Environmental Engineering Reference
In-Depth Information
where L
mecc
is the total mechanical energy required to reach the final desired
pressure value. Furthermore, the efficiency parameter g
c
includes also the addi-
tional contribution related to friction of moving parts (mechanical losses). Finally
the real power consumption of a compressor device can be calculated as:
P
real
¼
P
id
g
c
ð
4
:
5
Þ
On the other hand, a fraction of power consumption related to compressor can
be recovered from the electric work obtainable from exhaust gas by using a
turbine. This electric power can be calculated according to the following equation:
"
#
k
1
k
p
atm
p
ex
P
real
¼
m
ex
c
pex
T
ex
1
g
exp
ð
4
:
6
Þ
where g
exp
represents the turbine efficiency while the other parameters are strictly
related to FCS management. In particular, m
ex
is the exhaust flow rate, T
ex
and p
ex
correspond to stack temperature and pressure, while c
pex
is the specific heat of the
exhaust mixture. The choice of a proper high efficiency compressor is an important
criterion also when Compressor Expander Module (CEM) groups are selected in
high pressure plants [
4
].
In Fig.
4.3
two schemes of the air supply sub-system are reported, related to
both low and high pressure fuel cell plants. As the air supply sub-system implies
the highest power consumption among all BOP components, and could heavily
affect the overall system efficiency (see
Sect. 4.6
), the utilization of blowers
(Fig.
4.3
a) would permit to limit power losses, even if their impact on the overall
efficiency would not be negligible. In fact, blowers not only require a not negli-
gible electric power consumption especially at minimum load, but they can pro-
vide low air pressure values, then limiting cell voltages and thus stack efficiency
(see
Sect. 3.3
). In any case the low cost and simplicity make this solution more
appropriate for small size power trains (1-10 kW).
On the other hand, high pressure determines a higher energy consumption
associated to the compressor, then a CEM can be used to recover some energy
from the pressurised cathode exhaust stream (Fig.
4.3
b). This solution adds
complexity to the on-board power plant and can be usefully applied in medium
large-size fuel cell power trains (10-100 kW).
The air feeding strategy must minimize the disadvantages related to an excessive
parasitic power consumption and meanwhile to oxygen starvation. The last problem
exists when air is fed to cathode side instead of pure oxygen, and depends on
variable partial pressure of oxygen during stack dynamic operation. The control
strategy capable to overcome the O
2
starvation is based on a careful regulation of
inlet pressure to realise values of air flow rate higher with respect to stoichiometric
requirements. Oxygen concentration in the air intrinsically decreases crossing
through the reaction cells. The inert nitrogen moles contained in the air remain
unchanged during stack operation, and oxygen concentration drops at the catalyst
