Environmental Engineering Reference
In-Depth Information
the strategy N.4 allows the minimum compressor consumption to be assured,
especially in the field of low powers, it does not guarantee the reliable stack
operation in more elevated dynamic conditions (starting from 25 A/s). On the
other hand, the strategies N.1 and 2, based on the regular operation of the air
compressor, evidences the necessity of higher air flow rates during dynamic
phases, in particular at 50 A/s, but implies a strong efficiency loss, as indicated by
the steady state results of Fig.
7.41
. The best balance between efficiency optimi-
zation and dynamic response of the FCS is then reached by the management
strategy N.3 characterized by R values only slightly higher with respect to the
strategy N.4 up to 200 A. This strategy allows a regular stack voltage to be
maintained up to 50 A/s also in the field of high powers, while the overall effi-
ciency decrease with respect to the strategy N.4 could be considered not signifi-
cant, as suggested by the results obtained in stationary conditions (Fig.
7.41
).
A further improvement of efficiency and regular stack working in conditions of
high loads could be reached by increasing the reactant pressure, in particular this
could prevent too low cell voltages at the highest loads (\0.5 V), which are
observed in Fig.
7.55
[
3
].
7.5 Fuel Cell Power Train Tested on the R40 Driving Cycle
The propulsion system is powered in a hybrid configuration by using lead batteries
and the 20 kW PEM stack described in the above paragraphs. The experimental
tests in dynamic operation are carried out on a laboratory test bench utilizing the
European R40 driving cycle, varying both dynamics and maximum power of the
test cycle for different hybridization levels between FCS and batteries.
As previously discussed (
Sect. 5.5
), the battery pack of a fuel cell power train
can be either minimized assigning the role of generating most energy required by
the load to the fuel cell stack (soft hybrid), or sized in order to provide all dynamic
requirements of the vehicle allowing the utilization of a smaller FCS (hard hybrid).
The first experiment on the fuel cell power train is carried out imposing to the
R40 cycle an acceleration slope corresponding to the stack current the variation
profile of Fig.
7.14
(5 A s
-1
) and the air management strategy of Fig.
7.17
(minimum compressor consumption). The results obtained are shown in Fig.
7.56
,
where the power distribution between engine, battery pack, and DC-DC converter
is reported versus cycle length. The engine power reaches three maximum values,
which are 5, 10, and 15 kW, at the end of the three acceleration phases of the R40
cycle, then after the stationary phases diminishes up to negative power values
during the deceleration phases, when the engine operates as generator. The control
strategy adopted for this test is based on the hypothesis that the hybrid vehicle is
used as pure electric vehicle at the start up and for partial loads, in particular the
energy flows inside the power train are regulated to satisfy the engine requirements
only by batteries up to about half of the engine maximum power. For higher loads
the FCS satisfies all the engine demands following the same dynamic of the cycle,
