Last month, Siemens Wind Power started field testing one of two direct-drive wind turbines it has had under development since 2005. The company acknowledges the step is "a potential game changer" for the entire industry. The second machine is to be erected this autumn at the same site as the first in west Denmark, where permission has also been granted for a third turbine.
The purpose of the test program is to establish whether direct drive technology can be competitive with conventional drivetrains with gearboxes. Both test turbines are equipped with permanent magnet excited generators (PMGs), though from two separate suppliers, Siemens' industrial division and British company Converteam. They are among the largest ever built, says the company. Siemens is testing the generators over the next two years on adapted versions of its commercial 3.6 MW turbine. The recently erected turbine is located in a 13 metre long steel canopy mounted on a 90 metre tower. The PMG measures 5.5 metres in diameter and weighs 70 tonnes. The total tower head weight is more than 150 tonnes.
Although described as a prototype, the first turbine has been sold to a customer and will remain in place after the test period is over, says project leader Henrik Stiesdal from Siemens Wind Power, previously Bonus Energy. He explains that the main advantage of direct drive turbines is the simplicity that results from dispensing with a gearbox. Its job in a conventional turbine is to convert the high-torque, low speed rotation of the wind turbine's rotor into the low-torque, high speed rotation of the output shaft connected to a high speed generator.
As wind turbines have grown bigger, gearboxes have had difficulty withstanding the loads placed upon them. Large offshore turbines have been particularly troubled by gearbox failure in recent years.
Stiesdal confirms that the demands of the offshore market for large series of wind turbines that require a minimum of maintenance have prompted Siemens to examine whether a simpler technology in which a low speed, multi-pole generator is directly turned by the rotor at the same speed -- nine to 13 revolutions a minute -- might be the best option for lower cost of energy over the long term.
The drawbacks of direct drive machines are their extra weight, which makes them expensive, and the limited number of suppliers for low speed generators, which are much larger than conventional high speed generators, particularly when they need to be of offshore quality.
Neither Bonus, which first looked at direct drive turbines in 1999, nor Siemens, which took part in Norway's experiments with the ScanWind 3 MW project in 2003 (page 40) found their first tests with the technology convincing enough to take it further. But things have changed since.
Permanent magnet generators are now cheaper than they were in 1999 and Siemens believes their weight can be reduced in the design process. Even so, wind turbines without gearboxes remain more expensive than their conventional counterparts. That extra cost, however, could be offset by lower maintenance costs, particularly offshore, where servicing turbines is far more expensive. Unlike in the Enercon turbine, Siemens' generator is a completely sealed unit fully protected from damp, salty air.
Stiesdal describes direct drive PMG turbines as "an exciting technological option." But even though test-bed operation has validated the theory behind them, it will take two years of field testing for Siemens to complete its assessment. Until then it is too early to conclude "whether they will in the end prove to be a competitive alternative to geared turbines," says Stiesdal.