Environmental Engineering Reference
In-Depth Information
First of all it is important to analyze the response rate to transient or dynamic
requirements of all individual sub-systems, included the stack [
50
]. The electro-
chemical phenomena that occur inside the fuel cell stack are very fast, while water
and thermal management systems show an enormous inertia with respect to typical
vehicle requirements. In particular working temperature is a crucial parameter for
designing BOP components such as heat exchangers, condensers and humidifiers
devices to match dynamic requirements. Equation
4.8
could be used to demon-
strate that the thermal capacity of the whole stack limits the dynamic in temper-
ature change.
On the other hand, fuel supply system response (when hydrogen is stored in
high pressure tanks) results intrinsically faster than air supply system. The
dynamics of this last sub-system appear particularly significant for the evaluation
of dynamic performance of an overall FCS [
48
], as air compressor response is the
limiting step for an adequate response of stack to load requirement changes. In
particular the variations in air flow rates have to guarantee instantaneous stoi-
chiometric ratios always not much lower than 2 during fast accelerations [
49
].
A dynamic corresponding to a variation rate from 10 to 90% of maximum power
in about 5 s appears the best actual performance of a well-balanced FCS for
automotive application [
6
]. However, the reliability of the stack undergone to
several dynamic cycles based on the above power variation rate need to be verified.
The question is firstly to instantaneously avoid flooding at low temperature or
de-hydration at high temperatures. FCS management strategies have to consider the
low response rate to changes in working conditions of water/thermal sub-system.
A variation of operating temperature may occur only slowly, while dynamic
operation of FCS requires rapid shifting in electric power production.
Two crucial phases have to be considered:
1. Taking into account that an optimal strategy should expect a maximum tem-
perature level (about 75C), during minimum power working points at high
temperature the water produced inside the stack results very low, in spite of a
fast evaporation from the membrane. Humidifier operation is then essential to
assure saturated streams inside the stack. Fortunately the operative conditions
are beneficial (low air flow rate and high temperature) for exploiting the per-
formance of thermal/humidification devices.
2. During start up, when the stack temperature and then performance (voltage) are
low the passive BOP devices devoted to stream and stack humidification
(enthalpy wheels membrane humidifiers) do not work as they are essentially
based on the difference of temperature and then are practically excluded by the
FCS management; on the other hand, at low temperature the requirements for
stack humidification are strongly reduced and membrane could self-hydrate if a
sufficient water production (proportional to power requirement) occurs.
The presence of water in the system makes FCS susceptible to freezing, and
specific procedures are necessary to purge almost completely the stack when it
stops during winter seasons in cold countries [
43
]. A portion of catalyst surface
(especially at cathode side) could be covered with ice becoming not capable to carry
