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Now showing 1 - 8 of 8

Silicon Nanowires for Solar Thermal Energy Harvesting: an Experimental Evaluation on the Trade-off Effects of the Spectral Optical Properties
(2016-01-04)
Sekone, Abdoul Karim
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Chen, Yu-Bin
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Lu, Ming-Chang
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Chen, Wen-Kai
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Liu, Chia-An
Silicon nanowire possesses great potential as the material for renewable energy harvesting and conversion. The significantly reduced spectral reflectivity of silicon nanowire to visible light makes it even more attractive in solar energy applications. However, the benefit of its use for solar thermal energy harvesting remains to be investigated and has so far not been clearly reported. The purpose of this study is to provide practical information and insight into the performance of silicon nanowires in solar thermal energy conversion systems. Spectral hemispherical reflectivity and transmissivity of the black silicon nanowire array on silicon wafer substrate were measured. It was observed that the reflectivity is lower in the visible range but higher in the infrared range compared to the plain silicon wafer. A drying experiment and a theoretical calculation were carried out to directly evaluate the effects of the trade-off between scattering properties at different wavelengths. It is clearly seen that silicon nanowires can improve the solar thermal energy harnessing. The results showed that a 17.8 % increase in the harvest and utilization of solar thermal energy could be achieved using a silicon nanowire array on silicon substrate as compared to that obtained with a plain silicon wafer.
Scale Effect on Dropwise Condensation on Superhydrophobic Surfaces
(2014-08-27)
Lo, Ching-Wen
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Wang, Chi-Chuan
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Lu, Ming-Chang
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機械工程學系
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Department of Mechanical Engineering
Micro/nano (two-tier) structures are often employed to achieve superhydrophobicity. In condensation, utilizing such a surface is not necessarily advantageous because the macroscopically observed Cassie droplets are usually in fact partial Wenzel in condensation. The increase in contact angle through introducing microstructures on such two-tier roughened surfaces may result in an increase in droplet departure diameter and consequently deteriorate the performance. In the meantime, nanostructure roughened surfaces could potentially yield efficient shedding of liquid droplets, whereas microstructures roughened surfaces often lead to highly pinned Wenzel droplets. To attain efficient shedding of liquid droplets in condensation on a superhydrophobic surface, a Bond number (a dimensionless number for appraising drop-wise condensation) and a solid-liquid fraction smaller than 0.1 and 0.3, respectively, are suggested.
A solar-thermal energy harvesting scheme: enhanced heat capacity of molten HITEC salt mixed with Sn/SiOx core-shell nanoparticles
(2014)
Lai, Chih-Chung
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Chang, Wen-Chih
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Hu, Wen-Liang
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Wang, Zhiming M.
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Lu, Ming-Chang
We demonstrated enhanced solar-thermal storage by releasing the latent heat of Sn/SiOx core-shell nanoparticles (NPs) embedded in a eutectic salt. The microstructures and chemical compositions of Sn/SiOx core-shell NPs were characterized. In situ heating XRD provides dynamic crystalline information about the Sn/SiOx core-shell NPs during cyclic heating processes. The latent heat of similar to 29 J g(-1) for Sn/SiOx core-shell NPs was measured, and 30% enhanced heat capacity was achieved from 1.57 to 2.03 J g (1) K (1) for the HITEC solar salt without and with, respectively, a mixture of 5% Sn/SiOx core-shell NPs. In addition, an endurance cycle test was performed to prove a stable operation in practical applications. The approach provides a method to enhance energy storage in solar-thermal power plants.
Spatial Control of Heterogeneous Nucleation on the Superhydrophobic Nanowire Array
(2014-03-01)
Lo, Ching-Wen
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Wang, Chi-Chuan
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Lu, Ming-Chang
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機械工程學系
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Department of Mechanical Engineering
Condensation is a common phenomenon and is widely exploited in power generation and refrigeration devices. Although drop-wise condensation offers high heat and mass transfer rates, it is extremely difficult to maintain and control. In this study, the ability to spatially control heterogeneous nucleation on a superhydrophobic surface by manipulating the free energy barrier to nucleation through parameterizing regional roughness scale on the Si nanowire array-coated surface is reported. Water vapor preferentially condenses on the designed microgrooves on the Si nanowire surface and continuous shedding of the drop-wise condensate is observed on the surface. The nucleation site density can also be manipulated by tailoring the density of the microgroove on the surface. Moreover, the cycle time on the Si nanowire array with microgrooves is approximately ten times smaller than that on a plain Si surface. This suggests that potentially high heat and mass transfer rates can be achieved on the surface. The insight from this study has implications in enhancing energy efficiency in a wide range of thermal energy conversion systems.
