1. 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? 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. The fuel and oxidant gas flows are separated by a semi-permeable membrane, through which only specific ions can pass. 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 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. CFD Modelling of Fuel Cells 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: Fuel-side gas channel - Fuel-side diffusion layer Fuel-side catalyst layer - Membrane layer Air-side catalyst layer - Air-side diffusion layer - Air-side gas channel - Further considerations such as compressibility, the presence of a multi-phase species (H 2 O), 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. 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 Diffusion layers – allow reactant species to diffuse to reaction sites Catalyst layers – sites of reaction Membrane – allows transport of hydrogen ions only Measurement Planes (Side View) Serpentine Flow Path (Plan View) Gas In Gas Out 2e - H 2 2H + + 2e - ½ O 2 + 2e - + 2H + H 2 O - Convection & diffusion of H 2, - convection & conduction of heat. - Porous diffusion of H 2 to catalyst layer, - conduction of heat to fuel channel. - Porous diffusion & consumption of H 2, - 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 O 2, - production & transport of H 2 O, - production & conduction of heat. - Porous diffusion of O 2 + H 2 O, - conduction of heat to air-side gas channel. - Convection/diffusion of H 2 /H 2 O/O 2 /Air in - multi-species mixture, convection/conduction of heat. Computed velocity, species mass fraction and temperature profiles in gas channel/porous layer geometries Views of the assembled Optical Fuel Cell and during LDA experiments with the MEA in place