Environmental Engineering Reference
In-Depth Information
a short-transient period (about 5 s, Fig.
7.36
), R decreases below 1.5 at 7 kW
(time = 32 s in Fig.
7.37
), C
v
profile shows a sharp peak of about 2.5% at the
same instant (Fig.
7.38
). The stack dynamic behavior is better detailed in
Fig.
7.39
reporting the individual cell voltages at the moment corresponding to the
end of the acceleration phase (maximum C
v
). It can be observed that the cell
voltage distribution significantly worsens compared to results of Fig.
7.35
, but no
cell voltage drops below 0.4 V, which can represent a limit value for a reliable
stack operation, endurable only for few seconds. The results of Figs.
7.36
,
7.37
,
7.38
, and
7.39
represent the acceptable limit for the stack tolerance to dynamic
conditions of warm-up, and provide useful quantitative indications of the power
ratio between the fuel cell generator and the battery during the start up [
6
].
7.4 Effect of Different Air Management Strategies
The results shown in previous sections suggest the crucial role of the oxidant
supply system, in particular the effect of the stoichiometric ratio (air flow rate) on
FCS efficiency and dynamic performance. In order to give further information
about this issue, in this paragraph different air management strategies are closely
examined, with particular reference to their influence on cell voltage uniformity
and air compressor parasitic losses.
The behavior of the subsystem composed by stack and air compressor is studied
using four different management strategies, which allows different profiles of
stoichiometric ratio versus stack current to be obtained, as discussed in the
following analysis.
If the stack efficiency is calculated using the equation already shown in
Sect. 6.4
:
g
stack
¼
V
V
id
;
ð
7
:
1
Þ
the following coefficient (g
SC
) can be used to evaluate the effect of air compressor
power consumption on the net power provided by the stack:
c
SC
¼
P
stack
P
comp
P
stack
ð
7
:
2
Þ
;
then the efficiency of the sub-system composed by stack and compressor (SC) is
calculated by:
g
SC
¼
g
stack
c
SC
:
ð
7
:
3
Þ
The individual cell uniformity during the dynamic tests is evaluated by using
the coefficient of percentage variation C
v
.
In Table
7.2
, the relative uncertainties of the calculated experimental parame-
ters are reported. The results are analyzed focusing the attention on effects that air
