View source for Flywheel energy storage

{{Short description|Method of storing energy}}
[[File:G2 front2.jpg|right|thumb| [[#NASA_G2_flywheel_for_spacecraft_energy_storage|NASA G2 flywheel]]]]

'''Flywheel energy storage''' ('''FES''') works by accelerating a rotor ([[flywheel]]) to a very high speed and maintaining the energy in the system as [[rotational energy]].  When energy is extracted from the system, the flywheel's rotational speed is reduced as a consequence of the principle of [[conservation of energy]]; adding energy to the system correspondingly results in an increase in the speed of the flywheel.

Most FES systems use electricity to accelerate and decelerate the flywheel, but devices that directly use [[mechanical energy]] are being developed.{{why|date=December 2024}}<ref name="Torotrak">[http://www.xtrac.com/pdfs/Torotrak_Xtrac_CVT.pdf Torotrak Toroidal variable drive CVT] {{webarchive |url=https://web.archive.org/web/20110516084534/http://www.xtrac.com/pdfs/Torotrak_Xtrac_CVT.pdf |date=May 16, 2011 }}, retrieved June 7, 2007.</ref>

Advanced FES systems have rotors made of high strength carbon-fiber composites, suspended by [[magnetic bearing]]s, and spinning at speeds from 20,000 to over 50,000 rpm in a vacuum enclosure.<ref name="ScienceNews">{{Cite journal
| last1 = Castelvecchi
| first1 = Davide
| title = Spinning into control: High-tech reincarnations of an ancient way of storing energy
| doi = 10.1002/scin.2007.5591712010
| journal = Science News
| volume = 17
| issue = 20
| pages = 312–313
| date = May 19, 2007
| url = http://sciencewriter.org/flywheels-spinning-into-control/
| access-date = August 2, 2012
| archive-url = https://web.archive.org/web/20140606223717/http://sciencewriter.org/flywheels-spinning-into-control/
| archive-date = June 6, 2014
| url-status = dead
}}</ref> Such flywheels can come up to speed in a matter of minutes&nbsp;– reaching their energy capacity much more quickly than some other forms of storage.<ref name="ScienceNews"/>

{{toclimit|3}}

== Main components ==
[[File:Example of cylindrical flywheel rotor assembly.png|thumb|right|The main components of a typical flywheel]]
A typical system consists of a flywheel supported by [[rolling-element bearing]] connected to a [[motor–generator]]. The flywheel and sometimes motor–generator may be enclosed in a [[vacuum chamber]] to reduce friction and energy loss.

First-generation flywheel energy-storage systems use a large [[steel]] flywheel rotating on mechanical bearings. Newer systems use [[carbon-fiber]] composite [[flywheel|rotors]] that have a higher [[tensile strength]] than steel and can store much more energy for the same [[mass]].<ref>{{cite web |url=http://www.flybridsystems.com/F1System.html |title=Original F1 System - Flybrid Automotive |author=Flybrid Automotive Limited |access-date=2010-01-28 |archive-url=https://web.archive.org/web/20160303195458/http://www.flybridsystems.com/F1System.html |archive-date=2016-03-03 |url-status=dead }}</ref>

To reduce [[friction]], [[magnetic bearing]]s are sometimes used instead of [[bearing (mechanical)|mechanical bearings]].

===Possible future use of superconducting bearings===
The expense of refrigeration led to the early dismissal of low-temperature superconductors for use in magnetic bearings.  However, [[high-temperature superconductivity|high-temperature superconductor]] (HTSC) bearings may be economical and could possibly extend the time energy could be stored economically.<ref>{{Cite web |url=http://orbit.dtu.dk/fedora/objects/orbit:88109/datastreams/file_7728861/content |title=Superconducting bearings for flywheel applications |access-date=2017-02-04 |archive-date=2019-05-13 |archive-url=https://web.archive.org/web/20190513233812/http://orbit.dtu.dk/fedora/objects/orbit:88109/datastreams/file_7728861/content |url-status=dead }}</ref>  Hybrid bearing systems are most likely to see use first.  High-temperature superconductor bearings have historically had problems providing the lifting forces necessary for the larger designs but can easily provide a stabilizing force.  Therefore, in hybrid bearings, permanent magnets support the load and high-temperature superconductors are used to stabilize it. The reason superconductors can work well stabilizing the load is because they are perfect [[diamagnet]]s.  If the rotor tries to drift off-center, a restoring force due to [[flux pinning]] restores it. This is known as the magnetic stiffness of the bearing. Rotational axis vibration can occur due to low stiffness and damping, which are inherent problems of superconducting magnets, preventing the use of completely superconducting magnetic bearings for flywheel applications.

Since flux pinning is an important factor for providing the stabilizing and lifting force, the HTSC can be made much more easily for flywheel energy storage than for other uses.  HTSC powders can be formed into arbitrary shapes so long as flux pinning is strong.  An ongoing challenge that has to be overcome before superconductors can provide the full lifting force for an FES system is finding a way to suppress the decrease of levitation force and the gradual fall of rotor during operation caused by the [[flux creep]] of the superconducting material.

== Physical characteristics ==
{{see also|Flywheel#Physics}}

=== General ===
Compared with other ways to store electricity, FES systems have long lifetimes (lasting decades with little or no maintenance;<ref name="ScienceNews"/> full-cycle lifetimes quoted for flywheels range from in excess of 10<sup>5</sup>, up to 10<sup>7</sup>, cycles of use),<ref name="Investire">{{cite web|url=http://www.itpenergised.com/|title=Home|website=ITPEnergised}}</ref> high [[specific energy]] (100–130&nbsp;W·h/kg, or 360–500 kJ/kg),<ref name="Investire"/><ref name="pddnet">{{cite web |title=Next-gen Of Flywheel Energy Storage |url=http://www.pddnet.com/article-next-gen-of-flywheel-energy-storage/ |publisher=Product Design & Development |access-date=2009-05-21 |url-status=dead |archive-url=https://web.archive.org/web/20100710052927/http://www.pddnet.com/article-next-gen-of-flywheel-energy-storage/ |archive-date=2010-07-10 }}</ref> and large maximum power output. The [[energy conversion efficiency|energy efficiency]] (''ratio of energy out per energy in'') of flywheels, also known as round-trip efficiency, can be as high as 90%. Typical capacities range from 3&nbsp;[[kilowatt hour|kWh]] to 133&nbsp;kWh.<ref name="ScienceNews"/> Rapid charging of a system occurs in less than 15 minutes.<ref name="Distributed Energy">{{cite web |last=Vere |first=Henry |title=A Primer of Flywheel Technology |url=https://foresternetwork.com/weekly/energy-storage-solutions-weekly/a-primer-of-flywheel-technology/ |publisher=Distributed Energy |access-date=2008-10-06 |archive-date=2018-05-22 |archive-url=https://web.archive.org/web/20180522184525/https://foresternetwork.com/weekly/energy-storage-solutions-weekly/a-primer-of-flywheel-technology/ |url-status=dead }}</ref>  The high specific energies often cited with flywheels can be a little misleading as commercial systems built have much lower specific energy, for example 11 W·h/kg, or 40 kJ/kg.<ref name="Rosseta">[http://www.rosseta.de/texte/pdat-t4.pdf rosseta Technik GmbH, Flywheel Energy Storage Model T4], retrieved February 4, 2010.</ref>

=== Form of energy storage ===
: {|
|-
| [[Moment of inertia]]: || <math>J_m = \int_m r^2\,\mathrm dm</math>
|-
| [[Angular velocity]]: || <math> \omega_m = 2 \pi \cdot n_m</math>
|-
|  Stored [[rotational energy]]: || <math> W_{\text{kin}} = \frac{1}{2} J_m\omega^2</math>
|}
Here <math>m</math> is the integral of the flywheel's mass, and <math>n_m</math> is the [[rotational speed]] (number of revolutions per second).

