Flywheel energy storage (FES) works by spinning a rotor (flywheel) and storing energy as rotational energy. When energy is taken out of the system, the flywheel slows down because of the principle of conservation of energy. Adding energy to the system causes the flywheel to spin faster. Some systems use lightweight rotors that spin very fast, while others use heavy rotors (e.g., 200 tonnes) that spin much slower, called grid-scale flywheel energy storage. Other designs use rotors that spin at tens of thousands of revolutions per minute.
Most FES systems use electricity to make the flywheel spin faster or slower. Some systems are being developed to directly use mechanical energy instead.
Advanced FES systems use rotors made of strong carbon-fiber materials. These rotors are held in place by magnetic bearings and spin from 20,000 to over 50,000 revolutions per minute inside a vacuum container. These flywheels can reach their top speed in a few minutes, storing energy much faster than some other storage methods.
Main components
A typical system includes a flywheel held up by rolling-element bearings connected to a motor-generator. The flywheel and sometimes the motor-generator may be placed inside a vacuum chamber to lower friction and energy loss.
Early flywheel energy-storage systems used large steel flywheels spinning on mechanical bearings. More recent systems use carbon-fiber rotors that are stronger than steel and can store more energy with the same weight.
To reduce friction, magnetic bearings are sometimes used instead of mechanical bearings.
The cost of cooling made low-temperature superconductors unsuitable for use in magnetic bearings. However, high-temperature superconductor (HTSC) bearings might be more affordable and could allow energy to be stored for longer periods. Hybrid bearing systems are likely to be used first. High-temperature superconductors have historically struggled to provide enough lifting force for large designs but can easily provide stability. In hybrid bearings, permanent magnets support the weight, and high-temperature superconductors help keep the system stable. Superconductors work well for stability because they are perfect diamagnets. If the rotor moves off-center, a restoring force from flux pinning brings it back. This is called the magnetic stiffness of the bearing. Vibrations in the rotational axis can occur due to low stiffness and damping, which are problems with superconducting magnets. This limits the use of fully superconducting magnetic bearings in flywheel systems.
Since flux pinning is important for stability and lifting, HTSCs can be shaped more easily for flywheel energy storage than for other uses. HTSC powders can be formed into different shapes as long as flux pinning is strong. A challenge that must be solved before HTSCs can fully support a flywheel energy-storage system is finding a way to prevent the decrease in levitation force and the slow drop of the rotor during operation caused by flux creep in the superconducting material.
Physical characteristics
Flywheel energy storage systems (FES) have long lifetimes, lasting many years with little or no maintenance. They can store a lot of energy for their weight (100–130 watt-hours per kilogram) and can release energy quickly. These systems are very efficient, with up to 90% of the energy stored being usable again. Typical systems can store between 3 and 133 kilowatt-hours of energy. They can be charged rapidly in under 15 minutes. However, some commercial systems have lower energy storage capacity than the highest values reported, such as 11 watt-hours per kilogram.
The energy storage capacity of a flywheel depends on its shape and the material it is made from. The best possible shape for maximizing energy storage is a disc with even stress distribution. Other shapes, like a disc of constant thickness or a rod, have lower energy storage potential. Flywheels with a shaft typically have lower energy storage than those without a shaft.
Materials used in flywheels must be strong and lightweight. Composite materials, such as carbon fiber, are often used because they have a high strength-to-weight ratio. Some composites can store over 400 watt-hours per kilogram of energy. Examples of flywheels made from these materials include those from Beacon Power Corporation and Phillips Service Industries. Some systems use high-performance steel instead.
The energy storage capacity of a flywheel depends on the material’s strength, its shape, and how these factors interact. A key limit in flywheel design is the tensile strength of the rotor. Stronger materials allow the flywheel to spin faster and store more energy. If the material breaks, the flywheel can shatter, releasing energy suddenly. This is dangerous and can cause damage.
Flywheels made of metal can fail due to repeated stress, leading to cracks and unbalanced movement. This can damage the system and cause parts to break off. To protect against this, flywheels are often placed in strong containment vessels. Some systems use special materials inside the vessel to absorb energy if a failure occurs. Large systems are sometimes buried to prevent debris from escaping.
Flywheels with mechanical bearings can lose 20% to 50% of their energy over two hours due to friction. This happens because the flywheel resists changes in direction caused by Earth’s rotation, increasing friction. This can be reduced by aligning the flywheel’s rotation axis with Earth’s axis.
Flywheels used in vehicles act like gyroscopes, helping to keep the vehicle stable. However, this can also make handling more difficult during sharp turns or on rough roads. If the flywheel is used only for stability, it is called a reaction wheel or control moment gyroscope.
To prevent the flywheel from affecting the vehicle’s movement, it can be mounted in gimbals, which allow it to rotate freely. However, this adds complexity. Some systems use limited movement gimbals with shock absorbers to reduce space and complexity.
Flywheels in vehicles may need to rotate freely within a spherical space to move with the Earth’s rotation. If not controlled, they can shift position over time. Full-motion gimbals require special wiring to provide power, which adds to the design challenge. Limited-movement gimbals use shock absorbers to adjust the flywheel’s orientation gradually, reducing the space needed.
