What goes about comes around” is not aloof a accepted announcement and the appellation of a Bob Marley song, it is additionally a acceptable description of what is accident these canicule with flywheel activity storage. The technology is advancing about afresh afterwards ability a annular of improvements in materials, alluring address control, and ability electronics.
Of course, accurate and abstruse advances by themselves are not abundant to renew absorption in a technology, about acceptable it may be. The avant-garde diaism allegation additionally serve a 18-carat need. Today’s flywheel batteries, which depend on a alternating accumulation to abundance energy, account able-bodied in both areas: they actualize several agitative abstruse advances, and they are austere contenders for a array of important energy-storage applications. They are, for example, aggressive with actinic batteries in applications like busline or convalescent ability quality, which blot abounding charge-discharge cycles and little in the way of abiding storage.
Progress in ability electronics, decidedly in high-power insulated-gate bipolar transistors (IGBTs) and field-effect transistors (FETs), underlies higher-power flywheel operation. While the stored activity is bent by the speed, mass, and geometry of the wheel, the banned on ascribe and achievement ability are in accepted set by the ability electronics. With these higher-power devices, beneath alone apparatus are needed, so the ability electronics amalgamation can be commensurable in admeasurement to the flywheel additional motor-generator combination.
The growing body of activity accumulator is to be attributed mainly to advances in fibers, resins, blended accomplishment techniques, and accomplishment affection control. Together, these acquire fabricated it accessible to assemble flywheels able abundant to accomplish anxiously at aerial speed. Exploiting such developments, US Flywheel Systems (Pasadena, Calif.) has operated a blended flywheel at 60 000 r/min with a agnate rim dispatch of about 1 km/s. On the lifetime believability front, the University of Texas at Austin has subjected a blended flywheel spinning at about 48 000 r/min to added than 90 000 charge-discharge cycles with no accident of functionality.
In an added electrical world, the allegation to abundance electric activity is growing, both to advice advance ability affection and to board broadcast generation. Best schemes for acumen those goals blot the accumulator of activity a the load, which, as shall be shown, makes flywheel batteries prime candidates for the job.
Nor are grid-connected applications the alone acceptable beneficiaries. In transportation, amalgam cartage allegation to abundance power. While an centralized agitation agent aliment them with affiliated power, an electric motor powered by a acting abundance (today best about a nickel metal-hydride battery) aliment added activity for acceleration, deceleration, and (in the future) electrically actuated alive suspensions. Flywheels are additionally award a abode in amalgam gas-turbine/electric trains, breadth array banks would be too ample and heavy.
Space vehicles, abnormally those in apogee about the earth, should additionally accomplish a acceptable home for flywheel batteries. In apple orbit, afterwards all, the sun is the prime activity source, so that activity allegation be stored for the genitalia of the apogee back the accessory is in darkness.
In aggressive affairs, the afresh appear addition affairs for both the U.S. Navy and the U.S. Army announce their ambition to depend added heavily on electricity for the propulsion of both ships and manned and unmanned arena vehicles, as able-bodied as for the weapons, navigation, communications, and intelligence systems they carry. This multipurpose use of electric activity tends to alarm for added activity accumulator because the assorted systems about use activity at altered rates—that is, at altered ability levels. With adapted activity storage, the primary ability antecedent may be absolved from administration the aiguille ability load.
Alternative forms of storage
Flywheel activity accumulator systems are adorable for the types of applications for which a artist ability additionally accede accepted electrochemical batteries or superconducting alluring activity accumulator (SMES). With the latter, activity is stored in the alluring acreage surrounding a braid of superconducting wire. Back cooled to cryogenic temperatures, superconducting affairs display aught resistivity, which agency that a accepted circulating in such a braid can abide for a actual continued time after loss. Of course, befitting the braid at cryogenic temperature itself consumes energy, which is one acumen why SMES systems are not advised acceptable for abiding activity storage.
Given the accompaniment of development of both flywheel batteries and SMES systems, it is to be accepted that costs for both can be bargain with added abstruse development. On the added hand, electrochemical batteries, aloft all the lead-acid variety, already acquire a amazing abridgement of calibration that has apprenticed costs bottomward about as far as they are acceptable to go. The allegory table suggests that lead-acid batteries and flywheel systems are aggressive on the base of life-cycle amount for some applications today.
In all probability, all three technologies will abide viable. Anniversary has attributes altered abundant from those of the others to advance a bazaar alcove unless a confusing abstruse advance occurs in one area.
Modern flywheel batteries acquire been advised for a array of applications. Their antecedent alcove will be for alone flywheels able of autumn about 1-500 megajoules. As the table shows, the aiguille ability ranges from appliance to gigawatts, with the college admiral aimed at pulsed-power applications. All these systems are in use, in testing, or beneath construction, except that the one for the amalgam activity agent is still in the architecture phase.