=== Specific energy ===
The maximal [[specific energy]] of a flywheel rotor is mainly dependent on two factors: the first being the rotor's geometry, and the second being the properties of the material being used. For single-material, [[isotropic]] rotors this relationship can be expressed as<ref>{{cite book |last=Genta |first=Giancarlo |title=Kinetic Energy Storage |date=1985 |publisher=Butterworth & Co. Ltd. |location=London}}</ref>

: <math>\frac{E}{m} = K\left(\frac{\sigma}{\rho}\right),</math>
where
: <math>E</math> is kinetic energy of the rotor [J],
: <math>m</math> is the rotor's mass [kg],
: <math>K</math> is the rotor's geometric shape factor [dimensionless],
: <math>\sigma</math> is the tensile strength of the material [Pa],
: <math>\rho</math> is the material's density [kg/m<sup>3</sup>].

==== Geometry (shape factor) ====
The highest possible value for the shape factor<ref>{{cite web |title=Flywheel Kinetic Energy |url=http://www.engineeringtoolbox.com/flywheel-energy-d_945.html |website=The Engineering Toolbox}}</ref> of a flywheel rotor, is <math>K = 1</math>, which can be achieved only by the theoretical ''constant-stress disc'' geometry.<ref>{{cite journal |last=Genta |first=Giancarlo |title=Some considerations on the constant stress disc profile |journal=Meccanica |date=1989 |volume=24 |issue=4 |pages=235–248 |doi=10.1007/BF01556455|s2cid=122502834 }}</ref> A constant-thickness disc geometry has a shape factor of <math>K = 0.606</math>, while for a rod of constant thickness the value is <math>K = 0.333</math>. A thin cylinder has a shape factor of <math>K = 0.5</math>. For most flywheels with a shaft, the shape factor is below or about <math display="inline">K = 0.333</math>. A shaft-less design<ref>{{Cite journal|last1=Li|first1=Xiaojun|last2=Anvari|first2=Bahareh|last3=Palazzolo|first3=Alan|last4=Wang|first4=Zhiyang|last5=Toliyat|first5=Hamid|date=August 2018|title=A Utility-Scale Flywheel Energy Storage System with a Shaftless, Hubless, High-Strength Steel Rotor|journal=IEEE Transactions on Industrial Electronics|volume=65|issue=8|pages=6667–6675|doi=10.1109/TIE.2017.2772205|s2cid=4557504|issn=0278-0046}}</ref> has a shape factor similar to a constant-thickness disc (<math display="inline">K = 0.6</math>), which enables a doubled energy density.

==== Material properties ====
For energy storage, materials with high strength and low density are desirable. For this reason, composite materials are frequently used in advanced flywheels. The [[Specific strength|strength-to-density ratio]] of a material can be expressed in Wh/kg (or Nm/kg); values greater than 400 Wh/kg can be achieved by certain composite materials.

==== Rotor materials ====
Several modern flywheel rotors are made from composite materials. Examples include the carbon-fiber composite flywheel from Beacon Power Corporation<ref>{{cite web |title=Carbon Fiber Flywheels |url=http://beaconpower.com/carbon-fiber-flywheels/ |access-date=2016-10-07}}</ref> and the ''PowerThru'' flywheel from Phillips Service Industries.<ref>{{cite web |title=PowerThru flywheel |url=http://www.power-thru.com/flywheel_ups_technology.html |access-date=2012-04-29 |archive-url=https://web.archive.org/web/20120503071712/http://www.power-thru.com/flywheel_ups_technology.html |archive-date=2012-05-03 |url-status=dead }}</ref> Alternatively, Calnetix utilizes aerospace-grade high-performance steel in their flywheel construction.<ref>{{cite web |title=Kinetic Energy Storage Systems |url=https://www.calnetix.com/vdc-kinetic-energy-storage-systems |access-date=2016-10-27}}</ref>

For these rotors, the relationship between material properties, geometry and energy density can be expressed by using a weighed-average approach.<ref>{{Cite thesis |last=Janse van Rensburg |first=P. J. |title=Energy storage in composite flywheel rotors |date=December 2011 |publisher=University of Stellenbosch |hdl=10019.1/17864|type=Thesis }}</ref>

=== Tensile strength and failure modes ===
One of the primary limits to flywheel design is the tensile strength of the rotor.  Generally speaking, the stronger the disc, the faster it may be spun, and the more energy the system can store. (Making the flywheel heavier without a corresponding increase in strength will slow the maximum speed the flywheel can spin without rupturing, hence will not increase the total amount of energy the flywheel can store.)

When the tensile strength of a composite flywheel's outer binding cover is exceeded, the binding cover will fracture, and the wheel will shatter as the outer wheel compression is lost around the entire circumference, releasing all of its stored energy at once; this is commonly referred to as "flywheel explosion" since wheel fragments can reach kinetic energy comparable to that of a bullet. Composite materials that are wound and glued in layers tend to disintegrate quickly, first into small-diameter filaments that entangle and slow each other, and then into red-hot powder; a cast metal flywheel throws off large chunks of high-speed shrapnel.

For a [[cast metal]] flywheel, the failure limit is the binding strength of the [[grain boundaries]] of the [[polycrystalline]] molded metal. Aluminum in particular suffers from [[Fatigue (material)|fatigue]] and can develop [[fracture|microfracture]]s from repeated low-energy stretching. Angular forces may cause portions of a metal flywheel to bend outward and begin dragging on the outer containment vessel, or to separate completely and bounce randomly around the interior. The rest of the flywheel is now severely unbalanced, which may lead to rapid bearing failure from vibration, and sudden shock fracturing of large segments of the flywheel.

Traditional flywheel systems require strong containment vessels as a safety precaution, which increases the total mass of the device. The energy release from failure can be dampened with a gelatinous or encapsulated liquid inner housing lining, which will boil and absorb the energy of destruction. Still, many customers of large-scale flywheel energy-storage systems prefer to have them embedded in the ground to halt any material that might escape the containment vessel.

