Power Engineering

Direct Drive vs. Gearbox: Progress on Both Fronts

Six Northern Power 100 kW direct drive wind turbines produce power for Unalakleet, Alaska. The turbines are owned by Unalakleet Valley Electric Cooperative. Photo courtesy STG Inc.

Will the wind turbine technology showdown leave just one technology standing? 

By Lindsay Morris, Associate Editor

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, that is.

Henrik Stiesdal, chief technology officer of Siemens Wind Power, said that in the last two years direct drive “has gone from one thing to something else.” A number of developments, including changes in direct drive magnets and generator arrangements, have led to a lighter, more affordable direct drive model.

The nacelle on a Siemens direct-drive turbine. Photo courtesy Siemens AG.

Dr. Fort Felker, director of the National Wind Technology Center at the U.S. Department of Energy’s (DOE) National Renewable Energy Laboratory (NREL), said the industry perception a year or two ago was that direct drive machines were more expensive than gearboxes. “Both Siemens and (China-based) Goldwind are demonstrating that that’s not necessarily the case.”

Make Consulting, a wind energy consulting business, reported that in 2009, 15 percent of turbines were made with direct drive. In 2012, that number is expected to double, equaling almost an 80 percent increase in direct drive technology over a three-year span.

Does this mean that gearbox technology is becoming a thing of the past? Not necessarily, Felker said. With thousands of gearboxes currently in operation, this machinery will not be retired overnight. Wind turbine manufacturers continue to make strides in gearbox technology that lead to fewer outages and increased reliability.

Gearbox vs. Direct Drive

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.

Until recently, a wind turbine system that incorporated a gearbox was less expensive than a wind turbine that did not because of the cost savings that came from having a higher-speed generator, Felker said.

However, over the last two years, direct-drive machines have been demonstrated to not necessarily be heavier or more expensive than geared systems, said Siemens’ Stiesdal. This fluctuation has been caused by two technological advancements: the cost of the permanent magnets used in direct drives has declined significantly and the arrangement of the generator has become more streamlined.

The conversion of direct drive from coil-driven to permanent magnet systems has been paramount in driving down costs. Not only are the magnetic systems lighter, but they also require less copper, which has experienced a cost increase since early 2010. The reduction of weight seen by direct-drive machines is actually leading to a decrease in costs, especially in terms of manufacturing costs, said Parthiv Amin, president of the community wind business at Northern Power Systems.

Changes in the generator have also helped make direct-drive systems more affordable. Northern Power Systems, for example, has implemented a more efficient transition in its systems from the permanent magnet generator through the full power converter and onto the grid. Losses through power conversion with Northern Power’s 100 kW wind turbine are less than 2 percent, Amin said.

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.

“On our machine, the rotor is only a thin-walled pipe with permanent magnets,” Stiesdal said. “What is outside the air gap is only 20 millimeters of magnet and 50 millimeters of steel pipe.”

Felker said that developments made by wind turbine manufacturers are charging the market for direct-drive machines. “Clearly there’s been some real progress made or you wouldn’t see so many direct drive machines sold.”

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.

“We expect that to be a game-changing machine when it becomes available in bulk numbers four to five years from now,” Stiesdal said. The 6 MW machine is expected to be popular with offshore wind farms due to its robustness and lower-maintenance requirements, Stiesdal said. “Ten years from now, my estimate is that all the offshore machines from 6 MW and up will be fitted with direct drive.”

GE has found success at its offshore test site in Hundhammerfiellet, Norway, where the first ScanWind direct drive unit has been operating for more than five years. In 2005, GE installed 13 of the units at the test site. Those turbines have accumulated the equivalent of 50 years’ experience under some challenging conditions, said Milissa Rocker, global communications manager – wind for GE Power & Water.

Scanwind (recently acquired by GE) wind turbines operating on the Norwegian coast. Photo courtesy GE.

The success of these test units has led to the development of GE’s next generation wind turbine, a 4 MW machine that is the largest wind turbine in GE’s fleet. This turbine will incorporate advanced drive train and control technologies gained through GE’s acquisition of ScanWind.

