Environmental Engineering Reference
In-Depth Information
electrochemical approach for hydrogen oxidation assures intrinsically zero local
emissions, the potentialities of the fuel cell power generator play the crucial role in
determining the efficiency of a fuel cell propulsion system. On the other hand, the
expected fuel consumption of a vehicle depends not only on energy conversion
efficiency of the FCS but also on lightness and compactness of all propulsion
system components.
The FC power system could be practically applied in transportation field if it
demonstrated to own some essential qualifications, not limited to energy effi-
ciency, but extended to operation durability, also in terms of water neutrality, and
to FCS dynamic response during start-up and variable driving requirement phases.
Beside the characteristics of the main components involved in stack manage-
ment, already previously underlined, in this section the system integration con-
cerns involving dynamic behavior of individual sub-systems are analyzed,
detailing their impact on efficiency evaluation and stack response in automotive
conditions. The inter-connections between the stack and some crucial ancillary
components are specifically discussed to itemize the basic design issues of an
integrated FCS and to support the analysis of management strategies for vehicle
consumption optimization in several automotive conditions.
For a low pressure configuration the air supply system includes the com-
pressor preceded by a filter, while for a high pressure option a CEM group could
be adopted, adding an expander to the compressor and recovering the high
energy content of the outlet stream. In low pressure plant the hydrogen section is
based on dead-end configuration with a purge valve electronically controlled,
while in high pressure configuration the purged hydrogen should be used
optionally in a burner to enhance operation of the expander/turbine [
43
]. Higher
hydrogen exhaust flow rate increases the electric power recovered from turbine
reducing parasitic losses but the net efficiency results lower because of a minor
utilization of the fuel [
43
].
A representative scheme of a low pressure FCS plant for automotive application
is shown in Fig.
4.9
. The reactant supply sub-systems could directly interact with
thermal and water management sub-systems, by means of a simultaneous transfer
of heat and mass into the humidifier devices, which should be inserted at the
entrance of the stack for both reactants. Thermal sub-system includes an internal
coolant circuit that is essentially constituted by a liquid pump, a radiator necessary
to reject the stack waste heat, and a liquid reservoir. Other minor but equally
important components are the de-ionizer filters, thermostat, and valves.
However, the cooling sub-system should expect interactions with the humidi-
fication devices inserted in air and hydrogen feeding pipelines. In particular the
warm cooling liquid outgoing from the stack can transfer a fraction of its enthalpy
content to feeding streams for their humidification, before passing through the air
cooled radiator. Humidification/water sub-system is constituted by all humidifiers
inserted in the overall layout, and by a water recovery loop, which concur to
realize an overall process water circuit.
This sub-system includes in particular condensers and demisters at the outlet of
cathode and anode compartments to recover a significant part of the high water
