Short communication
Investigation on the influence of channel geometries on PEMFC performance

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Abstract

The performance of PEM fuel cells depends significantly on an appropriate flow field design. In this work, an investigation on the influence of channel geometries on stack performance has been carried out. A parallel flow field design was developed to widely eliminate the influence of other flow field design parameters, e.g. degree of parallelization and gas crossover effects at the turning points of the gas channels. Starting from a basic design, channel as well as rib dimensions were varied and their influence on stack performance were studied. An optimum between 0.7 and 1 mm was found for either channel or for rib widths. For wider dimensions, the influence on mass transport (ribs) or lateral conductivity (channels) has been found to become significant. For very small dimensions, the manufacturing effort becomes unacceptably high (ribs) and the probability of channel clogging by formation of water droplets increases. In general, narrow channel dimensions are preferred for high current densities, whereas wider dimensions are better at low current densities.

Introduction

PEM fuel cell stacks having an active area of 100 cm2 and a length up to 50 cells resulting in a power of 1.4 kW have been built and operated successfully since 2001 at ZSW. A power density of up to 0.3 W cm−2 has been obtained. However, it is well-known, that also the degree of fuel and air utilization is of great importance to the system efficiency. Despite showing acceptable power characteristic, the utilization of hydrogen (0.35–0.7) and air (0.15–0.3) were not satisfactory. Therefore, an investigation on the influence on flow direction, flow field geometry and channel geometries has been started having the objective of improved flow field design leading to better performance. In addition to general design improvements for the flow field and operating conditions of ZSW 100 cm2 stacks which are reported elsewhere [1], the influence of channel geometries on stack performance has been studied in this work. There are already data on this subject reported in the literature, e.g. [2]. Also, investigations of non-conventional fractal geometries are given, e.g. [3]. Since there was no superior performance of fractal geometries reported compared to serpentine or parallel flow fields, this investigation was limited to conventional designs, specially to the influence of channel and rib dimensions on the PEFC cell performance. The results published in the literature indicate that dimensions less than 1 mm for both channel and ribs could be advantageous. However, the published data are limited to a channel width of 1 mm, and the variation of rib widths is given in steps of 0.5 mm only. The same restrictions are valid for [4] where results on the modeling of gas diffusion layers (GDL) are reported. Therefore, it was of interest to find out whether channel and rib dimensions ranging from 1.5 mm to less than 1 mm could contribute to improved cell performance.

Section snippets

Experimental procedures

Using the basic layout of the ZSW 100 cm2 cell design, a parallel flow field including optimized inlet and outlet areas was designed which should guarantee an almost even flow rate throughout all channels. The flow field design process was accompanied by commonly used CFD-modeling using FLUENT™ software. In doing so, first approach was to verify experimental results by CFD-calculations. A measured distribution of static pressure inside a pattern flow field model of a PEM fuel cell with an active

Results and discussion

For all investigations, counter flow between the gaseous media was selected. The cells were positioned as such, that no flow direction opposite to the direction of gravity occurred in order to avoid artefacts caused by droplets not being purged out due to gravity reasons.

Summary and conclusion

Summarizing the results, a negative correlation between cell resistance and rib to channel ratio could be shown. In most cases, a corresponding positive correlation exists between the cell voltage at 0.7 A cm−2 and the rib to channel ratio for a range from 0.67 to 2. An optimum dimension between 0.5 and 1 mm was found as well for channel as for rib widths, if combined with a rib to channel ratio ≥1. For wider dimensions, the influence on mass transport (ribs) or lateral conductivity (channels)

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