The electromechanical battery (EMB) is considered as an alternative energy storage system for electric utilities and other applications. An EMB is a modular unit consisting of an evacuated housing containing a fiber-composite rotor. The rotor is supported by magnetic bearings and contains an integrally mounted permanent magnet array. Recent developments in high-strength fibers, in permanent-magnet materials, and in solid-state power electronics allow the design of small (1 kW-h) modular EMBs for full-or hybrid-electric vehicles, and larger size modules (25 kW-h) for a variety of stationary storage needs. Design issues for EMBs with rotors made up of nested cylinders are addressed. Rotational stability, stress distributions, generator/motor power and efficiency, power conversion, and cost are discussed. It is concluded that the use of EMBs in electric autos could result in a fivefold reduction (relative to the IC engine) in the primary energy input required for urban driving, with a concommitant major positive impact on the economy and on air pollution. >