=== Energy storage efficiency ===
Flywheel energy storage systems using mechanical bearings can lose 20% to 50% of their energy in two hours.<ref name="Rosseta2">[http://www.rosseta.de/srsy.htm rosseta Technik GmbH, Flywheel Energy Storage, German], retrieved February 4, 2010.</ref>  Much of the friction responsible for this energy loss results from the flywheel changing orientation due to the rotation of the earth (an effect similar to that shown by a [[Foucault pendulum]]).  This change in orientation is resisted by the gyroscopic forces exerted by the flywheel's angular momentum, thus exerting a force against the mechanical bearings.  This force increases friction.  This can be avoided by aligning the flywheel's axis of rotation parallel to that of the earth's axis of rotation.{{citation needed|date=December 2018}}

Conversely, flywheels with [[magnetic bearings]] and high vacuum can maintain 97% [[mechanical efficiency]], and 85% round trip efficiency.<ref name="Beacon2">[http://www.beaconpower.com/files/Flywheel_FR-Fact-Sheet.pdf Beacon Power Corp, Frequency Regulation and Flywheels fact sheet], retrieved July 11, 2011.  {{webarchive |url=https://web.archive.org/web/20100331042630/http://www.beaconpower.com/files/Flywheel_FR-Fact-Sheet.pdf |date=March 31, 2010 }}</ref>

=== Effects of angular momentum in vehicles ===
When used in vehicles, flywheels also act as [[gyroscope]]s, since their [[angular momentum]] is typically of a similar order of magnitude as the forces acting on the moving vehicle. This property may be detrimental to the vehicle's handling characteristics while turning or driving on rough ground; driving onto the side of a sloped embankment may cause wheels to partially lift off the ground as the flywheel opposes sideways tilting forces. On the other hand, this property could be utilized to keep the car balanced so as to keep it from rolling over during sharp turns.<ref>''Study on Rollover prevention of heavy-duty vehicles by using flywheel energy storage systems'', Suda Yoshihiro, Huh Junhoi, Aki Masahiko, Shihpin Lin, Ryoichi Takahata, Naomasa Mukaide, Proceedings of the FISITA 2012 World Automotive Congress, Lecture Notes in Electrical Engineering Volume 197, 2013, pp 693-701, [[doi:10.1007/978-3-642-33805-2 57]]</ref>

When a flywheel is used entirely for its effects on the attitude of a vehicle, rather than for energy storage, it is called a [[reaction wheel]] or a [[control moment gyroscope]].

The resistance of angular tilting can be almost completely removed by mounting the flywheel within an appropriately applied set of [[gimbal]]s, allowing the flywheel to retain its original orientation without affecting the vehicle (see [[gyroscope#Properties|''Properties'' of a gyroscope]]). This does not avoid the complication of [[gimbal lock]], and so a compromise between the number of gimbals and the angular freedom is needed.

The center axle of the flywheel acts as a single gimbal, and if aligned vertically, allows for the 360 degrees of yaw in a horizontal plane. However, for instance driving up-hill requires a second pitch gimbal, and driving on the side of a sloped embankment requires a third roll gimbal.

==== Full-motion gimbals ====
Although the flywheel itself may be of a flat ring shape, a free-movement gimbal mounting inside a vehicle requires a spherical volume for the flywheel to freely rotate within. Left to its own, a spinning flywheel in a vehicle would slowly precess following the Earth's rotation, and precess further yet in vehicles that travel long distances over the Earth's curved spherical surface.

A full-motion gimbal has additional problems of how to communicate power into and out of the flywheel, since the flywheel could potentially flip completely over once a day, precessing as the Earth rotates. Full free rotation would require [[slip ring]]s around each gimbal axis for power conductors, further adding to the design complexity.

==== Limited-motion gimbals ====
To reduce space usage, the gimbal system may be of a limited-movement design, using shock absorbers to cushion sudden rapid motions within a certain number of degrees of out-of-plane angular rotation, and then gradually forcing the flywheel to adopt the vehicle's current orientation. This reduces the gimbal movement space around a ring-shaped flywheel from a full sphere, to a short thickened cylinder, encompassing for example ± 30 degrees of pitch and ± 30 degrees of roll in all directions around the flywheel.

==== Counterbalancing of angular momentum ====
An alternative solution to the problem is to have two joined flywheels spinning synchronously in opposite directions. They would have a total angular momentum of zero and no gyroscopic effect. A problem with this solution is that when the difference between the momentum of each flywheel is anything other than zero the housing of the two flywheels would exhibit torque. Both wheels must be maintained at the same speed to keep the angular velocity at zero. Strictly speaking, the two flywheels would exert a huge [[torque]]ing moment at the central point, trying to bend the axle. However, if the axle were sufficiently strong, no gyroscopic forces would have a net effect on the sealed container, so no torque would be noticed.

To further balance the forces and spread out strain, a single large flywheel can be balanced by two half-size flywheels on each side, or the flywheels can be reduced in size to be a series of alternating layers spinning in opposite directions. However this increases housing and bearing complexity.

== Applications ==

=== Transportation ===

==== Automotive ====

In the 1950s, flywheel-powered buses, known as [[gyrobus]]es, were used in [[Yverdon]] ([[Switzerland]]) and [[Ghent]] ([[Belgium]]) and there is ongoing research to make flywheel systems that are smaller, lighter, cheaper and have a greater capacity.  It is hoped that flywheel systems can replace conventional chemical batteries for mobile applications, such as for electric vehicles.  Proposed flywheel systems would eliminate many of the disadvantages of existing{{when|date=December 2024}} battery power systems, such as low capacity, long charge times, heavy weight and short usable lifetimes. Flywheels may have been used in the experimental [[Chrysler Patriot]], though that has been disputed.<ref>{{cite web|url=http://www.allpar.com/model/patriot.html|title=Chrysler Patriot hybrid-electric racing car: 20 years early for F1 racing?|date=16 November 2020 }}</ref>
[[File:Gyrobus G3-1.jpg|thumb|One of the older gyrobuses parked in a museum in Antwerp.]]

