Flywheels
Flywheels are mechanical devices designed to store rotational energy, functioning as a key component in flywheel energy storage systems (FES). These systems offer an alternative to traditional batteries by harnessing energy through the conservation of energy principle and are particularly effective in scenarios where energy efficiency can be enhanced by utilizing existing rotational energy. Flywheels consist of a heavy rotor that spins at high speeds, with energy being added through torque and released similarly to power mechanical loads. An everyday example of a flywheel is a toy car that gains energy when pulled back and then propelled forward upon release.
Historically, flywheels played an essential role in steam engine designs, and their applications have expanded to include modern technology, such as data centers and transportation systems like Formula One race cars and rail systems. Flywheels are known for their longevity and lower maintenance costs compared to battery systems, although they are best suited for applications where energy storage is not required for extended periods. Key limitations include the rotor's tensile strength and energy loss due to friction, particularly with mechanical bearings, although advancements like magnetic bearings can mitigate these issues. Overall, flywheels represent a promising technology in energy storage with diverse applications across various sectors.
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Flywheels
Summary: A flywheel is a heavy rotating wheel, the key component of a flywheel energy storage system, and a method of storing rotational energy. They are used as alternatives to batteries or where energy use can be made more efficient by harnessing rotational energy already being exerted.
A flywheel is a mechanical device similar in appearance to a very large, heavy reel, used to store rotational energy in an amount proportional to the square of its rotational speed. A flywheel energy storage system works because of the principle of conservation of energy. Rotational speed and stored energy are increased by applying torque to the flywheel, while the flywheel releases its energy by applying torque to a mechanical load. The common toy car that is dragged backwards and then released, propelled by the energy stored by the initial rotation of the wheels, is a familiar example of a simple flywheel system.
Flywheels were a key component in James Watt’s steam engine design, and some of the flywheels used in the first generation of those steam engines remain in use today. Although flywheel energy storage systems using mechanical energy to accelerate the flywheel are in development, current systems use electricity. Apart from the superconductors used to power magnetic bearings, if applicable, flywheels are not as affected by ambient temperature or temperature change as battery systems are, and have a much longer working lifespan.
Flywheel energy storage (FES) systems usually consist of a vacuum chamber (in order to reduce friction), a combination motor/generator that accelerates the flywheel and generates electricity, and a heavy steel rotor suspended on ball bearings inside the vacuum chamber. More sophisticated systems may use magnetic bearings to further reduce friction, though powering them may not be economically efficient. The advantage of a flywheel system is that it provides continuous energy, even when the energy source is discontinuous. Energy is collected over a long period of time and released in a short spike, thus allowing the energy release to greatly exceed the ability of the energy source. Since 2001, FES systems have been able to provide continuous power with a discharge rate faster than batteries of equivalent storage capacity. Furthermore, flywheel maintenance is half as expensive as traditional battery system maintenance—even less than that when magnetic bearings are used. They are increasingly used as part of the integrated power design of large data centers. FES systems have also long been used in research and development laboratories where circuit breakers and other electrical equipment are tested.
FES systems have numerous applications in transportation. Formula One race cars sometimes use flywheels to recover energy from the drive train during braking, which is then re-deployed during acceleration. While the purpose of this application is to improve acceleration, and the economic efficiency of the system is considered by different criteria in competitive motor sports than in the design of passenger vehicles, a similar system can be used in other vehicles to increase fuel efficiency or to provide faster acceleration than the power source would otherwise be able to provide. Flywheel systems have been proposed to replace chemical batteries in electric vehicles; a small number of vehicle designs before the current generation of electric and hybrid vehicles incorporated flywheels into their designs, including flywheel-powered Swiss buses in the 1950s.
Flywheels have also been used in rail systems, especially electric rail systems, to provide boosts to power or to provide power when there is an interruption in the electrical supply. Since 2010, the London Midland train operator has operated two flywheel-powered railcars. New York’s Long Island Rail Road (LIRR) has begun a pilot project to explore the possibility of using flywheels lineside to generate some of the electricity used by the LIRR’s electric trains, to improve their acceleration and recover electricity during braking. Elsewhere in New York, Beacon Power opened a 20-megawatt flywheel energy storage plant in Stephentown, where power is purchased at off-peak hours and stored with fewer carbon emissions than traditional plants.
The biggest limitations to flywheel systems are the rotor’s tensile strength and the energy storage time. The tensile strength of the rotor determines how fast the rotor can rotate, and therefore the system’s capacity; if it is exceeded, the flywheel can shatter in an explosion, the risk of which requires a containment vessel which increases the system’s mass and cost. Because of the rotation of the Earth, flywheels change orientation over time, which is resisted by the flywheel’s gyroscopic forces exerted against the bearings. The resulting increase in friction results in a loss of energy of as much as 50 percent over two hours, when mechanical bearings are used, though careful design can reduce that amount by half. Magnetic bearings are much more efficient, losing only a few percent of their energy. Even so, until magnetic bearings are more cost-efficient, this limitation is one reason why flywheels are best used in applications where energy won’t need to be stored for long periods of time, and why they are well-suited for public transit applications, where they are in near constant use.
Bibliography
Bitterly, J. G. “Flywheel Technology: Past, Present, and 21st Century Projections.” Aerospace and Electronic Systems Magazine 13, no. 8 (August 1998).
Hebner, R. “Flywheel Batteries Come Around Again.” Spectrum 39, no. 4 (April 2002).
Leclercq, Ludovic, Benoit Robyns, and Jean-Michel Grave. “Control Based on Fuzzy Logic of a Flywheel Energy Storage System Associated With Wind and Diesel Generators.” Mathematics and Computers in Simulation 63, no. 3–5 (November 2003).
Li, Xiaojn and Alan Palazzolo. "A Review of Flywheel Energy Storage Systems: State of the Art and Opportunities." Journal of Energy Storage, vol. 46, Feb. 2022, doi.org/10.1016/j.est.2021.103576. Accessed 1 Aug. 2024.