## 2 Flywheel Theory and Structure
### Components of FESS
#### 2.2.3. Bearing Systems  
The bearing system supports the rotor and allows the flywheel to spin freely. The selection of bearing system depends on many factors but importantly on its lifetime, weight and level of acceptable losses [54]. There are two types of bearings commonly used in FESS: (1) mechanical bearings and (2) magnetic bearings. Mechanical bearings are simple in structure but have major losses due to friction and deterioration of lubricants and they require maintenance such as lubrication replacement. Magnetic bearing systems work using magnetic levitation and do not have any contact with the mechanical shaft of the flywheel. They are stable but low loss bearing systems due to less friction and are suitable for high-speed applications in FESS [55]. Magnetic bearings that use permanent magnets for levitation are called passive magnetic bearings whereas active bearings are the type that make use of a feedback system with electrical coils [56]. Active magnetic bearings require continuous power for energisation of the magnetic levitation system [13].  
The combined use of magnetic and mechanical bearing in FESS can make the system more stable, cost effective and less complex; an example of such system is discussed in [57]. Another type of bearing called spiral groove bearing (SGB) is low loss bearing system, used for low speed applications of less than 10 m/s. SGB is used in combination with magnetic bearing for flywheels with larger storage capacity and less frequency vibrations of the rotor [58]. A technique has been developed by Argonne National Laboratory to produce high-temperature superconducting (HTS) bearing system to reduce  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**SCIM** & **PMSM** & **SRM** \\ \hline \multirow{6}{*}{**Advantages**} & Rugged construction & Simple rotor design & Robust simple structure \\ \cline{2-3}  & No risk of & High power, high load & \\ \cline{2-3}  & demagnetization & and high torque density & High power density \\ \cline{2-3}  & Low cost & Easy to install and & \\  & & maintain due to & \\  & & smaller size & \\ \cline{2-3}  & High efficiency (93.4\%) & Very high efficiency & \\  & & (95.5\%) & High efficiency (93\%) \\ \cline{2-3}  & No idling losses due to & & \\  & its lower rotor magnetic & & \\  & field at zero torque value, & No field winding losses & \\  & consequently this & due to use of & No idling losses \\  & reduces a total electrical & permanent magnets & \\  & loss of IM & & \\ \cline{2-3}  & High torque and high & Suitable to operate in low & \\  & power capability & pressure vacuum with & Wide speed range \\  & negligible rotor losses & & \\ \hline \multirow{6}{*}{**Drawbacks**} & Difficult mechanical & & Significant rotor eddy \\  & design for high & Risk of sudden & current losses due to the \\  & speed applications & demagnetization & time varying \\  & & & magnetic field \\ \cline{2-3}  & Difficult to operate in & Due to use of permanent & \\  & absolute vacuum due to & magnets, PMSM have & High torque and \\  & complex heat & low tensile strength & current ripples \\  & transfer system & & \\ \cline{2-3}  & Limited speed and & High cost & Difficult to \\  & larger volume & & regulate speed \\ \cline{2-3}  & Copper losses in & & \\  & electromagnets during & Idling losses at zero & \\  & field excitation due to & torque due to presence of & High losses due to need \\  & high invush currents & magnetic field & of excitation \\ \hline \hline \end{tabular}
\end{table}
Table 1: Advantages and drawbacks of commonly used electrical machines in FESS [13; 25; 47; 52; 53].  
rotational losses to less than 2% per day of stored energy. Compared to magnetic and mechanical bearing, HTS bearings incur half of the rotational loss of conventional mechanical bearings [59].