Evaluation of the overall efficiency of a low pressure proton exchange membrane fuel cell power unit

Polymer electrolyte membrane fuel cells (PEMFC) at present can be considered the best choices to set up the auxiliary power unit of an hybrid electric vehicle. This paper describes how it is possible and convenient to manage a PEMFC at low pressure, using a blower instead of a compressor/expander. The polarization curve obtained from experimental tests on the stack confirms a typical behaviour at different values of pressure; although high pressure brings best performance, in the balance of the plant the air compressor generally absorbs more then 40% of the power generated from the stack. As a matter of fact, the overall efficiency of the whole FC apparatus can be improved by managing it at low pressure. In an FC/batteries hybrid electric vehicle, the FC is asked to operate not at partial load, but only at full load by using an ON-OFF control. Consequently it is possible to arrange a very compact and reliable apparatus in which the dynamic mass flow and pressure controls are not required and the mechanical and electric hardware is very simple.