Employing Hot Wire Anemometry to Directly Measure the Water Balance of a Proton Exchange Membrane Fuel Cell

Proton exchange membrane fuel cells (PEMFC’s) are currently being commercialized for various applications ranging from automotive to stationary such as powering telecom back-up units. In PEMFC’s, oxygen from air is internally combined with hydrogen to form water and produce electricity and waste heat. One critical technical problem of these fuel cells is still the water management: the proton exchange membrane in the center of these fuel cells has to be hydrated in order to stay proton-conductive while excessive liquid water can lead to cell flooding and increased degradation rates. Clearly, a fundamental understanding of all aspects of water management in PEMFC is imperative. This includes the fuel cell water balance, i.e. which fraction of the product water leaves the fuel cell via the anode versus the cathode.
Our research group is currently developing a novel technique to obtain an ad-hoc and real time electrical signal of the fuel cell water balance by employing hot wire anemometry. The hot wire sensor is placed into a binary mixture of hydrogen and water vapour, and the voltage signal received gives valuable insight into heat and mass transfer phenomena in a PEMFC. A central question in hot wire anemometry is the description of the Nusselt number as function of the Reynolds number and the Prandtl number, and in the current application it can be shown it is essential to employ a power-law equation instead of the commonly employed Churchill Bernstein equation. It will be shown that in the former case the relative voltage signal of the hot wire is then independent of the fuel cell current which is essential for this method to work.