# A Review of Flywheel Energy Storage System Technologies and Their Applications
## 2 Description of Flywheel Energy Storage System
### Structure and Components of FESS
#### Housing  
The housing has two purposes: to provide an environment for low gas drag and for the containment of the rotor in the event of a failure. The aerodynamic drag loss in an FESS increases with the cube of the rotational speed, if the system is operated in atmospheric pressure [23]. These losses are reduced by mounting the flywheel in a vacuum enclosure to improve the system performance and safety. The housing or enclosure is the stationary part of the flywheel and is usually made of a thick steel or other high strength material, such as composites. The container holds the rotor in a vacuum to control rotor aerodynamic drag losses by maintaining the low pressure inside the device, thus withstanding failures as a result of any possible rotor failures [28; 29; 59]. Operating the system in such a low pressure requires a vacuum pump and an efficient cooling system to handle the heat generated from MG and some other parts of FESS [23]. When the power into and out of the flywheel is via an electric machine, there are no rotary seals, so leakage can be very small. This means that the vacuum pump does not need to operate frequently, or can be eliminated with a sufficient sealing of the housing. The operation of the vacuum pump depends on the rotor type. Composite rotors have a very high tip speed requiring lower (harder) vacuum pressures and outgas, due to the nature of the polymer resin matrix materials in contrast to steel. An alternative approach [23] is to use a gas mixture of helium and air, which reduces both the aerodynamic drag loss and the system cooling requirements.  
In the event of rotor failure, composite rotors tend to break into numerous small fragments and its energy is dissipated by friction as the fragments rotate inside the casing. As this happens, pressure is built up inside the casing and the end plates of the casing. If air enters during failure, then a much stronger dust type explosion must also be contained, leading to the need for stronger containment. Single piece steel rotors can burst into several fragments which will be difficult for the enclosure to withstand, so require very large containment systems. This issue can be mitigated by making the rotor from a stack of thinner discs, as explained in the section on rotors. This is because the catastrophic failure would release a fraction of the energy contained in the flywheel rotor. The enclosure design for high-speed FESS will contribute to half of the flywheel weight, whereas this factor would be two and half times larger for low speed FESS, according to [18]. The authors hold a different view on this multiplier, as do most flywheel manufacturers who adopt a range of stances to justify safety including placing flywheels in bankers for large composite rotors. Published literature on what is required to contain both composite and steel rotors is not readily available and no officially adopted standards currently exist, to the author's knowledge. One recent publication has proposed a standard [60], which is a useful step forward.