Direct-Drive Wind Turbines

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A Different Drive Train Design That Eliminates The Gearbox Between A Turbine’s Rotor And Generator Is Attracting Wind Turbine Manufacturers In The Quest For Higher Power Output, Increased Offshore Reliability, And Potential Cost Savings Over The System’s Lifetime. 
The majestic, low-speed rotation of wind turbine blades, our usual first sight of a wind farm, is largely dictated by laws of physics—the need to limit detrimentally high blade tip speeds at blade lengths of 50 m or more. However, to produce electricity, traditional wind turbine design must introduce a gearbox (or speed incremental) between the turbine’s rotor, spinning at 4-20 rpm, and the standard generator that requires higher rotary speed. Gear ratios are on the order of 100:1 or more. A newer technology for utility-scale wind turbines eliminates the gearbox by use of a low-speed, permanent magnet generator, as the term "direct drive" suggests. Besides simplifying the turbine’s drive train, direct drive (DD) offers decreased system weight, potentially higher reliability due to fewer parts, and reduced maintenance. A growing number of wind turbine manufacturers produce DD machines, with current focus on higher power ratings and offshore applications. DD and geared wind turbines differ little from the controls viewpoint. Either approach employs pitch control of individual blades and yaw control of the nacelle relative to wind direction. Generator output is regulated to voltage and frequency requirements of the grid by the power converter and transformer. The turbine controller handles system functions and interacts with a supervisory control used in multiple turbine wind farms.

Direct drive wind generator

Virtually all direct drive wind turbine vendors also offer geared turbines, which are expected to remain competitive in the foreseeable future due to their large installed base and excellent overall performance. (See table of manufacturers, online.) Ultimate application sectors for geared and DD wind turbines will be determined by further operating experience and system cost.

“Direct-drive technology has an advantage that increases with size. Hence, it is likely that geared turbines will remain competitive at low power values,” said Henrik Stiesdal, CTO of Siemens Wind Power. Meanwhile, vendors are quite tight-lipped about comparative power generation costs and system reliability figures for the two design approaches.

No gearbox = 12 tons less !!!

Eliminating the gearbox, the heaviest element in the turbine nacelle, provides a definite weight advantage.

“Using the same 101-m rotor, the nacelle of Siemens’ 3.0 megawatt (MW) direct-drive turbine is 12 metric tons lighter than Siemens’ 2.3 MW geared turbine,” Stiesdal noted. The direct drive wind turbine (SWT-3.0-101) is shown in the Siemens photo. Gearbox reliability is a concern for the industry. The high dynamic load environment and number of moving parts require proactive monitoring and maintenance. While some recent gearbox failures have been reported, wind turbine manufacturers stand firmly behind the product’s reliability.

Direct drive generator

“Siemens geared turbines are currently setting the [reliability] benchmark for the wind industry, and it is not easy to improve on this,” explained Stiesdal. “However, in principle the direct-drive turbine with its much lower number of moving parts and 50% less components should offer yet a small nudge upwards on reliability.”

Some gain in drive-train efficiency comes from removing the gearbox. In addition, turbine nacelle length becomes noticeably shorter. One indication of efficiency gain and other benefits comes from MagnetDrives AG of Switzerland in a presentation at Motor Summit 2010 in Zurich. (For more on Motor Summit, see online reference 2.)

Dr. Stefan Berchten, principal of MagnetDrives, compared 2 MW gearless and geared wind turbine designs, showing five metric tons less weight and 94% versus 92.4% system efficiency in favor of DD. For an additional 5% direct drive project investment, Berchten forecasted substantial savings in maintenance cost, energy, and total cost over a seven-year payback period.

Low-speed generator
A different generator design is needed to produce electricity at low rotary speeds. Permanent magnet (PM) synchronous generators provide that need most efficiently and, for direct drive, they take a “doughnut” configuration rather than the cylindrical shape of traditional generators. A substantially larger diameter generator is necessary to increase the effective rotary motion of the PMs relative to the stator coils so that the required high torques can be developed.

Hub connection

The Switch Controls & Converters Inc., a provider of megawatt-class PM generator and full-power converter packages, noted the advantageous efficiency of PM synchronous generators for DD turbines. Importantly, PM generator efficiency remains high, close to nominal value, even at partial loads—where turbines must often operate due to wind inconsistency, explained Anders Troedson, vice president and GM of The Switch. For example, a generator rated 2.2 MW at 18 rpm (see photo) has full-load efficiency of 94.4%, while at 25% load its efficiency is a remarkable 92.9%. Similar values, even slightly higher 25% load efficiencies, apply to the company’s other DD generator models.