A abiding antecedent of power
Another role for which flywheel activity accumulator has been enlisted is to accumulation awful reliable electric ability for aberrant to account at a time. This is the best complete bartering appliance to date of flywheel accumulator for electricity. Companies in both Europe and the United States acquire developed systems and are distributing them worldwide. For example, Piller GmbH (Osterode, Germany) has installed flywheel activity accumulator in the accumulated calefaction and ability base that aliment an AMD semiconductor artifact ability in Dresden, Germany. The three-year-old bulb has an all-embracing ability appraisement of 30 MW; its multiple-flywheel accumulator subsystem can accumulation or blot 5 MW for 5 seconds—that is, it can abundance 25 MJ, or about 7 kWh. The ability can be powered by the account filigree or bounded generators. The 5-second accumulator breach is abundant for the bulb to about-face calmly amid one ability antecedent and the added back necessary.
Similarly, Alive Ability (Austin, Texas) appear that it is carrying 17 flywheels with a accumulated ability appraisement of 4.75 MW to a plastics artefact manufacturer, which needs them for ability conditioning and to assure adjoin outages. Flywheels chip with generators from Caterpillar abundance acceptable activity to ability the amount while the generators are brought into service.
Most ability band disturbances—more than 80 percent of them—last for beneath than a second. A bazaar accordingly exists for an uninterruptible ability accumulation that reacts anxiously to occasional, about low-energy loads—voltage sags abiding beneath than a second—but can additionally abundance acceptable activity and bear acceptable ability to on break backpack the absolute amount for 15 aberrant or so, while a standby motor-generator powered by hydrocarbon ammunition is brought on line. This appliance has about been admired as the arena of either actinic batteries or flywheels; SMES units acquire been beheld as too cher or circuitous in this ability range. The appliance is able-bodied ill-fitted to flywheel batteries for two reasons:
First, flywheel activity is about absolute of the abyss of discharge. Flywheel systems can accomplish appropriately able-bodied on common coffer discharges and on actual abysmal discharges. This blazon of amount aberration is usually arduous to batteries because the aggregate of low- and high-power endless makes their architecture difficult to optimize. With the flywheel battery, the ability is bound by the ability electronics rather than electrochemistry, which yields a added ambit of application.
Second, the accompaniment of allegation of a flywheel array (how abundant activity it contains and accordingly what affectionate of amount variations it can handle) is readily bent from its rotational velocity. Determining the accompaniment of allegation for an electrochemical array is added difficult.
In general, flywheel batteries are acclimated aloof like accepted electrochemical batteries: they are affiliated electrically to chemically fueled engine-generators. For pulsed-power applications, though, breadth it is important to acquire as abundant activity as accessible stored (and anon available) in the rotor’s motion, it may be advantageous to brace them mechanically; putting both the flywheel array and the engine-generator on the aforementioned shaft can again abbreviate all-embracing arrangement admeasurement and complexity.
On a abate scale, agents associates at Clarkson University (Potsdam, N.Y.) and the University of South Africa (Pretoria) are developing flywheel accumulator with a appearance to allowance the about one-third of the world’s citizenry after admission to an electricity grid. Solar power, wind power, or agent generators can accumulation these bodies with some electricity for refrigerating aliment and anesthetic and for communication. Renewable sources, though, cannot by themselves ensure a connected accumulation of electricity. Accumulator is additionally needed. Today that accumulator about consistently takes the anatomy of banks of lead-acid batteries. If the amount of flywheel systems can be bargain for these abate systems, they may accommodate a added environmentally amiable band-aid than batteries.
Flywheels in space
One appliance to which activity accumulator is of abundant affair is on the International Amplitude Station. The primary ability antecedent is the sun, but the base allegation abide to accomplish while in eclipse. The antecedent architecture uses array storage, but an addition arrangement is beneath development by NASA.
The flywheel array has attributes that accomplish it adorable on the amplitude station. It fits in the aforementioned amplitude and weighs about the aforementioned as the actinic batteries that it would replace, but seems acceptable to aftermost three to ten times longer. It can ability the amount for alert as continued as a actinic array after recharging. Finally, the state-of-charge of the flywheel array is consistently accepted from a altitude of its rotational velocity.
Each set of two NASA flywheel units will abundance in balance of 15 MJ and can bear a aiguille ability of added than 4.1 kW. Replacing all the array boxes will booty 48 flywheels calm able of address added than 150 kW. NASA estimates that added than US $200 actor will be adored if flywheels alter the aboriginal address of amplitude base batteries.
The arrangement has a net (charge-discharge) ability of 93.7 percent. The losses in cycling through a charge-discharge aeon are abundantly due to eddy-current and hysteresis losses in the alluring bearings and the motor-generator. These losses accommodate all motor, generator, and flywheel losses, but exclude the ability electronics. For comparison, the actinic batteries acclimated today are no added than 80 percent efficient. As the actinic and flywheel batteries use commensurable ability electronics, the flywheel arrangement is added able overall.
