## 4 Applications of FESS  
There are numerous uses for flywheels in industry, ranging from the large scale electrical grid level, to the small scale consumer level [8; 9]. Flywheels can be arranged in banks in order to achieve high power capacity and density instead of using larger machines [49]. But the most suitable flywheel applications concern the requirement for high power output over a short duration (e.g., 100s of kW/10s of seconds) [6] where there are frequent charge-discharge cycles [8]. The most common applications concern power quality, such as voltage and frequency regulation [50; 51; 52; 53; 54; 55], pulsed power applications for the military [48], attitude control in space craft [48], UPS [8; 12; 35], load levelling [2], transportation (such as trains and hybrid and electric vehicles) [7; 8; 56], and energy storage applications [48].  
Generally, FESS is classified as short-term response with high power density, with the ability to respond within seconds [57]. Storage duration can be prolonged to few hours without the loss of excessive amounts of energy as long as run-down losses are kept to a minimum. Flywheels can assist with the improvement of system stability where wind and solar energy penetrate an island system or are integrated into a power system. Due to flywheels having fast response characteristics, they are suitable in RES applications where grid frequency needs to be balanced. Power variations due to wind and solar fluctuations can be compensated by storing energy during low demand windy or sunny periods, and then supplied back during times of high demand [9; 10]. Flywheels can be used to improve the system frequency by rectifying wind oscillations; whereas in solar PV systems, they can be incorporated with chemical batteries to improve system output and prolong the battery's operational lifetime [9].  
As part of its applications in energy storage, flywheels are able to accomplish this task both at the grid as well as at the customer level. They can be used to supplement residential PV systems in islanded or grid connected mode. Energy storage systems in residential PV systems are highly dominated by chemical batteries and flywheels have not been widely tested for this application. However, there is a high demand for FESS operated PV systems in developing countries where the grids are not very stable and diesel backup generators are needed to call upon. Unreliability of the grid would push the storage system for increased number of cycles, and hence, battery lifetime would not be the same as it is for developed countries. Of course, the batteries are mass produced and are currently more cost effective compared to flywheels, but flywheels could offer a viable choice when it comes to numerous cycles per day.  
A recent publication discusses application of FESS for residential photovoltaic installation [58]. The paper provides the quantitative evaluation of a FESS for a residential PV system mounted on the roof of a one-family house. The conducted case study shows that a 3 kWh FESS can improve the self-sufficiency of the PV system from 36.8% to 51.9%. The self-consumption of the system is also improved from 30.2% to 61.4%.  
A detailed description of the flywheel applications and their industrial onsite installations is discussed in [38]. A brief overview of a number of commercial flywheel manufacturers and their applications is presented in [59], as shown in Table 2. There are no commercially available flywheel storage systems for residential applications and majority of the flywheel manufacturers are providing high power fast-response flywheels for power quality and UPS applications.  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline \hline  & **Filter Power** & **Active Power** & **Temporal Power** & **Beacon Power** & **Roseata T2** & **Vyoon** & **Kinetic Taction** & **Stystems** & **Sometic** & **Power/True** & **Gryoticity** & **Amber Kinetics** \\ \hline
**Origin** & Germany & USA & Canada & USA & Germany & USA & USA & Germany & USA & UK & USA \\