremov, N.I. Bochkareva, R.I. Gorbunov, D.A. Lavrinovich, Y.T. Rebane, D.V. Tarkhin, Y.G. Shreter, Effect of the joule heating on the quantum efficiency and choice of thermal conditions for high-power blue InGaN/GaN LEDs, Semiconductors 40 (2006) 605e610. [32] S.L. Chuang, A. Ishibashi, S. Kijima, N. Nakayama, M. Ukita, S. Taniguchi, Kinetic model for degradation of light-emitting diodes, IEEE J. Quantum Electron. 33 (1997) 970e979. [33] Philips and Lumileds Co. Ltd., Understanding Power LED Lifetime Analysis, in: Technology White Paper, Philips and Lumileds Co. Ltd., 2007. [34] E. Hong, N. Narendran, A method for projecting useful life of LED lighting systems, 3rd International Conference on Solid State Lighting, Proc. SPIE 5187 (2004) 93e99. [35] J. Hu, L. Yang, M.W. Shin, Mechanism and thermal effect of delamination in light-emitting diode packages, Microelectron. J. 38 (2007) 157e163. [36] M. Bürmen, F. Pernus, B. Likar, Prediction of intensity and color degradation of LEDs, Proc. SPIE 6486 (2007) 64860M1e10. [37] J.H. Choi, M.W. Shin, Thermal investigation of LED lighting module, Microelectron. Reliab. (2011). http://dx.doi.org/10.1016/j.microrel.2011.04.009. [38] D. Loe, Energy efficiency in lighting e considerations and possibilities, Lighting Res. Technol. 41 (2009) 209e218. [39] J.C. Maxwell, A Treatise on Electricity and Magnetism, third ed., vol. 1, Clarendon, Oxford, 1904. [40] R.L. Hamilton, O.K. Crosser, Thermal conductivity of heterogeneous two component systems, I&EC Fundam. 1 (1962) 182e191. [41] S. Garbe, S. Hanloh, Growth of potassium-filled bubbles in doped tungsten and its relation to hot spot development and intergranular fracture, Philips J. Res. 38 (1983) 248e262. [42] Publication Number TP-110Incandescent Lamps, General Electric Company, Nela Park, Cleveland, OH, 1964. [43] R. Kane, Heinz Sell Revolution in Lamps: A Chronicle of 50 Years of Progress, second ed., The Fairmont Press, Inc, 2001.