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. An LLNL program Module Conserves Energy 103 operating the EMB modules in pairs—one spinning clockwise, the other Methyl alcohol from 1978 to 1983 validated various counterclockwise—the net gyroscopic effect on the car would be nearly zero. Hydrogen internal flywheel design concepts using rotors The basic Livermore module The other special problem associated with EMBs for vehicles is failure combustion engine made of composites and yielded consists of a high-speed rotor containment. The limited understanding of rotor burst and containment is presently Gasoline valuable data on rotor failures (called integrated with a generator motor, the single most significant obstacle to implementing flywheel energy storage in NickelÐzinc bursts) and life spans. suspended by magnetic bearings, and vehicles. To acquire further understanding, the Livermore team is performing a series ZincÐair “In the intervening years, several housed in a sealed, evacuated chamber. of rotor burst tests using both integrated flywheel systems and isolated parts. In 102 addition, the team fires projectiles composed of rotor material at various containment LithumÐion critical technologies emerged, and new An artist’s concept of such a module, a LeadÐacid design principles were established that small one storing about 1 kWh of structures at speeds exceeding 1,000 meters per second. The tests show that a well-

Specific short-duration peak power, W/kg peak power, Specific short-duration designed rotor made of graphite fibers that is made to fail turns into an amorphous Lithium–metal– made it worthwhile to re-examine the energy and “about the size of a bread Ferrous disulfide mass of broken fibers. This failure mode is far more benign than that of metal Hydrogen fuel cell basic idea,” says Post. Out of this effort box,” is shown in the cutaway drawing flywheels, which typically break into shrapnel-like pieces that are difficult to contain. emerged the Livermore concept for the in Figure 2. The team is working toward the design of lightweight structures (made in large part of EMB, with far more economic promise Table 1 lists some of the attributes low-cost fiber composite) to completely contain rotors that fail for any reason. 10 and wider applications than the older of the basic module. Also listed for An array of small EMB modules, each with its own reinforced vacuum housing 10 102 103 prototypes (see box next page). comparison are typical values for the and an outer protective housing (Figure 2), offers a major advantage over the Specific energy, Wh/kg The current Livermore program common lead–acid battery. One can problems posed by a few large units. Not only is the energy that can be released by Various energy storage devices are compared for peak power and specific energy. began in 1992 under Laboratory see a substantial advantage of the EMB each unit reduced, but the twisting torque in the containment structure that might Batteries and fuels produce roughly the same range of peak power, about one Directed Research and Development over its lead–acid counterpart. result from a failed rotor is very small compared to that of rotors just two or three times larger. order of magnitude below advanced flywheels.

Science & Technology Review April 1996 Science & Technology Review April 1996 16 Electromechanical Battery Electromechanical Battery 17

The only difference between the example), the EMB is expected to have a energy back into the battery pack Calculations reveal that a high-strength fiber composites, materials of choice in flywheel rotors Livermore EMB, viewed as a “black useful service life measured in decades. whenever the vehicle slows down, is representative automobile powered by particularly graphite. The strength of for energy storage. A metal flywheel box” to store electrical energy, and an This longevity should be attainable even braked, or descends a hill. electricity using EMBs for storage graphite fibers, now used in everything does indeed store more energy than an electrochemical cell is that, instead of under repeated “deep-discharge” One way to express the resulting instead of an internal combustion from tennis racquets to sailboat masts, equivalent-size flywheel made of low- low-voltage direct current, the EMB cycling, an attribute not possessed by energy savings is through an energy engine would have an ECF of 4.0. That has increased by a factor of 5 over the density material and rotating at the “cell” accepts and delivers variable- any known electrochemical cell. conservation factor (ECF). This is the is, four barrels of oil delivered to a last two decades. same speed. However, a low-density frequency alternating current at an A typical gasoline-powered ratio of energy required to drive a refinery would yield the same number These fibers play a central role in wheel can be spun up to a higher speed operating voltage level chosen by the automobile in urban driving converts vehicle powered by a gasoline engine of urban driving miles in a gas-powered flywheel energy storage. The reason lies until it reaches the same internal tensile designer. When coupled to a power only about 12% of the heat energy of over a given urban cycle compared to the vehicle as one barrel of oil (or its in the laws dictating how much kinetic stresses as the metal one, where it stores converter, the EMB delivers its gasoline to useful drive power. In energy that would be required to drive a energy equivalent) delivered to a power energy can be stored in a rotating body the same amount of kinetic energy at a electrical energy at higher power addition, gas-powered vehicles have vehicle with the same weight and drag plant for a car powered by electricity (Figure 3). Any spinning rotor has an much lower weight. For example, levels per kilogram of mass than any no way to recover the energy that is coefficients equipped with an electric stored in EMBs. “The impact of such upper speed limit determined by the lightweight graphite fiber is more than known battery. wasted upon slowing down, braking drive system. (Of course, the ECF for an a major increase in the efficiency of tensile strength of the material from ten times more effective per unit mass Furthermore, like other electro- to a stop, or descending a hill. EMB electric vehicle must include the the transportation sector would be which it is made. On the other hand, at for kinetic energy storage than steel. mechanical equipment operating in a vehicles offer a simple way to efficiently efficiency with which the electric utility phenomenal in terms of reducing our a given rotation speed, the amount of Which modern fiber is optimum sealed environment (the household recoup this energy through “regenerative generates and delivers electricity to need for petroleum and also in terms of kinetic energy stored is determined by for an EMB depends on whether the refrigerator motor and compressor, for braking.” In this mode, the electric drive charge the batteries.) air pollution,” says Post. the mass of the flywheel. designer wants maximum energy motors are operated as generators to put When the same calculations are done This observation originally led to storage per unit mass (as in vehicular for a lead–acid electrochemical battery, the intuitive notion that high-density applications) or, for economic reasons, the ECF drops to about 2.5, owing to its materials, namely metals, are the the designer requires the maximum Basic technology components Applications lower energy recovery efficiency (60 to 70%). Post says that if for no other reason than superior efficiency, special Table 1. Comparison of attributes for battery modules. attention should be paid to exploiting EMB Lead–acid battery

