# Flywheel Energy Storage System

Electric energy is supplied into flywheel energy storage systems (FESS) and stored as kinetic energy. Kinetic energy is defined as the “energy of motion,” in this situation, the motion of a rotating mass known as a rotor, rotates in a near-frictionless environment. When utility power is lost or fluctuates, the inertia of the rotor permits it to continue spinning, converting the kinetic energy into electricity.

A huge spinning cylinder (a rim attached to a shaft) is maintained on a stator – the stationary element of an electric generator – by magnetically levitated bearings in most modern high-speed flywheel energy storage systems. The flywheel system is performed in a vacuum to diminish drag and maintain efficiency. The flywheel is coupled to a motor-generator that uses modern power electronics to communicate with the utility grid.

## How Flywheel Energy Storage Systems Work?

Flywheel energy storage systems employ kinetic energy stored in a rotating mass to store energy with minimal frictional losses. An integrated motorgenerator uses electric energy to propel the mass to speed. Using the same motor-generator, the energy is discharged by pulling down the kinetic energy. The quantity of energy stored is related to the object’s moment of inertia times its angular velocity squared.

The flywheel must spin at the fastest possible speed to maximize the energy-to-mass ratio. Rapidly rotating objects experience considerable centrifugal forces; yet, while dense materials may store more energy, they also experience higher centrifugal forces, making them more prone to failure at lower rotational speeds than low-density materials. As a result, tensile strength is more significant than material density. Low-speed flywheels are made of steel and rotate at speeds of up to 10,000 revolutions per minute.

Advanced FESS utilizes four main aspects to produce attractive energy density, high efficiency, and minimal standby losses (during time periods ranging from a few minutes to several hours):

1. Rotating mass built of high-strength-to-weight-ratio fiberglass resins or polymer compounds.
2. To reduce aerodynamic drag, a mass that functions in a vacuum is used.
3. Rotating mass with a high frequency of rotation.
4. To accommodate high rotational speeds, air or magnetic suppression bearing technology is used.

Advanced FESS have a rotational frequency of more than 100,000 RPM and tip speeds of more than 1000 m/s. FESS are ideal for high-power, low-energy applications with a lot of cycles.

They also have a number of advantages over chemical energy storage. They have a high energy density and a long lifespan, allowing them to be cycled frequently without losing performance. They also feature lightning-quick response and ramp times. They may even go from full discharge to full charge in a matter of seconds. FESS are becoming more relevant in high-power, low-energy applications. They’re especially appealing for applications that require a lot of cycling because they only have a small life reduction when used frequently (i.e., they can undergo many partial and full charge-discharge cycles with trivial wear per cycle).

Electric service power quality and dependability, ride-through while gen-sets start-up for longer-term backup, area regulation, quick area regulation, and frequency responsiveness are just a few of the areas where FESS excel. FESS could also be useful as a component of track-side or onboard regenerative braking systems in hybrid vehicles that stop and start frequently.

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## The Theory of Flywheel Energy Storage

With reduced frictional losses, FESS uses kinetic energy stored in a spinning mass. The following equation describes the kinetic energy of the system:

E=\frac{1}{2}I \omega^2

where E is the kinetic energy, I is the moment of inertia, and ω represents the angular velocity of the rotating disc; when I or ω increases, the energy of the system raises.

Flywheels, which were once composed of steel, are now built of a carbon fiber composite with high tensile strength and the ability to store significantly more energy. Higher rotational speeds are preferable since the quantity of energy stored in the flywheel is a function of the square of its rotating speed and mass. The best energy-to-mass ratio is achieved by spinning at the highest achievable speed. However, at lower rotational speeds, the flywheel is subjected to large centrifugal forces and may be more prone to failure than lesser density materials.

FESS are connected to a motor-generator that communicates with the utility grid via modern power electronics and run in a vacuum to reduce drag, friction, and energy loss. They are employed in energy grid storage as a reserve for momentary grid frequency adjustment and balancing unexpected variations in supply and demand, as well as when short-term backup power is necessary due to utility power fluctuation or loss.

The built-in motor-generator uses electric energy to propel the rotor up to speed; inertia permits the rotor to maintain spinning, and the kinetic energy is transferred to electricity. Using the same motor as a generator, energy is discharged by pulling down kinetic energy. However, it is not a primary source of energy. Extra grid power is diverted to the flywheels, which are then placed in motion. When the electricity is needed later, the flywheel’s motion is employed to generate power that is supplied back into the grid.

