Gas Flow Channel Design

As gas enters the gas diffusion layer through the gas contact area, it diffuses toward the reaction surfaces as well as into the land area to some extent. The overall performance of the fuel cell is directly proportional to the area of the gases in contact with the bipolar plate. 

A ratio of reactant gas contact area to the land area has to be selected based on a balance of effective heat and mass transport rate through the gas contact 

MEA and bipolar plate with integrated gas flow channels, (a) Open single gas flow channels, (b) Multiple separated gas flow channels, (c) Gas channel with direct contact with GDL and land area. 

Bipolar plates with gas flow channels in contact with anode and cathode electrodes, (a) Geometrical parameters for contact and land areas, (b) Different chaimel geometries. 

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One of the major losses in PEM fuel cells is the mass transfer loss, which is caused by the lack of reactant gas concentration distribution at the electrode-catalyst reaction surface. In order to reduce this resistance, a high-performance gas flow channel design has to be developed. 

Figure 3.36. Different design options for the gas flow channels (A) straight, (B) inter-digitated, (C) serpentine and (D) spiral. For the interdigitated design, the incoming flow must proceed through the gas diffusion layer to reach the outlet channel.

When a channel in the reactor Is to be inspected the standard TRIUMPH camera (see section 8.2.1) Is connected to the hose. A clamp xmit with expanding arms can be connected in series with the camera to hold it steady in the channel in the face of buffeting from channel gas flows. The design operating depth in reactor Is 25m. 

This variant of gas supply in a heavy-metal coolant was chosen because of design simplicity, the possibility of restarting the system after coolant entry into the gas flow channels, simplification of the system actuation and gas flow control, more modest hydraulic resistance,... 

A more detailed discussion of gas flow channel analysis and design is considered in Chapfer 10. 

Water produced by electrochemical reactions in a fuel cell needs to be removed for efficient operation of the cell. Water transport in electrode-gas diffusion layers, electrolyte, and gas flow channels plays a critical role in the design of a fuel cell. Figure 7.8 shows water generation at the electrodeelectrolyte interface and mechanisms of water transport in a fuel cell. 

One of the major challenges in the bipolar plate design is to house reduced-size and highly complex gas flow channels with complex patterns for both fuel and oxidant gas flows as well as house cooling/heating channels if required. Additionally, it has to integrate well with the internal supply and return manifolds. 

Straight parallel channel gas flow-field design, (a) Straight parallel flow channels, (b) Gas distribution primarily by diffusion in the gas diffusion layer. 

The interdigitated flow-field design consists of two sets of dead-ended gas flow channels as shown in Figure 10.16a. The first set of dead-ended inflow channels carries the gas stream from the inlet ports and transfers to the electrode gas diffusion layer. The gas stream is forced by advection through the porous gas diffusion layer toward the electrode-electrolyte interface and toward the second set of dead-ended outflow channels and moves toward the gas stream outlet port. 

Although mote expensive to fabricate than the pelleted catalyst, and usually more difficult to replace or regenerate, the honeycomb catalyst is more widely used because it affords lower pressure losses from gas flow it is less likely to collect particulates (fixed-bed) or has no losses of catalyst through attrition, compared to fiuidized-bed and it allows a mote versatile catalyst bed design (18), having a weU-defined flow pattern (no channeling) and a reactor that can be oriented in any direction. 

However, low velocities require towers with large cross-sectional areas to handle a given gas flow, and allow the wet gas to channel through the desiccant bed and not be properly dehydrated. In selecting the design velocity therefore, a compromise must be made between the tower diameter and the maximum use of the desiccant. Figure 8-22 shows a maximum design velocity. Smaller velocities may be required due to pressure drop considerations. 

The IIEC model was also used to study the importance of various design parameters. Variations in gas flow rates and channeling in the bed are not the important variables in a set of first-order kinetics. The location of the catalytic bed from the exhaust manifold is a very important variable when the bed is moved from the exhaust manifold location to a position below the passenger compartment, the CO emission averaged over the cycle rose from 0.14% to 0.29% while the maximum temperature encountered dropped from 1350 to 808°F. The other important variables discovered are the activation energy of the reactions, the density and heat... 

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