2.6.1 Specifying Model Options

The Model tab of the Fuel Cell and Electrolysis Models dialog allows you to turn on or off various options when solving a fuel cell problem. To model polymer electrolyte membrane fuel cells, enable the PEMFC option in the Model tab. Likewise, to model solid oxide fuel cells, enable the SOFC option in the Model tab. Finally, to model electrolysis, enable the Electrolysis option in the Model tab.

Figure 2.6.2: The Model Options in the Fuel Cell and Electrolysis Models Dialog - PEMFC Enabled

Several fuel cell model options are available in the Model tab of the Fuel Cell and Electrolysis Models dialog including:

• The Joule Heating option takes into account ohmic heating. This option includes the $I^2R$ term in the energy source term from Equation  1.4-1 in the calculations.

• The Reaction Heating option takes into account the heat generated by the electrochemical reactions, which includes the $h_{\rm reaction}$ term, and the product of transfer current and the over-potentials in the energy source term from Equation  1.4-1 in the calculations.

• The Electrochemistry Sources option allows the Fuel Cell and Electrolysis Model to take electrochemistry effects into account. If you are only interested in the basic flow field throughout the fuel cell, you can turn off the Electrochemistry Sources option in order to suppress most effects of the Fuel Cell and Electrolysis Model. To turn off all effects of the Fuel Cell and Electrolysis Model, you should also turn off the Membrane Water Transport and Multiphase options.

• The Butler-Volmer Rate option (the default) is used to compute the transfer currents inside the catalyst layers. If this option is turned off, the Tafel approximation (Equation  1.2-6) is used.

• The Membrane Water Transport option takes into account the transport of water across the membrane. This option is only available for the PEMFC model.

• The Multiphase option takes into account multiphase calculations. Use this option if you are solving for approximate liquid transport in the gas diffusion layer of the fuel cell. (PEMFC only)

• The Multicomponent Diffusion option is used to compute the gas species mass diffusivity using the full multicomponent diffusion method as described in Equation  1.6-2, as opposed to the default option that uses Equation  1.6-1.

• The Anisotropic E-Conductivity in Porous Electrode option is used to model the typically non-isotropical electrical conductivity. It is applicable only for porous electrodes (gas diffusion layers).

  Due to the fibrous structure of the porous material that is used for the electrodes (or gas diffusion layer), the electrical conductivity is typically non-isotropical, with the cross-plane components being orders of magnitude smaller than the in-plane components. This can be modeled using the Anisotropic E-Conductivity in Porous Electrode setting. When this option is enabled, the Electrical Conductivity for the solid material used in the electrolyte is no longer used. Instead, you need to specify, for this solid material, the electrical conductivity by choosing one of the three non-isotropical options for the UDS diffusivity ( UDS-0). The three options are: anisotropic; orthotropic; and cyl-orthotropic. For more information about these UDS Diffusivity options, refer to the ANSYS FLUENT User's Guide.

  For example, to use this feature, perform the following steps:

  • Select the Anisotropic E-Conductivity in Porous Electrode option in the Model tab of the Fuel Cell and Electrolysis Models dialog.

  • In the Materials task page, select defined-per-uds for UDS Diffusivity for the solid material that is to be used for the porous electrode.

  • Select one of the three options for UDS-0: anisotropic; orthotropic; or cyl-orthotropic and set the appropriate values.

    Note that, in this case, the Electrical Conductivity for this solid material is ignored.

• For PEMFC problems, you can use the Under-Relaxation Factors fields to influence the solution process.

  The saturation source term $r_{\rm w}$ in Equation  1.5-2 usually requires under-relaxation. You can change the default value for the under-relaxation factor by changing the value for Saturation Source.

  The water content, $\lambda$, in Equation  1.6-8 also may need under-relaxation. You can change the default value for the under-relaxation factor by changing the value for Water Content.

Nearly all options are turned on by default. You may wish to override the default values, depending on the problem you wish to model. For instance, if you are not concerned with the heat generated due to chemical reaction, then you may want to turn off the Reaction Heating option.