Thermo-mechanical stress in tubular solid oxide fuel cells: Part II - operating strategy for reduced probability of fracture failure

A spatially discretised thermo-electrochemical model is developed to calculate the temperature distribution in a tubular solid oxide fuel cell (SOFC). This is used in a mechanical model to compute the distribution of thermo-mechanical stress in the ceramic membrane-electrode assembly of the cell. The resulting risk of fracture failure is determined by means of Weibull analysis. Part I of this work covers the dynamic operating properties of the SOFC and the time scale of material creep in its ceramic components. This work, Part II, deals with the risk of fracture failure related to transient operating scenarios, discusses its dependency on the operating conditions and derives a low-risk operating strategy. Contrary to the common perception, thermal gradients are found to have little impact on thermo-mechanical stress in the studied SOFC. Failure-relevant stress levels arise merely due to thermal mismatch of the ceramic layers. Regarding the operating strategy, the dynamics of changes in operating conditions are of minor importance for the resulting risk of failure, while operating strategies aiming at a constant mean cell temperature prove to be advantageous. The consideration of material creep is shown to be essential for a sound analysis of thermo-mechanical stress and risk of fracture in the investigated SOFC.