Environmental Engineering Reference
In-Depth Information
(a)
(b)
AIR BLOWER / COMPRESSOR
AIR COMPRESSOR
EXHAUST
M
M
EXPANDER
H
2
O
H
2
O
BACK
PRESSURE
REGULATOR
EXHAUST
Fig. 4.3
Scheme of air feeding sub-system in low (a) and high (b) pressure plants. M compressor
motor
surface, requiring then an excess of air variable as function of load. In particular the
stoichiometric ratio R (defined in
Sect. 3.3
) needs to be about 2 in a wide range of
operative conditions (details about air management strategy and related R values
are reported in case studies,
Chaps. 6
and
7
). On the other hand, if the risk of
starvation is reduced, the energy consumption related to compression stage of
higher mass flow rates could increase dramatically, strongly limiting the overall
FCS efficiency (see also
Sect. 4.6
).
A further requirement to be satisfied by the oxidant supply device is represented
by dynamic performance in a wide range of air flow rates. This aspect is crucial in
some hybrid power train operative modes characterized by high stack dynamics
(full power or soft hybrid configurations, see
Sect. 5.5
).
A compressor should also match the other typical requirements of a road
vehicle, in particular noise, cost reduction, and compactness. A further consider-
ation derives from the quality of oxidant required by the stack. The air must be
very clean since the presence of few small oil droplets or traces of chemical
contaminants could damage dramatically the fuel cells, reducing their efficiency
and durability. Therefore, PEMFC systems require oil-free compressors or air
filtration for removing particulate and contaminants (sulfurs, salts, CO, and
hydrocarbons).
