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More OEMs Taking the Direct (Drive) Approach for Wind Turbines

by Jack McGuinn, Senior Editor

Siemens 6MW offshore turbines will use gearless direct-drive technology

Siemens 6 MW offshore turbines will use gearless direct-drive technology.

Anyone familiar with the workings of wind turbines knows that when one of these behemoths stops working—look to the gearbox as the source. More accurately, it is typically the bearings within the gearbox that fail, in turn gumming up the gearbox, but that's a story for another time. And so the overriding challenge for designers and builders of wind turbines is reliability coupled with reasonable lifetime and maintenance schedules. This is especially true for offshore installations, which are beginning to proliferate off coastlines around the world, as the following reports suggest.

With existing, gearbox-driven wind turbines, the rotor and blades are connected to the gearbox and generator. The gearbox converts the turning speed of the blades to one sufficient for enabling the generator to generate electricity. Given the often extreme conditions to which turbines are exposed—which are even more extreme for offshore installations—the gearbox is severely stressed and, eventually, rendered inoperable. The gearbox is easily the most maintenance-needy component in a turbine. Common sense therefore dictates—eliminate the gearbox, eliminate the problem.

Ergo, direct drive technology is making its move.

Following is a digest of recent news stories chronicling the developments and increasing market share of direct drive wind turbines. But for openers and comparison purposes, here are brief descriptions of gearbox-driven and direct drive systems for turbines.

First up—the gearbox-driven turbine:

With gearbox-driven wind turbines, its blades rotate a shaft linked via the gearbox to the generator. This enables the gearbox to convert the rotational speed of the blades, typically up to 20 rotations-per-minute for, say, a 1 MW turbine, into roughly 1,800 rotations-per-minute—i.e., the speed required for enabling the generator to produce electricity.

As for the direct drive approach, here is a description supplied by Germany-based Enercon on its website (enercon.com):

"The drive system of modern wind energy converters is based on a simple principle: fewer rotating components reduce mechanical stress and at the same time increase the technical service life of the equipment. Maintenance and service costs for the wind turbine are lower (fewer wearing parts, no gear oil change, etc.) and operating expenses are reduced.

"The rotor hub and annular generator are directly connected to each other as a fixed unit without gears. The rotor unit is mounted on a fixed axle, the so-called axle pin. Compared to conventional geared systems that have a large number of bearing points in a moving drive train, the direct drive system has only two slow-moving roller bearings. The reason for this is the low speed of the direct drive.

"The annular generator is of primary importance in the gearless system design. Combined with the rotor hub it provides an almost frictionless flow of energy, while the gentle running of fewer moving components guarantees minimal material wear. Unlike conventional asynchronous generators, the annular generator is subjected to minimal mechanical wear, which makes it ideal for particularly heavy loads and a long service life. Time-consuming repair work and the associated down periods are thus prevented."

So how pervasive is direct drive becoming vis-à-vis gearbox power? Judging by the following, quite a bit.

Andy Wickless of Navigant (navigant.com) reports that "Through strategic acquisitions, some Chinese OEMs have quickly gained access to formidable R&D capabilities, particularly in the area of direct-drive technology. While two of the top three direct-drive turbine manufacturers in 2010 were Chinese, global power leaders like GE and Siemens are rapidly testing and deploying gearless turbines of their own. Whether Chinese OEMs can maintain their position as leaders in the growing direct-drive segment will depend primarily on potential supply chain constraints for critical components and renewable energy targets set by the Chinese government."

At windpowerengineering.com, Paul Dvorak's July, 2011 story reports that "More (wind turbine) competition is on the way. Venture capital firm New Enterprise Associates is backing a Boulder, CO-based startup called Boulder Wind Power, which is developing a 1.5 MW direct-drive turbine. The firm was founded in December by Sandy Butterfield, who was chief engineer for the U.S. National Renewable Energy Laboratory's (NREL) wind technology center, where he led a major study of the gearbox design process."

The site also reports that "At the end of March (2011), GE announced it had entered into a roughly $3.2 billion agreement to acquire some 90 percent of the equity in Converteam from a controlling shareholder group. GE's announcement followed a similar move from American Superconductor Corp. (AMSC), which has agreed to acquire Finland-based The Switch Engineering Oy, a $265 million acquisition. AMSC said it expects The Switch's background in advanced synchronous generators to help commercialize superconductor generators for its direct drive 10MW Sea Titan turbine. GE also recently announced an investment of € 340 million for plants in Germany, Norway, Sweden, and the U.K. to produce 4-MW gearless wind turbines for offshore use in 2012. The company boosted its DD technology expertise with the acquisition of ScanWind of Norway in 2009."

And greencongress.com reports that "Siemens Energy launched its new 6-megawatt (MW) direct-drive wind turbine at the European Offshore Wind Energy Conference in Amsterdam, The Netherlands. The new SWT-6.0 will be available with rotor diameters of 120 and 154 meters and is designed for the most challenging offshore sites. The new turbine features the Siemens direct drive design with 50 percent fewer parts than comparable geared wind turbines, along with reduced infrastructure, installation and service costs, and a boost in lifetime energy output and profitability, Siemens claims.

