A flywheel is a heavy rotating body which acts as a reservoir of energy. It is used to store energy when available and supply it when required. Flywheels have low maintenance costs, and their lifespan can be long. There is no greenhouse emission or toxic material produced when flywheels are working, so it is very environment-friendly.

A flywheel can store energy as long as it keeps spinning. High-speed Flywheels store energy inside a vacuum chamber, called “Flywheel Energy Storage Systems” or FESS for short.

The flywheel spins at 16000 to 60,000 RPM or even higher, on frictionless magnetic bearings. Speed of flywheel increases when current is fed to the reversible motor inside the flywheel space. When energy is drawn out, the speed decreases and thus enery stored is reduced. The flywheel can keep running for many months, even years, of left alone. No other Storage system can store energy this long. Batteries undergo reduction of energy at up to 5% per month depending upon type of battery.

A flywheel is a very heavy wheel that takes a lot of force to spin around. It might be a large-diameter wheel with spokes and a very heavy metal rim, or it could be a smaller-diameter cylinder made of something like a carbon-fibre composite. Either way, it’s the kind of wheel you have to push hard to set it spinning. Just as a flywheel needs lots of force to start it off, so it needs a lot of force to make it stop.

As a result, when it’s spinning at high speed, it tends to want to keep on spinning (we say it has a lot of angular momentum), which means it can store a great deal of kinetic energy. You can think of it as a kind of “mechanical battery,” but it’s storing energy in the form of movement (kinetic energy, in other words) rather than the energy stored in the chemical form inside a traditional, electrical battery.

Flywheels come in all shapes and sizes. The laws of physics tell us that large diameter and heavy wheels store more energy than smaller and lighter wheels, while flywheels that spin faster store much more energy than ones that spin slower.

Modern flywheels are a bit different from the ones that were popular during the Industrial Revolution. Instead of wide and heavy steel wheels with even heavier steel rims, 21st-century flywheels tend to be more compact and made from carbon fibre or composite materials, sometimes with steel rims, which work out perhaps a quarter as heavy.

How does a flywheel store energy?

A flywheel system stores energy mechanically in the form of kinetic energy by spinning a mass at high speed. Electrical inputs spin the flywheel rotor and keep it spinning until called upon to release the stored energy. The amount of energy available and its duration is controlled by the mass and speed of the flywheel. 

In a rotating flywheel, kinetic energy is a function of the flywheel’s rotational speed and the mass momentum of inertia. The kinetic energy of a high-speed flywheel takes advantage of the physics involved resulting in exponential amounts of stored energy for increases in the flywheel rotational speed.

So a large flywheel with a very heavy steel rim might have a higher moment of inertia than a much smaller, solid flywheel because more of its mass is further from the point of rotation.

Once made of steel, flywheels are now made of a carbon fibre composite which has high tensile strength and can store much more energy. The amount of energy stored in the flywheel is a function of the square of its rotational speed and its mass, so higher rotational speeds are desirable. Spinning at the maximum possible speed results in an optimal energy-to-mass ratio. However, the flywheel is then subject to significant centrifugal forces and could be prone to failure at lower rotational speed than lower density materials.

To maintain efficiency, the Flywheel energy storage systems (FESS) is operated in a vacuum to reduce friction. The flywheel is connected to a motor-generator that interacts with the utility grid through advanced AC to DC converters. They are used in energy grid storage as a reserve for momentary grid frequency regulation and balancing sudden changes between supply and consumption, so when short-term backup power is required because the utility power fluctuates or is lost.

Electric energy input is used to accelerate the rotor up to speed using the built-in motor-generator; the inertia allows the rotor to continue spinning and the resulting kinetic energy is converted to electricity. Energy is discharged by drawing down kinetic energy using the same motor as a generator.

But it is not a primary source of power generation. Extra power in the grid is shunted to the flywheel and used to set them in motion. When the power is required later, the momentum of the flywheel is used to generate power fed back to the grid.

Is Flywheel energy storage expensive?

The development of the FESS is being accomplished under a technology development program that will result in a low-cost and high-efficiency solution, allowing for storage of almost six-kilowatt hours (5.8kWh) of electricity to be used at the exact moment of necessity as dictated by the consumer.

Carbon flywheels currently cost around $300/kWh, steel flywheels $250/kWh and concrete ones around a few dollars. The target retail price of the FESS is $150-250 per kWh, compared to $1000 per kWh for leading lithium-ion battery providers. The flywheel will have the charging time of 40 seconds which will help it to be commercially feasible, with efficiency exceeding 90%. The depletion time for the FESS will be approximately 12 hours, compared to months for chemical batteries.

What are some disadvantages of flywheel energy storage?

Even though the flywheel has an energy efficiency of 85-90%. It still has some disadvantages.

The flywheel is heavy, which increases fuel consumption. It has high rotational inertia, which reduces how fast engine revs can come up, or slow down, which later increases stress on the engine. It has to be exactly balanced to avoid severe vibration. It adds somewhat to the cost of materials and manufacture. Slowing a flywheel wastes the energy used previously to accelerate it unless a special provision is made to recapture the energy. Typically electrically powered propulsion can achieve that, and can be made to operate smoothly with a much less massive flywheel.

If a flywheel comes adrift while the engine is running, it can cause disastrous damage. That’s a rare event, but it has happened.

Top flywheel energy storage companies

Revterra – Low Loss Flywheel Energy Storage

The US-based startup Revterra provides low-cost flywheel energy storage solutions. With the help of materials like carbon fibre composites and active magnetic levitation, using magnetic bearings and superconductors, rotational energy is stored with minimal loss. By substituting a unique superconducting magnetic bearing into a traditional flywheel apparatus, Revterra can deploy this robust, long-lifespan method of storing energy to new markets and applications

Advanced flywheel systems include rotors, made with lightweight carbon fibre composites, and superconductors to efficiently store kinetic energy. To effectively store energy at lower costs, and to improve energy storage performance, startups and emerging companies work on innovative solutions in flywheel systems. As a result, this significantly improves the overall performance of energy storage, when compared to existing flywheel systems.

Company’s Website – https://www.revterra.io/

Amber Kinetics -Looking into the Future

Amber Kinetics is the industry leader in manufacturing grid-scale kinetic energy storage systems (KESS). As the provider of long-duration flywheel energy storage, Amber Kinetics extends the duration and efficiency of flywheels from minutes to hours—resulting in safe, economical and reliable energy storage.

Company’s Website – https://amberkinetics.com/

Beacon Power

Beacon Power built the world’s largest flywheel energy storage system in Stephentown, New York.  The 20-megawatt system marks a milestone in flywheel energy storage technology, as similar systems have only been applied in testing and small-scale applications. 

Company’s Website – https://beaconpower.com/

Conclusion

Flywheel energy storage systems are increasingly being considered a promising alternative to electrochemical batteries for short-duration utility applications. There is a scarcity of research that evaluates the techno-economic performance of flywheels for large-scale applications. Evaluating the capital cost, the Levelized cost of storage and the scale factor is crucial to making an informed decision in future development and deployment of the technology.