## 5 Conclusions  
Composite flywheels represent a challenging field of study due to factors, such as the technical properties of materials, design criteria, various geometric shapes, and manufacturing methods. As an innovation in this study, a system was designed with a metal core and a composite outer layer. Analyses were conducted for both steel and titanium as the metal core. Additionally, the effects of carbon composite layer thicknesses on the system's energy and speed values were observed.  
When the obtained data are evaluated, it is seen that manufacturing steel alone is sufficient for applications requiring a peripheral speed of up to 250 m/s. However, to reach a value of 500 m/s, a titanium alloy or a titanium-carbon composite multi-layer structure would be preferred. For peripheral speeds ranging from 500 m/s to 2500 m/s, a fully carbon composite rotor structure is required. According to the results, the hybrid use of titanium and carbon composite materials does not significantly contribute to an energy enhancement, as the increase is only around 15%. Taking the risks associated with a hybrid structure for a mere 15% energy increase is not considered worthwhile. However, if the goal is to enhance energy density, a 65% increase is observed in the hybrid structure. Furthermore, in steel rotor structures, the energy density increase can exceed 100%.  
In conclusion, carbon composite represents the highest level of performance in terms of lightness, strength, and energy density. However, high manufacturing and material costs make it challenging to choose for low-performance and cost-sensitive applications. In sectors where cost is a secondary concern, such as aerospace, aviation, and military technologies, carbon fiber rotor structures are frequently preferred. Due to their mechanical strength properties, steel rotors are disadvantaged compared to titanium and carbon composite materials in terms of performance and durability. However, owing to their ease of manufacturing and reasonable material costs, steel rotors can be used in grid-based  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Rotor Material** & **Maximum** & **Production** & **Material** & **Stored Energy** & **Total Rotor** & **Energy Density** \\  & **RPM** & **Cost** & **Cost** & **(Wh)** & **Weight (kg)** & **(Wh/kg)** \\ \hline Mild steel & 68,740 & 1 & 1 & 26.27 & 2.92 & 9 \\ Titanium alloy & 137,000 & 2 & 2 & 61.42 & 1.72 & 35.7 \\ Mild steel—20 mm CFRP & 99,650 & 3 & 3 & 32.32 & 1.71 & 18.9 \\ Titanium alloy—20 mm CFRP & 175,800 & 4 & 4 & 68 & 1.16 & 58.8 \\ CFRP (23 GPA) & 952,500 & 5 & 5 & 1156.63 & 0.67 & 1727 \\ \hline \hline \end{tabular}
\end{table}
Table 14: Comparison of systems according to different rotor materials.  
energy storage solutions and relatively low-speed applications. The multi-layer hybrid rotor structure examined in this study and presented as an original contribution is found to enhance energy density due to the resulting weight reduction.  
Methodology, C.Y.; Formal analysis, C.Y.; Investigation, K.E.; Writing--original draft, K.E.; Writing--review & editing, K.E.; Visualization, C.Y.; Supervision, K.E. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.  
The authors declare no conflict of interest.