"Since installing the first offshore wind power plant in 1991, Siemens has successfully installed more than 700 offshore wind turbines with a combined capacity of more than 2,000 MW in European waters. In offshore wind power alone, Siemens has an order backlog of more than 1,100 offshore wind turbines with a total capacity of more than 4,000 MW.

"Meanwhile," the site reports, the French energy conglomerate Alstom "has begun manufacture of its prototype 6 MW offshore turbine, also featuring direct drive technology…both Siemens and Alstom expect their wind turbines to reach serial production in 2014, according to the companies."

And it just gets better for Siemens, as renewableenergyfocus.com's Isabella Kaminski reports.

"Siemens will supply 17 direct drive turbines to Eneco Wind U.K. for the Lochluichart Wind Farm in the Scottish Highlands. According to Siemens, this is the U.K.'s largest order for direct drive turbines, and at 51 MW, is the largest U.K. investment for Eneco Wind U.K., a subsidiary of Dutch company Eneco."

But there do exist drawbacks to direct drive; for one, the uncertainty over the cost and availability of rare earth elements, a necessity for direct drive permanent magnets.

As Navigant's Wickless reports, "All of the emerging direct-drive concepts on the market include permanent magnet generators. While it may not be immediately evident, this could serve as a competitive advantage for Chinese OEMs, at least in the near term. In 2010, the Chinese mining industry accounted for more than 95 percent of the global supply of rare earth metals, including neodymium, a rare earth metal and the preferred material for permanent magnets."

Another reality is that the initial capital outlay for direct drive systems is another issue giving some OEMs second thoughts. Direct-drive generators currently cost more than geared systems and are 15 to 20 percent heavier. They require much more robust generators, for instance-and pricey motors.

Also, bloomberg.com reported recently that "Denmark-based Vestas is offering a traditional turbine design that employs gearboxes to convert the slow rotation of blades into faster revolutions to drive an electrical generator. That approach may attract financing more easily, analysts in London at Commerzbank AG (CBK) and Barclays Capital said.

"'It's the devil you know rather than the devil you don't know,' Ben Lynch at Commerzbank said in a telephone interview. 'Banks are going to want to reduce the risk, and technology is one of those risks. At the moment they'll favor the known technology, which is the Vestas route.'" The report also says that "Vestas has chosen a geared design for its prototype 7 MW offshore turbine announced earlier in 2011. However, prototype production of the machine with 164-m rotor diameter is not expected until 4Q 2012."

To close, we posed a few questions regarding direct drive to Ryan Greenfield, industry manager/windpower for Schaeffler Group USA, Inc. and George Holling, technical director for Montana-based Rocky Mountain Technologies.

We asked, for example, about the cost issue.

"Rare earth magnets are a very expensive commodity due to the majority ownership by China," Greenfield says. "These are usually in larger MW-class sizes and require much larger generators due to the elimination of the gearbox and stiffer, larger bearings."

Holling explains it this way, citing torque, RPM and material issues.

"The size of an electrical machine (generator) is determined by its torque rating (magnetic volume) and the output power is determined by its T*RPM. Thus, for a 4:1 gearbox the machine runs at 4x the ROPM and ¼ the torque, thus the magnetic volume of the geared is 1/4(roughly and we can argue about specific examples) of that of a direct drive, which translates into 1/4 magnets, copper, lamination steel etc., depending on the technology used."

One of the overriding complaints about gearbox-driven turbines is their maintenance needs—both scheduled and unscheduled. So what about direct drive?

"Some in the industry also believe that turbines have electrical issues and software problems that are harder to isolate and fix," says Greenfield. "Without condition monitoring and preventive maintenance, the gearbox has to be taken down via crane. (But) direct drive systems also would need a crane for main bearings and larger generators."

"There really should be none except, maybe, for bearing maintenance, and in certain cases brush replacements, although (the need for) brushes is disappearing if they have not already done so," says Holling.

And what about bearings?

"Typically, they are more expensive and are larger" for wind turbines, says Greenfield.

Adds Holling, "The RPMs are the same as the input side of the gear, but the torque is higher. Fortunately, this does not necessarily translate into higher side loading, but it requires larger-diameter shafts. So, in theory there will be little advantage on the input side, but the RPMs are lower than the output side of the gear and those bearings have been eliminated. In general, lower-speed bearings are typically more reliable than higher RPMs bearings."

Finally, revisiting the issue of investor confidence in direct drive over traditional gearbox technology, "We have not dealt with banks and they always play it safe," says Holling. "But as the installed base grows with positive showcases there will, over time, be less resistance as the technology becomes accepted. Other tech investors are typically more willing to takes a modest risk—especially if you play the reliability and up-time card."