Supercapacitor Sizing for Fast Power Dips in a Hybrid Supercapacitor-PEM Fuel Cell System

Polymer electrolyte membrane fuel cell (PEM FC) operation is likely to be characterized by voltage dips on timescales shorter than 1 s, arising from temporary flooding of gas channels or porous layers, particularly when the FC is operated at high humidity levels. If supercapacitors are employed in hybrid systems, they can make up for the temporary lack of energy produced by the FC. However, the steep slopes of the voltage dips affect the energy that can be actually delivered by the supercapacitor because of its series impedance, and this should be taken into account when sizing it. This paper presents a simplified approach for sizing the supercapacitor, based on some observed peculiar features of the FC dips, which allow a simple but accurate model for the evaluation of the supercapacitor response to such dips. The validity of such an approach is supported by simulation and experimental results performed on a single PEM FC and on a supercapacitor.