Advanced Automotive Technology: Visions of a Super-Efficient Family Car

polarized liquid layer that forms when voltage is applied between two

electrolyte. A key characteristic is their high power density--they can be

should be able to store and release electricity with high efficiency.

Flywheels, in contrast, store energy as the mechanical energy of a

electrodes immersed in

discharged rapidly, and

rapidly spinning mass,

rotating on virtually frictionless bearings in a near-vacuum environment to minimize losses. The

flywheel itself can serve as the rotor of a motor/generator, so that the flywheel can be accelerated

(to store more energy) when excess electricity is available (e.g., from regenerative braking), or it

can release its mechanical energy as electricity when a power boost is needed. The flywheel is also

expected to have high storage efficiency.

Both types of devices are viewed primarily as sources of peak power required during vehicle

acceleration or hill climbing, because they have very high specific power. Some advocates also

view flywheels as capable of providing basic energy storage, though most analysts consider both

devices to be impractical for this role because of their relatively low energy density and their

tendency to “self discharge,” that is, gradually lose energy when not in use. The DOE goals for

advanced ultracapacitors are 15 Wh/kg specific energy and 1600 W/kg specific power with round

trip efficiencies of 90 percent;^112 DOE has not yet set quantitative goals for flywheels.^113

Ultracapacitors are being developed for the DOE by several contractors and the technologies

include:

carbon/metal fiber composites,

monolith foamed carbon,

doped polymer layers on carbon paper,

thin-film lithium polymer, and

ceramic metal oxides on metal foil.

Ultracapacitor cells of the carbon/metal fiber type have been constructed by Maxwell Labs, and
their measured performance exceeds the near-term goals of the DOE program. Single cell organic
electrolyte capacitors have shown the capability of providing peak power in excess of 2 kW/kg
but have specific energy of about 7.5 Wh/kg (at 600 W/kg power)^114 --about 10 times more
powerful than lead acid batteries of equal weight, but with only one-quarter of the energy storage
capacity. Monopolar capacitor stacks are expected to be built in the near term, as there are no
problems with scaling or sealing, but these stacks are bulky and could reduce the power and
energy density by 25 percent or more from cell levels. Bipolar stacks offer lower internal
resistance and weight, but sealing is a major problem. The bipolar stack can attain energy and

(^112) U.S. Departmentof Energy, Office of Transportation Technologies ‘Hybrid Propulsion Program Plan,” October 1994.
(^113) Wi11iam Siegel, Department of Energy, personal communication, June 26, 1995.
(^114) C. Murphy and W. Kramer, “DOE Ultracapacitor Program Overview,” paper presented at the Automotive Technology Development
Contractors Coordination Meeting, U.S.Department of Energy, October 994.