Flywheels have also been proposed for use in [[continuously variable transmission]]s. Punch Powertrain is currently working on such a device.<ref>{{cite web|url=http://www.agoria.be/www.wsc/Punch-Powertrain-werkt-aan-revolutionaire-vliegwiel-hybride-transmissie-150269?vWebSessionID=32896&vUserID=999999&ENewsID=86139&TopicID=8420&TopicList=8420&ComingFromResultTopic=5306|title=Agoria>GoodNews!>Archieven 2012>Punch Powertrain werkt aan revolutionaire vliegwiel-hybride transmissie|access-date=2012-09-13|archive-url=https://web.archive.org/web/20130522031713/http://www.agoria.be/www.wsc/Punch-Powertrain-werkt-aan-revolutionaire-vliegwiel-hybride-transmissie-150269?vWebSessionID=32896&vUserID=999999&ENewsID=86139&TopicID=8420&TopicList=8420&ComingFromResultTopic=5306|archive-date=2013-05-22|url-status=dead}}</ref>

During the 1990s, [[Rosen Motors]] developed a [[gas turbine]] powered [[series hybrid]] automotive powertrain using a 55,000 rpm flywheel to provide bursts of acceleration which the small gas turbine engine could not provide.  The flywheel also stored energy through [[regenerative braking]].  The flywheel was composed of a [[titanium]] hub with a [[carbon fiber]] cylinder and was [[gimbal]]-mounted to minimize adverse gyroscopic effects on vehicle handling.  The prototype vehicle was successfully road tested in 1997 but was never mass-produced.<ref name="Wakefield">{{cite book |title=History of the Electric Automobile: Hybrid Electric Vehicles |last=Wakefield |first=Ernest |date=1998|publisher=SAE|isbn=978-0-7680-0125-9|page=332}}</ref>

In 2013, [[Volvo Cars|Volvo]] announced a flywheel system fitted to the rear axle of its S60 sedan. Braking action spins the flywheel at up to 60,000 rpm and stops the front-mounted engine. Flywheel energy is applied via a special transmission to partially or completely power the vehicle. The {{convert|20|cm|adj=on}}, {{convert|6|kg|adj=on}} [[carbon fiber]] flywheel spins in a vacuum to eliminate friction. When partnered with a four-cylinder engine, it offers up to a 25 percent reduction in fuel consumption versus a comparably performing turbo six-cylinder, providing an {{convert|80|hp}} boost and allowing it to reach {{convert|100|km/h}} in 5.5 seconds. The company did not announce specific plans to include the technology in its product line.<ref>{{cite web|url=http://www.gizmag.com/volvo-flywheel-kers-testing/27273/ |title=Volvo confirms fuel savings of 25 percent with flywheel KERS |date=26 April 2013 |publisher=Gizmag.com |access-date=2013-04-26}}</ref>

In July 2014 [[GKN]] acquired [[Williams Grand Prix Engineering|Williams]] Hybrid Power (WHP) division and intends to supply 500 [[carbon fiber]] ''Gyrodrive'' electric flywheel systems to urban bus operators over the next two years.<ref>{{cite web|url=http://www.automotiveworld.com/news-releases/gkn-go-ahead-group-using-f1-technology-improve-fuel-efficiency-london-buses/|title=GKN and the Go-Ahead Group using F1 technology to improve fuel efficiency of London buses|date=29 July 2014}}</ref> As the former developer name implies, these were originally designed for [[Formula one]] [[motor racing]] applications. In September 2014, [[Oxford Bus Company]] announced that it is introducing 14 ''Gyrodrive hybrid'' buses by [[Alexander Dennis]] on its Brookes Bus operation.<ref>{{cite web|url=https://oxfordbus.wordpress.com/2014/09/05/its-the-new-brookesbus/|title=It's the New BROOKESbus!|work=Oxford Bus Company|date=5 September 2014}}</ref><ref>{{cite news|url=https://www.bbc.co.uk/news/uk-england-oxfordshire-29030173|title=BBC News - Formula One race technology to power buses in Oxford|work=BBC News|date=2 September 2014}}</ref>

==== Rail vehicles ====
Flywheel systems have been used experimentally in small [[electric locomotive]]s for shunting or [[switcher|switching]], e.g. the [[Sentinel Waggon Works|Sentinel-Oerlikon Gyro Locomotive]].  Larger electric locomotives, e.g. [[British Rail Class 70 (electric)|British Rail Class 70]], have sometimes been fitted with flywheel boosters to carry them over gaps in the [[third rail]]. Advanced flywheels, such as the 133&nbsp;kWh pack of the [[University of Texas at Austin]], can take a train from a standing start up to cruising speed.<ref name="ScienceNews"/>

The [[Parry People Mover]] is a [[railcar]] which is powered by a flywheel. It was trialled on Sundays for 12 months on the [[Stourbridge Town Branch Line]] in the [[West Midlands (county)|West Midlands]], [[England]] during 2006 and 2007 and was intended to be introduced as a full service by the train operator [[London Midland]] in December 2008 once two units had been ordered. In January 2010, both units are in operation.<ref>{{cite web|url=http://www.londonmidland.com/index.php/news/news_items/view/23|title=Parry People Movers for Stourbridge branch line|date=2008-01-03|access-date=2008-03-19|publisher=[[London Midland]] |archive-url=https://web.archive.org/web/20080517110918/http://www.londonmidland.com/index.php/news/news_items/view/23 <!-- Bot retrieved archive --> |archive-date=2008-05-17}}</ref>

==== Rail electrification ====
FES can be used at the lineside of electrified railways to help regulate the line voltage thus improving the acceleration of unmodified electric trains and the amount of energy recovered back to the line during [[regenerative braking]], thus lowering energy bills.<ref>{{cite magazine|url=http://www.railwaygazette.com/nc/news/single-view/view/high-speed-flywheels-cut-energy-bill.html|title=High-speed flywheels cut energy bill|date=2001-04-01|access-date=2010-12-02|magazine=[[Railway Gazette International]]|archive-date=2011-06-15|archive-url=https://web.archive.org/web/20110615230024/http://www.railwaygazette.com/nc/news/single-view/view/high-speed-flywheels-cut-energy-bill.html|url-status=dead}}</ref>  Trials have taken place in London, New York, Lyon and Tokyo,<ref name=RG-KESS>{{cite magazine|url=http://www.railwaygazette.com/nc/news/single-view/view/kinetic-energy-storage-wins-acceptance.html|title=Kinetic energy storage wins acceptance|date=2004-04-01|access-date=2010-12-02|magazine=[[Railway Gazette International]]|archive-date=2011-05-28|archive-url=https://web.archive.org/web/20110528010941/http://www.railwaygazette.com/nc/news/single-view/view/kinetic-energy-storage-wins-acceptance.html|url-status=dead}}</ref> and [[New York MTA]]'s [[Long Island Rail Road]] is now investing $5.2m in a pilot project on LIRR's [[West Hempstead Branch]] line.<ref>{{cite magazine|url=http://www.railwaygazette.com/nc/news/single-view/view/new-york-orders-fly-wheel-energy-storage.html|title=New York orders flywheel energy storage|date=2009-08-14|access-date=2011-02-09|magazine=[[Railway Gazette International]]|archive-date=2011-05-28|archive-url=https://web.archive.org/web/20110528011001/http://www.railwaygazette.com/nc/news/single-view/view/new-york-orders-fly-wheel-energy-storage.html|url-status=dead}}</ref>
These trials and systems store kinetic energy in rotors consisting of a carbon-glass composite cylinder packed with neodymium-iron-boron powder that forms a permanent magnet. These spin at up to 37,800 rpm, and each {{convert|100|kW|hp|abbr=on}} unit can store {{convert|11|MJ|kWh}} of re-usable energy, approximately enough to accelerate a weight of {{convert|200|MT|ST LT|0|abbr=off}} from zero to {{convert|38|km/h|mph|abbr=on}}.<ref name=RG-KESS/>