## Main Components of Flywheel Energy Storage System

A flywheel is supported by a rolling-element bearing and is coupled to a motor-generator in a typical arrangement. To reduce friction and energy waste, the flywheel and sometimes the motor–generator are encased in a vacuum chamber.

A massive steel flywheel rotates on mechanical bearings in first-generation flywheel energy storage systems. Carbon-fiber composite rotors, which have a higher tensile strength than steel and can store significantly more energy for the same mass, are used in newer systems.

Magnetic bearings are occasionally used instead of mechanical bearings to reduce friction.

The figure below demonstrates the main components of a typical flywheel energy storage system.

### Superconducting Bearings Could Be Used in the Future

Low-temperature superconductors were initially dismissed for use in magnetic bearings due to the high cost of cooling. High-temperature superconductor (HTSC) bearings, on the other hand, may be cost-effective and expand the amount of time energy may be stored. The usage of hybrid bearing systems is most likely to come initially. Historically, high-temperature superconductor bearings struggled to deliver the lifting forces required for bigger designs, but they can easily supply a stabilizing force.

As a result, permanent magnets support the load in hybrid bearings, while high-temperature superconductors stabilize it. Because superconductors are ideal diamagnets, they can perform effectively in load stabilization. If the rotor attempts to drift off-center, it is restored by a restoring force caused by flux pinning. This is referred to as the bearing’s magnetic stiffness. Because of the poor stiffness and damping of superconducting magnets, rotational axis vibration might occur, restricting the use of entirely superconducting magnetic bearings for flywheel applications.

The HTSC can be produced considerably more readily for FESS than for other uses since flux pinning is essential in delivering the stabilizing and lifting force. As long as the flux pinning is strong, HTSC powders can be shaped into any shape. Finding a technique to prevent the reduction in levitation force and gradual fall of the rotor throughout service caused by the flux creep of the superconducting material is an ongoing difficulty that must be overcome before superconductors can offer the entire lifting force for FESS systems.

## The Applications of Flywheel Energy Storage

FEES have broad applications from transportation and power supplies to aircraft and even toys. Here we present a comprehensive overview of numerous applications of FEES.

### Transportation

#### Rail Vehicles

Flywheel systems have been employed for shunting and switching in tiny electric locomotives, such as the Sentinel-Oerlikon Gyro Locomotive. Flywheel boosters have been used on larger electric locomotives, such as the British Rail Class 70, to carry them over gaps in the third rail. Advanced flywheels, such as the University of Texas at Austin’s 133 kWh pack, can accelerate a train from a standstill to cruising speed.

The Parry People Mover is a railcar using a flywheel as its power source. It was trialed on the Stourbridge Town Branch Line in the West Midlands, England, for a year on Sundays in 2006 and 2007, with the intention of being implemented as a full service by the train operator London Midland in December 2008, once two units had been ordered. Both units are operational as of January 2010.

### Test Laboratories

Circuit breakers and similar device testing facilities have long been a niche market for flywheel power systems: even a simple domestic circuit breaker can be rated to interrupt a current of 10000 or more amperes, and bigger units can have interrupting ratings of 100000 or 1000000 amperes. If these tests were performed directly from building power, the massive transient loads produced by purposefully driving such devices to demonstrate their ability to terminate simulated short circuits would have undesirable impacts on the local grid. A laboratory like this will typically contain many huge motor-generator combinations that may be spun up to speed for several minutes before a circuit breaker is tested.

### Aircraft Launching Systems

Flywheels will collect energy from the ship’s power source and deliver it quickly into the electromagnetic aircraft launch system on the Gerald R. Ford-class aircraft carrier. The shipboard power system is unable to provide the high power transients required to launch aircraft on its own. At 6400 rpm, each of the four rotors will store 121 MJ (34 kWh). In 45 seconds, they can store 122 MJ (34 kWh) and release it in 2–3 seconds. The flywheel energy densities are 28 kJ/kg (8 Wh/kg), omitting the torque frame, and 18.1 kJ/kg (5 Wh/kg) with the stators and casings.

