• 한국신재생에너지학회:학술대회논문집
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• 2011.05a
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• pp.112.1-112.1
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• 2011

The back contact solar cell (BCSC) has several advantages compared to the conventional solar cell since it can reduce grid shadowing loss and contact resistance between the electrode and the silicon substrate. This paper presents the effect of the surface texturing of the silicon BCSC by varying the texturing depth or the texturing gap in the commercially available simulation software, ATHENA and ATLAS of the company SILVACO. The texturing depth was varied from $5{\mu}m$ to $150{\mu}m$ and the texturing gap was varied from $1{\mu}m$ to $100{\mu}m$ in the simulation. The resulting efficiency of the silicon BCSC was evaluated depending on the texturing condition. The quantum efficiency and the I-V curve of the designed silicon BCSC was also obtained for the analysis since they are closely related with the solar cell efficiency. Other parameters of the simulated silicon BCSC are as follows. The substrate was an n-type silicon, which was doped with phosphorous at $6{\times}10^{15}cm^{-3}$, and its thickness was $180{\mu}m$, a typical thickness of commercial solar cell substrate thickness. The back surface field (BSF) was $1{\times}10^{20}\;cm^{-3}$ and the doping concentration of a boron doped emitter was $8.5{\times}10^{19}\;cm^{-3}$. The pitch of the silicon BCSC was $1250{\mu}m$ and the anti-reflection coating (ARC) SiN thickness was $0.079{\mu}m$. It was assumed that the texturing was anisotropic etching of crystalline silicon, resulting in texturing angle of 54.7 degrees. The best efficiency was 25.6264% when texturing depth was $50{\mu}m$ with zero texturing gap in case of low texturing depth (< $100{\mu}m$).

• Journal of the Korean Society of Manufacturing Technology Engineers
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• v.25 no.6
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• pp.410-420
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• 2016

The geometric dimension and shape of microchannels that are formed during surface texturing are widely studied for applications in flow control, and drag and friction reduction. In this research, a new method for controlling the deformation of U channels during micro-rolling-based surface texturing was developed. Since the width of the U channels is almost constant, controlling the depth is essential. A calibration procedure of initial rolling gap, and proportional-integral PI controllers and a linear interpolation have been applied simultaneously to control the depth. The PI controllers drive the position of the pre-U grooved roll as well as the rolling gap. The relationship between the channel depth and rolling gap is linearized to create a feedback signal in the depth control system. The depth of micro channels is studied on A2021 aluminum lamina surfaces. Overall, the experimental results demonstrated the feasibility of the method for controlling the depth of microchannels.

• Tribology and Lubricants
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• v.25 no.5
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• pp.305-310
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• 2009

Laser surface texturing (LST) methods are applied recently to generate micro-dimples in machine components having parallel sliding surfaces such as thrust bearings, mechanical face seals and piston rings, etc. And it is experimentally reported by several researchers that the micro-dimpled bearing surfaces can reduce friction force. Until now, however, theoretical results for various dimple parameters are not fully presented. In this paper, a commercial computational fluid dynamics (CFD) code, FLUENT is used to investigate the effect of dimple depth on the lubrication characteristics of parallel thrust bearing. The results show that the pressure, velocity and density distributions within dimples are highly affected by dimple depths and cavitation conditions. Adoption of micro-dimple on the bearing surface can reduce the friction force highly and its levels are affected by dimple depth. The numerical methods and results can be use in design of optimum dimple characteristics to improve thrust bearing performance.

