1. Introduction
The gearbox is the second most expensive component in a wind turbine after the blades, its importance is self-evident. However, among the major components—blades, main bearings, gearbox, generator, and converter—the gearbox is also the most criticized. Because the gearbox is the only indispensable component in a wind turbine, direct drive technology, which eliminates the gearbox, has become another mainstream technological approach. In recent years, the rise of medium-speed transmission technology has reignited interest in gearboxes. Having experienced adoption, removal, and re-adoption, what will the future hold for gearboxes? It's worth discussing this in detail.
2. Gearbox in the Evolution of Technological Routes
2.1 From high-speed gearboxes to gearless gearboxes
The introduction of gearboxes into wind turbines is to match the characteristics of low-speed impellers and high-speed generators. From early fixed-axis gear trains to later combinations of planetary and fixed-axis gear trains, and then to multi-stage planetary gear trains; rated power has increased from hundreds of kilowatts to more than ten megawatts; and transmission ratios have increased from less than 100 to nearly 200.
However, gearboxes have also been the biggest problem in wind turbines. They have experienced the highest failure rates, dragged down turbine manufacturers, and been repeatedly targeted for complete elimination. This led to the development of alternative solutions, such as eliminating gearboxes from wind turbines and developing gearless direct drive technology and products. Just as direct drive turbines were gradually seen as the future replacements for high-speed doubly-fed turbines due to their unique advantages, their cost and efficiency disadvantages slowly became apparent, especially with the increasing size of the turbines.
2.2 From high-speed gearboxes and gearless gearboxes to medium-speed gearboxes
The disadvantages of direct drive technology have led to a shift in focus towards medium-speed transmission technology, which offers the advantage of lower gearbox speeds. Consequently, many turbine manufacturers that previously pursued high-speed doubly-fed and direct drive technologies have turned to medium-speed transmission. With the increasing size of offshore wind turbines, medium-speed transmission technology appears to be the current trend and the future direction, enjoying even greater popularity. Furthermore, numerous concepts have been packaged and enriched to describe medium-speed transmission technology, leading to what some have described as a golden age for medium-speed transmission, a scene of flourishing innovation. Against this backdrop, wind turbine gearboxes are gaining attention comparable to permanent magnet generators during the heyday of direct drive, becoming a new focal point.
3. Gearboxes in the trend towards larger sizes
The reason why early gearboxes often failed was not only due to the operating conditions of the wind turbine units, but also due to the inherent characteristics of the gearboxes themselves.
Gearboxes are mechanical components that transmit power, specifically divided into those that transmit motion (speeding up or slowing down) and those that transmit power (torque). The combined force of power transmission and the frequency of motion transmission make contact fatigue a fundamental failure mode, occurring in the two main gearbox components: gears (tooth surfaces) and bearings (raceways and rolling elements). Therefore, the basic causes of gearbox failure are twofold: excessive load and insufficient load-bearing capacity. Common issues in wind turbine gearboxes, such as pitting of tooth surfaces and rolling elements, broken gear teeth, and rolling bearing failure, are all related to this. Technological advancements in gearboxes have primarily focused on reducing load and increasing load-bearing capacity.
The increasing size of wind turbine generators necessitates larger gearboxes. The new challenges facing the scaling up of wind turbine gearboxes include the following issues:
The increase in transmitted power—the power transmitted by the gearbox is always higher than the rated power of the unit, and this difference becomes increasingly pronounced as the unit capacity increases. Although gearboxes capable of transmitting this level of power can be manufactured, the increasingly heavy engine nose brings more new challenges to the entire system, especially for offshore units.
Decreasing speed and increasing torque – Typically, larger turbines operate at lower rotor speeds, resulting in greater torque despite increased power output, which places higher demands on the gearbox's load-bearing capacity. This leads to a higher weight increase in the gearbox. Extended design life – Offshore wind turbines are designed for a lifespan of at least 25 years, requiring higher load-bearing capacity from the sequentially meshing gear teeth and the sequentially rolling bearings.
Increased support requirements – In wind turbines, the gearbox transmits torque while the outer casing needs to be supported by a reaction torque, thus increasing the requirements for support capacity.
Higher reliability requirements – offshore wind turbines have high reliability requirements, which in turn require even higher reliability from gearboxes. However, the loads and load-bearing capacity that affect reliability have not been effectively addressed.
In conclusion, whether the golden age of medium-speed transmission technology is coming does not depend on the variety of medium-speed transmission models, but rather on the gearbox. The gearbox will be a key factor in the future scaling up of wind turbine units; one could even say that without a viable gearbox, medium-speed transmission technology will not succeed.
As we all know, in the process of increasing the size of wind turbines, more and more large wind turbine components are becoming bottlenecks. A weakness in any component—blades, main bearings, or gearboxes—will hinder the scaling up of turbines and the development of offshore wind power. Given this, how far can we go by continuing down the conventional technological path? Furthermore, is the gearbox a necessary component for the scaling up of wind turbines?
In fact, since the rise of offshore wind power, domestic and international counterparts have not only followed conventional technological routes but have also been committed to developing new wind power equipment to meet the future needs of offshore wind power. If we return to the essential characteristics of wind power equipment, the most promising new wind power equipment is the one that can capture wind energy to the maximum extent and convert it into electricity in the most economical way. A natural question then arises: Is there a technology that eliminates the limitations of blades, main bearings, and gearboxes in achieving these fundamental goals?
There must be someone trying to solve this problem.
4. Conclusion
The gearbox in a wind turbine has a unique role—it acts as a bridge, transmitting motion and power. It's important when the turbine is included, but it can also be removed.
The increasing size of wind turbines places new and higher demands on large components such as blades, main bearings, and gearboxes. While the gearbox plays an increasingly important role, it will also gradually become one of the shortcomings of large-scale wind turbines.
Following the natural course of development, after the gearbox has gone through a process of existence, non-existence, and re-existence in wind turbine units, will it face a new selection in larger-scale units?