=== Uninterruptible power supplies ===
Flywheel power storage systems in production {{as of|2001|lc=y}} had storage capacities comparable to batteries and faster discharge rates.  They are mainly used to provide load leveling for large battery systems, such as an [[uninterruptible power supply]] for data centers as they save a considerable amount of space compared to battery systems.<ref>{{cite web |url=http://www.datacenterknowledge.com/archives/2007/06/26/flywheels-gain-as-alternative-to-batteries/ |title=Flywheels gain as alternative to batteries |website=www.datacenterknowledge.com |date=26 June 2007 |access-date=17 September 2024}}</ref>

Flywheel maintenance in general runs about one-half the cost of traditional battery UPS systems. The only maintenance is a basic annual preventive maintenance routine and replacing the bearings every five to ten years, which takes about four hours.<ref name="Distributed Energy"/> Newer flywheel systems completely levitate the spinning mass using maintenance-free [[magnetic bearing]]s, thus reducing bearing maintenance and failures.<ref name="Distributed Energy"/>

Costs of a fully installed flywheel UPS (including power conditioning) were (in 2009) about $330 per [[kilowatt]] (for 15 seconds full-load capacity).<ref name=Andrews2009>{{cite web|url=http://www.claverton-energy.com/active-power-article-flywheel-energy-storage.html|title=Active Power Article - Flywheel energy storage - Claverton Group |website=www.claverton-energy.com |date=21 June 2009 |access-date=17 September 2024}}</ref>

=== Test laboratories ===
A long-standing niche market for flywheel power systems are facilities where [[circuit breaker]]s and similar devices are tested: even a small household circuit breaker may be rated to interrupt a current of {{val|10,000|fmt=commas}} or more amperes, and larger units may have [[breaking capacity|interrupting ratings]] of {{val|100,000|fmt=commas}} or {{val|1,000,000|fmt=commas}} amperes. The enormous transient loads produced by deliberately forcing such devices to demonstrate their ability to interrupt simulated short circuits would have unacceptable effects on the local grid if these tests were done directly from building power. Typically such a laboratory will have several large motor–generator sets, which can be spun up to speed over several minutes; then the motor is disconnected before a circuit breaker is tested.

===Physics laboratories===
[[Tokamak]] fusion experiments need very high currents for brief intervals (mainly to power large electromagnets for a few seconds). 
*JET (the [[Joint European Torus]]) has two {{convert|775|MT|ST LT|0|abbr=on}} flywheels (installed in 1981) that spin up to 225 rpm.<ref>{{cite web |url=https://www.euro-fusion.org/news/detail/week-20-jet-experiments-sensitive-to-tv-schedules/ |title=Week 20: JET Experiments: sensitive to TV schedules |access-date=2018-05-03 |archive-date=2020-07-31 |archive-url=https://web.archive.org/web/20200731192830/https://www.euro-fusion.org/news/detail/week-20-jet-experiments-sensitive-to-tv-schedules/ |url-status=dead }}</ref> Each flywheel stores 3.75 GJ and can deliver at up to {{convert|400|MW|hp|abbr=on}}.<ref name=JETFW>{{cite web |url=https://www.euro-fusion.org/fusion/jet-tech/jets-flywheels/ |title=Power supply |access-date=2015-12-07 |archive-date=2016-01-05 |archive-url=https://web.archive.org/web/20160105221442/https://www.euro-fusion.org/fusion/jet-tech/jets-flywheels/ |url-status=dead }}</ref> 
*The [[Helically Symmetric Experiment]] at the [[University of Wisconsin-Madison]] has 18 one-ton flywheels, which are spun to 10,000 rpm using repurposed electric train motors.
*[[ASDEX Upgrade]] has 3 flywheel generators.
*[[DIII-D (tokamak)]] at [[General Atomics]]
*the [[Princeton Large Torus]] (PLT) at the [[Princeton Plasma Physics Laboratory]]

Also the non-tokamak: [[Nimrod (synchrotron)|Nimrod synchrotron]] at the [[Rutherford Appleton Laboratory]] had two 30 ton flywheels.

===Aircraft launching systems===
The [[Ford class aircraft carrier|''Gerald R. Ford''-class aircraft carrier]] will use flywheels to accumulate energy from the ship's power supply, for rapid release into the [[EMALS|electromagnetic aircraft launch system]]. The shipboard power system cannot on its own supply the high power transients necessary to launch aircraft. Each of four rotors will store 121&nbsp;MJ (34&nbsp;kWh) at 6400&nbsp;rpm. They can store 122&nbsp;MJ (34&nbsp;kWh) in 45 secs and release it in 2–3 seconds.<ref name=EMALS>{{cite report |url=http://carcamal.ele.cie.uva.es/CEM/articulos/Catapult.pdf |archive-url=https://web.archive.org/web/20030708024628/http://carcamal.ele.cie.uva.es/CEM/articulos/Catapult.pdf |url-status=dead |archive-date=2003-07-08 |author=Michael R. Doyle |author2=Douglas J. Samuel |author3=Thomas Conway |author4=Robert R. Klimowski |name-list-style=amp |title=Electromagnetic Aircraft Launch System - EMALS |date=1994-04-15 }}</ref> The flywheel energy densities are 28 kJ/kg (8 W·h/kg); including the stators and cases this comes down to 18.1 kJ/kg (5 W·h/kg), excluding the torque frame.<ref name=EMALS/>

=== NASA G2 flywheel for spacecraft energy storage ===
This was a design funded by [[NASA]]'s Glenn Research Center and intended for component testing in a laboratory environment. It used a carbon fiber rim with a titanium hub designed to spin at 60,000 rpm, mounted on magnetic bearings. Weight was limited to {{convert|250|lbs|kg|abbr=off}}. Storage was {{convert|525|Wh|MJ|abbr=on}} and could be charged or discharged at {{convert|1|kW|hp|abbr=on}}, leading to a specific energy of 5.31 W⋅h/kg and power density of 10.11 W/kg.<ref>[https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20060028492.pdf G2 Flywheel Module Design]</ref> The working model shown in the photograph at the top of the page ran at 41,000 rpm on September 2, 2004.<ref>{{cite journal|url=https://ntrs.nasa.gov/search.jsp?R=20050217267|title=NASA Technical Reports Server (NTRS)|journal=Research and Technology 2004|date=June 2005|last1=Jansen|first1=Ralph H.|last2=McLallin|first2=Kerry L.}}</ref>

=== Amusement rides ===
The [[Montezooma's Revenge|Montezooma's Revenge roller coaster]] at [[Knott's Berry Farm]] was the first flywheel-launched roller coaster in the world and is the last ride of its kind still operating in the United States. The ride uses a 7.6 tonnes flywheel to accelerate the train to 55 miles per hour (89&nbsp;km/h) in 4.5 seconds.

