Focus Variation for Fuel Cell Technology: How the Optical 3D Measurement of Bipolar Plates Can Be Used to Ensure Functionality and Performance of Fuel Cell Systems

Abstract

Fuel cell technology research is becoming increasingly important as the technology contributes to energy supply in a wide range of applications. From portable devices such as laptops, to stationary applications for powering critical infrastructure, fuel cells can be used. Another area of applications is the mobile sector, as fuel cell systems make it possible to supply energy not only to passenger cars, but also to larger vehicles with high power requirements and longer drives as trucks or construction machinery, as the recharging times can be reduced to minutes.

Energy is generated by the reaction of oxygen and hydrogen in the fuel cell stack. Due to the high demand on functionality, performance and lifetime of a fuel cell system, the quality assurance of its components plays an essential part in research and production. A component of special interest is the bipolar plate. Bipolar plates are made of metal or graphite and typically have a thickness of some hundred µm, while the area of the flow field can also have a size of e.g., 300×600 mm. Bipolar plates are installed by the hundreds in a fuel cell stack and they are responsible for the gas and coolant flow. One decisive factor in quality assurance is the measurement of the flow field geometry of the bipolar plate itself and its stamp as the geometry of the channels determines the currents and thus the performance of the system. Moreover, the geometry of the welding position is essential for the welding process and thus for the structure of the fuel cell stack. Thereby the tolerances are in the µm range. Another interesting factor is the measurement of the surface texture of the stamp to ensure the optimal forming process.

In this work, the optical inspection of geometry and surface texture of bipolar plates and their stamps by the use of a pure optical coordinate measurement instrument based on focus variation is presented. Focus variation is an optical, area-based measurement method which uses the small depth of focus in combination with a vertical scanning process to obtain a 3D dataset of the sample. In contrast to other optical measurement instruments, which often focus on surface metrology, the focus variation technology offers advantages in terms of the ability to measure form and surface texture at the same time. Therefore, focus variation became indispensable for many coordinate metrology applications in the last years.

The performance of the coordinate measurement instrument is shown by several 3D measurements of bipolar plates and their stamp including their geometrical inspection by parameters like channel depths, flank angles, radii, or welding position parameters as well as area and profile-based deviations to the CAD data. In addition, profile and area-based roughness measurements on the stamp surface show the capability for further quality assurance. Moreover, the performance of the coordinate measurement instrument is shown. The results in this work demonstrates the high potential of the focus variation technology for high-precision coordinate applications in fuel cells.