Applying hot-wire anemometry to directly measure the water balance in a proton exchange membrane fuel cell for a pre-humidified hydrogen stream

In a recent publication it has been shown how the water balance in a proton exchange membrane fuel cell can be determined employing hot wire anemometry. The hot wire sensor has to be placed into the anode outlet pipe of the operating fuel cell, and the voltage signal E that is read from the sensor has to be divided by a pre-determined voltage signal E0 that has been obtained for a stream of dry hydrogen where the molar flow rate corresponds to a total current I of the fuel cell stack and a stoichiometric flow ratio, ξ. Because the last two properties are usually continuously known in fuel cell experiments, E0 is also continuously known. There is a one-to-one correlation between the relative voltage signal E/E0 and the fuel cell water balance, and therefore the fuel cell water balance can be a continuous output signal similar to the fuel cell voltage and the high frequency resistance. This method was originally believed to be limited to the fuel cell anode operating on dry hydrogen. In the current work, it is expanded for the case of a pre-humidified hydrogen stream. In addition, useful correlations are derived that link the fuel cell water balance with the anode side inlet and outlet thermodynamic state. Finally, it will be shown how previously developed dew point diagrams for the anode side in a fuel cell can be corrected for a humidified hydrogen inlet stream.