The [[Incredible Hulk Coaster|Incredible Hulk roller coaster]] at [[Islands of Adventure|Universal's Islands of Adventure]] features a rapidly accelerating uphill launch as opposed to the typical gravity drop. This is achieved through powerful [[traction motor]]s that throw the car up the track. To achieve the brief very high current required to accelerate a full coaster train to full speed uphill, the park utilizes several motor-generator sets with large flywheels. Without these stored energy units, the park would have to invest in a new substation or risk [[Brownout (electricity)|browning-out]] the local energy grid every time the ride launches.

=== Pulse power ===
The compensated pulsed alternator ([[compulsator]]) is one of the most popular choices of pulsed power supplies for fusion reactors, high-power pulsed lasers, and hypervelocity electromagnetic launchers because of its high energy density and power density.<ref>{{cite journal|title=Analytical Investigation and Scaled Prototype Tests of a Novel Permanent Magnet Compulsator|first1=H.|last1=Wang|first2=K.|last2=Liu|first3=B.|last3=Zhu|first4=J.|last4=Feng|first5=P.|last5=Ao|first6=Z.|last6=Zhang|date=1 August 2015|journal=IEEE Transactions on Magnetics|volume=51|issue=8|pages=2415466|doi=10.1109/TMAG.2015.2415466|bibcode=2015ITM....5115466W|s2cid=24547533}}</ref> 
Instead of having a separate flywheel and generator, only the large rotor of the low-[[inductance]] alternator stores energy. (See also [[Homopolar generator]].)<ref>{{cite web|url=http://orbitalvector.com/Power/COMPULSATORS.htm|title=COMPULSATORS|website=orbitalvector.com|access-date=31 March 2018}}</ref>

=== Motor sports ===
{{main|Kinetic energy recovery system}}
[[File:Flybrid Systems Kinetic Energy Recovery System.jpg|thumb|right|A Flybrid Systems Kinetic Energy Recovery System built for use in Formula One]]
Using a [[continuously variable transmission]] (CVT), energy is recovered from the drive train during braking and stored in a flywheel. This stored energy is then used during acceleration by altering the ratio of the CVT.<ref>{{cite web|url=http://www.flybridsystems.com/Technology.html|title=Technology - Flybrid Automotive|author=Flybrid Automotive Limited|access-date=2007-11-09|archive-url=https://web.archive.org/web/20100713210303/http://www.flybridsystems.com/Technology.html|archive-date=2010-07-13|url-status=dead}}</ref> In motor sports applications this energy is used to improve acceleration rather than reduce carbon dioxide emissions{{spaced ndash}}although the same technology can be applied to road cars to improve [[fuel efficiency]].<ref>{{cite web|url=http://www.flybridsystems.com/Roadcar.html|title=Road Car Systems - Flybrid Automotive|author=Flybrid Automotive Limited}}</ref>

[[Automobile Club de l'Ouest]], the organizer behind the annual [[24 Hours of Le Mans]] event and the [[Le Mans Series]], is currently "studying specific rules for [[Le Mans prototype|LMP1]] which will be equipped with a kinetic energy recovery system."<ref>{{cite web|url=http://www.lemans.org/sport/sport/reglements/ressources/auto_2008/cdc_reglement_lmp_fr_gb_2008.pdf |title=ACO Technical Regulations 2008 for Prototype "LM"P1 and "LM"P2 classes |page=3 |publisher=Automobile Club de l'Ouest (ACO) |date=2007-12-20 |access-date=2008-04-10 |url-status=dead |archive-url=https://web.archive.org/web/20080517114143/http://www.lemans.org/sport/sport/reglements/ressources/auto_2008/cdc_reglement_lmp_fr_gb_2008.pdf |archive-date=May 17, 2008 }}</ref>

Williams Hybrid Power, a subsidiary of Williams F1 Racing team,<ref>{{cite web |title=Williams Hybrid Power Motorsports Applications |url=http://www.williamshybridpower.com/#%2Fapplications%2Fmotorsport |access-date=2014-03-05 |archive-url=https://web.archive.org/web/20140209191443/http://www.williamshybridpower.com/#%2Fapplications%2Fmotorsport |archive-date=2014-02-09 |url-status=dead}}</ref> have supplied Porsche and Audi with flywheel based hybrid system for Porsche's 911 GT3 R Hybrid<ref>{{cite web |title=911 GT3 R Hybrid Celebrates World Debut in Geneva |url=http://www.porsche.com/usa/aboutporsche/pressreleases/pag/?pool=international-de&id=2010-02-11 |website=www.porsche.com |date=2010-02-11 |access-date=17 September 2024}}</ref> and Audi's R18 e-Tron Quattro.<ref>{{cite web|title=Audi R18 e-Tron quattro|url=http://www.ultimatecarpage.com/car/5040/Audi-R18-e-tron-quattro.html |website=www.ultimatecarpage.com |date= |access-date=17 September 2024}}</ref> Audi's victory in 2012 24 Hours of Le Mans is the first for a hybrid (diesel-electric) vehicle.<ref>{{cite magazine |last=Beer|first=Matt |title=Audi #1 crew claims first hybrid Le Mans 24 Hours win |url=http://www.autosport.com/news/report.php/id/100465 |magazine=Autosport}}</ref>

=== Grid energy storage ===
{{anchor|Frequency regulation}}<!-- old section name after rename -->
{{main|Flywheel storage power system|Grid energy storage}}
Flywheels are sometimes used as short term [[Operating reserve|
spinning reserve]] for momentary grid [[Utility frequency|frequency regulation]] and balancing sudden changes between supply and consumption. No carbon emissions, faster response times and ability to buy power at off-peak hours are among the advantages of using flywheels instead of traditional sources of energy like natural gas turbines.<ref>[http://www.beaconpower.com/products/EnergyStorageSystems/docs/BeaconPower_solutions_grid.pdf Flywheel-based Solutions for Grid Reliability] {{webarchive |url=https://web.archive.org/web/20070712140640/http://www.beaconpower.com/products/EnergyStorageSystems/docs/BeaconPower_solutions_grid.pdf |date=July 12, 2007 }}</ref> Operation is very similar to batteries in the same application, their differences are primarily economic.

