OPTIMAL STORAGE SYSTEM DESIGN
It is well known that a series HEV provides all driving power to the
wheels through electric motor(s). the internal combustion engine is
disconnected from the wheels and therefore can run in a narrow torque
and speed range with higher efficiency and less emissions. This
advantage is seriously diminished, though, by the fact that energy must
be converted two times before it reaches the wheels. Such conversions
lower HEV efficiency to not more than 85%. [2]
HEV's are currently attracting a great deal of attention due to their
ability to save fuel and reduce emissions. They are also perceived as
transitional vehicles on the move from internal combustion engines to
Fuel Cells. To date, many HEV's have been built and tested. Most of
them would fall into two major categories: Engine-dominated HEV, and
HEV dominated by the storage system. If some all-electric operation
were required then the storage system would dominate the design
criteria. If the all-electric capability is not an issue, but fuel
efficiency is (as in PNGV goals), then the engine dominated HEV would
be more suitable. The design idea here is to keep the energy storage as
small, as possible while achieving the best fuel economy of the
internal combustion engine. Numerous studies have proven that, to be
more efficient, vehicles must generate their power on-board rather than
store it. Therefore, the HEV's curb weight and drive train efficiency
must be as close as possible to the conventional diesel-powered vehicle
or the bonus of regenerated braking energy will be negated.
Optimal storage systems return energy to storage by recouping energy
that would normally be lost in breaking. Kinetic energy is saved during
deceleration of the vehicle. However, only 25% of the kinetic energy of
the moving vehicle can be recouped and returned to the storage system.
Many designers of HEV's have ignored this fact. The confusion might
have come from a false belief that almost all of the energy of braking
could be saved in the storage system.
Due to their robust construction and durability the capacitors are most
suitable to be positioned within the vehicle's chassis (to keep center
of gravity low). There are some buses in operation where as much as
1818 kg (4000 lbs.) of lead batteries are located within the roof
superstructure. Super-capacitors would allow the same bus to achieve
comparable performance to the batteries with only 650 kg (1430 lb) of
capacitors. The actual fuel savings would be even higher due to lower
curb weight and the capacitors higher efficiency. The super-capacitor
system can be delivered either loose or assembled on a special light
skid.
Design analyses has revealed that, for best results, engine/generator
peak power must be not less than 80% of the peak power consumed by the
traction motor, the storage system must provide 20% of the traction
motors peak power. The above mentioned 80/20 relationship has been
calculated from the following criteria:
The Energy storage capacity is rated to accept 25% of the
vehicle's kinetic energy of braking, including the efficiencies of the
drive train and the capacitors.
The Engine/generator must be powerful enough to provide
satisfactory climbing ability without the assist from the storage
system (when energy storage is discharged).
The Traction motor peak power must be sufficient to provide the required acceleration. [5]
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