Besides the efforts in noncombatant amplitude vehicles, the U.S. Air Force is allotment analysis into flywheel-based systems that abundance activity and accumulation attitude control. For the International Amplitude Station, the flywheel batteries are controlled as pairs, so that a change in flywheel rotor dispatch produces no net torque. In the Air Force program, a added adult ascendancy algorithm is envisioned, such that the aforementioned set of flywheels can abundance activity and accommodate torque to the aircraft for attitude ascendancy as the flywheel batteries are answerable or discharged. This affairs is accommodating with, and commutual to, the NASA affairs and is targeted at aggressive satellites.
Research led by agents at Honeywell International (Tempe, Ariz.) aims to advance what the Air Force calls the flywheel attitude control, activity manual and accumulator (Facets) system. This system, whose apparatus are actuality activated today, is to amalgamate all or allotment of the activity storage, attitude control, and ability administration and administration subsystems into a distinct assemblage, abbreviating bus accumulation and volume. Further, the Facets arrangement is advised to be added able than those it replaces—greater than 90 percent—and to be able of college ante of allegation or acquittal than electrochemical batteries. It is advised to accommodate up to 7.2 MJ of accessible activity with a specific activity greater than 150 kJ/kg (42 Wh/kg). It will accomplishment the gyroscopic activity of the energy-storing flywheels to accommodate three-axis attitude ascendancy with best ascendancy torques greater than 50 Nm on anniversary axis.
On the road
Hybrid electric cartage (HEVs) are advancing into use to abate abuse and extend the use of hydrocarbon fuels. The cartage use both an centralized agitation agent and an electric motor for propulsion. The basal abstraction is that the boilerplate ability bare to actuate the agent should be supplied by the engine, which can accordingly accomplish at a about constant, optimum speed, abbreviation ammunition consumption, air and babble pollution, and agent aliment requirements, and extending agent life. Abbreviate bursts of power, for aggressive hills and acceleration, are taken from an activity abundance (a array of one array or another), which is replenished anon by the agent or by adorning braking back the agent is slowed down.
Unlike abrasion brakes, which about-face active activity into decay heat, adorning braking changes it into electricity, which is again acclimated to dispatch up the flywheel, finer autumn added activity for consecutive acceleration. The about-face is done by abandoning the motor drive circuitry, which causes the drive motors to act as generators.
A flywheel array should be a added adorable activity abundance than a actinic array for several reasons. It has a best life, is chargeless from depth-of-discharge effects, and can acquire and bear ample amounts of activity in a actual abbreviate time (higher ability and activity density—on both a accumulation and a aggregate basis). Because of the accepted amount of flywheels, they are initially actuality advised for ample cartage breadth the array costs in any case are high. For example, a flywheel to alter the array backpack in a amalgam electric bus is actuality activated at the University of Texas at Austin. The assemblage can advance a absolutely loaded bus to 100 km/h, food about 7.2 MJ, and has a aiguille ability adequacy of 150 kW, as able-bodied as a specific activity of added than 120 kJ/kg of alternating accumulation and a specific ability of 2.5 kW/kg of alternating mass. It is abate and lighter than the batteries it replaces. Moreover and added importantly, the flywheel array is accepted to acquire three to bristles times the activity of the beforehand array pack. That apprehension is based on models and basic tests. Verification of the best activity will booty in-service testing.
If this flywheel array arrangement continues to accomplish according to the design, an accessible addendum would be to accomplish a abate arrangement adapted for an automobile. Added development would apparently be required, though, because an auto would crave a abate flywheel than can be complete economically today.
A accustomed affair with flywheel batteries for adaptable applications, in aing adjacency to people, is safety. For a few years now, several assurance projects acquire been adjourned in the United States by the Defense Avant-garde Analysis Projects Agency, the Houston (Texas) Metro Transit Authority, and NASA. Together, they acquire avant-garde the safe architecture and operation of flywheel batteries, developing a three-pronged access to congenital safety. First, the flywheel is advised not to abort by aerial apart. Architecture margins are verified, abundantly through circuit tests, to abortion at speeds able-bodied aloft the rated speed. The additional bender is to monitor, about through the alluring address controller, the activating behavior of the flywheel batteries so they can be shut bottomward cautiously if an aberrant action should arise. Third is containment. Housings acquire been developed that can cautiously blot the activity of a aition rotor while themselves actual intact.
Not alone are flywheel batteries actuality developed for bus applications, but the U.S. Federal Railroad Administration additionally has a affairs to advance flywheel batteries for accelerated abuse applications. Diesel-powered trains are admired by abounding in the industry as too abundant for operation at speeds aloft about 180 km/h. Electric trains, conversely, are able-bodied ill-fitted for accelerated operation but electrification of the right-of-way costs millions of dollars per kilometer.
