Experimental and numerical vibration analysis of a vertical axis wind turbine shaft

Wind turbines are a rapidly growing, sustainable, clean, and cost-effective energy source that generates electricity by converting wind's kinetic energy into mechanical energy. Energy generated by small wind turbines is used in applications such as small businesses, homes, schools, field hospitals, temporary tent cities, military camps, greenhouses, and agriculture. Variable wind loads acting on the turbine's main components, such as the shaft, body, blades, gears, and bearings, can cause unwanted vibrations. These vibrations can lead to mechanical failures in wind turbines. The vibrations generated in the shaft also affect the surrounding turbine system structure. Therefore, understanding the dynamic behavior characteristics of the turbine shaft is important for controlling vibrations in the turbine system. Excessive instability and vibrations can have negative effects on the performance and stability of the entire wind turbine system. In this study, the effects of the geometric and material properties of the shaft on the vibration behaviour of a vertical axis wind turbine were investigated using experimental methods. In the tests, shafts made of different geometries and materials, including AISI 1040 Solid, AISI 4140 Solid, AISI 1040 Hollow, and carbon fiber reinforced plastic (CFRP) Hollow, were used, tested, and analyzed for their suitability for wind turbine applications. Experimental Modal Analysis (EMA) was conducted to understand the dynamic behavior of the turbine shaft, including its natural frequencies, mode shapes, and damping. The results obtained from EMA were validated using the Finite Element Method (FEM). Additionally, an Operational Deflection Analysis (ODS) was performed to evaluate the excessive vibrations that the turbine shafts are exposed to under general vibration levels within the wind turbine structure. Furthermore, to gain a more detailed understanding of the dynamic properties of the shafts, Order Analysis (OA) was performed at wind speeds of 5, 9 and 12 m/s using a wind tunnel. The findings show that the vibration amplitude of the turbine shaft is the lowest at all wind speeds in the AISI 4140 solid shaft. Composite shafts exhibit behaviour more prone to resonance due to their low rigidity, while solid steel shafts offer more stable dynamic performance. These findings highlight the critical importance of selecting the appropriate material and cross-section for the turbine shaft in terms of vibration reduction and system stability.