the EMB for designing “real-world” Specific power 5–10 kW/kg 0.1–0.5 kW electric vehicles. Energy recovery 90–95% 60–70% Specific energy 100 Wh/kg 30–35 Wh/kg Service lifetime >10 years 3–5 years Fiber Is Key Self-discharge time Weeks to months Many variables Magnetic bearings (temperature, usage, etc.) Integrated system Hybrid vehicle Hazardous chemicals None Lead, sulfuric acid The Livermore effort to design and build an EMB takes advantage of recent advances in materials such as

1000

800 Power electronics

Figure 3. Steel was Power quality 600 originally used in flywheels; but graphite, which is lighter,

Wh/kg stores kinetic energy better. 400

200 Composite materials Bulk energy storage

Figure 2. Concept of the flywheel battery 0 system and its applications. Carbon Alloy Glass Graphite fibers Graphite fibers steel steel fibers (current) (projected)

Science & Technology Review April 1996 Science & Technology Review April 1996 18 Electromechanical Battery Electromechanical Battery 19

Bearings Top view Halbach array derived early in the nineteenth century. (a) (b) This theorem asserts the impossibility of stably levitating a charged body by Lithium– aluminum– NickelÐiron using electrostatic forces arising from iron sulfide other fixed, electrically charged bodies. By extension, the theorem also applies Figure 5. Various to magnets and magnetic bearings. LeadÐacid LeadÐacid energy storage Figure 4. The Commercial magnetic bearings, now devices are Halbach permanent in use in specialty applications, must compared for (a) magnet array is an employ complex and expensive power density and integral part of our Battery type Zinc–

