In our increasingly electrified society there is a growing need for ef
ficient cost-effective means for storing electrical energy. The electr
ic auto is a prime example. Storage systems for the electric utilities
, and for wind or solar power, are other examples. While electrochemic
al cells could in principle supply these needs, existing E-C batteries
have well-known limitations. This article addresses an alternative-th
e electromechanical battery (EMB). 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, i
n permanent-magnet materials, and in solid-state power electronics ope
n up new possibilities, allowing the design of small (1 kW-h) modular
EMB's for full- or hybrid-electric vehicles, and larger size modules (
25 kW-h), for a variety of stationary storage needs. The article addre
sses design issues for EMBs with rotors made up of nested cylinders. I
ssues addressed include: rotational stability, stress distributions, g
enerator/motor power and efficiency, power conversion, and cost. An im
portant conclusion; the use of EMB's in electric autos could result in
a five-fold reduction (relative to the IC engine) in the primary ener
gy input required for urban driving, with a concomitant major positive
impact on our economy and on air pollution.