impact of reduced generator mass for fixed-bottom and floating offshore wind turbine design

Wind turbine rotor sizes have increased significantly in the past decade, driven by the ongoing demand for a lower levelized cost of energy (LCOE). This trend aligns with the increasing capacity of offshore wind farms with fixed-bottom foundations [1, See Figure 35]. Larger turbines require heavy, high-cost direct-drive generators compared to their geared counterparts. But they offer key offshore advantages. Without a gearbox, they have fewer mechanical components, improving efficiency, reliability, and reducing downtime risks [2, 3, 4]. Reducing generator mass could lower operational costs and unlock drivetrain, tower, and foundation redesign opportunities [5, 6], delivering broader economic benefits.
A 2014 study [6] found that lighter generators reduce monopile and transition piece mass if the monopile’s natural frequency is maintained. Another study suggested an 11% tower mass reduction for a 10 MW turbine using a superconducting generator [7], though details of their models were lacking. More recently, Barter et al. [8] compared generator technologies for 15-25 MW offshore turbines, optimizing 30 generator designs based on the IEA 15 MW reference turbine [9]. Their results showed that permanent-magnet and superconducting direct-drive generators scaled better than geared systems, but rotor thrust loads—not tower-top mass—most influenced support structure design. This suggests that lighter generators do not always lead to lower tower and foundation costs. Instead of a sequential design, an integrated redesign of the turbine and substructures could lead to lower costs.
In this work, we investigate the effects of incorporating a 50% lighter direct-drive generator on the drivetrain, tower, and offshore structure designs for both fixed-bottom monopiles and floating foundations. Our preliminary analysis, based on the IEA 15 MW reference turbine, utilizes different software tools, optimization approaches, and cost models compared to the study by Barter et al. [8]. While Barter et al. [8] highlighted the potential of superconducting materials for lighter generators, we investigate a more compact and lighter alternative. The Swedish research spin-off Hagnesia develops a novel direct-drive permanent magnet machine for low-speed applications. Their patented innovation allows for multiple airgaps to be stacked in a very compact and modular structure, thus radically increasing the torque density compared to both conventional radial flux machines and state of the art axial flux machines. While using the same materials, the technology enables up to 90 % weight reduction compared to conventional direct drive generators for wind power, while at the same time improving conversion efficiency.