The Electromechanical Battery
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12 13 A New Look at an Old Idea TheThe ElectromechanicalElectromechanical BatteryBattery Laboratory researchers PINNING at 60,000 revolutions “charged” by spinning its rotor to lead–acid battery. Power densities can S per minute, a cylinder about the maximum speed with an integral soar to 5 to 10 kW/kg, several times size of a large coffee can may hold the generator/motor in its “motor mode.” that of a typical gasoline-powered are integrating innovative key to the long-awaited realization of It is “discharged” by slowing the rotor engine and up to 100 times that of practical electric cars and trucks. The of the same generator/motor to draw out typical electrochemical batteries. And materials and designs to graphite, fiber-composite cylinder the kinetically stored energy in its because of its simple design and belongs to a new breed of LLNL- “generator mode.” The advanced design advanced materials, an EMB is developed, flywheel-based, energy features a special array of permanent expected to run without maintenance develop highly efficient storage systems with new materials, magnets (called a Halbach array) in the for at least a decade. new technologies, and new thinking generator–motor to perform these Livermore researchers envision about the most efficient ways to charging and discharging functions several small, maintenance-free and cost-effective energy store energy. efficiently. modules, each with a kilowatt-hour of Called an electromechanical battery The EMB offers significant energy storage, for use in electric or (EMB) by its Laboratory creators, the advantages over other kinds of energy hybrid-electric vehicles. See the storage. modular device contains a modern storage systems (see box, next page). prototype in Figure 1 (also see box, flywheel stabilized by nearly For example, the efficiency of energy p. 15). Larger modules with 2 to frictionless magnetic bearings, recovery (kilowatt-hours out versus 25 kWh of storage capacity could be integrated with a special ironless kilowatt-hours in) is projected to employed by electrical utilities for more generator motor, and housed in a sealed exceed 95%, considerably better than efficient use of their transmission lines vacuum enclosure. The EMB is any electrochemical battery such as a and by factories for power conditioning. These larger units could also be used in wind and solar-electric power systems Figure 1. Prototype to enable them to deliver power of the LLNL whenever it is needed, rather than only electromechanical when it is generated. battery, which is The exceptional potential of the based on the Laboratory design has not gone flywheel concept of unnoticed by American industry. energy storage. Left Trinity Flywheel Batteries, to right: high-speed Westinghouse Electric, and General rotor, rotor in motion, Motors have all sponsored research at and enclosed battery Livermore for vehicular and industrial (20 cm in diameter by applications. The efforts, which include 30 cm high). tapping the expertise of researchers throughout the Laboratory, involve Science & Technology Review April 1996 14 Electromechanical Battery Electromechanical Battery 15 solving challenging problems in funding. The program drew Storing Energy motor/generator design, composite considerable interest from the private EMB Applications for Vehicles rotors, magnetic bearings, containment, sector and eventually direct Since the introduction of electricity into society, stored electrical energy has played and integrated system design. sponsorship of development work by Except that their output is alternating current rather than direct current, EMB a critical role in the development of electrical devices. Before the turn of the century three companies. Trinity Flywheel modules would power an electric car in the same way as a bank of electrochemical electrochemical storage cells were used to power the telegraph and the telephone. Old Invention, New Use Batteries Inc. and Westinghouse batteries. If each module stored about 1 kWh, as is currently projected, some 20 to 30 Some of the earliest automobiles were powered, not by an internal combustion engine, Electric Corp. continued to develop modules might be needed to provide the 200-mile-plus range for a vehicle required but by an electrical motor that drew energy from lead–acid storage batteries. Before Despite its current high-tech EMBs to smooth out the flow of by the public. At the same time, the fast charge (5 to 10 minutes) that could be the 1920s, electric cars were as common as gasoline-powered ones. designed into such a car would answer the challenge of long-range trips, provided appearance, the flywheel is one of electricity for factories, computer Today, concern for the air pollution from the gasoline-powered automobile has there was a “charging station” infrastructure, (which could also use EMB modules intensified the development of electric-powered cars and power to run them. However, society’s oldest inventions. (Its kin, the centers, and other facilities; General for peak power demand). along with the concern for less pollution come the plaguing shortfalls of current potter’s wheel, is mentioned in The Motors Corp. has evaluated EMBs Although these possibilities are intriguing for long-range planning purposes, they electric autos: sluggish acceleration, limited driving range, and too-short battery Bible.) Even the “modern” idea of as part of a future automobile may not be very realistic in the short term. Fortunately, there is another possibility: a service lifetime. The figure below illustrates the vast differences in present power coupling a flywheel to a generator/ propulsion system. “hybrid” internal combustion–electric car. One kind of hybrid would feature a small, storage strategies. Today the push is on to develop a vehicular “super battery” to motor to emulate a battery for use in “This unusual technology transfer constant-speed internal combustion engine (piston or a gas turbine) to provide overcome these limitations. electric vehicles is at least four decades arrangement offers several advantages. average-power requirements, with one or two EMB modules providing peak power- The electric car is only one example of the need to store energy. Others include old. It dates to the Swiss “Gyrobus,” an It places significant emphasis on the handling capabilities and recouping energy otherwise lost through braking or “load leveling” for electrical utilities, which must make more efficient use of their urban bus that used a steel flywheel to end use of EMBs and addresses the descending a hill. Such a hybrid would fit well with the present vehicle infrastructure transmission lines and base-load generating plants. Also, wind and solar-electric power a generator/motor and drive it flywheel system as an interdependent while also significantly reducing air pollution and fuel consumption. power systems, owing to the intermittent nature of their power outputs, urgently between stops, where a charging trolley whole, rather than as a collection of Another type of EMB hybrid would use electrochemical batteries, with EMB units need energy storage systems that can deliver power when it is needed, not just when again providing peak power demands. (See the article on zinc–air batteries in Science was engaged. Too cumbersome, too subsystems,” Post says. Indeed, the it is generated. & Technology Review, October 1995.) Besides providing snappier performance, the Thus far, virtually the entire effort to develop improved batteries for storage has expensive, and too limited by 1950s-era primary thrust of the present program EMB would reduce wear and tear on conventional batteries and improve the centered on hoped-for extensions of the electrochemical art. The Laboratory’s power electronics, the Gyrobus never is to test complete prototype EMB efficiency of a regenerative braking system. electromechanical battery (EMB), however, may be a better way to go or, at the very caught on, but a few researchers have systems. Operation at over 100 kW of Compared to stationary EMB applications such as with wind turbines, vehicular least, be an important piece in the evolving energy storage infrastructure. not let the concept die. power and storage of more than 1 kWh applications pose two special problems: gyroscopic forces and containment in Livermore has been involved in of energy have been demonstrated the case of failure. Solving both problems is made much simpler by the choice of developing flywheels made of using compact rotors and integrated small modules. Gyroscopic forces come into play whenever a vehicle departs from a straight-line 104 composite materials since a new way of containment structures. Prototype thinking about such flywheels was rotors have been tested at 60,000 rpm course, as in turning or in pitching upward or downward from road grades or bumps. Advanced flywheels published in a 1973 seminal article in and have exceeded specific power of The effects can be minimized by vertically orienting the axis of rotation (as in Scientific American. It was written by 8 kW/kg with a measured energy Figure 2, p. 16), which is also a desirable orientation for the magnetic bearing system. The designer can also mount the module vacuum chamber in limited- Ultracapacitors Richard Post, Livermore fusion scientist recovery efficiency of more than 92%. excursion gimbals or provide restoring forces in the magnetic bearing system (or in and current EMB program leader, and a mechanical backup bearing) to resist the torque from the vehicle’s movements. By his son Stephen.