Applications
In the 1950s, flywheel-powered buses, called gyrobuses, were used in Yverdon, Switzerland, and Ghent, Belgium. Scientists continue to study how to make flywheel systems smaller, lighter, cheaper, and more powerful. Researchers hope these systems might replace traditional chemical batteries in vehicles like electric cars. Flywheel systems could solve problems with current batteries, such as low energy storage, long charging times, heavy weight, and short lifetimes. Some believe the Chrysler Patriot car tested a flywheel, but this is not certain.
Flywheels have also been suggested for use in car transmissions. A company called Punch Powertrain is working on this idea.
In the 1990s, Rosen Motors made a car powered by a gas turbine and a flywheel that spins at 55,000 revolutions per minute. The flywheel helped the car accelerate quickly and stored energy from braking. The flywheel had a titanium center and a carbon fiber shell, and it was mounted to reduce effects on the car’s handling. A test version of the car worked in 1997, but it was never sold to the public.
In 2013, Volvo added a flywheel to the rear of its S60 sedan. When the car brakes, the flywheel spins up to 60,000 revolutions per minute and stops the front engine. Energy from the flywheel powers the car through a special transmission. The flywheel is 20 centimeters wide, weighs 6 kilograms, and spins in a vacuum to reduce friction. When paired with a four-cylinder engine, the system uses 25% less fuel than a similar six-cylinder engine and can reach 100 kilometers per hour in 5.5 seconds. Volvo did not say if it would use this technology in other cars.
In 2014, GKN bought a company called Williams Hybrid Power and plans to provide 500 flywheel systems for city buses. These systems were first made for Formula One racing. Oxford Bus Company announced it would use 14 of these hybrid buses in its operations.
Flywheels have been tested in small electric trains for moving trains around tracks, like the Sentinel-Oerlikon Gyro Locomotive. Larger trains, such as the British Rail Class 70, sometimes use flywheels to help cross gaps in power lines. A flywheel system at the University of Texas can store enough energy to accelerate a train from a stop to its cruising speed.
The Parry People Mover is a train powered by a flywheel. It was tested on a railway in England from 2006 to 2007 and was supposed to start full service in 2008. Both units are now in use.
Flywheel systems can help regulate electricity on railways, improving train performance and saving energy. Tests have been done in cities like London, New York, Lyon, and Tokyo. In New York, a project is using flywheels to store energy from braking and reuse it for train movement. These systems use rotors made of carbon and glass with magnets that spin at 37,800 revolutions per minute. Each unit can store enough energy to accelerate 200 metric tons to 38 kilometers per hour.
By 2001, flywheel energy storage systems had similar power to batteries but could release energy faster. They are often used in places like data centers to provide backup power without taking up much space.
Flywheel systems usually cost about half as much as traditional battery systems to maintain. They require little care, such as annual checks and replacing parts every five to ten years. Newer systems use magnetic bearings to float the spinning parts, reducing wear and failure.
In 2009, a fully installed flywheel system for emergency power cost about $330 per kilowatt.
Flywheels are used in places where electrical devices like circuit breakers are tested. These tests create large electrical surges that could harm the power grid if done directly. Special equipment, like motor-generator sets, is used to handle these tests safely.
Tokamak fusion experiments need large amounts of electricity for short times. For example, the Joint European Torus (JET) uses two large flywheels to store and release energy. Other projects, like the Helically Symmetric Experiment and ASDEX Upgrade, also use flywheels.
The Gerald R. Ford-class aircraft carrier will use flywheels to store energy for launching planes. Each flywheel can store 121 megajoules of energy and release it quickly.
A NASA-designed flywheel for testing used carbon fiber and titanium, spinning at 60,000 revolutions per minute. It stored 525 watt-hours of energy and could charge or discharge at 1 kilowatt.
The Montezooma's Revenge roller coaster at Knott's Berry Farm was the first flywheel-launched ride in the world. It is still operating in the United States. The ride uses a 7.6-ton flywheel to launch the cars.
Comparison to electric batteries
Flywheels are less affected by changes in temperature and can operate in a much wider range of temperatures than chemical rechargeable batteries. They are also less likely to experience common problems that batteries often face. Flywheels are made mostly of materials that are not harmful to the environment. A key benefit of flywheels is that the exact amount of energy they store can be determined by measuring how fast they spin.
Most batteries can only work for a limited time, such as about 10 years for lithium iron phosphate batteries. In contrast, flywheels can potentially last for a very long time. Some flywheels from James Watt’s steam engines have been working continuously for over 200 years. Examples of ancient flywheels used in milling and pottery can still be seen in many places across Africa, Asia, and Europe.
Modern flywheels are often sealed and require little maintenance during their lifetime. Flywheels with magnetic bearings inside vacuum enclosures, like the NASA model shown, do not need any maintenance for their bearings. This makes them better than batteries in terms of lifespan and energy storage because their long-term performance is still unknown. Flywheels with mechanical bearings may have shorter lifespans due to wear over time.
High-performance flywheels can explode, sending fast-moving pieces that may harm people nearby. To reduce this risk, flywheels can be placed underground. While batteries can catch fire and release harmful substances, people usually have time to move away and avoid injury.
Batteries can be shaped in many different ways to fit various designs. Flywheels, however, must take up a certain amount of space because the energy they store depends on their size and how fast they spin. If a flywheel becomes smaller, its mass decreases, so it must spin faster. This increases the stress on the materials used. In situations where space is limited, such as under the chassis of a train, flywheels may not be a practical option.