
In the 1950s, flywheel-powered buses, known as , were used in () and () and there is ongoing research to make flywheel systems that are smaller, lighter, cheaper and have a greater capacity. It is hoped that flywheel systems can replace conventional chemical batteries for mobile applications, such as for electric vehicles. Proposed flywh. The amount of energy stored, E, is proportional to the mass of the flywheel and to the square of its angular velocity. It is calculated by means of the equation (1) E = 1 2 I ω 2 where I is the moment of inertia of the flywheel and ω is the angular velocity. [pdf]
The application of flywheel energy storage systems in a rotating system comes with several challenges. As explained earlier, the rotor for such a flywheel should be built from a material with high specific strength in order to attain excellent specific energy .
Flywheel energy storage (FES) works by accelerating a rotor (flywheel) to a very high speed and maintaining the energy in the system as rotational energy.
The main components of a flywheel energy storage system are a rotor, an electrical motor/generator, bearings, a PCS (bi-directional converter), a vacuum pump, and a vacuum chamber . During charging, the rotor is accelerated to a high speed using the electrical motor.
The German company Piller has launched a flywheel energy storage unit for dynamic UPS power systems, with a power of 3 MW and energy storage of 60 MJ. It uses a high-quality metal flywheel and a high-power synchronous excitation motor.
Flywheel Energy Storage Systems (FESS) play an important role in the energy storage business. Its ability to cycle and deliver high power, as well as, high power gradients makes them superior for storage applications such as frequency regulation, voltage support and power firming [, , ].
The low-speed rotors are generally composed of steel and can produce 1000s of kWh for short periods, while the high-speed rotors produce kWh by the hundreds but can store tens of kWh hours of energy . Figure 17. Flywheel energy storage system in rail transport, reproduced with permission from .

Compared with other ways to store electricity, FES systems have long lifetimes (lasting decades with little or no maintenance; full-cycle lifetimes quoted for flywheels range from in excess of 10 , up to 10 , cycles of use), high (100–130 W·h/kg, or 360–500 kJ/kg), and large maximum power output. The (ratio of energy out per energy in) of flywheels, also known as round-trip efficiency, can be as high as 90%. Typical capacities range from 3 to 1. The FESS is capable of generating several mW of power for brief periods. Flywheels are best suited to produce high power outputs of 100 kW to 2 mW over a short period of 12-60 seconds. The peak output, at 125 kW for 16 seconds, is sufficient to provide 2 mW for one second. [pdf]
The flywheel goes through three stages during an operational cycle, like all types of energy storage systems: The flywheel speeds up: this is the charging process. Charging is interrupted once the flywheel reaches the maximum allowed operating speed. The flywheel energy storage system is now at capacity.
Flywheel Energy Storage Systems (FESS) are found in a variety of applications ranging from grid-connected energy management to uninterruptible power supplies. With the progress of technology, there is fast renovation involved in FESS application.
Flywheels, one of the earliest forms of energy storage, could play a significant role in the transformation of the electrical power system into one that is fully sustainable yet low cost.
Our flywheel energy storage calculator allows you to calculate the capacity of an interesting type of battery!
Assuming a 28 in wheel with mass m = 2.87 lb, the energy stored is 3.25 J. To find this result: I = 2.87 × 1 × 14² = 3.9 lb·ft². How does a flywheel store energy? A flywheel can store energy thanks to the conservation of angular momentum.
To connect the Flywheel Energy Storage System (FESS) to an AC grid, another bi-directional converter is necessary. This converter can be single-stage (AC-DC) or double-stage (AC-DC-AC). The power electronic interface has a high power capability, high switching frequency, and high efficiency.

Energy storage flywheels can be dangerous due to the risk of "flywheel explosion"1. When the tensile strength of a composite flywheel's outer binding cover is exceeded, the wheel can shatter, releasing all of its stored energy at once1. An example of such an accident occurred at Quantum Energy Storage, where an "out-of-control" 11,000-pound metal flywheel caused an explosion2. [pdf]
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