NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Contract No. DE-AC36-08GO28308

Boulder Wind Power Advanced 

Gearless Drivetrain

Cooperative Research and Development 

Final Report

CRADA Number: CRD-12-00463

NREL Technical Contact: Jason Cotrell

CRADA Report

NREL/TP-7A10-57183

April 2013
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1 

Cooperative Research and Development Final Report

In accordance with Requirements set forth in Article XI.A(3) of the CRADA document, this document is 

the final CRADA report, including a list of Subject Inventions, to be forwarded to the Office of Science 

and Technical Information as part of the commitment to the public to demonstrate results of federally 

funded research.

CRADA Number: CRD-12-00463

CRADA Title: Boulder Wind Power Advanced Gearless Drivetrain

Parties to the Agreement: Boulder Wind Power

Joint Work Statement Funding Table showing DOE commitment: 

Estimated Costs NREL Shared Resources

Year 1 $ 253,000.00

Year 2 $ 00.00

Year 3 $ 00.00

TOTALS $ 253,000.00

Abstract of CRADA work: 

The Boulder Wind Power (BWP) Advanced Gearless Drivetrain Project explored the application of BWP’s 

innovative, axial-gap, air-core, permanent-magnet direct-drive generator in offshore wind turbines. The

objective of this CRADA is to assess the benefits that result from reduced towerhead mass of BWP’s 

technology when used in 6 MW offshore turbines installed on a monopile or a floating spar foundation.

Summary of Research Results: 

In general, the reduced tower-top mass contributed to a reduction of extreme loads on the 

substructure, which is estimated in potential mass savings of 5-15% (for both monopile and spar-buoy 

cases). This result assumes a fixed design philosophy (based on modal approach) and the adopted 

geometrical constraints. Results may vary if these assumptions change. In particular for the spar, for a 

given heel angle equal to that of the baseline case, the BWP system draft could potentially be reduced 

up to 10%, with additional savings in material and cost, but only if a larger heave displacement can be 

accepted, or mitigated via a new mooring design.

Extreme Loads on the rotor and blades are slightly lower for the floating cases compared to the monpile, 

possibly due to the reduced stiffness of the platform in pitch; however, the towers are significantly 

‘overloaded’ in bending due to the additional gravity effect associated with platform heel when 

compared to the monopile conditions.

Subject Inventions Listing: None

Report Date: 04/06/12 Responsible Technical Contact at Alliance/NREL: Jason Cotrell

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