Power sources V-8 engine electronic servo systems to overcome bromide (b) energy recovery. electromechanical this constraint. battery. Stator The Livermore team is working to wires achieve levitation by using a magnetic EMB EMB bearing energized by permanent module module magnets to support the spinning mass of the flywheel against gravity, at Magnetic field 0 2 4 6 8 10 0 0.2 0.4 0.6 0.8 1.0 present supplemented by a Power density, kW/kg Energy recovery efficiency, % conventional bearing to stabilize the system. For the longer term, the team is Halbach Vacuum Composite aiming its main effort on rotor Laboratory team has adapted them for 95%, while specific power climbs to 10 electric cars (or hybrid internal magnet array barrier rotor dynamics effects to achieve stable use in EMBs. Figure 4 shows an end kW/kg. Figure 5 illustrates these values combustion engine/electric-drive cars) is levitation with so-called “passive” view of the array. for a modern V-8 gasoline engine and a being delayed by the lack of a magnetic bearings, in which no servo Noncontacting magnetic bearings small EMB module. satisfactory energy storage system. energy storage per unit cost (as in most second. The Livermore approach is to system is required. The team’s novel eliminate wear and minimize rotational Post says that the Laboratory’s EMB stationary applications, such as load achieve lowest cost and tolerate modest approach to passive magnetic bearings, drag losses, and ironless generator motor development program can make a major Key Words: electromechanical battery (EMB), energy efficiency, flywheel, storage leveling for electric utilities). Vehicular penalties in energy density. As a result, unique in the magnetic bearing designs eliminate hysteretic losses. If contribution toward solving a critical cells. uses call for graphite fibers, even the team uses rotors made of material community, takes advantage of the there were no losses from aerodynamic societal problem—finding less expensive though these are more than ten times as costing $26 per kilogram ($12 a pound) expertise within Livermore’s magnetic drag, the rundown, or self-discharge and more efficient ways to store Reference expensive as the most cost-effective that operate with tip speeds on the order fusion program staff. lifetime, of the module supported by electrical energy. This need, he says, 1. K. Halbach, “Design of Permanent fiber for EMB stationary applications. of 800 to 1,000 meters per second, as An integral part of the rotor is the optimized magnetic bearings would be appears in many aspects of the nation’s Multipole Magnets with Oriented Rare Earth Cobalt Material,” Nuclear Post emphasizes that using opposed to top-performing fibers costing generator motor, composed only of a very long. Rundown times in excess of use of electricity, from homes and Instruments and Methods 169 (1980), composite fibers has required the team $130 per kilogram ($60 a pound). rotating array of permanent magnet two years for magnetically levitated factories to the needs of electric utilities pp. 1–10. to rethink the entire flywheel concept, bars that produce a rotating magnetic high-speed rotors operated in vacuo were and wind-electric and solar-electric which was based on metal flywheels. Designing for Tomorrow field. This field couples through a demonstrated 40 years ago. power generators. It is felt most keenly, For further information contact Because steel is an isotropic material, vacuum-tight, glass–ceramic cylinder As in those early tests, Livermore however, in the transportation sector, Richard F. Post (510) 422-9853 its strength against rupture is the same With rotor design and materials to three-phase copper-wire windings researchers put the rotor in an evacuated where the development of practical ([email protected]). in every direction. Composites are problems largely solved, the most located inside this cylinder (and thus enclosure to minimize the losses from typically anisotropic materials; i.e., important challenges facing EMB outside the evacuated region). This aerodynamic friction. Fortunately, the they are strong in the direction of their designers are the two issues of bearings ironless design minimizes hysteretic degree of vacuum required to satisfy About the Scientist fibers but up to 100 times weaker in the and rotor dynamics. In current tests, losses from fluctuations in the even the most demanding vehicular other direction. Laboratory researchers have been using magnetic field, which would limit the needs is well within commercial In 1951, RICHARD F. POST received his Ph.D. in Physics Laboratory flywheel designs use a mechanical bearings. In future tests, rundown times and generate heat. practice. Computer models show from Stanford University, Stanford, California. In 1940, he basic geometry of a cylinder, with the they plan to incorporate a virtually This generator motor is the first aerodynamic rundown times of several received his B.S. from Pomona College, Claremont, California. fiber orientation that of a tight-wound frictionless, magnetic bearing system battery application of what is called a months and corresponding losses from A specialist in fusion research, plasma physics, and energy spring, i.e., essentially perpendicular in which the rotor is suspended by Halbach magnetic array. These aerodynamic drag of a fraction of storage, Post has been at Livermore since 1951. Currently he is a to the axis of the cylinder. In this way magnetic forces derived from uniquely arranged magnet designs a watt. senior scientist in Energy, Manufacturing, and Transportation they achieve maximal strength in the permanent magnets. were pioneered in the 1980s by Klaus Together, the ironless design, the Technologies within LLNL’s Energy Program. Since 1963, he outward centrifugal direction. The Although the concept of levitating Halbach1 of Lawrence Berkeley Halbach array, and the very high rotation also has been affiliated with the University of California, Davis, rotor’s highest tip speeds attained using magnetic bearings dates to the 1940s, National Laboratory. Although in a sealed, evacuated enclosure give where he now is a Professor Emeritus. Recent publications by the strongest available composite fibers every designer of such bearings must Halbach’s work related to magnet extremely high efficiency and specific Post include book chapters, journal articles, and conference proceedings on topics range from 1,400 to 2,000 meters per contend with Earnshaw’s Theorem, arrays for particle accelerators, the power. As noted, efficiencies exceed such as magnetic mirror fusion research and the electromechanical battery.

Science & Technology Review April 1996 Science & Technology Review April 1996