Running PEM Fuel Cells without Extra Humidification

On the other hand, relative humidity greater than 100% is basically impossible, and the airstream wiU contain condensed water droplets. This can be dealt with in moderation, with the water blown out by the air. However, if the theoretical humidity is greater than 100% the electrodes will become flooded. The result of this two-way constraint is a fairly narrow band of operating conditions. Nevertheless, if we stay below about 60°C, then there will be an air flowrate that will give a suitable humidity, that is, around 100%. Some of these conditions are shown in Table 4.2. 

A key to correct PEM fuel cell water balance is clearly the air flowrate, and hence stoichiometry. At lower temperatures, say 30°C, this will be about X = 24. This may seem a wildly extravagant airflow, but in practice it is not. Let us take the example of the small type of fuel cell to which this sort of technique applies. The smallest fan from the catalogue of a major manufacturer has a flow rate of 4.7cfm (= ft min , the manufacturer is in the United States). If we assume that only 2.0 cfm of this air actually flows over the cathodes, and that the stack operates with an average cell voltage of 0.6 V, then, using equation A2.4 (adapted for cfm), we find that this airflow is suitable for a fuel cell of power 

The electrical power of the fan in question is just 0.7 W, so the very high stoichiometry for air circulation is supplied using less than 1% of the fuel cell power. This is a very modest parasitic power loss. 

One of the best sources of data and further discussion of the issues of running a PEM fuel cell without humidification of the gases is Biichi and Srinivasan (1997). Here it is shown that even if the air flowrate and temperature are correctly set, then the unhumidified cell will still perform about 40% worse than an identical cell with humidified reactants. This is because, although the overall water balance may be correct, there will be areas within the ceU, especially near the gas entry points, where the humidity will be too low. Thus, although not having any humidification greatly reduces system complexity, size, and cost - it is quite rare for PEM fuel cells not to use one of the techniques in the section that follows. 

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