# Flywheel Power Systems Information

Flywheel power systems, also known as flywheel energy storage (FES) systems, are power storage devices that store kinetic energy in a rotating flywheel. The flywheel rotors are coupled with an integral motor-generator that is contained in the housing. The motor-generator is used to store and then harness energy from the rotating flywheel. The use of a flywheel power system can improve the overall life and reliability of an uninterruptible power supply (UPS), harness kinetic energy in high load or cyclic braking operations, replace or augment batteries, regulate power frequency, and ultimately provide a sustainable means for energy conservation.

## Operation

The flywheel rotor is typically suspended by either mechanical or magnetic bearings in a vacuum chamber. This reduces friction allowing the rotor to spin at a near constant velocity until acted upon be the motor-generator. As electricity is fed into the system the motor transfers the electrical energy to stored kinetic energy in the flywheel thereby increasing the rotational speed. This stored energy can then be harnessed by the generator when there is a demand for an electrical current and in turn the flywheel slows down.

## Energy Density

The energy density of the flywheel power system is primarily dependent on the properties of the materials used and the geometry of the rotor. For isotropic rotors composed of a single material the energy density can be expressed by the following formula:

### Where:

= Kinetic energy of rotor (J)

= Mass of rotor (kg)

= Shape factor constant

= Tensile strength of rotor material (Pa)

= Density of rotor material (kg/m3)

## Applications

Flywheel energy storage systems are designed for regenerative braking applications, to supplement DC power in uninterruptible power systems (UPS), or for energy storage applications in power grids.

### Flywheel Braking Systems

Flywheel braking systems utilize a regenerative brake or other hybrid kinetic energy recovery system (KERS). The regenerative brake or KERS recovers the kinetic energy from the rotor of a motor or moving wheel. As the brake is applied the kinetic energy from the rotor is transferred to the flywheel. Flywheel braking systems greatly reduce power consumption in mobile cranes, rail transport, automobiles, and in other significant load-bearing rotary motors.

Image Credit: ASME Intl.

### Flywheel UPS / Continuous Power Systems

A flywheel uninterruptible power system (UPS) is used to supply continuously clean, regulated electrical power to a critical load. They are used to supply a short-term power source when there is a disruption in the mains supply (or when the mains supply is lost) and until a back-up power source, such as a generator, is up and running.

Video Credit: VYCONEnergy / CC BY-SA 4.0

### Flywheel Energy Storage (FES) Systems

Flywheel energy storage systems provide highly responsive clean power that increases the reliability of an energy grid. FES systems may use several flywheels integrated into a single system that are used to store energy from non-continuous power sources, supplement energy supply during periods of high demand, and provide grid-scale frequency regulation services. The fast response of the FES systems prevent load unbalances, as they are able to ramp up power supply much faster than conventional gas-fired power plants.

## Components of a Flywheel Power System

Flywheel power systems have five main components: the housing, flywheel, magnetic bearings or electromagnets, motor-generator, and vacuum chamber.

Image Credit: PowerTHRU

Flywheel housings contain and isolate the bearings, rotor, and electromechanical components of the flywheel power system. By providing a controlled environment the housing allows the flywheel to spin with little to no interference from any external forces that would otherwise dampen its motion.

Flywheels are the primary load-bearing device inside flywheel power systems. The flywheel is commonly fabricated from steel or carbon fiber. Carbon fiber flywheels have a higher energy density due to their light weight and high strength. Steel, aluminum, and composite rotors are an economical alternative to the carbon fiber rotor.

Magnetic bearings, which may be assisted with permanent magnets or levitation systems, allow the rotor and flywheel to spin about a frictionless touchdown surface. Only a negligible amount of friction is experienced as the magnetic bearing assembly provides a non-contact surface about which the rotor spins.

Motor-generators are used to transfer energy to and from the flywheel. The coil is typically the stator component that acts on a ferrous or magnetic rotor that is coupled to the flywheel.

Vacuum chambers are used to remove any amount of air resistance that would otherwise interfere with the flywheel.

Flywheel DC energy storage systems are rated in either kilowatt-hours (kWh) or megawatt-hours (MWh), which equates to the amount of power that is available over a given time frame. Large-scale flywheel energy storage systems are measured in MWh's and use several flywheels that are coupled together. Flywheel power systems store energy very efficiently and have the potential for very high specific power when compared to batteries.

## Standards & Regulations

Industrial standards and regulations are critical references used in the design, manufacture, testing, installation, and maintenance of flywheel power systems. Following the guidelines set forth by industrial standards and regulations minimalizes liability, ensures compliance, and collectively lowers overall cost by eliminating wasted time and effort.

SAE J1240

The Society of Automotive Engineers (SAE) is a standards development organization for the engineering of all kinds of powered vehicles. This specific standard was developed in order to provide a uniform test procedure for flywheel assemblies. The test is designed to determine the rotational speeds at which a flywheel will either burst or withstand a specified limiting speed.