Investigation of the two-phase convective boiling of HFO-1234yf in a 3.9 mm diameter tube
(2013-10-01)
Lu, Ming-Chang
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Tong, Jing-Rei
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Wang, Chi-Chuan
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機械工程學系
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Department of Mechanical Engineering
In this study, the influences of heat flux and mass flux on the two-phase convective boiling heat transfer performance are reported for refrigerants HFO-1234yf and HFC-134a in a 3.9 mm smooth diameter tube. Tests are performed with a saturation temperature of 10 degrees C. It is found that at lower vapor quality region the nucleate boiling is the dominant heat transfer mechanism while the convective evaporation mechanism takes control at the higher vapor quality region. Both HFC-134a and HFO-1234yfshows similar trend and the difference in heat transfer coefficient between HFO-1234 and HFC-134a is quite small. The comparable heat transfer performance between HFC-134a and HFO-1234yf is attributed to similar physical properties and nucleate boiling contribution. The present test results are in line with some existing reports but are inconsistent with one other study having a tube diameter of 1.1 mm. It is found that the departure of heat transfer coefficients between the available publications is mainly attributed to the different flow phenomena caused by the difference of the channel size and channel geometry. A noticeable deterioration of the heat transfer coefficient for HFO-1234yf is encountered in the microchannel. The pressure drops for HFC-134a is about 5-15% higher than that of HFO-1234yf. (C) 2013 Elsevier Ltd. All rights reserved.
Specific heat capacity of molten salt-based alumina nanofluid
(2013-06-21)
Lu, Ming-Chang
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Huang, Chien-Hsun
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機械工程學系
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Department of Mechanical Engineering
There is no consensus on the effect of nanoparticle (NP) addition on the specific heat capacity (SHC) of fluids. In addition, the predictions from the existing model have a large discrepancy from the measured SHCs in nanofluids. We show that the SHC of the molten salt-based alumina nanofluid decreases with reducing particle size and increasing particle concentration. The NP size-dependent SHC is resulted from an augmentation of the nanolayer effect as particle size reduces. A model considering the nanolayer effect which supports the experimental results was proposed.
Enhanced Heat Transfer in Biporous Wicks in the Thin Liquid Film Evaporation and Boiling Regimes
(2012-10-01)
Coso, Dusan
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Srinivasan, Vinod
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Lu, Ming-Chang
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Chang, Je-Young
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Majumdar, Arun
Biporous media consisting of microscale pin fins separated by microchannels are examined as candidate structures for the evaporator wick of a vapor chamber heat pipe. The structures are fabricated out of silicon using standard lithography and etching techniques. Pores which separate microscale pin fins are used to generate high capillary suction, while larger microchannels are used to reduce overall flow resistance. The heat transfer coefficient is found to depend on the area coverage of a liquid film with thickness on the order of a few microns near the meniscus of the triple phase contact line. We manipulate the area coverage and film thickness by varying the surface area-to-volume ratio through the use of microstructuring. Experiments are conducted for a heater area of 1 cm(2) with the wick in a vertical orientation. Results are presented for structures with approximately same porosities, fixed microchannel widths w approximate to 30 mu m and w approximate to 60 mu m, and pin fin diameters ranging from d = 3-29 mu m. The competing effects of increase in surface area due to microstructuring and the suppression of evaporation due to reduction in pore scale are explored. In some samples, a transition from evaporative heat transfer to nucleate boiling is observed. While it is difficult to identify when the transition occurs, one can identify regimes where evaporation dominates over nucleate boiling and vice versa. Heat transfer coefficients of 20.7 (+/- 2.4) W/cm(2)-K are attained at heat fluxes of 119.6 (+/- 4.2) W/cm(2) until the wick dries out in the evaporation dominated regime. In the nucleate boiling dominated regime, heat fluxes of 277.0 (+/- 9.7) W/cm(2) can be dissipated by wicks with heaters of area 1 cm(2), while heat fluxes up to 733.1 (+/- 103.4) W/cm(2) can be dissipated by wicks with smaller heaters intended to simulate local hot-spots. [DOI: 10.1115/1.4006106]
Critical heat flux of pool boiling on Si nanowire array-coated surfaces
(2011-12-01)
Lu, Ming-Chang
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Chen, Renkun
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Srinivasan, Vinod
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Carey, Van P.
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Majumdar, Arun
Pool boiling of saturated water on plain Si surfaces and surfaces covered with a dense array of Si nanowires (SiNWs) has been studied. Measured CHF and heat transfer coefficient (HTC) values of about 223 +/- 5.61 W/cm(2) and 9 +/- 1.60 W/cm(2) K. respectively, on the nanowire array-coated surface are among the highest reported in boiling heat transfer. Meanwhile, the CHFs on both nanowire-coated and Plain Si surfaces show a similar heater size dependence - the CHF increases as heater size decreases. The measured CHFs on both types of surfaces are approximately following the prediction of the hydrodynamic theory assuming the Helmholtz wavelength equal to the corresponded heater length. It suggests that the CHFs on both types of surfaces might be limited by pool hydrodynamics. (C) 2011 Elsevier Ltd. All rights reserved.

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