Experimental Investigation of Channel Aspect Ratio on Interdigitated PEMFC Performance 2014-01-1828
Novel water management and reactant distribution strategies are critical to next generation polymer electrolyte membrane fuel cell systems (PEMFCs). Improving these strategies in PEMFCs leads to higher power density and reduced stack size for vehicle applications, which reduces weight and improves the price competitiveness of these systems.
Interdigitated flow fields induce convective transport (cross flow) through the porous GDL between adjacent channels and are superior at water removal beneath land areas, which can lead to higher cell performance. However, the head loss due to flow, among other factors, may cause cross flow maldistribution of reactants down the channel. Such maldistribution may lead to areas of low or areas of excess cross flow. This, in turn, can cause areas of low oxygen concentration and water build up, and therefore higher pressure losses and uneven membrane hydration, all of which reduce overall cell performance. This research seeks to examine the affect varying channel dimensions have on cross flow and water distribution in an interdigitated flow field.
A modeling study of a PEMFC at various aspect ratios and flow rates was completed using the multiphysics package COMSOL. Interdigitated flow fields at varying lengths and conditions were simulated in 3D. Power and cross flow trends were the main focus of the work. Reaction by-product water is a factor; however, no models can currently account for this completely. The cross flow and current distribution was shown to be more variable in the 25 cm cell than in the 5 cm cell. The central region of the 25 cm cell, where the cross flow rate is lowest, may be subject to reduced oxygen concentration and increased water buildup. Regions toward the entrance and exit, while receiving higher cross flow rates, may be oversupplied and contributing to increased pressure drop due to higher velocities.
For experimental validation, a PEMFC was designed that could be configured to run under varying interdigitated channel length configurations through the use of separate manifold exit ports. The lengths studied were 5 cm, 15 cm and 25 cm. The flow field is comprised of 10 channels with width/depth/land dimensions all of 1 mm. To isolate the effect of the cathode interdigitated flow field, the anode was run in parallel. Polarization and inlet pressure curves were produced for each length case at two different stoichiometries. Reduced cell performance with increasing channel length is a trend that was observed.
The even cross flow rate in the short cell appears to be more advantageous when these results are compared to the experimental data and suggests that interdigitated designs are sensitive to maldistribution along the channel length. Neutron radiography is being conducted to study the water removal characteristics of interdigitated cells of varying length. Through-plane images of in-situ cell operation should allow for comparison of inlet and outlet water content as well as comparison of GDL water distribution. The effect of width and depth of channels are currently being examined. Channel dimensions of interest are (width x depth, in millimeters) 1x1, 0.5x0.5, 0.25x0.25, 0.5x1, 1x0.5 and 0.25x0.5.