An active filter for fuel cell applications

Fuel cells are becoming a more attractive option for many remote power applications. One of the main well-known problems of a fuel cell system is its slow dynamic response: the fuel cell system needs significant time to reach a new steady-state condition after a load change. Notice that fuel cell dynamics, in terms of time-constants, may be in the order of seconds while electrical loads are usually in the millisecond range. This slow dynamic response should be compensated to allow operation under quick load variation conditions. A separate energy storage device, such as a battery, can be used to supply power to the load during all transient periods in which the fuel cell is adapting its state according to the load request. This paper proposes a bidirectional converter to compensate for the fast load changes while the fuel cell system adjusts the fuel flow to deliver the necessary energy. Two control strategies are proposed: a current feed-forward control and a voltage feedback control. Simulink simulations have been developed to obtain insight about these control strategies, as well as about the behavior of the bi-directional converter. A 35W hardware prototype has been built to verify the control performance. Experimental results of both controls are in agreement with their simulations and therefore validate the proposed control strategies. Both control strategies perform well. The current feed-forward control has the advantage of being extremely fast - with responses of the order of one time step of the digital controller - but has the disadvantage of requiring a load current sensor.