Response of a solid oxide fuel cell to load change

The transient behaviour of a solid oxide fuel cell (SOFC) caused by load changes is investigated. For this purpose a step function of the current density is set to the fuel cell and the response of the cell voltage is calculated by applying a special SOFC simulation code. The relaxation time is strongly related to the transient temperature distribution of the solid cell structure. Therefore, it is in the order of some minutes depending on the design parameters and the operating conditions. As a remarkable result the cell voltage undershoots beyond the new steady-state value from where the relaxation process starts. This effect is discussed and explained. The transient process is generalized by introducing two dimensionless groups, the modified Fourier number, Fo, and the source term number, So. In terms of these characteristic numbers a simple relationship is derived to determine the relaxation time of arbitrary cell designs under arbitrary operating conditions.