## Flywheels as Mechanical Batteries

Flywheel Energy Storage (FES) is a relatively new concept that is being used to overcome the limitations of intermittent energy supplies, such as Solar PV or Wind Turbines that do not produce electricity 24/7.

A flywheel energy storage system can be described as a mechanical battery, in that it does not create electricity, it simply converts and stores the energy as kinetic energy until it is needed. In a matter of seconds, the electricity can be created from the spinning flywheel making it the ideal solution to help regulate supply in the electrical grid.

It is based on a really old concept and is very similar to an old-fashioned pottery wheel where the potter moves his feet to make the wheel spin. As the potter works, he removes energy from the system, so to keep the wheel spinning; he needs to keep moving his feet.

## So how exactly does it work?

A flywheel is a heavy shaft mounted rotating disc that speeds up when electrical energy is applied to it. When energy is needed, the flywheel is slowed and the kinetic energy is converted back to electrical energy, where it can be transmitted to where it is required.

The energy a flywheel contains is a function of the speed that it is spinning multiplied by the moment of inertia.

The moment of inertia states that the effective mass of a spinning object is not dependant on how much actual mass the spinning object contains. Instead, it is dependant on where the mass is located in relation to the central point that it is rotating around.

For example, if spinning at the same speed, a solid flywheel will store less energy than a flywheel of the same mass that has spokes and its weight situated around the rim of the wheel.

## A high moment of inertia is good, but speed of rotation is better!

The speed that the flywheel rotates has a larger effect on the energy stored within it compared to the moment of inertia. If you have a flywheel with a rim weighing 1kg and replace it with a flywheel with a 2kg rim, it has the potential to store double the energy. If you take the original flywheel and double the speed at which it spins, you quadruple the potential energy that it can store.

## Innovation of the Flywheel

Historically, flywheels have been huge steel structures with the majority of the weight distributed towards the rim of the wheel. However, over the last 30 years, scientific innovation has meant that flywheels can store more energy in less weight and volume, increasing their potential for energy storage. Newer flywheels are made from very strong composite materials and are operated on a bed of near frictionless magnetic bearings housed in a vacuum enclosure. This allows the flywheels to be spun at incredible speeds helping maximise the energy that they can store. In fact NASA scientists have managed to get flywheels to spin in excess of 60,000 revolutions per minute, which is nearly 2.5 times the speed of sound. The amount of kinetic energy that can be stored at this speed makes them ideal for replacing chemical batteries in the future.

There is also potential to use magnetic levitation as a way of prolonging the life of the flywheel energy storage systems. Since there is no friction on a system that is magnetically levitated there will be no wear on the system, so it is thought that these systems could last fifteen years or more as opposed to a chemical battery that may only last five years.

## Flywheel Energy Storage in Action

In June 2011, the Beacon Power Corporation completed the Company’s first flywheel energy storage plant in Stephentown, New York at a cost of \$60m. The plant utilises 200 flywheels spinning at a maximum speed of 16000 rpm to store excess energy and help regulate the supply to the local grid.

On 7th March 2012, Rockland Capital acquired the assets of the Beacon Power Corporation and put up funding to develop a second 20 MW flywheel regulation plant in Pennsylvania.

Flywheel could be one of the solutions to provide mass scale storage of electricity during excess supply and provide the release of energy during excess demand.