Page 297 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

2 74 High Temperature Solid Oxide Fuel Cells: Fundamentals, Design and Applications

Table 10.1 Contributions to ASR for a Rise-type anode-supported cell (Ni-YSZ/YSZ/LSM-
YSZ) at 850°C tested in a plug flow-type configuration at 5 and 85% fuel utilisation (FU).
is
Rely* is calculated using a specific conductivity of YSZ of 0.045 S/cm, REOnneEt an
estimation, Ry.Plchem is the sum of typical anode and cathode polarisation resistances
measured in separate electrode experiments, Rp,dftr is calculated using a diffusion
coefficient of 10 cm2/s, 30% porosity, a tortuosity factor of 3 and a thickness of 0.1~~ and
R,,,, is calculated using Eq. (10) with i = 0.5 A/cmZ
Resistance type Contribution to ASR (a an2)
5% FU 85%FU
0.06 0.06
-0.1 -0.1
0.15 0.15
0.06 0.02
0.06 0.31

results from operation demands, and it is thus of special interest to be able to
correct ASR for the effect of fuel conversion.
If a significant amount of fuel is consumed in the cell (or stack) under test, a
resistance derived on the basis of Emf of the inlet gas (cf. Eq. (1)) will be an
overestimation of the 'true' cell resistance. The larger the fuel utilisation the
larger will be the overestimation and results of cell tests performed with different
fuel (and air) utilisations are thus not directly comparable. A comparison
between cell test results obtained under different and non-negligible fuel
utilisations must thus, to be meaningful, be based on a resistance measure,
ASRcOr, where the effects of changes in gas composition over the cell area have
been taken into account. How the correction is applied depends on how the gases
are fed to the cell. Here, two idealised cases are considered, namely the case
where the fuel compartment may be considered a continuously stirred tank
reactor (CSTR) or a plug flow reactor.
If the fuel compartment can be considered CSTR-like due to effective mixing
because of a turbulent gas stream and fast gas diffusion, ASR,,, can be calculated
from the expression

where Emf,,, signifies the average Emf, which in this case is the same as the Emf
of the outlet gas. An example of such conditions is reported in reference [42].
The plug flow case is slightly more complex. Under the assumptions that the
local area-specific resistance is independent of the position along the fuel and air
flow channels, and the flow pattern is co-flow, ASR,,, may be calculated from the
expression [43]