The solution: a amalgam access in which a gas agent active a accelerated architect aliment the boilerplate power, while a flywheel array handles ability leveling and accumulator applications. A architect actuality developed for this appliance has a continuous-power appraisement of 3 MW and fits in a beneath than 1.5 meters continued and 0.75 beat in diameter.
A flywheel array for a accelerated commuter alternation with nine cars would abundance 470 MJ and would accommodate 2 MW of aiguille power. The ability and activity requirements are set by the adapted dispatch and the best weight of the alternation sets. Although the flywheel is advantageous in ability leveling for the agent in grades, the primary architecture disciplinarian is endlessly and starting the alternation set at stations.
This aspect does accessible up an absorbing achievability that has been advised in a basic set of tests appear by London Underground, Urenco Ability Technologies, Balfour Beatty Abuse Ability Systems, and Seeboard Powerlink. They begin that by installing three flywheels, provided by Urenco, in a substation bartering a analysis track, they could abate the voltage bead at the substation from 180 V to 100 V. They additionally appear that they could balance about 30 percent of the braking energy. Siemens has appear agnate after-effects on a ablaze abuse band in Cologne, Germany.
Flywheel-powered activity vehicles
Hybrid electric ability is an capital enabling technology for abounding approaching activity vehicles, accustomed the cardinal of electrically powered subsystems planned for those conveyances. The U.S. Department of Defense envisions acceptable approaching activity cartage as accepting electric propulsion as able-bodied as suspension, communications, weapons, and arresting systems—all defective electric power. The amalgam electric ability arrangement will abide of an engine/alternator, sized for boilerplate ability demand; ammunition beef and their associated baby batteries for stealth (quieter) operation; added power-averaging energy-storage components, like flywheels and supercapacitors, to accommodate both connected and pulsed ability for the assorted systems on the vehicle; administration networks; subsystem controls; and power-conditioning devices.
This access promises to aerate arrangement activity by operating the prime ability sources at about affiliated ability levels. It additionally reduces the all-embracing agent weight because the prime ability units are sized for alone the boilerplate (not the peak) ability demand. For those activity systems that allegation be provided with ability in beneath than about 10 µs, the flywheel batteries would aboriginal allegation a coffer of supercapacitors, which would again ability the accelerated systems.
A flywheel advised for this appliance food 25 MJ and produces 5-10 MW of pulsed ability and 350 kW of connected power. A arrangement that can abundance 60 MJ and aftermath 6 GW is not abundant larger, actuality about 1.5 meters aerial and beneath than 1 beat in diameter.
Another acceptable appliance of flywheel batteries is in the ablution of aircraft from carriers. Today, barrage catapults are apprenticed by beef systems, which use beef accumulators to abundance abundant activity for the job. The U.S. Navy is developing electromagnetic systems in which flywheels could alter the beef accumulators so that the power-generating arrangement would not acquire to be sized for the aiguille ability load.
Clearly, flywheel batteries are an arising technology. Bartering versions are accessible for bound applications today, but the analysis and development now beneath way may activate abundant added use.
—Michael J. Riezenman, Editor
MICHAEL J. BASS and CLAYTON M. CHRISTENSEN met in 1999 while Bass, an R&D activity administrator at Hewlett-Packard Co., was alive on a master’s amount in technology administration at the Sloan School of Administration at the Massachusetts Institute of Technology. Christensen is the Robert and Jane Cizik Professor of Business Administration at the Harvard Business School, with a collective arrangement in technology and operations administration and accepted management.
A acceptable allegory of these batteries and superconducting alluring activity accumulator systems can be gleaned from A Summary of the Accompaniment of the Art of Superconducting Alluring Activity Accumulator Systems, Flywheel Activity Accumulator Systems, and Compressed Air Activity Accumulator Systems, by P. Taylor and others. For the 1999 Sandia National Laboratories Report, SAND 99-1854, address the National Abstruse Information Service, Commerce Department, 5285 Port Royal Rd., Springfield, VA 22161.
To apprentice how the Electric Ability Analysis Institute (EPRI) angle bartering possibilities, see Flywheel Array Commercialization Study, by T. Aanstoos and others. The 1999 Abstruse Address 113541, EPRICSG (Palo Alto, Calif.), may be acquired from the EPRI Administration Center, 207 Coggins Dr., Box 23205, Pleasant Hill, CA 94523; phone, 1 800 313 3774.
The hybrid-electric agent (HEV) abstraction is alien in the appropriate address in IEEE Spectrum, July 1995, pp. 16-31. And “Hybrid Electric Cartage Booty to the Streets,” IEEE Spectrum, November 1998, pp. 48-52, provides capacity of a commercially accessible HEV—the Toyota Prius
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