Environmental Engineering Reference
In-Depth Information
8.4 Energy storage system
Hybrid propulsion systems require energy storage systems having turn-around
efficiency greater than 90% to be effective. If energy is exchanged in a system
having lower efficiency, the benefits of hybridization become blunt and at some
point there are no benefits. This is why many investigators have and continue to
explore means of incorporating ultra-capacitors into the propulsion system since an
ultra-capacitor can be sized to deliver 95% efficiency in each direction, or a round
trip efficiency of 90%.
Most battery systems are simply incapable of meeting such high energy
cycling efficiency targets. In fact, any system in which energy is not stored in the
same medium as it is consumed or delivered is ill suited as an energy storage
system because there will be one or more energy conversion steps to access the
available energy. A battery for instance must go through a chemical to electrical
conversion in order for its energy to be accessed.
In an electric drive system, all modes of energy storage except capacitive and
inductive, result in one or more energy conversion steps before the energy can be
put to use. It is unlikely that inductive energy storage systems will be used in hybrid
vehicles, but such systems do exist for utility energy storage in the form of super-
conducting magnetic energy storage (SMES) systems. However, in these utility
systems a power electronic converter is necessary, not because the energy must
change form, but it must be conditioned from ac at the grid to dc to feed the storage
inductor.
8.5 Efficiency mapping
In this final section, it is insightful to illustrate some examples of ac drive system
efficiency mapping. When system simulation is performed, the first order of busi-
ness is determining an efficiency map of the various electric components from
energy storage system, to distribution system, to the M/G components.
The most ubiquitous hybrid M/G component is the synchronous generator, or
Lundel alternator, used today for high power generation and belt connected starter-
generator in low end hybridization. The Valeo and Hitachi corporations have each
made major strides in up-rating this machine technology for idle-stop hybrids in
power ratings of 5-8 kW at 42 V and about 3-4 kW at 14 V [15]. The efficiency
map of a Lundel alternator is characterized by open contours of efficiency unlike
classical synchronous or permanent magnet machines. Figure 8.7 is included here
to illustrate the Lundel alternator efficiency as it is today.
The Lundel alternator in Figure 8.7 is rated 180 A
dc
at 14 V regulated output.
The machine is a standard 137 mm OD frame and is liquid cooled. Peak efficiency
occurs at 2,000 rpm and for output current of 40-100 A
dc
.
High power M/Gs for electric traction are typically rated 30 kW and higher. In
Figure 8.8, the efficiency map of Unique Mobility Corporation's integrated electric
