Environmental Engineering Reference
In-Depth Information
On the other hand, the use of a fuel processor on-board of a vehicle makes it too
complex for a practical commercialization in the automotive field, independently
on carbon emissions associated to utilization of not renewable fuels.
Moreover, PEMFC systems fed by pure hydrogen show the highest relative
performance in terms of system dynamics, costs of fuel cells (the precious metal
loading of anode is minimum), and in terms of stack and system power densities,
which result 1.3 and 0.6 kW/l, respectively [
2
,
3
].
In this chapter the discussion is limited to the so-called direct hydrogen fuel cell
system (H
2
FCS) excluding from the analysis alternative configurations utilizing
hydrogen containing carriers (hydrocarbon mixtures, methanol) instead of pure
hydrogen as feed.
4.1 Hydrogen Fuel Cell Systems: Preliminary Remarks
The main operative characteristics of PEMFC stacks can be summarized in the
following points:
1. Different fuels can be used but hydrogen is the best reducing agent for an
efficient and reliable operation of PEMFC.
2. Oxygen is the ideal reactant to feed cathode side of the stack, however also air
feed is suitable, but in this case an excess of oxidant is required.
3. Water is the product of the electrochemical reaction.
4. Pressure and temperature increase the individual cell performance.
5. The electrolytic membrane needs to be maintained properly hydrated in all
operative conditions of pressure and temperature.
6. Heat is a by-product of fuel cell reaction and progressively boosts the stack
temperature.
7. The stack temperature cannot exceed 90C.
Thus, the optimization of PEMFC performance in terms of efficiency and
reliability requires a proper design and management of reactant feeding sections,
as well as of cooling and humidification sub-systems [
4
].
The selection and sizing of auxiliary devices, also called 'balance of plant'
(BOP) components, depends on their interactions with the stack and on all other
possible inter-connections inside the overall system.
In Fig.
4.1
a scheme of a direct hydrogen FCS is reported. The scheme points
out all input and outputs of the plant: the oxidant is ambient air, while the fuel
enters into the FCS coming from the selected H
2
storage device (see
Sect. 2.3
).
The electrochemical hydrogen oxidation produces electric energy, water and heat,
the last two can be partially recovered for stack management. The same Fig.
4.1
indicates the main FCS subsystems necessary to manage reactant feeding, control
stack temperature, and assure adequate membrane humidity level during stack
operation. The interconnections between all sub-systems, shown in Fig.
4.1
,
underline the necessity of a complex integration among BOP components and
