Fluent Inc. is developing a CFD-based model for polymer electrolyte membrane fuel cells (PEMFC) to satisfy the needs of the automotive and power generation industries as this new technology takes hold. Detailed sub-models describing the physics specific to the PEM fuel cell are being implemented within FLUENT and a complete PEM model is to be validated for complex fuel cell configurations. Some of the submodels under development include:

• PEM electrochemical submodel to predict local current density and voltage at the membrane-electrode assembly (MEA);
• Electrical submodel for current and voltage in all porous and solid regions that conduct electricity;
• PEM MEA submodel for predicting electrical losses and water transport through the MEA, based on local values of temperature, current density, and species concentration;
• Porous multiphase submodel to model the liquid water flow in the porous diffusion layers; and
• Multiphase thin film model for liquid water flow in the gas flow passages

FLUENT simulation geometry – serpentine PEMFC geometry

FLUENT readily calculates the flow and energy distribution within the fuel cell. The PEMFC submodels describing electrochemistry, electrical field, MEA behavior, and multiphase flows are fully coupled to the flow, species, and energy transport calculations performed in FLUENT. In adapting this model, the MEA components-membrane, catalyst regions, and electrodes have been lumped into a single layer for the PEMFC calculation. The MEA layer is assumed thin (one-dimensional) for electrochemical modeling purposes, though it can be represented as a finite thickness region in the FLUENT simulation.

The electrochemical submodel accounts for the three loss mechanisms present in fuel cells: ohmic overpotential, concentration overpotential, and electrochemical overpotential. In performing the simulation, a total current is specified for the cell as an initial boundary condition. Using an iterative process between the flow solver and electrochemical and electrical conduction submodels, a converged solution produces local values of current density and voltage on the MEA surface. Upon convergence of the model, the results show PEM cell behavior corresponding to the specified total current output. The current density and cell voltage is calculated based on local species concentrations and temperature at the MEA surface during the iterative solution. These conditions then impose species and heat fluxes on t he FLUENT CFD calculation. Through repeated iteration between the flow solver and PEMFC submodels, a steady state converged solution is obtained.

Basic PEM model geometry

To date, the electrochemical and electrical submodels have been fully implemented in FLUENT. Preliminary results for a simple serpentine PEMFC geometry are shown below. Also illustrated is the co-flow cell geometry where serpentine channels direct fuel and oxidizer over the respective anode and cathode diffusion layers. The fuel is a hydrocarbon fuel reformate, composed of hydrogen, carbon dioxide, and water vapor. The oxidizer is humidified air and the average cell current density is 5000 A/m 2. Contours of current density on the cathode-diffusion layer are shown, where the scale has been selected to accentuate the current density gradients. Development and validation of the PEMFC model will be completed during the first quarter of 2002.

Contours of current density on cathode-diffusion layer interface in serpentinechannel geometry

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