With the growing traction of wind energy in the power sector, many components within the wind turbine are undergoing technology advancements in a bid to improve efficiency thereby increasing the output of generated electricity. A wind turbine looks rather simple in appearance. However, it has several complex components that cumulatively build and support its overall functioning. These components have immense scope for technology advancements. These components include drivetrains and gearboxes that are witnessing radical changes in their technology as this article highlights…
Drivetrains
A drivetrain is an essential part of the wind turbine as it helps in converting the rotational energy of the wind turbine’s blades into electrical energy. A drivetrain consists of a gearbox and generator wherein the drivetrain connects the turbine’s rotor’s low-speed shaft to the generator’s high-speed shaft, effectively transmitting mechanical power and increasing rotational speed to generate energy efficiently.
A range of wind turbine drivetrains with various costs, technologies, physical features, efficiencies, degrees of reliability and materials currently exist in the market. However, with wind turbines constantly undergoing technological upgradation to keep pace with the growing demands of the global clean energy transition, there is an increasing emphasis on innovations in drivetrains. Furthermore, the wind energy market, globally, is witnessing a move towards offshore wind, creating further scope for adaptation and improvement in drivetrains. Offshore wind projects differ from land-based projects in terms of not only their geographical environment but also their ability to deploy wind turbines of larger size and capacity. Thus, a considerable increase in the capacity of projects may be supported by creating more robust drivetrains.
As per a research paper titled “Wind turbine drivetrains: state-of-the-art technologies and future development trends” published in 2021, a drivetrain’s technological drivers differ from other components of wind turbines. For towers, there are site-specific solutions that are dependent on factors like wind conditions or other site features, which significantly affect the cost. However, for the drivetrain, factors affecting their cost are the number and size of components, rather than site or region-specific features.
Drivetrain design has been constantly upgraded for greater efficiencies in cost as well as performance. Since the drivetrain is one of the most complex components of a wind turbine with a large number of moving parts, it is quite prone to breakdowns and faults. Thus, many of the advancements in drivetrains are focused on having fewer moving parts and weight reduction to have more compact, lighter and simpler systems.
The digitalisation of drivetrains is also emerging as a key focus area due to the importance of real-time data management in supporting wind turbine functioning and efficiency. Digitalisation in drivetrains is primarily associated with the sensors and actuators installed on them but may be extended to other turbine systems to create a more robust control and monitoring system. Digitalisation has also been cited as an enabler for the adoption of digital twin models in wind turbines, which utilise real-time data along with cloud computing to support decision-making.
Gearboxes and direct drives
A gearbox in the wind turbine connects the low-speed shaft connected to the turbine blades to the high-speed shaft connected to the generator. It translates to the spinning of the outer blades with the help of a succession of gears of varied diameters. Its primary function is to increase the rotational speed of the low-speed shaft coming from the wind turbine rotor to a higher speed suitable for the generator which is connected to the high-speed shaft. This increase in speed allows the generator to produce electricity efficiently. Gearboxes of three kinds namely planetary, parallel-shaft, and helical are employed in wind turbines. These types are differentiated by the gear types utilised and the operational configuration.
The gearbox works on the principle of gear ratios, using a set of gears with different sizes to achieve the desired speed increase. The low-speed shaft, connected to the wind turbine rotor, typically rotates at a relatively slow speed due to the large size of the rotor and the low wind speeds needed to capture energy effectively. The high-speed shaft connected to the generator needs to rotate at much higher speeds to produce electricity efficiently. By incorporating the gearbox, the low-speed but high-torque rotation of the wind turbine rotor can be converted into high-speed rotation with lower torque. This mechanical transformation is important because most of the electricity grids operate at a fixed frequency. In addition, the rotational speed of the wind turbine rotor varies with wind speed.
As supported by the literature studies, the majority of wind turbine failures are due to failures within the gearbox that add to their costly repairs and downtime. It is crucial to note that not all wind turbines use gearboxes. In recent years, there has been an emerging trend towards direct-drive turbines which do not require gearboxes. Direct-drive turbines connect the rotor’s low-speed shaft directly to the generator, eliminating the complexity and upkeep of gears. Direct-drive turbines are preferred over gear-driven turbines because they have lower maintenance costs and better reliability, although they have slightly higher upfront expenditures in comparison to their counterparts.
When it comes to gearbox wind turbines, wind turbulence can create huge stress to the wheels and bearings in the gearbox. This can further lead to faults in the turbine components and eventually lead to its breakdown. In lieu of all these factors, the technologies for the critical component used in the turbine development are evolving to tackle the ongoing challenges and produce more power effectively.
Overall, with time, the number of possible drivetrain configurations has increased, based on various factors such as loading and costs. The most prominent difference that has emerged over the years is whether a wind turbine has a gearbox or not. Thus, broadly, two different categories of turbines dominate the market. The first one is dependent on a mechanical transmission system to increase the rotational speed of the shaft of the turbine rotor so as to drive a generator. In the second category, the generator directly uses the high torque to generate power without any mechanical transmission for increasing speeds.
In the case of geared wind turbines, the speed conversion ratios are higher, which means higher operations and maintenance requirements due to various gearbox components. On the contrary, slow-rotating electric generators are larger and heavier. Thus, these days many manufacturers opt for hybrid drivetrains, which have a gearbox that transfers the slow rotational speed of the shaft to a medium or high-speed generator, coupled with a full converter. Whether a drivetrain is geared, non-geared or hybrid depends on financial factors as well as other considerations such as weight, reliability and the expertise of the manufacturer. Moreover, there is no consensus regarding the type of drivetrain that is best suited for all project sites in all geographies.
Future outlook
Going forward, improvements in drivetrains are expected to emerge in tandem with the overall growth in the wind turbine sector. New developments may include single-stage gearboxes, direct drive systems and permanent magnet generators.
The advancements in direct-drive magnets and generator designs for wind turbines have resulted in low-cost and lighter wind turbines. The cost of permanent magnets used in direct-drive turbines has also decreased which has further increased the prospect of direct-drive turbines.
Further, considering a long-term view over the entire project life cycle, wind turbines and their critical components such as drivetrains must be designed in a manner so as to optimise O&M costs. Thus, the focus of wind turbine manufacturers is to also decrease the complexity of conventional drivetrains, as in many cases these manufacturers are in charge of the O&M of wind plants. All these factors are leading to rapid innovations in the drivetrain space as the race to design the most optimal, efficient and lighter drivetrain continues.
While digitisation of the drivetrain technology is still at a nascent stage, it is critical to investigate how it can be used to address challenges in drivetrain systems. Also, it is necessary to address typical faults with gearboxes to make them more efficient. Furthermore, advanced operations and maintenance techniques with data analytics and artificial intelligence-enabled tools can help improve the cost efficiency of wind energy plants.