[[Beacon Power]] opened a 5 MWh (20 MW over 15 mins)<ref name="Beacon2"/> flywheel energy storage plant in [[Stephentown, New York]] in 2011<ref>http://www.sandia.gov/ess/docs/pr_conferences/2014/Thursday/Session7/02_Areseneaux_Jim_20MW_Flywheel_Energy_Storage_Plant_140918.pdf {{Bare URL PDF|date=March 2022}}</ref> using 200 flywheels<ref name=BS>{{cite web|url=http://beaconpower.com/stephentown-new-york/|title=Stephentown, New York - Beacon Power|website=beaconpower.com|access-date=31 March 2018}}</ref> and a similar 20 MW system at [[Hazle Township, Pennsylvania]] in 2014.<ref name=BH>{{cite web|url=http://beaconpower.com/hazle-township-pennsylvania/ |title=Hazle Township, Pennsylvania - Beacon Power|website=beaconpower.com|access-date=31 March 2018}}</ref>

A 0.5MWh (2 MW for 15 min)<ref>{{cite web|url=http://www.ieso.ca/Documents/caa/CAA_2013-EX688_Final_Report.pdf |title=IESO Expedited System Impact Assessment - MINTO FLYWHEEL FACILITY|website=ieso.ca|access-date=31 March 2018|archive-url=https://web.archive.org/web/20160129064802/http://www.ieso.ca/Documents/caa/CAA_2013-EX688_Final_Report.pdf |archive-date=29 January 2016|url-status=dead|df=dmy-all}}</ref> [[Minto flywheel facility|flywheel storage facility in Minto]], [[Ontario, Canada]] opened in 2014.<ref name=Willis2014>{{cite web|url=http://storage.pv-tech.org/news/canadas-first-grid-storage-system-launches-in-ontario |title=Canada's first grid storage system launches in Ontario - PV-Tech Storage|work=PV-Tech Storage|access-date=2014-07-30|archive-date=2014-08-31|archive-url=https://web.archive.org/web/20140831005958/http://storage.pv-tech.org/news/canadas-first-grid-storage-system-launches-in-ontario |url-status=dead}}</ref> The flywheel system (developed by [[NRStor]]) uses 10 spinning steel flywheels on magnetic bearings.<ref name=Willis2014/>

[http://www.AmberKinetics.com Amber Kinetics, Inc]. has an agreement with [[Pacific Gas and Electric Company|Pacific Gas and Electric]] (PG&E) for a 20 MW / 80 MWh flywheel energy storage facility located in Fresno, CA with a four-hour discharge duration.<ref>{{Cite web|url=https://www.pge.com/en/about/newsroom/newsdetails/index.page?title=20151202_pge_presents_innovative_energy_storage_agreements_ |title=PG&E Presents Innovative Energy Storage Agreements {{!}} PG&E|website=www.pge.com|access-date=2017-03-10}}</ref>

A 30 MW flywheel grid system started operating in China in 2024.<ref>{{cite web |title=China connects its first large-scale flywheel storage project to grid |url=https://www.ess-news.com/2024/09/13/china-connects-its-first-large-scale-flywheel-storage-project-to-grid/ |website=Energy Storage |date=13 September 2024}}</ref>

=== Wind turbines ===
Flywheels may be used to store energy generated by wind turbines during off-peak periods or during high wind speeds.

In 2010, [[Beacon Power]] began testing of their Smart Energy 25 (Gen 4) flywheel energy storage system at a wind farm in [[Tehachapi, California]]. The system was part of a wind power and flywheel demonstration project being carried out for the California Energy Commission.<ref>{{cite web|url=http://www.renewableenergyworld.com/rea/news/article/2010/03/beacon-connects-flywheel-system-to-california-wind-farm|title=Beacon Connects Flywheel System to California Wind Farm|date=26 May 2023 }}</ref>

=== Toys ===
[[Friction motor]]s used to power many [[toy car]]s, trucks, trains, action toys and such, are simple flywheel motors.

=== Toggle action presses ===
In industry, [[toggle action press]]es are still popular. The usual arrangement involves a very strong [[crankshaft]] and a heavy duty connecting rod which drives the press. Large and heavy flywheels are driven by electric motors but the flywheels turn the crankshaft only when clutches are activated.

== Comparison to electric batteries ==

Flywheels are not as adversely affected by temperature changes, can operate at a much wider temperature range, and are not subject to many of the common failures of chemical [[rechargeable battery|rechargeable batteries]].<ref>{{cite web |url=http://www.mpoweruk.com/lithium_failures.htm |title=Lithium Battery Failures |publisher=Mpoweruk.com |access-date=2013-04-26 |archive-date=2021-05-01 |archive-url=https://web.archive.org/web/20210501050357/http://www.mpoweruk.com/lithium_failures.htm |url-status=dead }}</ref> They are also less potentially damaging to the environment, being largely made of [[Chemically inert|inert]] or benign materials. Another advantage of flywheels is that by a simple measurement of the rotation speed it is possible to know the exact amount of energy stored.

Unlike most batteries which operate only for a finite period (for example roughly 10<ref>{{Cite web |title=How to Optimize Your LiFePO4 Battery's Lifespan |url=https://goalzero.com/blogs/news/optimize-lfp-battery-lifespan |website=Goal Zero|date=16 January 2024 }}</ref> years in the case of [[Lithium iron phosphate battery|lithium iron phosphate batteries]]), a flywheel potentially has an indefinite working lifespan. Flywheels built as part of [[James Watt]] [[steam engines]] have been continuously working for more than two hundred years.<ref>{{cite web |url=http://www.powerhousemuseum.com/collection/database/?irn=7177 |title=Boulton and Watt steam engine |author=Powerhouse Museum |access-date=2 August 2012 |publisher=Powerhouse Museum, Australia}}</ref> Working examples of ancient flywheels used mainly in milling and pottery can be found in many locations in Africa, Asia, and Europe.<ref>{{Cite journal
 | last1 = Donners | first1 = K.
 | last2 = Waelkens | first2 = M.
 | last3 = Deckers | first3 = J.
 | title = Water Mills in the Area of Sagalassos: A Disappearing Ancient Technology
 | journal = Anatolian Studies
 | volume = 52
 | pages = 1–17
 | date = 2002
 | doi = 10.2307/3643076
| jstor = 3643076
 | s2cid = 163811541
 }}</ref><ref>{{Cite journal
 | last1 = Wilson | first1 = A.
 | title = Machines, Power and the Ancient Economy
 | journal = The Journal of Roman Studies
 | volume = 92
 | pages = 1–32
 | date = 2002
 | doi = 10.2307/3184857
| jstor = 3184857
 | s2cid = 154629776
 }}</ref>

Most modern flywheels are typically sealed devices that need minimal maintenance throughout their service lives. Magnetic bearing flywheels in vacuum enclosures, such as the NASA model depicted above, do not need any bearing maintenance and are therefore superior to batteries both in terms of total lifetime and energy storage capacity, since their effective service lifespan is still unknown. Flywheel systems with mechanical bearings will have limited lifespans due to wear.

High performance flywheels can explode, injuring bystanders with high-speed fragments.<ref>{{Cite web |date=2015-12-10 |title=11K pound flywheel caused Poway explosion |url=https://www.10news.com/news/11k-pound-flywheel-caused-poway-explosion |access-date=2024-12-04 |website=ABC 10 News San Diego KGTV |language=en}}</ref> Flywheels can be installed below-ground to reduce this risk.  While batteries can catch fire and release toxins, there is generally time for bystanders to flee and escape injury.