Direct-drive systems are also being used onshore. Working in conjunction with local utilities to improve the cost effectiveness of powering remote communities, Northern Power Systems began deploying direct-drive systems in Alaskan communities near the Bering Sea in 2001. “The communities were not mechanically trained to fix equipment like gearboxes, so the challenge was making it maintenance-free,” said Amin.

Decreased maintenance is one reason to use direct drive machines instead of gearboxes in remote communities, Amin said. While most gearboxes require maintenance such as oil changes every six months, Amin said a Northern Power direct drive system is not expected to require “any maintenance beyond annual inspection and lubrication in its first five years.”

The company now has 39 100 kW permanent magnet direct-drive turbines operating in several remote communities in Alaska. The median availability of the company’s turbines installed worldwide is 98.7 percent, Amin said, while the availability of a traditional wind farm is 93 to 95 percent. Northern Power Systems has sold over 200 direct drive machines in Europe and the U.S.

While improvements have been made to models that are hitting the market, more testing is being done to further improve direct drive. NREL hosts a dynamometer facility in which a generator is tested to perform under different wind conditions. Currently, Northern Power Systems is testing a 2.2 MW generator at the facility.

MathWorks, a developer of software solutions for model-based design for engineers and scientists, is also finding ways to improve direct drive systems. MathWorks tools branded Simulink and Matlab can model a particular control system to adapt to the changing direction and speed of the wind. Through generating code and building what the company calls real-time prototypes, MathWorks presents an alternative route for implementing direct-drive design.

“These products allow you to build a system-level design of your turbine system, a multi-domain view to control the algorithms,” said Tony Lennon, marketing manager.

This software allows for simulation of direct drives that can lead to more accurate power conversion. “Because the systems are so expensive, anything you can do to make the right size choice, you’re saving a lot of money in the simulation process,” Lennon said.

Gearbox Breakthroughs

Although strides are being made in direct-drive machines, advancements in gearbox technology are also taking place, particularly aimed at decreasing the frequency of gearbox failures. Gearbox failures have “deviled the industry for some time now,” Felker said, and it is difficult and expensive to exchange a gearbox, requiring a crane to remove and exchange it. Many older machines contain gearboxes that were installed with a fraction of the current knowledge that is now available on gearboxes, Felker said. Some of those systems will not last their full 20-year design life.

Siemens, however, prides itself on the reliability of its older gearbox turbines. From 1983 to 1989, Siemens supplied 1,106 turbines to the U.S. During a review in 2009, 1,045 or 96 percent that could be accessed were still operating, Stiesdal said. The rest had mainly been taken down due to non-technical issues. Of the 1,045 operating turbines, the mean age was 23 years.

And while the machines were designed for a 20-year lifespan and to be serviced twice a year, most older models are serviced annually. “They had grown more robust with time,” he said.

Testing by the Gearbox Reliability Collaborative at NREL is providing new insight on how to design and operate gearboxes that will last even longer. The collaborative tests instrumented gearboxes to identify weaknesses in design and seeks to find ways to improve initial designs and retrofit packages. The project identifies equipment failures that are common throughout the industry and addresses deficiencies in the design process contributing to these issues.

In general, Felker said, most gearbox failures do not begin as gear failures or gear-tooth design deficiencies. The failures seen by the collaborative appear to start at several specific bearing locations. These initial failures may later advance into the gear teeth as bearing debris. Excess clearances that result can lead to surface wear and misalignments.

Addressing these problems is leading to greater gearbox reliability, Felker said. Machines that are coming into service today have “tremendous reliability” and are up and running “virtually all the time,” Felker said. “Availability is in the high 98 to 99 percentile.”

Gearboxes are undergoing a number of improvements, but are likely to face pressures from falling direct drive prices in the years to come.

“Gearbox technology will not completely disappear, but its rotating parts will have to be improved as the cost curve in direct drive continues to be driven down,” said Amin of Northern Power Systems.

For the time being, improvements in direct drive and gearboxes are not forcing each other into extinction. Instead, they appear to be working to make each other stronger.

“The improvements in direct drive are going to push gearbox technology to be more reliable,” Felker said. “The industry is making progress on both fronts.”

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