• Tribology and Lubricants
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• v.39 no.1
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• pp.21-27
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• 2023

Surface texturing is widely applied to friction surfaces of various machine elements. Most of the theoretical studies have focused on isothermal (ISO) analyses which consider constant lubricant viscosity. However, there have been limited studies on the effect of oil temperature increase owing to viscous shear. Following the first part of the present study that investigated the effects of film-temperature boundary condition (FTBC) and groove number on the thermohydrodynamic (THD) lubrication characteristics of a surface-textured parallel thrust bearing with multiple rectangular grooves, this study focuses on the effect of groove depths. Current study numerically analyzes the continuity, Navier-Stokes, and energy equations with temperature-viscosity-density relations using a commercial computational fluid dynamics (CFD) software, FLUENT. The results of variation in temperature, velocity, and pressure distributions as well as load-carrying capacity (LCC) and friction force indicate that groove depth and FTBC significantly influence the temperature distribution and pressure generation. The LCC is maximum near the groove depth at which the vortex starts, smaller than the ISO result. For intense grooves, the LCC of THD may be larger than that from ISO. The frictional force decreases as the groove becomes deeper, and decreases more significantly in the case of THD. The study shows that groove depth significantly influences the THD lubrication characteristics of surface-textured parallel thrust bearings.

• Tribology and Lubricants
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• v.30 no.5
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• pp.303-310
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• 2014

In order to reduce friction and improve reliability, researchers have applied various surface texturing methods to highly sliding machine elements such as mechanical seals and piston rings. Despite extensive theoretical research on surface texturing, previous numerical results are only applicable to isothermal and iso-viscous conditions. Because the lubricant flow pattern of textured bearing surfaces is much more complicated than that for non-textured bearings, the Navier?Stokes equation is more suitable than the Reynolds equation for the former. This study carries out a thermohydrodynamic (THD) lubrication analysis to investigate the lubrication characteristics of a single micro-dimpled parallel thrust bearing cell. The analysis involves using the continuity, Navier?Stokes, energy, temperature?viscosity relation, and heat conduction equations with the commercial computational fluid dynamics (CFD) code FLUENT. This study discretizes these equations using the finite volume method and solves them using the SIMPLE algorithm. The results include finding the streamlines, pressure and temperature distributions, and variations in the friction force and leakage for various dimple radii and depths. Increasing the dimple radius and decreasing the depth causes a recirculation flow to form because of a strong vortex, and the oil temperature greatly increases compared with the non-textured case. The present numerical scheme and results are applicable to THD analysis of various surface-textured sliding bearings and can lead to further study.

• Tribology and Lubricants
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• v.33 no.6
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• pp.288-295
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• 2017

In order to improve the efficiency and reliability of the machine, the friction should be minimized. The most widely used method to minimize friction is to maintain the fluid lubrication state. However, we can reduce friction only up to a certain limit because of viscosity. As a result of several recent studies, surface texturing has significantly reduced the friction in highly sliding machine elements, such as mechanical seals and thrust bearings. Thus far, theoretical studies have mainly focused on isothermal/iso-viscous conditions and have not taken into account the heat generation, caused by high viscous shear, and the temperature conditions on the bearing surface. In this study, we investigate the effect of dimple depth and film-temperature boundary conditions on the thermohydrodynamic (THD) lubrication of textured parallel slider bearings. We analyzed the continuity equation, the Navier-Stokes equation, the energy equation, and the temperature-viscosity and temperature-density relations using a computational fluid dynamics (CFD) code, FLUENT. We compare the temperature and pressure distributions at various dimple depths. The increase in oil temperature caused by viscous shear was higher in the dimple than in the bearing outlet because of the action of the strong vortex generated in the dimple. The lubrication characteristics significantly change with variations in the dimple depths and film-temperature boundary conditions. We can use the current results as basic data for optimum surface texturing; however, further studies are required for various temperature boundary conditions.

