Mehrdad Shahnam and Michael Prinkey,Senior Consulting Engineers, Fluent

Fuel cell technology promises to provide an environmentally friendly source of power with broad applications in many industries, such as transportation and the military. Among the current issues surrounding the continued development and deployment of this technology is that of manufacturing costs. Reduction of manufacturing costs can only be realized by optimizing the efficiency of the devices, and this can only happen through detailed analysis of the complex electrochemical and mass transport phenomena taking place. Fluent has developed modeling tools for FLUENT to help meet this need, so that engineers can optimize fuel cell design as well as performance. (See the Partnerships section on page 27.) As part of this ongoing effort, a user-defined function (UDF) has been developed recently with detailed models for Solid Oxide Fuel Cells (SOFC), a variety that is being targeted for distributed power applications, portable power generation (for the military), and auxiliary power units (or APUs, for commercial aircraft).

The geometry of the tubular SOFC shows the interconnects in green (used to electrically connect a stack of fuel cells), the air inflow and oxidizer channel in red, and the electrolyte in blue. The anode and cathode are cylindrical surfaces on the inside and outside of the electrolyte, respectively.

Temperature contours on the cathode side of the electrolyte

The current density and voltage on a
surface through the electrolyte

The SOFC module works in tandem with a FLUENT calculation that includes species transport and heat transfer. Species and temperature fields are passed to the SOFC model, which uses them to compute the current density, cell voltage, and heat flux at the electrodes. This information is then passed back to FLUENT, where it is used to update the species and temperature fields. The process continues in an iterative manner until convergence is reached. In addition to the fuel cell geometry, the operating characteristics include the total current output for the fuel cell, which is set as an initial condition. The comprehensive SOFC model, which is fully parallelized, address the following important processes:

Electrochemistry

Appropriate chemical reactions for H₂ and CO are used to predict the local current density and voltage distributions at the electrolyte surfaces. The electrolyte layer is assumed thin for electrochemical modeling purposes (the ionic transport across the electrolyte is assumed to be one dimensional), but a finite thickness region in the FLUENT simulation can be used to represent it. The electrochemical model takes into account the losses due to activation overpotential (kinetic losses), ohmic overpotential (losses due to ionic transport in the electrolyte), and concentration overpotential (losses due to to inadequate diffusion of species through the electrodes). Binary diffusion coefficients are used to calculate the molecular diffusion of the (gaseous) species throughout the domain.

Potential field

This model predicts the current and voltage in all conducting solid and porous regions of the SOFC. Heat generated as a result of ohmic losses in the conducting regions is also predicted.

The model has been applied recently to a tubular SOFC, where hydrogen and air are used as the fuel and the oxidizer, respectively. Fuel utilization is about 80% and the oxidizer utilization is about 25%. The total cell current is 11 Amp and the average current density is 1850 A/m².The figures illustrate the non-uniformity in several of the fuel cell variables that could not be captured by a more simplified approach.

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