flywheels with energy densities of over 130 Wh/kg, and power densities that can be tailored to
over 1 kW/kg. AFS also claims that it could mass produce such flywheels for a cost of $250/kWh
or less. Independent confirmation of AFS’s claims is not available; AFS has displayed a prototype
system on a car, but its performance is not reported publicly.^120
Other flywheel manufacturers do not support AFS cost claims, but their own technology
indicates that flywheels with similar performance can be built, though at high cost. For example,
SatCon Technology Corporation is providing a special flywheel for Chrysler’s Patriot race car,
and has delivered a complete flywheel system (with conventional bearings) that weighs 59 kg and
can store 4.3 kWh of energy,^121 while delivering very high-power pulses of 100 kW^122 (i.e., 73
Wh/kg and 1.7 kW/kg). Its engineering staff confirmed that this was an extremely costly system
developed only for racing use. Its flywheel operates with tip speeds of 2,000 m/see, which
requires very expensive, ultrastrong fibers. SatCon stated that commercial models (available in
perhaps 5 to 10 years) would utilize much cheaper materials but operate at tip speeds of only
1,400 m/see, reducing the specific energy by 50 percent to about 35 Wh/kg.^123 Peak power could
still be very high, in excess of 2 kW/kg, but this is a function of power system design. SatCon
believes that, although magnetic bearings are desirable, they are not necessary for a short-term
power storage device.
Not all the stored energy in a flywheel is recoverable; SatCon’s flywheel operates between
30,000 and 60,000 rpm so that 75 percent of the total energy at 60,000 rpm is recoverable.^124
SatCon did not provide a cost figure but claimed that it could eventually meet USABC goals--a
claim advanced by virtually all storage device developers, which makes it difficult to evaluate.
Researchers at Idaho National Engineering Laboratory and Argonne National Laboratory, as
well as several automanufacturers, are substantially more pessimistic about the flywheel’s
prospects. They contend that rotor dynamics problems are very complex, and that maintaining
rotor balance in a vehicle environment poses extreme challenges. After much advance publicity,
the SatCon flywheel for the Patriot car has not yet been capable of sustained performance Mass
production of rotors to extremely critical balance accuracy levels is also a difficult challenge, and
several researchers believe that rotors operating at 100,000 rpm or more will never be
commercially mass produced.
Electric Motors .-
An electric drive system uses a motor to convert electrical power to
direct current (DC) motors were used for variable speed applications,shaft power. Traditionally,
but the rapid development
of power electronics now allows the use of alternating current (AC) motors in these applications.
DC motors can further be classified into series-wound, shunt-wound, and separately excited, or