Energy Overview

Gearbox vs. Direct Drive

In traditional gearbox-operated wind turbines, the blades spin a shaft that is connected through a gearbox to the generator. The gearbox converts the turning speed of the blades—15 to 20 rotations per minute for a 1 MW turbine—into about 1,800 rotations per minute that the generator needs to generate electricity.

The multiple wheels and bearings in a gearbox suffer tremendous stress because of wind turbulence and any defect in a single component can bring the turbine to a halt. This makes the gearbox the highest-maintenance part of a turbine. Gearboxes in offshore turbines, which face faster wind speeds, are even more vulnerable than those in onshore turbines.

Removing the gearbox from the wind turbine eliminates the technically most complicated part of the machine, therefore improving reliability. But the downside to using direct drive instead of a gearbox system has been twofold: cost and weight. Until recently, a wind turbine system that incorporated a gearbox was less expensive than a wind turbine that did not because of the cost savings that came from having a higher-speed generator, Felker said.

However, over the last two years, direct-drive machines have been demonstrated to not necessarily be heavier or more expensive than geared systems, said Siemens’ Stiesdal. This fluctuation has been caused by two technological advancements: the cost of the permanenelectrical power. Windmills are used for their mechanical power, windpumps for water pumping, and sails to propel ships. Wind power as an alternative to fossil fuels, is plentiful, renewable, widely distributed, clean, produces no greenhouse gas emissions during operation, and uses little land. The net effects on the envirt magnets used in direct drives has declined.