Wind turbines – Direct drive options challenge gearboxes

At the end of last year, near the Danish headquarters of Siemens Wind Power, a new prototype 3 MW wind turbine started to generate power. Its signifi cance came from the fact that for the first time, the world’s sixth largest wind turbines manufacturer had chosen to dispense with a gearbox.

Could this represent a shift towards gearless direct drive technology across the wind energy industry?

The concept of direct drive is not new, but its attractions have been emphasised in recent years by two factors. One is the issue of gearbox reliability, with a number of prominent failures in one of the most expensive components in a wind turbine.

The other is the development of new, cheaper and more efficient permanent magnet generators (PMGs), ideally suited to direct drive systems.

More than 85% of grid-integrated wind turbines currently sold worldwide still use a gearbox to convert the relatively slow rotational speed of the rotor into the high speed required to drive a generator and produce grid-compatible electricity.

In a direct drive configuration, on the other hand, there is no gearbox and the rotor is directly connected to a low speed multi-pole generator which rotates at the same speed.

These generators are traditionally heavy and bulky, hence the distinctive bulbous shape of the nacelle in wind turbines such as those produced by German manufacturer Enercon.

Enercon has employed direct drive technology virtually from the company’s inception, and its wind turbines have been successfully installed around the world in their thousands.

The company uses a traditional synchronous generator rather than a PMG, and has developed direct drive systems for models up to a capacity of 6 MW.

But although a number of new entrants to the wind power market have attempted to follow a similar route, none have achieved anything approaching the same level of commercial success. Most of the other large wind turbine suppliers have stuck rigidly to the gearbox.

What has altered the direct drive landscape is the emergence of a new generation of permanent magnet generators, says Peter Jamieson, Principle Engineer at GL Garrad Hassan.

High strength rare earth magnets used in PMGs have significantly reduced in price, he explains. This has helped make the PMG comparable in weight to, or lighter than, the Enercon type design, in which a wound rotor is excited to create a magnetic field.

PMGs also have the advantage that they operate more efficiently when the wind turbine is working at partial load. This is in addition to the underlying bonus of direct drive that it avoids the necessity of having a gearbox.

“It’s a secure hard fact that if you don’t have a component you don’t have any maintenance or failures associated with it,” says Jamieson.

This is particularly significant when wind turbines are placed far out at sea, where maintenance costs are higher. All this has encouraged one of the largest new Chinese wind turbine manufacturers to follow the direct drive route.

Using technology adopted from the German company Vensys, both of the large wind turbine models manufactured by Goldwind – of 1.2 and 1.5 MW capacity – use direct drive with PMGs. So will its 2.5 MW model when it reaches serial production. The company was the second largest wind farm supplier in China in 2008, and has just seen its first wind turbines installed in the United States.

The major issue with direct drive continues to be the weight it will potentially add to the platform at the top of a wind turbine’s tower – and the cost involved.

“As a broad rule of thumb, manufacturers talk about an additional capital cost of about 10% for using direct drive,” says Jamieson. “But there’s a lot of development going on to produce lighter designs, some by more innovative structural solutions, some by different confi gurations of the electrical machinery.”

This is where Siemens says it has made a breakthrough. “We have managed to solve the weight problem to such an extent that the nacelle of the new 3 MW DD has a lower weight than our standard 2.3 MW nacelle,” says Henrik Stiesdal, Chief Technology Officer of Siemens Wind Power.

Siemens started seriously investigating direct drive options several years ago. “The obvious advantage is a simplified design,” says Stiesdal.

“With fewer moving parts direct drive technology has the potential to reduce maintenance costs, which could result in higher wind turbine availability. “The main drawback of a classical geared wind turbine is the complexity of the gearbox – a typical gearbox for a large wind turbine has three phases, two planetary phases and one helical phase, with a total of 13 gears and pinions and 22 bearings.

To ensure trouble-free performance for 20 years for such equipment, complex supporting systems are required, including a sophisticated oil conditioning system that maintains clean, cool and water-free lubrication at all times.

“Siemens has been producing geared wind turbines since 1980 (originally under the brand name Bonus) and despite occasional issues with gearboxes the company has always been happy with the technology. However, we also realised that for large wind turbines direct drive generators might in serial production become competitive with geared solutions. If this were to be the case then the simplicity and robustness of the direct drive technology would have a decisive advantage offshore.”

By 2008 the company’s engineers had come up with two alternative “proof-of-concept” 3.6 MW designs, using two different types of generator.

Research on these designs has now been translated into the 3 MW DD commercial machine, scheduled to be launched on the market later this year.

Siemens says that it opted for this size in the end because its development work showed that a direct drive concept would be commercially viable for the “high end, high volume” market, not just offshore. “We expect that the 3 MW DD wind turbine will prove competitive with our 2.3 MW volume wind turbine series using geared technology,” says Stiesdal.

The new 3 MW design will therefore be available in both onshore and offshore wind farm versions. As importantly, according to Stiesdal, it will be competitive with other wind turbines on the market.

If it does prove a success, this will add further weight to the forecast by Danish consultancy BTM Consult, in its latest supply chain report, that direct drive could increase its share of the wind energy market from the current 12-13% to 20% by 2013.

For more information:

By Crispin Aubrey,