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Short Description: PINNING at 60000 revolutions per minute, a cylinder about the size of a large coffee can may hold the key to the long-awaited realization of ...
Content Inside: 12 13 A The Electromechanical Battery New Look at an Old Idea The Electromechanical Battery Laboratory researchers are integrating innovative materials and designs to develop highly efficient and cost-effective energy storage. S PINNING at 60,000 revolutions per minute, a cylinder about the size of a large coffee can may hold the key to the long-awaited realization of practical electric cars and trucks. The graphite, fiber-composite cylinder belongs to a new breed of LLNLdeveloped, flywheel-based, energy storage systems with new materials, new technologies, and new thinking about the most efficient ways to store energy. Called an electromechanical battery (EMB) by its Laboratory creators, the modular device contains a modern flywheel stabilized by nearly frictionless magnetic bearings, integrated with a special ironless generator motor, and housed in a sealed vacuum enclosure. The EMB is "charged" by spinning its rotor to maximum speed with an integral generator/motor in its "motor mode." It is "discharged" by slowing the rotor of the same generator/motor to draw out the kinetically stored energy in its "generator mode." The advanced design features a special array of permanent magnets (called a Halbach array) in the generatormotor to perform these charging and discharging functions efficiently. The EMB offers significant advantages over other kinds of energy storage systems (see box, next page). For example, the efficiency of energy recovery (kilowatt-hours out versus kilowatt-hours in) is projected to exceed 95%, considerably better than any electrochemical battery such as a Figure 1. Prototype of the LLNL electromechanical battery, which is based on the flywheel concept of energy storage. Left to right: high-speed rotor, rotor in motion, and enclosed battery (20 cm in diameter by 30 cm high). leadacid battery. Power densities can soar to 5 to 10 kW/kg, several times that of a typical gasoline-powered engine and up to 100 times that of typical electrochemical batteries. And because of its simple design and advanced materials, an EMB is expected to run without maintenance for at least a decade. Livermore researchers envision several small, maintenance-free modules, each with a kilowatt-hour of energy storage, for use in electric or hybrid-electric vehicles. See the prototype in Figure 1 (also see box, p. 15). Larger modules with 2 to 25 kWh of storage capacity could be employed by electrical utilities for more efficient use of their transmission lines and by factories for power conditioning. These larger units could also be used in wind and solar-electric power systems to enable them to deliver power whenever it is needed, rather than only when it is generated. The exceptional potential of the Laboratory design has not gone unnoticed by American industry. Trinity Flywheel Batteries, Westinghouse Electric, and General Motors have all sponsored research at Livermore for vehicular and industrial applications. The efforts, which include tapping the expertise of researchers throughout the Laboratory, involve Science & Technology Review April 1996