Environmental Engineering Reference
In-Depth Information
6.3 FCS Characterization: Energy Losses in Steady-State
Conditions
The Fig.
6.4
shows the characteristic curves of the stack, in terms of voltage and
power versus stack current. The stack output voltage decreases from 32 V, at low
load, to about 22 V, at the highest load tested (115 A), when the stack power
reaches the peak value of 2.5 kW. The polarization curve presents a linear rela-
tionship between voltage and current in the working range of 10-90 A, with a
voltage decrease from 27 to 24 V.
The experimental conditions to obtain these characteristics are the following:
stack temperature of 60C, hydrogen pressure ranging from 20 to 50 kPa, air
pressure ranging from 10 to 16 kPa, and stoichiometric ratio (R) ranging from 2 to
6 (for definition of R see
Sect. 3.3
). As regarding the purge intervention strategy,
the opening frequency of the purge valve adopted for all tests reported in this
chapter is proportional to stack power (and then to the water amount produced by
the reaction), corresponding to a constant value of g
util
(0.90).
The Fig.
6.5
shows the relation between R and the stack power, which is used for
all the tests reported in this chapter. This figure evidences that the selected
air management strategy determines R values decrease from R = 9 at open circuit
to about R = 2 for a stack power between 500 and 2000 kW. These values
of R are specifically selected to minimize the air compressor energy losses (see
Sect. 4.3
).
As regarding the humification strategy, the quantity and frequency of water
injected is chosen according to the criterion of regulating both water injection and
fuel purge (see
Sect. 4.2
) according to individual cell voltage uniformity (see
Sect. 6.4
). Apart from this general criterion, the injection pump is automatically
activated when outlet air temperature is higher than 333 K, with water flow rate of
0.17 l/min and injection time of 1 s every 10 s [
1
], to assure sufficient membrane
hydration also in conditions of high temperatures.
Fig. 6.4 Stack characteristic
curves (R = 2-6, T = 333 K,
P
H
2
\ 50 kPa, P
air
\ 20 kPa)
