Cooling Using the Cathode Air Supply

PEM fuel cells are, of course, not 100% efficient. In converting the hydrogen energy into electricity, efficiencies are normally about 50%. This means that a fuel cell of power X watts will also have to dispose of about X watts of heat. More precisely, it is shown in Appendix 2, Section A2.6, that the heat produced by a fuel cell, if the product water is 

The way this heat is removed depends greatly on the size of the fuel cell. With fuel cells below lOOW it is possible to use purely convected air to cool the cell and provide sufficient airflow to evaporate the water, without recourse to any fan. This is done with a fairly open cell construction with a cell spacing of between 5 and 10 mm per cell (Daugherty, 1999). The fact that damp air is less dense than dry air (perhaps counterintuitive, but true) aids the circulation process. However, for a more compact fuel cell, 

However, when the power of the fuel cell rises, and a lower proportion of the heat is lost by convection and radiation from and around the external surfaces of the cell, problems begin to arise. In practice, this simplest of all methods of cooling a fuel cell can only be used for systems of power up to about 100 W. 

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