Wind Turbines: Direct Drive vs. Gearbox

Direct Drive

For years, wind turbine manufacturers have been searching for ways to make direct drive turbines competitive with gearbox turbines. Direct drive technology has been praised for its design, which is less complex than gearbox technology, leading to easier operations and maintenance. This appeal has made direct drive especially coveted for use in offshore wind developments. However, the stigma of a bigger price tag and a heftier machine had disqualified direct drive from being a major competitor in the wind turbine sector – until recently. In the last two years direct drive undergone a number of developments, including changes in direct drive magnets and generator arrangements, that have led to a lighter, more affordable direct drive model.

Gearbox

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.

GE has incorporated a 6-meter diameter permanent magnet generator in its 4 MW direct drive units. The two main bearings transfer axial and bending loads from the rotor to the bedplate for a higher reliability than the previous generator.

Siemens has also made a change in regards to the generator. In the past, direct-drive machine rotors have been located inside the system. Siemens is now inverting the machine so the rotor is on the outside.  As a benchmark, Siemens plans to release its 6 MW direct-drive prototype later this year, which will be one of the largest permanent magnet machines ever built. It could be a game-changing machine when it becomes available in bulk numbers four to five years from now. The 6 MW machine is expected to be popular with offshore wind farms due to its robustness and lower-maintenance requirements.

The direct drive machines were designed for a 20-year lifespan and to be serviced twice a year, most older models are serviced annually. (Power Gen Worldwide, 3/1/2011)