Steady state and transient thermal stress analysis in planar solid oxide fuel cells

Resulting from elevated temperatures the major structural problem foreseen with planar SOFCs is their thermal stress. Due to the brittle nature of ceramic material, operation in or near the material plastic limit can be very critical. Therefore stress levels must always be kept below the tensile and shear limits. The analysis is focused on determination of the stress caused by the difference in thermal expansion coefficients when high temperature gradients occur in the SOFC layers during steady state and transient operation (heat-up, start-up and shut-down). Utilizing an in-house developed tool for assessment of the electrochemical and thermal performance of a bipolar planar cell the input temperature profiles are generated for a finite element analysis code to predict thermal component of the stress. The failure criterion adopted is based on the strength of the cell materials and the principal stresses developed by the thermal loading. To visualize the stress concentration in the fuel cell layers, maximum principal stress is calculated and compared with the yield strength of the SOFC materials found in the literature. The in-house code is capable to predict both steady state and dynamic temperature profiles. Of particular importance is the knowledge gained of the transient stress in the cell, which can be used to establish control parameters during transient operations.