Environmental Engineering Reference
In-Depth Information
Fig. 7.41 Stack and
stack ? compressor system
(SC) efficiencies as function
of stack current, for the four
strategies of Fig.
7.40
a
0,8
0,6
stack
SC law n.1
SC law n.2
SC law n.3
SC law n.4
0,4
0,2
0
0
50
100
150
200
250
Stack current, A
For the tests carried out with the compressor management strategy N.4 a simple
current cycle is used, whose dynamic is characterized by an acceleration ramp
(2, 10, and 25 A/s), a stationary phase at 160 A and a deceleration step at the same
rate of acceleration. In Fig.
7.42
a-c, the acquisitions of the main operative
parameters during cycle length are shown for the test at 2 A/s. In Fig.
7.42
a, the
current cycle is reported together with the total stack voltage. This drops down
from 71 to about 49 V at the end of the acceleration step, then slowly increases up
to 51 V during the stationary phase (110 s), and rises again during the deceleration
step. The R and air flow rate profiles (Fig.
7.42
b) show that the compressor
working is not stressed for the slow dynamic adopted, as their values are exactly
corresponding to those expected by the imposed management strategy for every
instant of the cycle. The slight voltage increase, observed in Fig.
7.42
a during the
stationary phase, can be correlated with the stack temperature increase, which is
possible to observe in Fig.
7.42
c. However, the initial phase of this voltage
increase (between 80 and 100 s) can be also related to the thermal management
strategy adopted during the test. In particular, the flow of the external water
through the heat exchanger is stopped just before the beginning of the cycle, when
the stack output temperature is decreasing. This justifies the initial stack cooling
shown in Fig.
7.42
c, and the voltage behavior between 80 and 100 s. Figure
7.42
c
also evidences the constant values of inlet air humidity and temperature (100% and
303 K, respectively).
The results obtained at 10 A/s are shown in Fig.
7.43
, in particular the voltage
profile reported in Fig.
7.43
a is practically coincident with that related to 2 A/s,
with a more regular behavior also at the beginning of the stationary phase, because
during this test the stack is not previously cooled by external water through the
heat exchanger.
The temperature profiles are reported in Fig.
7.43
c, where it is evidenced that
output air and cooling water temperatures slightly increase during the test fol-
lowing almost the same values. The imposed values of air humidity and inlet
temperatures are also reported in Fig.
7.43
c. The faster dynamic of this test
determines an appreciable effect on the stoichiometric ratio (Fig.
7.43
b), whose
profile decreases below R = 2 in the range 15-20 s at the end of the acceleration
phase. These effects are more evident in the test effected at 25 A/s (Fig.
7.44
b),
