Environmental Engineering Reference
In-Depth Information
immune from problems. In particular a proper cooling of the warm gaseous
mixture containing hydrogen, nitrogen and steam, is required to maintain a sat-
isfactory durability of the hydrogen recirculation blower. Furthermore, the addi-
tion of other components determines an increase of parasitic power consumption
of the overall FCS [
8
].
Finally the possibility to utilize electrolysers to elevate hydrogen pressure and
recirculate it inside anode flow fields in H
2
FCS power sources has been recently
considered [
10
]. This electro-chemical approach for hydrogen pumping could be
effected with an external additional smaller PEMFC component, connected to the
stack devoted to electric power generation, or alternatively using in electrolysis
mode a group of cells of the main PEMFC stack.
In flow-through operation a slight voltage decrease of individual cells is observed
with respect to the simple dead-end configuration, but also a more stable fuel cell
performance, i.e., due to a minimum intervention of parasitic purge. The role of purge
in an integrated humidification strategy is further discussed in
Sect. 4.5
.
4.3 Air Feeding Systems
The sub-system described in this section has the function to feed the oxidant to the
cathodic compartment. Air is the oxidant commonly used in PEMFC, while the use
of pure oxygen in high pressure cylinders, in spite of higher stack performance, is
generally not preferred for a practical application on fuel cell vehicles since the
addition of oxygen tanks in fuel cell propulsion systems would limit the space
available for the hydrogen fuel storage devices, which is one of the most critical
issues for a practical application of hydrogen fuel cell vehicles. Moreover, oxygen
must be produced, concurring to a well-to-wheel efficiency decrease.
The key parameters to be controlled in this sub-system are air mass flow rate
and pressure. Therefore, this section is focused on the characteristics of devices
capable to realize useful pressure and flow rate of the oxidant feed (blowers and
compressors), postponing the discussion about the integration of air supply system
with the other FCS sub-systems to the successive three sections, which are dedi-
cated to thermal and water management strategies and to overall system perfor-
mance optimization.
Free convection mode is not sufficient to assure a satisfactory oxygen con-
centration at the cathode surface and then an acceptable fuel cell power [
11
]. Thus,
the most adopted solution for the design of oxidant feeding section is represented
by the utilization of a compact air compressor device [
1
]. The overall air supply
system is based on the air compressor, but could also include an expander in high
pressure plants (pressure higher than 2 bar), to recover a part of pressure energy.
An important aspect related to air supply sub-system for the attainment of
optimal FCS operation is the selection of a compressor highly efficient in all
operative conditions suitable for automotive application.
