Environmental Engineering Reference
In-Depth Information
cathode flow field through the electrolyte to the anode side. As fuel is consumed in
the fuel cell, the concentration of nitrogen in the anode compartment increases,
thus accumulating therein and negatively impacting the performance of the fuel
cell [
5
]. Moreover, the humidification control (see
Sect. 4.5
) could create operative
phases characterized by a dangerous stagnation of liquid droplets on surface of
electrodes in cathode and anode sides, favoring flooding of the compartments and
fuel starvation, and then interfering with the access of the hydrogen fuel during
stack power requirement. Then the purge action, draining out the possible excess
of nitrogen and water accumulated in anode compartment, can eliminate most of
liquid molecules from the catalyst surface and refresh the feed. As a consequence
the overall strategy for maintaining the well-hydrated cell membranes, assuring a
reliable and efficient stack operation with minimization of nitrogen crossover and
simultaneously avoiding the flooding phenomena, should involve the periodic
acting of the purge valve. This is normally closed but, when necessary, the control
strategy expects to open it by managing the opening frequency for a specified
opening time, sufficient to drain the contaminants but not too long for undesired
leaking of useful fuel. In order to evaluate the effect of purge operation on FCS
efficiency, it is possible to define a coefficient expressing the ratio of fuel
converted to fuel supplied (g
util
in
Sect. 6.2
), which can reach values higher than
90% in optimized realizations [
6
,
7
].
On the other hand, in flow-through mode a system layout based on fuel recir-
culation by means of a pump or ejector-diffusers can be adopted in the anode
circuit [
8
]. In this case a portion of the exhausted fuel is recirculated from the
outlet to the inlet section through a loop and mixes with fresh fuel from the fuel
supply. An important advantage of this solution is that the mixed fuel entering the
fuel cell stack is at least partially humidified, and this aspect can bring strong
benefits to an overall humidification management strategy, avoiding or minimizing
external humidification of anode stream (see
Sect. 4.5
). However, nitrogen could
cross-over from the cathode side through the conductor membrane to the anode
side and then could affect the fuel purity in the recirculation loop. This behavior
reduces the performance of the fuel cells and the efficiency of the recirculating
device. In addition, contaminants from the fuel source can also be accumulated in
the anode flow fields (and/or in anode recirculation loop) as fuel is consumed.
Thus, this approach does not exclude the use of a purge valve; indeed it is highly
recommended that a remainder portion of anode stream is purged periodically, in
order to limit fuel purity decrement during FCS operation. However, the proper
management of this layout of fuel feeding section could improve the control of
anode humidity and purity, minimizing hydrogen escape. In ejector systems, based
on Bernoulli's principle, the supply flow passes down through a nozzle and
Venturi tube. They are passive devices that need no parasitic power, however, their
actual performance, in terms of optimal regulation of anode humidity with recir-
culating flow, should be improved to match all automotive FCS requirements [
9
].
Instead of ejectors a blower could be inserted in the anode loop to increase the
pressure and recirculate hydrogen. This solution permits sizing and controlling the
sub-system adding an independent parameter for FCS management, but it is not
