COMPUTATIONAL MODELLING AND EXPERIMENTAL ANALYSIS OF FLOWS IN PEM FUEL CELLS
Robert A. Blewitt and Dr John S. Shrimpton
Thermofluids Section, Department of Mechanical Engineering, South Kensington Campus, Imperial College London, SW7 2AZ
What is a Fuel Cell?
CFD Modelling of Fuel Cells
A fuel cell is an electrochemical device that directly combines fuel
and oxidant in the absence of any combustion processes to produce
useful electrical power.
Computational Fluid Dynamics (CFD) is a useful method for modelling a fuel cell numerically because it allows all the inter-dependent physical, thermal,
electrochemical and electrostatic processes to be modelled concurrently without the need for separate models or assumed boundary conditions between
the different layers of the fuel cell. Specifically, the following processes can be modelled simultaneously:
The fuel and oxidant gas flows are separated by a semi-permeable
membrane, through which only specific ions can pass.
Fuel-side gas channel
Fuel-side diffusion layer
In the case of Proton Exchange Membrane (PEM) fuel cells, the fuel
is hydrogen and the oxidant air or oxygen. The membrane is
typically made of Nafion and is permeable only to hydrogen ions,
which are encouraged to form through the use of a platinum-based
catalyst. During operation electricity is produced and the only
reaction product generated is water.
Fuel-side catalyst layer
Membrane layer
Air-side catalyst layer
H2
2H+ + 2eAir-side diffusion layer
2eAir-side gas channel
½O2 + 2e- + 2H+
H2O
Fuel cells offer an attractive alternative to traditional methods of
power generation, particularly for automotive applications due to
environmental considerations and their increased efficiency over
combustion engines. They are also well suited to a variety of other
applications, ranging from industrial-scale power generation to
mobile phones and laptop computers.
Low-temperature fuel cells are only just starting to be used in
commercial applications, however, since they are expensive to
manufacture. It is therefore important to continue to develop our
understanding of fuel cells in order to bring improvements in design
and efficiency that will help fuel cells reach commercial maturity.
Structure of a Fuel Cell
A fuel cell is made up of multiple layers, as shown in the diagram above:
Gas layers – contain the fuel and oxidant gas streams
- Convection & diffusion of H2,
convection
&
conduction
of
heat.
- Porous diffusion of H2 to catalyst layer,
- conduction of heat to fuel channel.
- Porous diffusion & consumption of H2,
-- production of H+ ions, production of e-’s,
- conduction & absorption of heat.
- Electrostatic transport of charge (H+) through
- an electric field, conduction of heat.
- Porous diffusion & consumption of O2,
-- production & transport of H2O,
- production & conduction of heat.
- Porous diffusion of O2 + H2O,
-- conduction of heat to air-side gas channel.
- Convection/diffusion of H2/H2O/O2/Air in
-- multi-species mixture, convection/conduction of
heat.
Further considerations such as compressibility, the presence of a multi-phase species (H2O), reaction ‘hot-spots’ and time-dependent operation require far
more detailed analysis than is possible through other modelling methods, and of particular interest is the effect that one process (e.g. gas-stream fluid
mechanics) may have on another (e.g. thermal, electrostatic or electrochemical behaviour). This multi-disciplinary approach makes fuel cell modelling a
particularly challenging and interesting application for CFD.
The Optical Fuel Cell – LDA Measurements
A working fuel cell has been constructed in co-operation with the Chemistry epartment at Imperial
College London, using Perspex gas channels and therefore allowing optical access to the gas flows
inside. This allows us to measure 3D velocity profiles of the gas mixtures at planes along the gas
channels using Laser Doppler Anemometry (LDA).
The measured velocity profiles of the gases within a working fuel cell are very useful both for
comparison with other electrochemical experiments, such as measurement of current density along the
surface of the Membrane Electrode Assembly (MEA), and for validation of numerical models such as
the CFD models described above.
Because of the small geometry of PEM fuel cell gas channels (typically 1mmx1mm in cross-section),
LDA is ideally suited to velocity measurement in this environment due to its small spatial resolution
(around 80 microns). The flows are seeded with water droplets to scatter light as water is already
present in the system in the form of reaction product and so poses no risk to the delicate MEA.
Gas In
Gas Out
Diffusion layers – allow reactant species to diffuse to reaction sites
Catalyst layers – sites of reaction
Membrane – allows transport of hydrogen ions only
Computed velocity, species mass fraction
and temperature profiles in gas
channel/porous layer geometries
Measurement Planes (Side View)
Serpentine Flow Path (Plan View)
Views of the assembled Optical Fuel Cell and
during LDA experiments with the MEA in place