# A Review of Flywheel Energy Storage System Technologies and Their Applications
## 2 Description of Flywheel Energy Storage System
### Structure and Components of FESS
#### 2.2.4 Bearings  
Bearings are required to keep the rotor in place with very low friction, yet provide a support mechanism for the flywheel [3]. The bearing system can be mechanical or magnetic, depending on the weight, lifecycle life, and lower losses [7]. Gas bearings cannot be used due to the vacuum within the enclosure. Traditionally mechanical ball bearings have been used, but these have a higher friction compared to magnetic bearings and also require higher maintenance as a result of lubricant deterioration [7]. These difficulties may be mitigated by using a hybrid system of magnetic and mechanical bearings. A magnetic bearing has no friction losses and does not require any lubrication but, if active, requires power to energize it. It stabilizes the flywheel by supporting its weight using permanent magnets [9]. Permanent (passive) magnetic bearings (PMB), active magnetic bearings(AMB), and superconducting magnetic bearings (SMB) are the main types of magnetic bearing systems [16]. A PMB has high stiffness, low cost, and low losses, due to lack of a current. However, it has limitations in providing stability and is usually considered as an auxiliary bearing system [16; 18]. An AMB is operated by the magnetic field produced from current carrying coils controlling the rotor position. It positions the rotor through a feedback system by applying variable forces which are determined based on the deviation of the rotor position, due to external forces. An AMB has a high cost, a complicated control system, and consumes energy to operate, which in turn, adds to system losses [16]. FESS standby loss is affected by the AMB mass. As a result, increasing the rotational speed and AMB mass add to the AMB iron and copper loss [35]. To ensure a good efficiency of the overall system, a compromise between speed and losses has to be made. An SMB provides a high speed, frictionless, long life, compact, and stable operation. It is the best magnetic bearing for a high speed operation as it can stabilize the flywheel without electricity or a positioning system, according to [9; 16]. However, an SMB requires a cryogenic cooling system as it operates at a very low temperature; but recently, it has been improved by using high temperature superconductors (HTS). The main drawback of an SMB system is its very high cost [7; 9; 16]. The parasitic losses of mechanical bearings are about 5% of the total storage capacity per hour unless hybrid systems are used. This factor is about 1% for electromagnetic bearings [14] and can be further lowered to 0.1% by using HTS bearings [55]. The use of hybrid bearings will reduce the losses and complexity of the control system and also provide a stable and cost effective solution [56]. A compact flywheel energy storage system assisted by hybrid mechanical-magnetic bearings is proposed in [57]. The magnetic levitation in the vertical orientation is maintained by the magnetic bearing, while the translational and rotational levitation is assisted by mechanical bearing. In [58], a combination of SMB and PMB have been analysed to reduce the cost of the cooling system. The position of the flywheel rotor is controlled by the PMB and the SMB is mainly used to suppress the rotor vibrations. However, the capability of PMB in restraining the system during high speeds still remains in question.