Environmental Engineering Reference
In-Depth Information
4. H
2
O molecules in cathode compartment are removed by continuous flow of not
saturated air stream; a similar event, but of minor magnitude, occurs also in the
anode compartment in recycling hydrogen configuration or during purge in
dead end configuration.
The electro-osmotic drag could limit the effect of back diffusion mechanism by
transfer of water from the anode to cathode side, thanks to the drag of hydrated
protons, with a linear relationship with current density [
27
].
Finally water removal capacity of reactant streams, especially air stream,
withdraws definitively a part of the water from the cell module. These quantities
could become very significant at high temperature, exceeding the water produced
by electrochemical reaction. This happens because evaporation rate and saturated
vapor pressure of water increase with temperature in a non-linear relationship [
1
].
In Table
4.1
saturation pressure of water vapor and related maximum humidity
ratio are reported as function of temperature in the range of interest for PEMFC
utilization and atmospheric pressure. The humidity parameter is defined as ratio
between the mass of water vapor present in saturated air to the mass of the dry air.
At high temperatures air streams could contain large water amounts as vapor
phase, maintaining a continuous strong potential of drawing water molecules by
electrode surface, as relative humidity value (RH, defined as ratio between
gravimetric density of actual vapor to gravimetric density of saturated vapor at the
same temperature) remains always well below 100% along the entire cathode
feeding channel. The temperature of wet air stream typically remains lower than
local surface temperature of electrode, thus evaporation rate is fast concurring to
drying out of membrane that represents one of the main problems to be faced for
an optimal operation of FCS [
1
], especially at low pressure. The above criticality is
well evidenced if we consider that the electro-osmotic drag causes a movement of
water molecules from anode to cathode side, only partially balanced by the
counterflow related to diffusion by concentration gradient. On the other hand, the
working pressure could strongly affect the maximum humidity amount of air
stream, as pressures higher than atmospheric value reduce the water requirement
necessary to reach the saturation.
Table 4.1 Effect of
temperature on saturated air
parameters, P = 1 atm
Temperature (C)
Saturated pressure of
water vapor (kPa)
Maximum humidity
ratio (g
H
2
O
kg
dry air
)
0
0.6
3.8
10
1.2
7.5
20
2.3
15
30
4.2
27
40
7.4
49
50
12
78
60
20
120
70
31
190
80
47
270
