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NREL Next Generation DrivetrainMechanical Design and Test Plan

Christopher Halse, Roma

Jon Keller, National Renewable E

The information contained in this poster is subject to a government license.

2014 American Wind Energy Association (AWEA) Windpower Conference and Exhibition • Las Vegas, Nevada • May 5-8, 2014 • NREL/PO-5000-61599

The U.S. Department of Energy and industry partners are sponsoring a $3-million project for design and testing of a next generation wind turbinedrivetrain at the National Renewable Energy Laboratory (NREL). This posterfocuses on innovative aspects of the gearbox design, completed as part of anend-to-end systems engineering approach incorporating technology advancesthat minimize capital costs and increase drivetrain reliability, efficiency, andtorque density.The gearbox features a unique combination of hydrodynamic journalbearings used in the rotating planet pins and “flex-pins” for the planet gearsupport. Because rolling-element planet bearings are typically the life-critical

part of a gearbox, journal bearings that perform effectively reduce thegearbox size, decreasing weight and increasing life. Wind turbine operatingloads, however, are challenging for journal bearings and the risks associatedwith their use at low speeds and frequent starts-stops will be the subject ofdynamometer testing. The gearbox utilizes four planets to strike a balance

between achievable ratio and torque density. The flex-pins allow each planetto deflect a small amount circumferentially to permit equal load share. Thisremoves the “oversizing” that is required in conventional designs to accountfor unequal load share caused by manufacturing variability.

Justification of the gearbox design is presented, followed by a description ofthe large-scale prototype and the test program that will be used for simulatingrealistic operating conditions in NREL’s 2.5-MW dynamometer.

Drivetrain capital costs were estimatedfor the single-stage, medium-speedconfiguration and compared to othercommon configurations. Both directdrive and traditional three-stagegearboxes with induction generatorconfigurations had higher capital costs.The levelized cost of energy was alsoestimated for each configuration. Theexpected maintenance costs of asingle-stage gearbox are much lowerthan those of a three-stageconfiguration, resulting in the lowestcost of energy.

Efficiency is much improved over three-stage doubly fed drivetrains becausefewer gear meshes are generating heat. The medium-voltage approachallows for lower currents, which reduce losses. The silicon carbide switchesoperate more efficiently than typical silicon switches. The test article will beadapted from an earlier prototype built for a 2005 WindPACT project. Themain bearing and generator will be reused.

Photo by Lee Jay 

Fingersh, NREL 29199

CAD of the drivetrain showing the fo

The gearbox design utilizes flexpins to support the planets. Thesehave been used in Romax andother designs. Additional parts andinterference fits are added to thegearbox, but the benefit is inequalizing the load share betweenthe planets. The flex pins arecarefully designed to bend in amanner to prevent misalignment ofthe tooth flanks.Miba is supplying the journal bearings andcarrying out elasto-hydrodynamic simulation ofthe oil film. The particular concern is with start-upand low-speed operation. These operatingregimes will be tested thoroughly in thedynamometer.

A further focus of the dynamometertesting will be on transient electricalloading of the drivetrain with grid-faultconditions. NREL’s controllable gridinterface enables this testing.

Consortium Team:

Bill Erdman, Doug Blodgett (DNV)

Dave Grider (CREE)

William Gobeli (GE)

Janne Ukonsaari (Vattenfall)

The prototype drivetrain will be tested in NREL’s 2.5-MW dyn