Environmental Engineering Reference
In-Depth Information
the dominant mechanism and water is transported away from the anode and hence
the anode will dry out while cathode will become flooded. Then temperature and
stack power are the key parameters to maintain wet the membranes in self-
humidification conditions.
The membrane hydration is related also to air flow management. Air stream
passes through cathodic compartment becoming progressively warm and wet, and
favors a drying effect, that is strongly non linear in its relationship with temper-
ature. At temperature higher than 60C the relative humidity of outlet air can result
easily lower than saturation (100%). By controlling the R parameter it is possible
to manage RH close 100% of exit cathode streams and then to control hydration of
membranes. Small size FCS (\3 kW) could operate in reliable self-humidification
condition if a proper working temperature (\60C) was adopted in FCS man-
agement, correlated to adequate stack powers during the execution of pre-defined
driving cycles [
6
]. The maximum power drops of about 40% but the efficiency
remains high.
A further ''internal'' technique, known as ''internal membrane humidification''
uses liquid water injection directly inside the stack, specifically designed to receipt
''water droplet spray''. A portion of the membrane is set aside to humidify the inlet
gas and liquid water is injected directly into this inactive portion of the stack,
permitting then the possibility for a more flexible water management thanks to an
additional governable parameter [
31
,
32
]. Internal humidification strategies gen-
erally reduce the complexity of the FCS design, and present a further important
advantage: the gases are conditioned inside the stack, and their temperature is very
close to membrane temperature itself, avoiding fast evaporation and then dehy-
dration. However, a portion of membrane is not used for reaction and the power
density decreases.
On the other hand, the use of external humidification is essential at high tem-
peratures, because the concentration gradient of H
2
O in the membrane of indi-
vidual cells would be more uniform if both air and hydrogen streams were
humidified externally. The external water supply helps to balance the combined
effects of electro-osmotic drag and back diffusion permitting to maintain the
performance of the membrane. External humidification is practically useful also
below 60C, at least for medium large-size FCS.
A possibility is to saturate at different temperatures the reactants before they
enter into the stack [
33
]. This approach can be accomplished by several procedures
based on external dewpoint, external evaporation, steam injection with down-
stream condensers, or flash evaporation. High temperature values allow to absorb
significant water amount in gas streams and then transport it inside the stack
compensating the water losses due to internal fast evaporation. However, the main
problem with external humidification is that the gas cools after the humidifier
device, the excess of water could condense and enter the fuel cell in droplet form,
which floods the electrodes near the inlet, thereby preventing the flow of reactants.
On the other hand, ''internal liquid injection'' method appears preferable for
example with respect to the steam injection approach because of the need of large
energy requirement to generate the steam.