In addition, PM generators eliminate the need for separate excitation, slip rings, and rotor windings with associated losses, and require less m

aintenance compared to double-fed induction generators (DFIGs). While the most numerous generators in service, DFIGs aren’t applicable to DD operation because of power factor restrictions and lack of fault ride-through capability, among other reasons. Over a 20-year turbine lifespan, PM generators are reportedly also more cost effective.

“Direct-drive generator design requires special magnet shapes and arrangements to match specific wind conditions and optimize efficiency,” said Troedson. Another feature is a patented stator construction with several independent segments that are controllable by different converters—which translates to drive redundancy. “The turbine may even remain operable in case of minor faults in one of the segments,” he stated.

As for other generator types, “the separately excited synchronous generator is a viable but less efficient type of generator, which can be used for direct drive,” added Troedson. “Largest user of this technology is German wind turbine manufacturer, Enercon. In addition to low-speed generators, The Switch offers medium- and high-speed units optimized to work efficiently and reliably with its full-power converters.

More power offshore, onshore

Intended for higher power output, including offshore applications, DD generators rely on rare-earth PM materials—typically neodymium iron boron (Nd-Fe-B). However, technology offers no free lunches. A trade-off for eliminating the gearbox is the need for large quantities of these costly magnet materials, which are becoming subject to supply shortages. Approximately 650 kg of PMs is needed per MW wind turbine capacity, according to Siemens, of which 25%-30% is rare-earth magnet material.

“The cost of these materials is accounted for in the competitive price of the direct-drive concept.” Stiesdal noted. Recent strategic sourcing issues for rare-earth PMs have caused user companies to consider alternatives.

“Generators currently offered use permanent magnets with neodymium and dysprosium elements,” added Stiesdal. “A variant design magnetized with a system not using rare-earth magnets will be available in case of shortage of these rare-earth elements.” (See more on supply and cost issues of rare-earth magnets in an online article extension.)

Growing power output of DD wind turbines is reflected in recent introductions and announcements. GE Energy, reportedly the leading producer of wind turbines in the U.S. and the second worldwide, introduced a 4-MW turbine in March 2011, optimized for offshore use. Design of the 4.1-113 direct drive turbine is said to have built-in redundancy and partial operation capability for its major components. DD technology focuses on “keeping turbines operating reliably at sea…and relies on a modular approach to maximize in-situ repair and reduce the need for large repair vessels,” according to the company.

The 4.1-113 has rated wind speed of 14 m/s (31.3 mph) and 113 m rotor diameter. It builds on the evolution of GE’s 3.5 MW DD turbine, and its base design that has operated since 2005 in the high wind speed and high turbulence environment of a Norwegian coastal site. The turbine is going to Sweden’s Göteborg Energi for installation in Gothenburg harbor in the second half of 2011.

Offshore direct drive

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.

Siemens Energy and Alstom are among other companies that have announced large DD wind turbines. In June 2011, Siemens installed a prototype of its latest offshore turbine with DD rotor technology at a Danish site. Trial operation has started for the 6 MW machine with a 120-m rotor diameter.

Meanwhile, Alstom has begun manufacture of its prototype 6 MW offshore turbine also featuring direct drive technology (see photo). The first prototype machine is to be installed before year-end 2011 on the west coast of France. Both Siemens and Alstom expect their wind turbines to reach serial production in 2014, according to the companies. (These developments are covered further online.)

In contrast, Denmark’s Vestas—reputed to have built the most wind turbines—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.

Not for all applications

While higher power is now the “sweet spot” for DD wind turbines, sheer physical size can impose mechanical constraints.

“Several challenges arise as we approach 8 to 10 MW rating with direct drive,” said Troedson. “The generator grows rapidly larger and heavier, due to high torque requirements.” Besides design limits such as material strength, manufacturing tolerances, and bearing requirements, Troedson mentioned the following constraints:

• Generator weight and size—must be limited to avoid upgrading the tower structure to handle the higher nacelle weight
• Air gap design—to handle flux density requirements and prevent rotor-stator rubbing under all operating conditions
• Dynamic short circuit forces—must be accommodated within limits of the minimum air gap.

Some of these factors can become too dominant and affect generator choice, Troedson explained. “A single-stage gearbox enables a much smaller, lighter generator and still offers some of the same advantages as a direct-driven generator,” he stated. “Therefore we see some of our customers for very large generators favoring a single-stage gearbox over direct drive.”

In view of the above, Troedson emphasized the need for generator and turbine designers to work together to optimize the entire turbine design, rather than a particular component.

Exciting times appear to lie ahead for wind turbine technology.