Flow-Field Design
In fuel cells, flow field plates are sized so that a certain amount of reactants (hydrogen and oxygen) reaches the catalyst surface and gas diffusion layer (GDL) while minimizing pressure drop. The most common channel geometries or channel configurations for PEM fuel cells are parallel flow, serpentine flow, and interdigital flow. Some fuel cells do not use a flow field to distribute hydrogen and air, relying on diffusion processes from the environment. Usually, a serpentine arrangement is chosen for the anode for smaller fuel cells because the hydrogen reaction is not velocity limited and hydrogen blockage can occur in the anode.
The geometrical structure of a serpentine flow field is clearly shown in Figure 1. From beginning to end, the flow path is continuous to achieve efficient distribution of flow over the electrode area of the fuel cell. There may be pressure losses in the flow channel with this design due to the long flow path. Alternative serpentine designs are used for operation at high current densities, large plates, or when air is used as the oxidant. The serpentine flow path means that obstructions in the flow path cannot impede all downstream activity, which is an advantage. A disadvantage of the serpentine geometry is that the chemical reactant is needed along the entire length of the flow channel, so enough gas must be provided. As soon as air is used as oxidant, problems with the cell water management and with the distribution of the cathode gas flow can occur. As soon as the fuel cell is operated for a longer period of time, water will accumulate at the cathode. To remove the water, pressure is required to flush the water out of the channels.
Figure 2 shows a schematic diagram of multiple serpentine flow channels. In this model, multiple flow channels are used to limit the pressure drop and reduce the amount of energy required to pressurize the air through a single flow channel. In addition, this design prevents the formation of a stagnant region at the cathode surface due to water accumulation in the channels. The pressure drop of the reactant through the flow channels is lower than when using one flow channel (like Figure 1). However, due to the long flow path, the pressure drop is still significantly high.
Figure 3 shows a parallel arrangement of the flow channels. In this variation, the flow channels require a lower mass flow per channel and produce a uniform gas distribution with less pressure drop. When air is used as the oxidant, low and unstable cell voltages can occur after prolonged operation due to water accumulation and cathode fuel distribution. Water accumulation occurs in the flow channels during continuous operation of the fuel cell. A disadvantage of parallel arrangement is that a blockage in one flow channel results in a change of flow in the other channels. This results in a dead zone downstream of the location of the blockage. The amount of water in each channel may be different, resulting in uneven gas distribution. Another disadvantage of this channel design is that the flow channels are very short with almost no change in direction. This leads to an increased pressure drop in the manifold. In addition, the first cells near the inlet of the manifold have a higher flow rate than the cells located at the end of the manifold.
Reactant flow is parallel to the electrode surface for interdigitated flow fields. In most cases, the flow channels are not continuous from the inlet to the outlet. The channels are designed as dead ends so that the reaction flow must pass through the porous reactant layer under pressure to achieve effective flow distribution with the flow channels. Using this design, water is removed from the electrode structure, which increases performance. The interlocking structure of the flow fields results in forced convection, thus flooding as well as gas diffusion restrictions are avoided. The layout of the ducts sometimes surpasses conventional flow field design. The interdigital layout of the ducts is graphically illustrated in Figure 4.
Channel shape, dimensions and spacing
In general, fluid flow channels are rectangular, circular, triangular, or may have other shapes. The geometry of the channel shape can affect the water accumulation in the cells and thus the oxidant and fuel flow rate. In rounded channels, the condensing forms a thin film at the bottom of the flow channel. In contrast, water droplets form in other channel shapes. The size and shape of the water droplets are determined by the hydrophilicity and hydrophobicity of the porous media and the channel walls. The channel dimensions are usually about 1 mm, but there is a wide range (0.1 mm to 3 mm) for micro to large fuel cells. From simulations, the optimal channel dimensions for macro fuel cell stacks (not MEM fuel cells) are 1.5, 1.5, and 0.5 mm for channel depth, width, and web width (distance between channels), respectively. The dimensions given depend on the overall design of the stack and the size of the stack. The dimensions of the channels also affect oxidant and fuel flow rates, pressure drop, heat and water generation, and power generated in the fuel cell. Wider flow channels result in better contact between the catalyst layer and the fuel, have lower pressure drop and efficient water removal. If the channels are too wide, the MEA layer is not sufficiently supported. A too large distance between the flow channels, leads to lower contact area to the reactants and thus to a higher accumulation of water.
Final considerations
The flow field plates are designed to transport liquids and gases to the catalysts. Channel configurations used are: Serpentine, the parallel flow, and the interlocking flow. There are different flow field designs which depend on the size, design and configuration of the stacks.