# A Review of Flywheel Energy Storage System Technologies and Their Applications
## 2 Description of Flywheel Energy Storage System  
### Background  
The flywheel as a means of energy storage has existed for thousands of years as one of the earliest mechanical energy storage systems. For example, the potter's wheel was used as a rotatory object using the flywheel effect to maintain its energy under its own inertia [21]. Flywheel applications were performed by similar rotary objects, such as the water wheel, lathe, hand mills, and other rotary objects operated by people and animals. These spinning wheels from the middle ages do not differ from those used in the 19 th or even 20 th centuries. In the 18 th century, the two major developments were metals replacing wood in machine constructions and the use of flywheels in steam engines. Developments in cast iron and the production of iron resulted in the production of flywheels in one complete piece, with greater moment of inertia for the same space [21]. The word 'flywheel' appeared at the beginning of the industrial revolution (namely in 1784). At the time, flywheels were used on steam engine boats and trains and as energy accumulators in factories [22]. In the middle of 19 th century, as a result of the developments in cast iron and cast steel, very large flywheels with curved spokes were built. The first three-wheeled vehicle was built by [PERSON] in 1885 and can be named as an example [21]. Over time, several shapes and designs have been implemented, but major developments came in the early 20 th century, when rotor shapes and rotational stresses were thoroughly analysed, and flywheels were considered as potential energy storage systems [23]. An early example of a flywheel system used in transport was the Gyrobus, powered by a 1500 kg flywheel, produced in Switzerland during the 1950s [24]. In the 1960s and 1970s, FESS were proposed for electric vehicles, stationary power back up, and space missions [9; 10]. In the following years, fibre composite rotors were built and tested. In the 1980s, relatively low-speed magnetic bearings started to appear [25].  
Despite major developments during their early stages, the utilization of flywheels has not been significant and has declined with the development of the electric grid. However, due to the recent improvements in materials, magnetic bearings, power electronics, and the introduction of high speed electric machines, FESS have been established as a solid option for energy storage applications [7; 8; 9; 26; 27].  
A flywheel stores energy that is based on the rotating mass principle. It is a mechanical storage device which emulates the storage of electrical energy by converting it to mechanical energy. The energy in a flywheel is stored in the form of rotational kinetic energy. The input energy to the FESS is usually drawn from an electrical source coming from the grid or any other source of electrical energy. The flywheel speeds up as it stores energy and slows down when it is discharging, to deliver the accumulated energy. The rotating flywheel is driven by an electrical motor-generator (MG) performing the interchange of electrical energy to mechanical energy, and vice versa [28; 29]. The flywheel and MG are coaxially connected, indicating that controlling the MG enables control of the flywheel [30; 31; 32; 33].