Page 222 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

Cell and Stack Designs 199

(iv) Mechanical/structural integrity. This requirement means that any
planar SOFC stack must be designed to have adequate mechanical
strength for assembly and handling. Thus, mechanical and thermal
stresses must be kept to minimum to prevent cracking, delamination, or
detachment of the components under the variety of operating conditions
the stack is expected to experience (e.g., normal operating temperature
gradients, off-design temperature gradients, thermal shock conditions
such as sudden power change and cold start-up, and mechanical loading
expected during installation, moving, and vibration loading).

Table 8.1 summarises the design requirements for planar SOFCs.

Table 8.1 Design requirements
Property requirement Design target
Electrical Minimal ohmic loss Short current path
performance Good electrical contact and sufficient
contact area
Current collector design for uniform and
short current path
Electrochemical Full open circuit voltage Insignificant gas leakage or cross-leakage
performance (no or minimal sealing)
No electrical short
Low polarisation loss Uniform gas distribution between cells
and across cell
Easy gas access to reaction sites
Thermal Cooling and uniform Simple and efficient means for cooling
management temperature distribution Appropriate gas flow configuration
Highest possible Design to withstand thermal stress
temperature gradient
across stack
Mechanical/structural Mechanical strength for Minimal mechanical stress
integrity assembly and handling

The most important design feature of the planar SOFC relates to gas flow
configuration and gas manifolding which can be arranged in several ways:

(i) Gas flow configurations. Fuel and oxidant flows in planar SOFCs can be
arranged to be cross-flow, co-flow, or counter-flow. The selection of a
particular flow configuration has significant effects on temperature and
current distribution within the stack, depending on the precise stack
design. Various flow patterns can be implemented in the different flow
configurations including Z-flow, serpentine, radial, and spiral patterns
(Figure 8.2). Flowfields (flow channels) are used in planar SOFCs to
increase uniformity of gas distribution and to promote heat and mass
transport in each cell. In addition, the flowfield is often designed to have
sufficient pressure drop through the cell to promote cell-to-cell flow