Anode-supported intermediate-temperature direct internal reforming solid oxide fuel cell: II. Model-based dynamic performance and control

Abstract

In operation, solid oxide fuel cells ( SOFCs) can be subjected to frequent load changes due to variable power demand. Knowledge of their dynamic behaviour is thus important when looking for suitable control strategies. The present work investigates the open and closed-loop transient response of a co-flow planar anode-supported intermediate-temperature direct internal reforming solid oxide fuel cell to load step-changes. A previously developed dynamic SOFC model, which consists of mass and energy balances and an electrochemical model that relates the fuel and air gas compositions and temperature to voltage, current density, and other relevant fuel cell variables, is used. A master controller that imposes a current density disturbance representing a change in power demand and sets the fuel and air flow rates proportional to that current (keeping the fuel utilisation and air ratio constant) and a typical feedback PID temperature controller that, given the outlet fuel temperature, responds by changing the air ratio around the default set by the master controller, have been implemented. Two distinct control approaches are considered. In the first case, the controller responds to a fixed temperature set-point, while in the second one the set-point is an adjustable parameter that depends on the magnitude of the load change introduced. Open-loop dynamic simulations show that, after a positive/negative load step-change, the overall SOFC temperature increases/decreases and the intermediate period between the disturbance imposed and the new steady-state is characterised by an undershoot/overshoot of the cell potential. Closed-loop simulations when load step-changes from 0.5 to 0.3, 0.4, 0.6, and 0.7 A cm −2 are imposed show that the proposed fixed set-point PID controller can successfully take the outlet fuel temperature to the desired set-point. However, it is also shown that for load changes of higher magnitude, an adjustable set-point control strategy is more effective in avoiding oscillatory control action, which can often lead to operation failure, as well as in preventing potentially damaging temperature gradients that can cause excessive stresses within the SOFC components and lead to cell breakdown.