Nonbattery Energy Storage: Ultracapacitors and Flywheels.
Ultracapacitors
Ultracapacitors are devices that can directly store electrical charges—unlike batteries, which store electricity as
chemical energy. A variety of ultracapacitor materials and designs are being investigated, but all share some basic
characteristics-very high specific power, greater than 1 kW/kg, coupled with low specific energy. The U.S.
Department of Energy mid-term goal is only 10 Wh/kg (compared to the U.S. Advanced Battery Consortium mid-
term battery goal of 100 Wh/kg). Other likely ultracapacitor characteristics are high storage efficiency and long life.
Ultracapacitors’ energy and power characteristics define their role. In electric vehicles, their high specific power
can be used to absorb the strong power surges of regenerative braking, to provide high power for brief spurts of
acceleration, and to smooth out any rapid changes in power demand from the battery in order to prolong its life. In
hybrids, they theoretically could be used as the energy storage mechanism; however, their low specific energy limits
their ability to provide a prolonged or repeatable power boost. Increasing ultracapacitors’ specific energy is a critical
research goal.
Flywheels
A flywheel stores energy as the mechanical energy of a rapidly spinning mass, which rotates on virtually
frictionless bearings in a near-vacuum environment to minimize losses. The flywheel itself can serve as the rotor of
an electrical motor/generator, so it can turn its mechanical energy into electricity or vice versa, as needed. Like
ultracapacitors, flywheels have very high specific power ratings and relatively low specific energy, though their
energy storage capacity is likely to be higher than ultracapacitors. Consequently, they may be more practical than
ultracapacitors for service as the energy storage mechanism in a hybrid. In fact, the manufacturer of the flywheel
designed for Chrysler’s Patriot race car, admittedly a very expensive design, claims a specific energy of 73 Wh/kg,
which would make the flywheel a very attractive hybrid storage device. Mass-market applications for flywheels
depend on solving critical rotor manufacturing issues, and, even if these issues were successfully addressed, it is
unclear whether mass-produced flywheels could approach the Patriot flywheel’s specific energy level.