Many people consider fuel cells to be the most promising power generationtechnology of the future. This is mainly because the energy conversion efficiencyof a fuel cell device can be much higher than that of current technologies, whilstits emissions are less harmful than those from, for example, internal combustionengines.

However, fuel cell developers must still overcome some technical problemsbefore their devices can penetrate the energy market. Commercial success willdepend on the degree of optimisation of fuel cellperformance, design and fabrication, not only toachieve performance goals but also to reduce the costof the device. For this reason it is important to be ableto predict and control the parameters that govern theperformance of the cell.

With this aim in view, researchers in the field haveover the past decade learned to enhance theperformance of fuel cells, both by experimental workand by developing models to predict their performance.These models often make it possible to study the effect of different operationalconditions as well as design parameters. In recent years this development hasaccelerated as the research effort to introduce fuel cells has risen dramatically,and today, most academic and industrial researchers consider that CFD is themost promising way of conducting design and optimisation studies in this field.

Aalborg University, Institute of Energy Technology, and IRD Fuel Cell A/S,Denmark, are collaborating in a research project which aims to model the

complex coupling between mass transport and the electrochemical reaction thattakes place in a fuel cell. The design considered in the project is a PEM (ProtonExchange Membrane) fuel cell. It consists of a membrane electrode assemblysandwiched between a pair of bipolar plates; this is called a cell element andhas a potential of about 1V. The bipolar plates are connected to a flow supplythat supports the feed of reactants and removal of products. In addition, thebipolar plates collect the electrons generated at the anode (fuel) side and supply

the electrons to the cathode (air) side via an externalcircuit (the power grid). The cell elements are thenstacked in series to form a fuel cell stack with a highpotential (50V).

For this work, we preferred CFX-4 above otherCFD codes as the modelling platform. One of thereasons is the enormous flexibility that its user Fortranand utility routines provide. Together with the largenumber of physical models available, this makes CFX apowerful tool for this application.

We first developed a model for the simulation of the cathode compartmentof the cell. The model uses several of the standard user routines available inCFX-4 coupled with an electrochemical module developed especially for thepurpose. Since the structure consists of many parallel channels, we appliedsymmetry boundary conditions at the centreline of the channel. The catalystlayer is modelled in 3-D and consists of multiple mesh elements in which theelectrochemical conversion between species occurs.

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No 22 Autumn 2002

by Mads Bang, IRD Fuel Cell A/S and Aalborg University,Institute of Energy Technology, Denmark

CFX ADVANCES FUEL CELL

CFX solution domain of the fuel cell.

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No 22 Autumn 2002

For the transport of reactant species in the system, we solve for thestandard CFX mass fraction equations together with the momentum equations.Since diffusion plays a dominant role in fuel cell modelling, we use binarydiffusion coefficients for each of the chemical species to calculate the speciesdiffusivity. The electrochemical reaction rates in the catalyst layer are modelledto predict the local current fluxes in this region. The electrochemical model takesinto account losses due to: activation of the electrochemical reaction (activationenergy); transport of electrons in the porous layer (Ohmic losses), and changesin concentration of the active species involved in the reaction(concentration losses).

The current flow in the porous layer and the catalyst layer isthen calculated by incorporating the predicted current fluxes intothe source term of a user-defined scalar equation. In this way, themodel solves the current transport and potential field in all theelectrically conducting parts of the computational domain.

This model provides valuable information about the utilizationof the catalyst layer (where the maximum reaction rates occur),such as gradients of the different variables, that cannot beobtained experimentally because of the small size and restrictedaccess of the fuel cell.

TECHNOLOGY

Cell elements are stacked in series to form afuel cell stack with a high potential (50V).

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POWER

HEAT

AIR HYDROGEN

‘The user Fortran’s enormous flexibility and the many physical models available make CFX a powerful tool.’

Below:

Figures 1 and 2 show the calculated current fluxes athigh and medium load respectively.

Figures 3 and 4 show the oxygen concentration at highand medium load respectively.

Figures 5 and 6 show the calculated potential field forthe electrode at high and medium load respectively.

Left: Design principle of a PEM fuel cell.