Environmental Engineering Reference
In-Depth Information
with high non-idling times result in battery energy drain from SOC = 0.8 to 0.35.
Referring to Table 4.14 it can be seen that the first five events have the highest
passenger loading, the highest stop times and modest acceleration (and braking).
Since the ultra-capacitor in this architecture relies on the availability of braking
energy to replenish its SOC, the fact that it becomes fully depleted means that
energy balance between the generator, battery and ultra-capacitor is suboptimal.
Simply increasing the ultra-capacitor size will not remedy the situation. We saw
earlier in (4.16) and (4.17) that the battery charge acceptance is low, so most of
the regeneration energy is already being directed into the ultra-capacitor bank.
There are simply not enough regeneration events to maintain its SOC. The
strategy would therefore require further manipulation to include opportunity
charging during decelerations so that instead of shutting off the engine it would
continue to run at some lower, and more efficient, power level, with that output
being directed into the ultra-capacitor bank. The variations on control strategy of
such multiple source hybrid systems are too great to explore here. The salient
point is that beyond component sizing based solely on physics there must be a
control algorithm designed around anticipated usage and projected passenger
occupancy in the case of a city bus, to further refine the system.
4.4.1 Lead-acid technology
The most cost effective secondary storage battery is the flooded lead-acid battery.
This technology costs approximately $0.50/Ah for a 6-cell module. Maintenance
free, valve regulated, lead-acid, valve regulated lead acid (VRLA), and absorbant
glass mat (AGM) lead-acid batteries are capable of higher cycle life than
the flooded lead-acid type. The main disadvantage of lead-acid for hybrid vehicle
traction application is its low cycle life. Even deep discharge lead-acid batteries
such as those used in BEV traction applications are not capable of much beyond
400 cycles (to 80% depth of discharge (DOD)).
Table 4.15 lists the differences between BEV and hybrid vehicle batteries. In
this table a thin-metal-foil (TMF) lead-acid battery is listed that was developed
during the mid-1990s by Johnson Controls and Bolder Technologies Corp. as a
very high power (thin electrode) secondary cell. A typical 1.2 Ah, 2.1 V cell in
cylindrical package,
f
22.86
L
72.26 mm, has a foil thickness of 0.05 mm, a plate
thickness that is less than 0.25 mm and a plate to plate spacing when spiral wound
of less than 0.25 mm. The cell ESR is
<
1.5 m
W
and weighs approximately 82 g. In
Table 4.15 the TMF specific power and specific energy are listed when this cell is
packaged into a 315 V module (150 cells in series, 4 strings in parallel) with a
capacity of 4.8 Ah and an active mass of 49 kg.
Combinations of secondary batteries, principally VRLA, with ultra-capacitors
in the presence of dc/dc converter interface represent a good application. This is
because the VRLA can provide the energy storage while the ultra-capacitor handles
all the transient power as was done in the hybrid city bus example. That is, if the
ultra-capacitor and its attendant dc/dc converter can be sized and implemented at