### NASA G2 Flywheel for Spacecraft Energy Storage

This was a NASA Glenn Research Center-funded design intended for component testing in a laboratory setting. It had a carbon fiber rim placed on magnetic bearings with a titanium hub designed to spin at 60,000 rpm. The maximum weight allowed was 250 pounds. The battery had a capacity of 525 W-hr (1.89 MJ) and could be charged or discharged at a rate of 1 kW. On September 2, 2004, the operational model in the photograph at the top of the page ran at 41,000 rpm.

### Pulse Power

Flywheel Energy Storage Systems are used in a wide range of applications, including grid-connected energy management and uninterruptible power supply. With the advancement of technology, the FESS application is undergoing rapid renovation. High-powered weapons, airplane powertrains, and shipboard power systems are examples of systems that require a lot of power for a small amount of time, such as a few seconds or even milliseconds.

Because of its tremendous energy and power densities, the compensated pulsed alternator (compulsator) is one of the most common options of pulsed power supply for fusion reactors, high-power pulsed lasers, and hypervelocity electromagnetic launches. Compulsators (low-inductance alternators) work in a similar way to capacitors in that they can be spun up to produce pulsed power for railguns and lasers. Only the huge rotor of the alternator stores energy rather than a separate flywheel and generator. A homopolar generator is another term for a homopolar generator.

### Grid Energy Storage

Flywheels are occasionally utilized as a short-term spinning reserve for grid frequency management and balancing supply and demand fluctuations. Flywheels provide a number of advantages over traditional energy sources such as natural gas turbines, including no carbon emissions, faster response times, and the flexibility to purchase power during off-peak hours. The operation is quite similar to that of batteries in the same application; the main distinction is cost.

In 2011, Beacon Power installed a 5 MWh (20 MW in 15 minutes) flywheel energy storage plant in Stephentown, New York, and a similar 20 MW system in Hazle Township, Pennsylvania, in 2014.

In 2014, Minto, Ontario, Canada, opened a 2 MW (for 15 minutes) flywheel storage plant. The NRStor flywheel system uses ten rotating steel flywheels on magnetic bearings.

Amber Kinetics, Inc. has signed a deal with Pacific Gas and Electric (PG&E) to build a 20 MW/80 MWh flywheel energy storage plant in Fresno, California, with a four-hour discharge time.

### Toys

Simple flywheel motors are used to power numerous toy vehicles, trucks, railroads, action toys, and other toys.

### Wind Turbines

Flywheels can be utilized to store energy generated by wind turbines during off-peak periods or when wind speeds are particularly high.

Beacon Power started testing their Smart Energy 25 (Gen 4) flywheel energy storage device at a wind farm in Tehachapi, California, in 2010. The system was built for the California Energy Commission as part of a wind power/flywheel demonstration project.

### Toggle Action Presses

Toggle action presses are still widely used in industry. An extremely powerful crankshaft and a heavy-duty connecting rod are typically used to drive the press. Electric motors drive big and heavy flywheels, but the flywheels only turn the crankshaft when clutches are engaged.

## Comparison to Electric Batteries

Flywheels are less sensitive to temperature fluctuations, can function over a far larger temperature range, and are immune to common problems disturbing chemical rechargeable batteries. They’re also less likely to harm the environment because they’re mostly constructed of inert or benign materials. Another advantage of flywheels is that the exact quantity of energy stored may be determined simply by measuring the rotation speed.

Unlike conventional batteries, which have a limited lifespan (about 36 months in the case of lithium-ion polymer batteries), a flywheel has the potential to function indefinitely. For more than two centuries, flywheels created as part of James Watt steam engines have been in continuous use. Many working examples of ancient flywheels, which were mostly employed in milling and pottery, may be found in Africa, Asia, and Europe.

The majority of current flywheels are sealed devices that require little maintenance over their service life. Magnetic bearing flywheels in vacuum enclosures do not require bearing maintenance and hence outperform batteries in terms of total lifetime and energy storage capacity. Due to wear, flywheel systems with mechanical bearings will have a limited lifespan.

High-speed shrapnel from high-performance flywheels can kill bystanders. While batteries can catch fire and spew chemicals, bystanders usually have enough time to evacuate and avoid harm.

Because the energy stored by a flywheel is related to its angular mass and the square of its spinning speed, the physical arrangement of batteries can be built to fit a wide variety of layouts, whereas a flywheel must occupy a minimum area and volume. Because the mass of a flywheel reduces as it gets smaller, the speed must increase, putting more stress on the components. A flywheel may not be a practical solution where space is limited (for example, under the chassis of a train).