Flywheel energy storage

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.^[1]

Advanced FES systems have rotors made of high strength carbon-fiber composites, suspended by magnetic bearings, and spinning at speeds from 20,000 to over 50,000 rpm in a vacuum enclosure.^[2] Such flywheels can come up to speed in a matter of minutes – reaching their energy capacity much more quickly than some other forms of storage.^[2]

Main components

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

To reduce friction, magnetic bearings are sometimes used instead of 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,

General

Compared with other ways to store electricity, FES systems have long lifetimes (lasting decades with little or no maintenance;

The highest possible value for the shape factor[10] of a flywheel rotor, is ${\displaystyle K=1}$, which can be achieved only by the theoretical constant-stress disc geometry.^[11] A constant-thickness disc geometry has a shape factor of ${\displaystyle K=0.606}$, while for a rod of constant thickness the value is ${\displaystyle K=0.333}$. A thin cylinder has a shape factor of ${\displaystyle K=0.5}$. For most flywheels with a shaft, the shape factor is below or about ${\textstyle K=0.333}$. A shaft-less design^[12] has a shape factor similar to a constant-thickness disc (${\textstyle K=0.6}$), 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 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^[13] and the PowerThru flywheel from Phillips Service Industries.^[14] Alternatively, Calnetix utilizes aerospace-grade high-performance steel in their flywheel construction.^[15]

For these rotors, the relationship between material properties, geometry and energy density can be expressed by using a weighed-average approach.^[16]

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

Flywheel energy storage systems using mechanical bearings can lose 20% to 50% of their energy in two hours.[17] 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]}

Conversely, flywheels with

When used in vehicles, flywheels also act as gyroscopes, 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.^[19]

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 gimbals, allowing the flywheel to retain its original orientation without affecting the vehicle (see Properties of a gyroscope). This doesn't 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 rings 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 torqueing 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

Flywheels have also been proposed for use in continuously variable transmissions. Punch Powertrain is currently working on such a device.^[21]

During the 1990s,

Uninterruptible power supplies

Flywheel power storage systems in production as of 2001^[update] 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.^[31]

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.^[7] Newer flywheel systems completely levitate the spinning mass using maintenance-free magnetic bearings, thus eliminating mechanical bearing maintenance and failures.^[7]

Costs of a fully installed flywheel UPS (including power conditioning) were (in 2009) about $330 per

A long-standing niche market for flywheel power systems are facilities where circuit breakers and similar devices are tested: even a small household circuit breaker may be rated to interrupt a current of 10,000 or more amperes, and larger units may have interrupting ratings of 100,000 or 1,000,000 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 775 t (854 short tons; 763 long tons) flywheels (installed in 1981) that spin up to 225 rpm.^[33] Each flywheel stores 3.75 GJ and can deliver at up to 400 MW (540,000 hp).^[34]
• 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 at the Rutherford Appleton Laboratory had two 30 ton flywheels.

Aircraft launching systems

The

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 250 pounds (110 kilograms). Storage was 525 Wh (1.89 MJ) and could be charged or discharged at 1 kW (1.3 hp), leading to a specific energy of 5.31 W⋅h/kg and power density of 10.11 W/kg.^[36] The working model shown in the photograph at the top of the page ran at 41,000 rpm on September 2, 2004.^[37]

Amusement rides

The

Flywheel Energy Storage Systems (FESS) are found in a variety of applications ranging from grid-connected energy management to uninterruptible power supplies. With the progress of technology, there is fast renovation involved in FESS application. Examples include high power weapons, aircraft powertrains and shipboard power systems, where the system requires a very high-power for a short period in order of a few seconds and even milliseconds. 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, which is generally designed for the FESS.[38]

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.^[40] In motor sports applications this energy is used to improve acceleration rather than reduce carbon dioxide emissions – although the same technology can be applied to road cars to improve fuel efficiency.^[41]

Williams Hybrid Power, a subsidiary of Williams F1 Racing team,[43] have supplied Porsche and Audi with flywheel based hybrid system for Porsche's 911 GT3 R Hybrid^[44] and Audi's R18 e-Tron Quattro.^[45] Audi's victory in 2012 24 Hours of Le Mans is the first for a hybrid (diesel-electric) vehicle.^[46]

Grid energy storage

Flywheels are sometimes used as short term spinning reserve for momentary grid 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.^[47] 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)^[18] flywheel energy storage plant in Stephentown, New York in 2011^[48] using 200 flywheels^[49] and a similar 20 MW system at Hazle Township, Pennsylvania in 2014.^[50]

A 0.5MWh (2 MW for 15 min)

Amber Kinetics, Inc. has an agreement with 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.^[53]

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/flywheel demonstration project being carried out for the California Energy Commission.^[54]

Toys

Toggle action presses

In industry, toggle action presses 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.