The physical arrangement of batteries can be designed to match a wide variety of configurations, whereas a flywheel at a minimum must occupy a certain area and volume, because the energy it stores is proportional to its rotational inertia and to the square of its rotational speed. As a flywheel gets smaller, its mass also decreases, so the speed must increase, and so the stress on the materials increases. Where dimensions are a constraint, (e.g. under the chassis of a train), a flywheel may not be a viable solution.{{Citation needed|date=March 2019}}

== See also ==

{{portal|Energy}}

* {{annotated link|Beacon Power}}
* {{annotated link|Compensated pulsed alternator}}
* {{annotated link|Electric double-layer capacitor}}
* {{annotated link|Energy storage}}
* {{annotated link|Grid energy storage}}
* {{annotated link|Inverter}}
* {{annotated link|Launch loop}}
* {{annotated link|List of energy storage projects}}
* {{annotated link|List of energy topics}}
* {{annotated link|Plug-in hybrid}}
* {{annotated link|Rechargeable battery}}
* {{annotated link|Regenerative brake}}
* {{annotated link|Rotational energy}}
* {{annotated link|STATCOM}}
* {{annotated link|United States Department of Energy International Energy Storage Database}}

== References ==
{{Reflist|30em}}

== Further reading ==
* Beacon Power Applies for DOE Grants to Fund up to 50% of Two 20 MW Energy Storage Plants, Sep. 1, 2009 [http://phx.corporate-ir.net/phoenix.zhtml?c=123367&p=irol-newsArticle&ID=1326376&highlight=]{{dead link|date=October 2017 |bot=InternetArchiveBot |fix-attempted=yes }}
* {{cite book
 |last=Sheahen |first=Thomas P. |date=1994
 |title=Introduction to High-Temperature Superconductivity
 |url=https://archive.org/details/introductiontohi00shea_181 |url-access=limited |publisher=Plenum Press |location=New York |pages=[https://archive.org/details/introductiontohi00shea_181/page/n90 76]–78, 425–431
 |isbn=978-0-306-44793-8
}}
* {{cite book
 |last=El-Wakil |first=M. M. |date=1984
 |title=Powerplant Technology
 |url=https://archive.org/details/powerplanttechno00melw |url-access=limited |publisher=McGraw-Hill |pages=[https://archive.org/details/powerplanttechno00melw/page/n700 685]–689
|isbn=9780070192881 }}
* {{cite journal
 |display-authors=4|last=Koshizuka |first=N.
 |author2=Ishikawa, F.
 |author3=Nasu, H.
 |author4=Murakami, M.
 |author5=Matsunaga, K.
 |author6=Saito, S.
 |author7=Saito, O.
 |author8=Nakamura, Y.
 |author9=Yamamoto, H.
 |author10=Takahata, R.
 |author11=Itoh, Y.
 |author12=Ikezawa, H.
 |author13=Tomita, M.
 |date=2003
 |title=Progress of superconducting bearing technologies for flywheel energy storage systems
 |journal=Physica C
 |volume=386 |issue=386 |pages=444–450
|doi=10.1016/S0921-4534(02)02206-2 |bibcode=2003PhyC..386..444K }}
* {{cite journal
 |last=Wolsky |first=A. M. |date=2002
 |title=The status and prospects for flywheels and SMES that incorporate HTS
 |journal=Physica C
 |volume=372 |issue=372–376 |pages=1495–1499
|doi=10.1016/S0921-4534(02)01057-2 |bibcode=2002PhyC..372.1495W }}
* {{cite journal
 |display-authors=4|last=Sung |first=T. H.
 |author2=Han, S. C.
 |author3=Han, Y. H.
 |author4=Lee, J. S.
 |author5=Jeong, N. H.
 |author6=Hwang, S. D.
 |author7=Choi, S. K.
 |date=2002
 |title=Designs and analyses of flywheel energy storage systems using high-Tc superconductor bearings
 |journal=Cryogenics |volume=42 |pages=357–362
 |doi=10.1016/S0011-2275(02)00057-7
 |bibcode=2002Cryo...42..357S
 |issue=6–7
}}
* {{cite web
 |url         = http://infoserve.sandia.gov/cgi-bin/techlib/access-control.pl/1997/970443.pdf
 |title       = Cost Analysis of Energy Storage Systems for Electric Utility Applications
 |date        = February 2007
 |first       = Abbas
 |last        = Akhil
 |author2     = Swaminathan, Shiva
 |author3     = Sen, Rajat K.
 |publisher   = Sandia National laboratories
 |url-status     = dead
 |archive-url  = https://web.archive.org/web/20070621121838/http://infoserve.sandia.gov/cgi-bin/techlib/access-control.pl/1997/970443.pdf
 |archive-date = 2007-06-21
}}
* {{cite web
 |first=David |display-authors=4 |last=Larbalestier
 |author2=Blaugher, Richard D.
 |author3=Schwall, Robert E.
 |author4= Sokolowski, Robert S.
 |author5= Suenaga, Masaki
 |author6= Willis, Jeffrey O.
 |url=http://www.wtec.org/loyola/scpa/04_02.htm
 |title=Flywheels
 |work=Power Applications of Superconductivity in Japan and Germany
 |publisher=World Technology Evaluation Center
 |date=September 1997
}}
* {{cite journal
 |title=A New Look at an Old Idea: The Electromechanical Battery
 |url=http://www.llnl.gov/str/pdfs/04_96.2.pdf
 |pages=12–19
 |journal=Science & Technology Review
 |date=April 1996
 |access-date=2006-07-21
 |archive-url=https://web.archive.org/web/20080405222654/https://www.llnl.gov/str/pdfs/04_96.2.pdf
 |archive-date=2008-04-05
 |url-status=dead
 }}
* {{Cite thesis|last=Janse van Rensburg
 |first=P.J.
 |title=Energy storage in composite flywheel rotors
 |date=December 2011
 |publisher=University of Stellenbosch, South Africa
|hdl=10019.1/17864
 |type=Thesis
 }}
* {{cite web
 |last=Devitt
 |first=Drew
 |title=Making a case for flywheel energy storage
 |url=http://www.renewableenergyworld.com/rea/news/article/2010/03/making-a-case-for-flywheel-energy-storage
 |date=March 2010
 |publisher=Renewable Energy World Magazine North America
}}
* Li, X., & Palazzolo, A. (2022). A review of flywheel energy storage systems: State of the art and opportunities. ''Journal of Energy Storage'', ''46'', 103576. <nowiki>https://doi.org/10.1016/j.est.2021.103576</nowiki>

==External links==
{{Commons category|Flywheel energy storage devices}}
*[https://web.archive.org/web/20140306043403/http://www.edsenerji.com.tr/userfiles/files/Federal%20Alert.pdf Federal Technology Alert, Flywheel Energy Storage]
*[http://www.magnetal.se/GESS.pdf Magnetal Whitepaper for its Green Energy Storage System&nbsp;– GESS]
*[http://www.magnetal.se/MagnetalGyro.pdf Magnetal analysis on gyro forces induced by flywheel energy storage]

{{emerging technologies|energy=yes}}

{{DEFAULTSORT:Flywheel energy storage}}
[[Category:Flywheels]]
[[Category:Energy storage]]