Browsing by Author "Yan, WM"

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Analysis of thermal and water management with temperature-dependent diffusion effects in membrane of proton exchange membrane fuel cells
(2004-04-22)
Yan, WM
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Chen, FL
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Wu, HY
;
Soong, CY
;
Chu, HS
In the present work, the detailed thermal and water management in the membrane of proton exchange membrane fuel cells (PEMFC) is investigated numerically. The coupling effects of mass diffusion and temperature gradient on the water distribution in the membrane are taken into account with consideration of the temperature-dependent diffusivity. Thermal and water transport equations with various boundary conditions are solved by the control volume finite difference method. Predictions show that under the conditions of fixed water concentration at the cathode side, the effect of cathode temperature, T-c, on the water concentration is significant. Increases in T-c may lead to an increase in membrane dehydration. At the water-flux condition on the cathode side, the influence of the operating temperature on the water distribution in the membrane shows a similar trend. The effects of the anode temperature, T-a, on the water management in the membrane are also examined. It is found that T-a has considerable impact on the water content in the membrane. In addition, high current density may cause non-uniformity of the temperature distribution in the membrane. (C) 2003 Elsevier B.V. All rights reserved.
Effects of flow distributor geometry and diffusion layer porosity on reactant gas transport and performance of proton exchange membrane fuel cells
(2004-01-02)
Yan, WM
;
Soong, CY
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Chen, FL
;
Chu, HS
;
機械工程學系
A study has been conducted on the effects of flow distributor and diffusion layer geometries and morphologies on the transport phenomena of reactant gas from flow channels to gas diffuser, and on the performance of a proton exchange membrane fuel cell. The emphasis of the analysis is placed on the effects of the cross-section of the channel of the anode flow distributor and the porosity of the gas-diffusion layer. By applying a two-dimensional mass transport model, the effects of the channel width fraction, lambda= l(c)/l(b) the channel number, N, the porosity of the diffuser layer, E, and the surface overpotential of the catalyst layer, eta, on the resultant current density are investigated. The cell performance is evaluated by the predicted voltage-current density curves. It is disclosed that an increase in either;, Nor E may lead to a better cell performance. At relatively low overpotential, better uniformity in current density distribution along the width of the cell (x-direction) can be attained. Based on the present results, a correlation of overpotentials is proposed, namely, eta(cr) = 0.165lambda(0.0935) N-0.2033 epsilon(0.0856), which may serve as a guideline to justify the validity of a simplified one-dimensional model. (C) 2003 Elsevier B.V. All rights reserved.
Transient analysis of reactant gas transport and performance of PEM fuel cells
(2005-04-27)
Yan, WM
;
Soong, CY
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Chen, FL
;
Chu, HS
;
機械工程學系
The dynamic performance of PEM fuel cells is one of the most important criteria in the design of fuel cells with application to mobile systems. To study this issue, we extend our previous steady model of gas reactant transport to an unsteady one and employ it to examine the transient transport characteristics and the system performance of the PEM fuel cells. With the assumption of the two-dimensional mass transport in the cathode side of PEMFC, the effects of the channel width fraction, lambda = l(c)/l(b), the porosity of the gas diffuser layer, epsilon(1), and the surface overpotential of the catalyst layer, eta, on the transient characteristics of the resultant current density and mass transport are focused in this work. It is disclosed that an increase in lambda, or eta may lead to a faster dynamic response for the fuel cell when the PEM fuel cell system is started up. Results of a typical case demonstrate that, although the dynamic response time may be as long as 10 s due to the mass transport lag, the fuel cell system needs only less than 0.4 s to reach the 90% response. (c) 2005 Elsevier B.V. All rights reserved.
Transient behavior of water transport in the membrane of a PEM fuel cell
(2004-05-01)
Chen, FL
;
Su, YG
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Soong, CY
;
Yan, WM
;
Chu, HS
The dynamic performance of a PEM fuel cell is one of the most important criteria in the design of fuel cells, especially when the application of the fuel cell in mobile systems is concerned. To attack this issue, we extend the theoretical model developed by Okada (J. Electroanal. Chem. 465 (1999) 1,18) to an unsteady state model and investigate the transient behavior of water transport across the membrane as well as the influences of several physical parameters on the characteristic time to reach the steady state. We also consider the influence due to the presence of foreign impurity ions, which turn Out to be a crucial factor affecting the unsteady state features of water transport across the membrane. The results suggest that a higher initial current density to start the operation, a higher water flux from the anode side, it smaller operational current density, and a lower level of contamination in the membrane (especially at the cathode side) can all result in a shorter time for the water transport to reach the steady state, and thus a better dynamic performance of the fuel cell can be obtained. (C) 2003 Elsevier B.V. All rights reserved.