Page 340 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

3 16 High Temperature Solid Oxide Fuel Cells: Fundamentals, Design and Applications

and hl/hz = 60, the residual stress is about 6 MPa. For cathode-supported cells,
however, the anode residual stress could be one order of magnitude higher and is
very undesirable. The anode residual stress is the highest in the electrolyte-
supported cells.
When temperature distribution in the cell structure is known, the finite-
element structure model can analyse the thermal stresses. The thermal stresses
dictate the process of heating and cooling required in SOFC applications.
Thermal stresses also dictate how uniform the temperature should be in
steady-state operations. Modelling results indicate that temperature gradient
makes the largest contribution to overall stress. Moreover, it is important to
maintain a uniform temperature gradient along the cell in minimising the
anode/electrolyte/cathode stress. Figure 11.7 demonstrates the dramatic
reduction in stress going from a parabolic temperature distribution to uniform
temperature gradient.

-I-cmpcl-atlll-e Y.: i Slress (El)
( h) I ltiifortn 'I'emperiit~ire trndicnt
(
Figure 11.7. EfSect of temperatureprofile on SOFCstresx

The probability of failure under stress (3 can be calculated from the Weibull
function:
W = 1 - exp[-(a/~o)~] (31)