The size of wind turbines in terms of rotor diameter, hub height and rated power has increased dramatically in the past two decades. Figure 1 shows UK offshore wind turbine size evolution from 2002 (2MW, 93m) to 2022-23 (12MW, 236m). Figures 2 and 3 show the number of newly installed wind turbines and the average power rating of onshore and offshore wind turbines between 2017 and 2022. The onshore wind turbines had an average power rating of 4.3 MW, while the offshore wind turbines had an average power rating of 8.6 MW in 2022.  

Types of Generators

The generator is the key component of wind turbines. There are different types of generators. The transmission is typically classified into direct-drive and gearbox mechanisms with different machine types, rotors and speeds. Leveraging a classification into different sub-technologies with typically dedicated material and component configurations provides an opportunity to estimate future release rates and the current composition of the material stock in use.

To provide a more comprehensive classification, the types of wind turbines can be classified based on the drive train configuration, speed, and onshore and offshore application. In Figure 4, the onshore applications are in green, the offshore applications are in dark blue, and those generators used in both onshore and offshore applications are in purple.

Conventional generators (e.g., SCIG and WRIG) developed before PMSG do not include permanent magnets, and tend to use in onshore wind turbines rather than offshore installations. The black dashed line represents the types of generators that are widely adopted in the past decade, but no longer dominate the market. DFIG currently has a higher market share but is only used for less than 3MW capacity, while the penetration rate of PMSG has continuously been increasing due to its better energy efficiency. PMSG is currently the main R&D focus of all generator types regarding reducing cost and REE content while increasing turbine capacity and efficiency. Direct drives based on High-Temperature Superconductors (HTS) are a new type of generator at the R&D stage and could potentially be used in the future to reduce the use of REE.