Time Characterization of the Anodic Loop of a Pressurized Solid Oxide Fuel Cell System

A dynamic solid oxide fuel cell (SOFC) model was integrated with other system components (i.e., reformer, anodic off-gas burner, anodic ejector) to build a system model that can simulate the time response of the anode side of an integrated $250kW$ pressurized SOFC hybrid system. After model description and data on previous validation work, this paper describes the results obtained for the dynamic analysis of the anodic loop, taking into account two different conditions for the fuel flow input: in the first case (I), the fuel flow follows with no delay the value provided by the control system, while in the second case (II), the flow is delayed by a volume between the regulating valve and the anode ejector, this being a more realistic case. The step analysis was used to obtain information about the time scales of the investigated phenomena: such characteristic times were successfully correlated to the results of the subsequent frequency analysis. This is expected to provide useful indications for designing robust anodic loop controllers. In the frequency analysis, most phase values remained in the $0–180deg$ range, thus showing the expected delay-dominated behavior in the anodic loop response to the input variations in the fuel and current. In Case I, a threshold frequency of $5Hz$ for the pressure and steam to carbon ratio and a threshold frequency of $31Hz$ for the anodic flow were obtained. In the more realistic Case II, natural gas pipe delay dominates, and a threshold frequency of $1.2Hz$ was identified, after which property oscillations start to decrease toward null values.