This is an excerpt from the Third Quarter 2012 edition of the Wind Program Newsletter.

DOE Funds Advanced Magnet Lab and NREL to Develop Next-Generation Drivetrains

Investing in next generation drivetrains can help lower the cost and improve the reliability of wind turbines, particularly in larger offshore applications. This includes both improving current drivetrain configurations, as well as creating innovative drivetrain designs. For these reasons, the U.S. Department of Energy (DOE) recently awarded Advanced Magnet Lab (AML) in Palm Bay, Florida, and the National Renewable Energy Laboratory (NREL) in Golden, Colorado, additional funding to continue work in developing their proposed next-generation drivetrains.

AML — Smaller and Lighter Direct-Drive Generators for Large Wind Turbines

AML and its partners (Emerson Electric Corporation, Creare Inc., DNV USA, and DOE's Argonne National Laboratory) are developing a 10-megawatt (MW) direct-drive fully superconducting generator for use in next-generation wind turbines. As wind turbines continue to increase in size, particularly in the offshore market, AML's drivetrain concept has the potential to out-perform competing concepts, ultimately reducing the cost of wind energy.

Key potential advantages of the AML direct-drive generator include improved scalability, reduced weight, and coils that are free of rare-earth materials. AML's design does not require a gearbox, which may lead to improved reliability and reduced maintenance costs. While this may also be true for contemporary, gearbox-free direct-drive generator designs, the AML generator makes a magnetic field using superconducting windings that are more powerful and compact than copper-based alternatives. They are also constructed of more readily available and lower-cost materials than permanent-magnet-based generators, which are sensitive to cost fluctuations in the volatile rare-earth magnet market. In addition, AML calculates that its generator will weigh up to 50% less than a comparable permanent-magnet rare-earth generator with a 10-MW power rating. A lower generator mass has major system benefits, including a lighter—and thus less expensive—tower, and reduced installation costs through the use of smaller cranes and offshore vessels.

To reduce commercial risk and validate the key technical assumptions made during the design of the generator in Phase I of the project, AML will use the Phase II funding awarded by DOE in July 2012 to build and test the generator's key subcomponents.

NREL — Next-Generation Drivetrain

Building on its role as a world-class DOE research facility, NREL and its industry partners (BEW Engineering, Brad Foote Gear Works, Clipper Windpower, CREE, Danotek Motion Technologies, McCleer Power, DOE's Oak Ridge National Laboratory, QuesTek, Romax Technology, Texas Tech University, and Vestas) have developed an innovative drivetrain system designed to increase reliability, decrease mass, improve efficiency, and reduce costs. In addition, NREL's concept for a medium-speed drivetrain will facilitate the scaling of generator design up to ratings as high as 10 MW while maintaining the lowest possible cost.

NREL's concept takes a system approach to improving the conventional wind turbine drivetrain design, focusing on all three of its major components: a single-stage gearbox, a medium-speed permanent-magnet generator, and high-efficiency power electronics. Traditional three-stage high-speed gearbox designs have been plagued with reliability issues caused by the large and unpredictable loads imparted on the gears and bearings from the wind acting on the rotor. The NREL design eliminates the last two stages of the traditional gearbox (the lower-reliability, higher-speed stages); uses a more compliant gear system of flex-pins and journal bearings in the remaining low-speed planetary stage that improves load distribution and increases the overall reliability; and is constructed from premium steels, which increases capacity. This single-stage gearbox connects to a medium-speed generator that uses a fraction of the rare-earth magnets used in a standard gearbox-free direct-drive permanent-magnet generator of similar power.

An illustration comparing a conventional Drivetrain to a medium-speed drivetrain. The different parts of the drivetrains are represented by cylinders of different size and color. The generator, represented by a red cylinder, is much larger in the medium-speed drivetrain and the medium-speed drivetrain has fewer parts. The medium-speed has one single-stage gearbox and the conventional has a three-stage gearbox.

This illustration compares a medium-speed drivetrain to a conventional drivetrain. NREL's medium-speed drivetrain is designed to increase reliability, improve efficiency, and reduce costs.
Source: NREL

Additionally, the generator operates at medium voltage, rather than the traditional low-voltage designs, which reduces cooling system requirements and the copper mass and cost of the power cables running down the tower.  Efficiency improvements in the power electronics are derived from advanced materials and improved circuit design. These combined innovations result in increased reliability, capacity, and efficiency; and thus, more energy generation and a lower cost of energy.

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