Beyond energy storage

Flywheels can be used for attitude control. There is also some research into motion control,^[55] mostly to stabilize systems using the gyroscopic effect.

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 batteries.^[56] They are also less potentially damaging to the environment, being largely made of 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^[

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, killing bystanders with high-speed fragments.^[citation needed] 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]}

See also

• US DoE International Energy Storage Database
• Energy storage
• List of energy topics
• Beacon Power company
• Compensated pulsed alternator
• Electric double-layer capacitor
• Inverter
• Grid energy storage
• Launch loop
• List of energy storage projects
• Plug-in hybrid electric vehicle
• Rechargeable battery
• Regenerative brake
• Rotational energy
• STATCOM

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Further reading

• Beacon Power Applies for DOE Grants to Fund up to 50% of Two 20 MW Energy Storage Plants, Sep. 1, 2009 [1]^{[permanent dead link]}
• Sheahen, T., P. (1994). Introduction to High-Temperature Superconductivity. New York: Plenum Press. pp. 76–78, 425–431.
  ISBN 978-0-306-44793-8.{{cite book}}: CS1 maint: multiple names: authors list (link
  )
• El-Wakil, M., M. (1984). Powerplant Technology. McGraw-Hill. pp. 685–689.
  ISBN 9780070192881.{{cite book}}: CS1 maint: multiple names: authors list (link
  )
• Koshizuka, N.; Ishikawa, F.; Nasu, H.; Murakami, M.; et al. (2003). "Progress of superconducting bearing technologies for flywheel energy storage systems". Physica C. 386 (386): 444–450.
  doi:10.1016/S0921-4534(02)02206-2
  .
• Wolsky, A., M. (2002). "The status and prospects for flywheels and SMES that incorporate HTS". Physica C. 372 (372–376): 1495–1499.
  doi:10.1016/S0921-4534(02)01057-2.{{cite journal}}: CS1 maint: multiple names: authors list (link
  )
• Sung, T. H.; Han, S. C.; Han, Y. H.; Lee, J. S.; et al. (2002). "Designs and analyses of flywheel energy storage systems using high-Tc superconductor bearings". Cryogenics. 42 (6–7): 357–362.
  doi:10.1016/S0011-2275(02)00057-7
  .
• Akhil, Abbas; Swaminathan, Shiva; Sen, Rajat K. (February 2007). "Cost Analysis of Energy Storage Systems for Electric Utility Applications" (PDF). Sandia National laboratories. Archived from the original (PDF) on 2007-06-21.
• Larbalestier, David; Blaugher, Richard D.; Schwall, Robert E.; Sokolowski, Robert S.; et al. (September 1997). "Flywheels". Power Applications of Superconductivity in Japan and Germany. World Technology Evaluation Center.
• "A New Look at an Old Idea: The Electromechanical Battery" (PDF). Science & Technology Review: 12–19. April 1996. Archived from the original (PDF) on 2008-04-05. Retrieved 2006-07-21.
• Janse van Rensburg, P.J. (December 2011). Energy storage in composite flywheel rotors (Thesis). University of Stellenbosch, South Africa.
  hdl:10019.1/17864
  .
• Devitt, Drew (March 2010). "Making a case for flywheel energy storage". 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. https://doi.org/10.1016/j.est.2021.103576

External links

Wikimedia Commons has media related to Flywheel energy storage devices.

• Federal Technology Alert, Flywheel Energy Storage^{[permanent dead link]}
• Magnetal Whitepaper for its Green Energy Storage System – GESS
• Magnetal analysis on gyro forces induced by flywheel energy storage