Environmental Engineering Reference
In-Depth Information
Table 6.5
Stack and FCS efficiency, and C
v
for the experimental points reported in Fig.
6.18
Experiment
No.
Stack
temperature (K)
Stack power
(W)
Stack
efficiency
FCS
efficiency
Coefficient of variation
C
v
(%)
1
293
245
0.655
0.171
0.32
2
298
252
0.661
0.173
0.36
3
303
255
0.667
0.175
0.35
4
303
460
0.633
0.398
0.41
5
308
465
0.635
0.400
0.42
6
310
465
0.649
0.408
0.44
7
313
650
0.638
0.452
0.43
8
313
855
0.614
0.472
0.51
9
318
820
0.619
0.472
0.55
10
318
1010
0.608
0.472
0.59
11
318
1460
0.579
0.461
0.94
12
323
1290
0.617
0.480
0.96
13
323
1460
0.588
0.466
1.04
14
323
2050
0.554
0.446
1.72
15
328
1502
0.588
0.470
1.44
16
328
2240
0.556
0.448
1.88
17
333
1815
0.578
0.464
1.96
18
333
2395
0.553
0.449
1.92
19
338
2220
0.563
0.456
1.98
20
338
2405
0.541
0.439
1.97
this important issue. In Fig.
6.20
, the relation between stack power and temper-
ature is reported for those experimental points corresponding to the reliable fuel
cell system operation (C
v
B 2%). As previously discussed, the purge strategy is
based on the criterion to increase the purge frequency in proportional way with the
stack power, maintaining constant the fuel utilization coefficient (about 0.90),
while the external humidification is used only when temperature is above 333 K.
The region upward of the upper dotted line corresponds to operative conditions
characterized by flooding phenomena (region A), while that downward of the
lower line comprises working points characterized by the tendency to dehydration
of cell membranes (region B).
This diagram gives useful indications about the control strategy to be adopted
during dynamic operations, in particular regarding humidification, purge, and
cooling procedures. For instance, if during a particular transient operation the stack
working point falls inside the region A, the control system has to react by
increasing purge frequency and stopping the heat removal from cooling circuit,
taking into account the slow stack heating dynamics with respect to automotive
demands. On the other hand, if the working point falls inside the region B, it is
necessary to reduce the stack temperature and increase the quantity of external
water injected.
In Table
6.5
the C
v
values for all experimental points of Fig.
6.20
are reported.
These data, calculated as average on the entire length of acquisition runs, increased
