Finite element analysis and modelling of thermal stress in solid oxide fuel cells

摘要

Durability and reliability of anode supported SOFC stacks have proven unsatisfactory in large scale trials, showing rapid failure, thermal cycling in tolerance and step change in electrochemical performance most likely related to mechanical issues. Monitoring and understanding the mechanical conditions in the stack especially during temperature changes can lead to improvements of the design and of the operating regime targeting maximum durability. Within this project modelling and simulation of thermal stresses within the different parts of the cells and the stack and the validation of this models play a key role and were performed in this work. The modelling and simulation of stress and strain have been carried out using the FEA software ABAQUSTM. Model variations documented the importance of exact knowledge of material properties like Young’s modulus, Poisson’s ratio, thermal expansion coefficient, thermal conductivity and creep viscosity. The benefit of literature data for these properties is limited by the fact that all these properties are highly dependent on the composition of materials but also on details of the fabrication process like mixing, fabrication technique and sintering temperature and duration. The work presented here is an investigation into the modelling techniques which can be most efficiently applied to represent anode supported solid oxide fuel cells and demonstrates the temperature gradient and constraint on the stresses experienced in a typical design. Comparing different meshing elements representing the cell parts thin shell elements (S4R) provided the most efficiently derived solution. Tensile stress is most significant in the cathode layers reaching 155 MPa at working conditions. The stress relieving effect of creep led to a reduction of stress by up to 20% after 1000 hours at 750°C, reducing the tensile stress in the cathode area to maximal 121 MPa. Constraint between bipolar plates increases the tensile stress, especially in the cathode layers leading to a peak value of 161 MPa.