An improved small-signal model of the dynamic behavior of PEM fuel cells

The dynamic behavior of a fuel cell is integral to the overall stability and performance of the power system formed by the fuel cell source, power conditioner and electrical load. Present-day fuel cells have transient (dynamic) responses that are much slower than the dynamic responses of the typical power conditioner and load to which they are attached. As such, the fuel cell´s inability to change its electrical output (current) as quickly as the electrical load changes has significant implications on the overall power system design. In particular, some form of energy storage with quick charge/discharge capability is needed to function as firm power backup during electrical load increases if the fuel flow to the fuel cell is not being kept constant at its maximum level (which is wasteful and inefficient). The slower the fuel cell´s response, the larger the amount of energy storage that is needed with the attendant increases in its size, weight and cost; it also reduces the number of suitable energy storage options (ultracapacitor, flywheel, battery, etc). Hence, this paper describes an effort to model a PEM fuel cell´s dynamic behavior as an initial step to prescribe internal design modifications and/or external controller designs to improve its transient behavior. Simulation results obtained from this model are presented along with corresponding test results, which show generally good agreement with each other.