Environmental Engineering Reference
In-Depth Information
Fig. 6.10 FCS warm up
from 30 to 45C, with
200 W s
-1
up to 1.2 kW
stack power. FCS efficiency,
stack efficiency, and temper-
ature versus time
that energy losses associated with the warm-up phase in these conditions decrease
to about 2%.
An important point to consider about the stack management, with reference to
an electric power train operating in dynamic conditions, as determined by road
requirements, is the regulation of the stack temperature together with the other
control parameters of water and reactants to avoid mass transfer limitations and
membrane drying out or flooding. Moreover, the interaction between stack and
auxiliaries has to be balanced taking into account the optimization of fuel cell
system efficiency and reliability (see
Sect. 4.6
).
A statistical indicator can be used for the evaluation of the individual cell uni-
formity, which is selected as representative of a reliable operation of the stack [
3
].
This indicator is a coefficient of percentage variation, C
v
, that can be calculated by
the following equation:
t
P
1
2
V
i
V
V
C
v
¼
100
;
ð
6
:
9
Þ
n
where V
i
is the voltage of individual cells, V is the average cell voltage and n is the
number of cells. A value of C
v
of 2% is considered acceptable for a regular
working of the stack in tests described in the following [
3
].
The results reported in Figs.
6.11
,
6.12
,
6.13
,
6.14
,
6.15
,
6.16
, and
6.17
[
3
]
refer to experiments performed using a series of power steps up and down, each
of them characterized by a rate of about 150 W/s. The Fig.
6.11
shows stack
power, stoichiometric ratio, and temperature acquired as a function of time,
whereas the coefficient of variation C
v
related to individual cells is reported in
Fig.
6.12
.
Two time acquisition windows (in the range of 0-1040 s and 1820-2120 s) are
specifically analyzed in Figs.
6.13
and
6.14
. In the first window, the stack power
increases up to 1.8 kW, while no temperature control is adopted until 335 K.
