Environmental Engineering Reference
In-Depth Information
Table 7.2 Relative
uncertainties of calculated
experimental parameters
Parameter
Uncertainty (%)
R
3.8
P
comp
1.4
P
stack
1.4
g
stack
0.7
c
SC
4.0
g
SC
4.7
Calculated
starting
from
the
sensor
accuracies
reported
in
reference [
3
]
compressor management strategies have on the stack performance, for different
power variation rates. The dynamic tests of the FCS are conducted on different
working cycles, changing the stack current variation rate from 2 to 50 A/s. These
values are selected with the aim of simulating the acceleration requirements of a
fuel cell vehicle of size and power compatible with the 20 kW FCS, for instance,
the power requirements of a city car or an urban minibus equipped with a nominal
30 kW electric drive running on the European R40 driving cycle are considered
[
7
]. During the dynamic phases of this cycle, which lasts about 20 s with accel-
erations of about 1 m/s
2
, peak power requirement of about 50 kW for the vehicle
selected are certainly expected. Evaluating an average efficiency for the electric
drives and DC-DC converters of about 0.8 and FCS average total voltage of about
55 V, the above value of engine peak power means a stack current derivative of
about 50 A/s.
During experiments all the main parameters characteristic of stack operation,
such as stack current and individual cell voltage, reactant pressure, air and cooling
water temperature, and humidification level are continuously monitored. Stack
temperature and reactant pressure during the tests are maintained below 330 K and
150 kPa, respectively.
Figure
7.40
a shows the profiles of stoichiometric ratio versus stack current for
different compressor management strategies, which can be obtained by imposing
different references of the compressor motor speeds as function of stack current,
while Fig.
7.40
b reports the air flow rate values corresponding to the different
strategies. The curves 1 and 2 of Fig.
7.40
a refer to compressor motor speed
regulations which determine two constant air flow rates of 33 and 24 Nm
3
/h,
respectively, and are obtained with constant voltage reference of the motor speed
(4 and 3 V). The other curves in Fig.
7.40
b refer to variable air flow rates, in
particular the curve 3 is related to R values comprised between 2 and 3 in the range
of 100-200 A, obtained with a linear voltage reference increase up to 4 V (140 A
as stack current), corresponding to an air flow rate almost linearly increasing from
12 to 33 Nm
3
/h in the range 0-150 A, and constant at this value up to 200 A.
Finally, the curve 4 is obtained with a control which allowed the compressor to
reach R = 2 already at 100 A and to keep it constant at higher loads, with an linear
flow rate increase from 16 to 33 Nm
3
/h in the range 100-200 A (Fig.
7.40
b). This
management is realized imposing a first increase of compressor voltage reference
