Power Density Analysis of Wind Turbine Main Bearing Units by Holistic Optimization of Material, Manufacturing and Design of the Main Shaft

Given the continuous increases in wind turbine (WT) rated power and size, the nacelle weight and logistic handling costs increases significantly. To support heavier nacelles, stronger towers are needed which again increases material costs, so a need for nacelle power density increase arises. One solution to this problem is to increase the power density of the cast or forged WT main shaft. The power density in cast main shafts is limited by the low tensile strength of cast iron. High tensile strength steels, which theoretically increase power density, are used in state-of-the-art forged main shafts. However, their inner shaft diameter is kept small to reduce drilling costs. Since the loads of WT main shafts are dominated by the bending moments of the rotors, a high section modulus corresponds to a high power density. Material near the centre of the shaft therefore decreases the shaft power density. Hollow forging combines high tensile strength steel with a variable inner shaft diameter, enabling shaft designs with increased power density. Additionally, the use of air-hardening ductile (AHD) steel eliminates the need for costly heat treatment if the wall thickness is thin enough. The paper presents a holistic system model for the predesign of main bearing units (MBU) considering various materials and manufacturing methods. The model enables a feasibility assessment of hollow forged main shafts by comparing the resulting MBU weights across a wide range of WT power ratings. The MBU is selected instead of solely analysing the main shaft to account for the bearing and bearing housing weights, which depend on the main shaft geometry. The results show increased MBU power density of up to 23% for hollow forged shafts compared to forged shafts of the same material. Furthermore, when the shaft is hollow forged from AHD steel, the increase is even greater, up to 52%.