## 1 Introduction  
The need for energy storage is increasing with technological developments. Energy storage can be classified as small, medium and large scale. There are many energy storage solutions, from consumer electronics to supporting the electrical grid. The most commonly used energy storage technologies are battery energy storage (BES), ultracapacitor energy storage (UESS) and flywheel energy storage (FESS) [1, 2]. Each of these storage technologies has its own areas of use, advantages and disadvantages. When choosing the storage technology to be used, parameters, such as system life, cost, energy density, power density, safety, performance, cycle life, and maximum system power, are taken into consideration. When FESS is examined structurally, it stands out with parameters, such as low maintenance cost, high power density, high cycle life, and long system life [1, 2]. When the studies are examined, it is seen that FESS is preferred, especially in the fields of space and aviation, military technologies, electric and hybrid electric vehicle (HEV) applications [3], renewable energy production facilities, grid-based energy storage systems, and distributed energy production systems. FESS, which works in parallel to BES in electric vehicles, is used for the storage of regenerative braking energy. For short-term high-energy transfers, kinetic energy storage systems are more advantageous than batteries. In addition, hybrid storage reduces the electrical stress on the system [4]. FESS has been preferred by NASA for many years in space applications. The most important reasons for this are its long service life, durability and wide operating temperature range. For similar reasons, it is also preferred in military applications. In addition, the increasing utilization of renewable energy sources has made FESS a compelling and appealing option for both implementation and further investigation. The main reason for this is that renewable energy sources have an intermittent energy production profile [1; 5]. Storage technologies compensate for this intermittent situation in energy production and enable a continuous and balanced energy output. FESS is a significant technology that regulates wind-related speed fluctuations, especially in wind power plants. Energy storage systems play an important role in distributed generation areas where there is no electricity grid and in order to perform the load shift operation required for peak demand management in the grid [6; 7]. It is emphasized that FESS is a storage system that increases power quality when used in a grid-connected manner [8]. Grid-connected FESS units are used as commercial products for over 10 years, in large-scale power stations in the USA, to compensate for events such as instantaneous voltage drops, voltage spikes, short-term interruptions and black-outs, which occur in the electrical grid and deteriorate power quality [2]. In this context, in 2024, China commissioned a 30 MW FESS energy storage power plant called the Dinglun flywheel energy storage power station in Shanxi province. Thus, China broke the record of the 20 MW FESS storage plant located in Stephentown, New York, belonging to the American company Beacon Power [9]. FESS works on the principle of kinetic energy storage on a rotating mass [10]. Although it is an industrial technology that has been used for over a century, today, the development of manufacturing technologies and materials science has made it possible to produce high-speed energy storage systems. In the last 50 years, high-speed FESSs have increased from 7000 RPM to 400,000 RPM. In addition, energy and power density values have increased by more than 10-times [2; 11]. Despite the increase in speed and stored energy, there has been a decrease in system weight. The most important parameter that allows for the decrease in system weight is the new-generation composite materials used in the rotor [12; 13; 14; 15; 16]. Carbon fiber or glass fiber materials with higher tensile strength than steel and titanium are preferred in high-speed FESSs.