Flywheel energy storage1 consists in storing kinetic energy via the rotation of a heavy wheel or cylinder, which is usually set in motion by an electric motor, then recovering this energy by using the motor in reverse as a power generator
A Long History
The concept of flywheel energy storage goes back a long way. In Antiquity, potter’s wheels worked using a wooden disc, which regulated and facilitated the spinning movement the craftsman produced with his foot. The same technique was used in many 19th century steam engines. In the 1920s, some Belgian and Swiss streetcars ran between stations without electricityForm of energy resulting from the movement of charged particles (electrons) through a conductor..., powered by heavy cast iron discs weighing more than a ton located under the floorboards. At each station, the disc was connected to the powerIn physics, power is the amount of energy supplied by a system per unit time. In simpler terms, power can be viewed as energy output... grid, which once more set it in motion. On a much smaller scale, the same technique is applied to push-and-go friction-powered toys.
Today, interest in flywheels has led to further developments. Storing electricity has now become an issue of strategic importance in overcoming the intermittent nature of wind and solar power. Technological progress in materials and motors has resulted in systems that are more compact and efficient.
How Flywheels Work
Modern flywheel energy storage systems generally take the form of a cylinder, known as a rotor, enclosed in a sealed vacuum chamber to eliminate air friction.2 The rotor is often made from new materials, such as carbon or glass fibers, or Kevlar, which withstand very high speeds better than traditional metals. Velocity can exceed 10,000 revolutions per minute (RPM), with magnetic levitation to reduce friction.
When the wheel spins at its maximum speed, its kinetic energyThe energy of an object due to its motion.3 can be recovered by using the motor as a power generator. This gradually reduces the rotational speed of the flywheel.
Advantages and Disadvantages
- Highly efficient, with 80% of the stored energy able to be recovered.
- Very quick to set in motion and convert stored energy.
- Pollution-free, with a very long useful life.
One Major Disadvantage
- Limited energy storage time of around 15 minutes, making flywheels only suitable for quick, timely applications.
Flywheels are therefore mainly used for regulating and optimizing systems, rather than for ensuring long-term autonomy like batteries and pumped-storage systems.
Due to their great weight, subway trains release considerable amounts of energy when breaking and absorb just as much when accelerating. Several networks, including the Hanover, Hamburg, Los Angeles and Rennes subway systems, use flywheels to store and recover this energy.
In Rennes, for example, a huge spinning top of sorts weighing 2.5 metric tons has been installed at the center of an 8-kilometer subway line. When a train slows down, the energy released from breaking does not dissipate as heatIn the field of statistical thermodynamics today, heat refers to the transfer of the thermal agitation of the particles making up matter... but is instead converted into electricity by the train’s motor. The electricity is then transmitted through the third rail and used to increase the rotational speed of the flywheel. This energy is then recovered to power the train when it pulls out of the station. Carefully managed train synchronization and “smart” digital technology are used to efficiently coordinate the energy flows. With this system, the Rennes subway saves roughly the equivalent of ten days of electricity per year.
The weight and size of flywheels are a major hurdle in small vehicles. Some systems can boost engine power using cylinders weighing a few kilograms spinning at very high speeds of 60,000 RPM. Mainly fitted to Formula One race cars, they provide drivers with extra power, when accelerating out of bends, for example.
Regulating Power Grids
Inertial storage stores kinetic energy and then releases it as electricity within about 15 minutes.
Flywheels are sometimes used to stabilize power grids by offsetting drops in voltage. In New York, for example, 200 flywheels at a small 20-megawatt power plant are capable of providing sufficient energy within a few seconds to contribute to maintaining steady supply throughout the grid.
On a smaller scale, the same technology is used in niche markets to secure an uninterrupted electricity supply to operating theaters or server rooms, for example. In the event of prolonged power outages, flywheels can provide electricity while waiting for other, more powerful means of electrical production to kick in, such as dieselDiesel is the name of an internal combustion engine that works by compression-ignition... generators.
Smoothing Solar and Wind Power Output
Solutions have been developed to “smooth” sporadic output from wind and solar power plants. However, they are only effective over short timespans. Some projects aim to pair solar photovoltaic panels with flywheel “fields”, for example using concrete masses in underground caverns. But technical feasibility, and above all cost issues, have so far hampered these plans.
(1) The technology is referred to as a flywheel energy storage system (FESS).
(2) See Beacon Power website – https://beaconpower.com/carbon-fiber-flywheels/
(3) The amount of energy stored is proportional to the mass of the rotor, the square of its rotational speed and the square of its radius.