Environmental Engineering Reference
In-Depth Information
For the values given in (4.23) and (4.26) the required number of series connect
ultra-capacitor cells is
580
2
:
7
¼
214
:
8
N
uc
¼
ð
4
:
28
Þ
which means the ultra-capacitor bank will consist of a series string of 215 cells.
To complete the hybrid strategy used in the energy management controller, we
constrain the engine driven generator to only those periods for which the vehicle
engine is required to run. The engine is not running during regenerative braking and
during stops. The generator power is zero when the engine is off. During such key-
ON stops, the engine remains off (idle-stop) and the battery plus ultra-capacitor
support the continuous loads. When the vehicle accelerates from a stop, the engine
is started and participates in acceleration and recharging of the battery and ultra-
capacitor packs. The engine control strategy participates with the EMS to maintain
the system bus voltage within the prescribed working voltage swing minimum
because the ESS requires some unfilled capacity to absorb regeneration energy.
It should also be understood at this point that the statistics of the customer
drive cycle will have a dramatic impact on storage system component rating. The
reason for this is that stopped periods vary considerably for different drive cycles
and the storage system must maintain all connected loads during engine off. For
passenger cars and light duty trucks this is not so much an issue. For instance, the
time spent stopped for existing standard drive cycles has been quantified and illu-
strated in Figure 4.32.
Truck/police/
fleet
Suburban/light
commuter
0
1
10
Stop time (min)
Figure 4.32 Statistics of stop time for various drive cycles (courtesy Ford
Motor Co.)
The trend line in Figure 4.32 represents a compilation of standard drive cycles
for North American passenger vehicles averaged over several geographical loca-
tions. For light commuters as suburban dwellers, the trend curve will move left to
