Environmental Engineering Reference
In-Depth Information
Table 6.6
Efficiency of the power train and its subsystems during R40 cycle [
2
]
Fuel cell system
DC-DC converter
Electric engine
Power train
Soft hybrid
0.48
0.80
0.75
0.29
Hard hybrid
0.51
0.87
0.75
0.33
demanding phase of the driving cycles, and to about 0.65 at medium load con-
ditions. With regard to the FCS efficiency in soft hybrid approach, it varies
between 0.13 and 0.48 (Fig.
6.30
) and follows the load curve of the cycle repro-
ducing the same efficiency values obtained for each power in the corresponding
steady-state operation (see
Sect. 6.3
). In the soft hybrid, the stack operates for
several minutes at the minimum load, for this reason the system energy losses
determine a strong decrease of the instantaneous system efficiency. This fact
negatively affects the average efficiency calculated on the entire cycle
(see Table
6.6
). In the hard hybrid configuration, the instantaneous efficiency of
the system is almost equal to the highest steady-state value for most of the time
lasting the cycle (Fig.
6.29
), providing this way a better average efficiency.
The comparison between soft and hybrid approach can be further accomplished
by analyzing the energy flow values involved within the power train during the
same cycle (R40 cycle). In Fig.
6.31
a, the electric energy profiles instantaneously
exchanged by electric drive, battery pack, and FCS with the electric bus are
reported versus cycle length for a hard hybrid configuration, on the assumption
that the energy at the beginning of the driving cycle is zero. In this test, the value
of power fixed at the DC-DC converter output is selected corresponding to the
average power of R40 cycle (about 300 W).
Up to 150 s, the energy flow provided by FCS is always higher than the electric
drive requirements, implying that the battery SOC remains always higher than its
initial value, while after 150 s the storage system compensates the difference
between electric drive requirements and the energy provided by the FCS. In
Fig.
6.31
a, the negative derivative for battery energy indicates that energy is
entering into the storage system, coming mainly from FCS and also from electric
drive during braking phases of the cycle, when the motor operates as generator.
In Fig.
6.31
b, the histogram corresponding to the monitoring of individual cell
voltages during the test is reported, evidencing the very uniform cell working
expectable for a stack managed in steady-state conditions (most cells showed
voltages comprised between 0.77 and 0.79 V).
In Fig.
6.32
, the results of the experiment on the scooter power train in soft
hybrid configuration are reported. The power at the DC-DC converter output is
controlled in order to supply the electric drive with all the power instantaneously
required during the R40 cycle. In Fig.
6.32
a, the electric energy profiles instan-
taneously exchanged by electric drive, battery pack, and FCS with the electric bus
are reported versus cycle length. The FCS profile follows almost completely the
electric drive energy requirements, and the limited contribution of batteries is
evident during all the cycle length. The dynamic performance of the stack is
