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Two-dimensional numerical simulation of a DMFC (Direct Methanol Fuel Cell)

Collaborator: J. Fuhrmann , K. Gärtner

Cooperation with: J. Divisek, H. Dohle (Forschungszentrum Jülich GmbH, IWV-3)

Supported by: IWV-3 Jülich

Description: During the third year of this project (see Annual Research Report 2001 ) we focused on:

1.

The verification of the model and the calibration of some of its parameters with respect to experiments. This process was partially automated by the application of a least-squares fit of IV-curves or other parameter-dependent functionals.

2.

The numerical study of the influence of geometry variations on the fuel cell performance;

3.

A model taking into account the influence of contact plate designs on the fuel cell performance.

Summary of the status reached:

The essential problem is the precision, some of the model parameters are known. On the other hand one has a very limited amount of experimental data. The experimental setups aim on the normal working conditions of a fuel cell. Parameter verification would be supported much better by an enlarged range of parameter variations that clearly addresses different limiting conditions and special nonlinear effects.

The computations show a significant influence of the fuel cell geometry on the cathodic reaction rates, especially due to temperature feedback effects. This points to carefully designed experiments which could be verified by two-dimensional computations, and a possible need for further model improvements regarding the heat conduction model (boundary conditions, temperature coefficients of the oxygen kinetics), and the concurrency for free catalytic sites at the cathode (oxygen/H⁺ versus the parasitic methanol oxidation).

Another topic of interest is to estimate the influence of contact plate designs on the fuel cell performance. Mechanical and production constraints limit the area exposed to oxygen and fuel while the contact plate material has to collect the electrons on the other hand. Due to the typical geometric situation (the height of the contact plate (z direction) is approximately two orders of magnitude smaller than the other dimensions ($x,\;y$)) the following simplified (height-integrated) model was solved numerically:

$$-\nabla \cdot \tilde \sigma_i(x) \nabla \phi_i = (-1)^{i+1}\tilde \rho(\phi_1- \phi_2,x),$$

i=1,2, 1: anode, 2: cathode, $\phi_i(x,y)$ electrostatic potential, $\tilde \rho(\phi_1-\phi_2,x)$ IV-characteristics of the MEA (Membrane-Electrode Assembly, corrected with respect to horizontal losses in the graphite). The functions describing the effective conductivities are given for the metal-covered zone by $\tilde \sigma=h_{metal} \sigma_{metal} + h_{graphite} \sigma_{graphite}$, and the fuel or air region $\tilde \sigma=h_{graphite} \sigma_{graphite}$ respectively. The reaction rate (electron source/sink) in a region covered by metal was assumed to be zero. The following Figure 1 shows the basic features, a grid and a potential pattern. Due to the Dirichlet boundary conditions imposed on different segments of the anodic and cathodic contact plate the possibly existing symmetry is broken, and both plates have to be computed.

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