Environmental Engineering Reference
In-Depth Information
Fig. 7.31 Individual cell
voltage versus time during a
sequence of eight fast load
variations from 0.7 to 8.5 kW
1
0.9
0.8
0.7
0.6
0.5
0
150
300
450
600
750
900
Time [s]
reported for the entire acquisition time, respectively. The coefficient of variation
C
v
is mostly lower than 1% for the entire range of time considered, while few
sharp peaks up to 2.2% are observed during the acceleration phases (R \ 1.5),
evidencing an acceptable regularity of stack working. Moreover, a slight voltage
recovery is observed for those cells more affected by incoming flooding phe-
nomena (Fig.
7.31
), specifically when the hydrogen pressure drops due to the
purge intervention.
7.3.2 Behavior of the FCS During Warm-up
In this section, the experimental results regarding the analysis of the FCS during
the warm-up phase are presented. The tests are carried out starting from 18C, for
two different acceleration values (150 and 1,500 W/s). Regarding the stack
humidification, the inlet air is saturated at the same temperature adopted for the
stack at the beginning of the tests, to avoid membrane de-hydration.
The individual cell uniformity is monitored by using the coefficient of per-
centage variation C
v
as statistical indicator already introduced in
Sect. 6.3
. For the
analysis reported in this paragraph a value of C
v
equals 2.5% is considered
acceptable for a regular working of the stack.
In order to study transient response of the stack during the warm-up phases the
following experiments are carried out connecting the FCS to a resistive load
electronically controlled. Starting from the temperature of 18C the stack power is
gradually increased up to about 8 kW by using two power accelerations (150 and
1,500 W/s). After each acceleration the system is left in steady state condition
until the temperature of about 45C is reached for the stack.
In Fig.
7.32
, the stack current, voltage, and temperature are shown versus time
for the test at 150 W/s, while Fig.
7.33
shows a magnification of the stack power
and stoichiometric ratio during the acceleration phase. It can be observed that the
stack temperature reaches its final value in about 10 min, while the stack voltage
reaches the minimum value of 53 V at the end of acceleration slope (55 s), then
increases up to about 60 V at 45C. The stack current reaches the pick value of