• 한국전산유체공학회:학술대회논문집
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• 2009.04a
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• pp.363-367
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• 2009

Recently, the manufacturing of micro-cavity by means of laser surface texturing (LST) technique and low friction study by the LST have been in great progress. Most of current works have been dealing with the effect of cavity on friction and wear. The main objective of the present study was to investigate numerically two-dimensional lubrication characteristics of micro-dimple shapes fabricated on solid surfaces, and this study utilized the commercial CFD code (Fluent V.6.3). For the evaluation, preliminary simulation was conducted and numerical predictions were compared with the analytic solution obtained from the Reynolds's equation. Mainly, the present study investigated the influence of dimple depth, pattern shapes, and film thickness on lubrication characteristics related to the reduction of friction. It is found that the existence of micro-dimpled surface makes it possible to substantially reduce the friction forces exerted on the surfaces. In particular, substantial decrease in shear stresses was observed as the lubricant film thickness decreases. For instance, in the case of the film thickness of 0.01 mm, the estimated shear stress decreases up to about 40%. It indicates that the film thickness would be important factor in designing the micro-dimpled surfaces. Furthermore, it was observed that such a optimum dimple depth would be present because the dimple depth larger than the optimum value did no longer affect the reduction in shear stresses.

• Journal of the Korean Society of Manufacturing Process Engineers
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• v.20 no.2
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• pp.23-31
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• 2021

Surface texturing is a promising route to reduce the friction forces between two surfaces in sliding contact. To this end, the fabrication of micro dimples is one of the most widely used surface texturing methods. According to published results, textured surfaces with elliptical micro dimples offer the best friction performance. Therefore, we fabricated elliptical micro dimples on carbon steel (SM45C) by using dual frequency vibration assisted machining. High and low frequencies of 16.3 kHz and 230 Hz were applied to the 3D resonant elliptical vibrator. The 3D resonant elliptical vibrator with a triangular cubic boron nitride insert was assembled on a computer numerically controlled turning lathe. Oval micro dimples of various profiles were manufactured on carbon steel. In terms of the profile of the elliptical micro dimples, the experimental results indicated that the average micro dimple width and depth were 112 ㎛ and 7.7 ㎛. These dimensions are closely related to the cutting conditions and can be easily controlled.

• Journal of the Korean Vacuum Society
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• v.20 no.3
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• pp.225-232
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• 2011

Reactive ion etching (RIE) technique for maskless surface texturing of mc-silicon solar wafers has been applied and succeed in fabricating a grass-like black-silicon with an average reflectance of $4{\pm}1%$ in a wavelength range of 300~1,200 nm. In order to investigate the optimized texturing conditions for mass production of high quantum efficiency solar cell Surface characteristics such as the spatial distribution of average reflectance, micrscopic surface morphology and minority carrier lifetime were monitored for samples from saw-damaged $15.6{\times}15.6\;cm^2$ bare wafer to key-processed wafers as well as the mc-Si solar cells. We observed that RIE textured wafers reveal lower average reflectance along from center to edges by 1% and referred the origin to the non-uniform surface structures with a depth of 2 times deeper and half-maximum width of 3 times. Samples with anti-reflection coating after forming emitter layer also revealed longer minority carrier lifetime by 40% for the edge compared to wafer center due to size effects. As results, mc-Si solar cells with RIE-textured surface also revealed higher efficiency by 2% and better external quantum efficiency by 15% for edge positions with higher height.

• The Transactions of the Korean Institute of Electrical Engineers C
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• v.52 no.11
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• pp.496-500
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• 2003

A low-cost, large area, random, maskless texturing scheme independent of crystal orientation is expected to significantly impact terrestrial photovoltaic technology. We investigated silicon surface microstructures formed by reactive ion etching (RIE) in Multi-Hollow cathode system. Desirable texturing effect has been achieved when radio-frequency (rf) power of about 20 Watt per one hollow cathode glow is applied for our RF Multi-Hollow cathode system. The black silicon etched surface shows almost zero reflectance in the visible region as well as in near IR region. The etched silicon surface is covered by columnar microstructures with diameters from 50 to 100 nm and depth of about 500 nm. We have successfully achieved 11.7% efficiency of mono-crystalline silicon solar cell and 10.2% multi-crystalline silicon solar cell.