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

This report is available at no cost from the National Renewable Energy

Laboratory (NREL) at www.nrel.gov/publications.

Contract No. DE-AC36-08GO28308

An Assessment of U.S. 

Manufacturing Capability for 

Next-Generation Wind Turbine 

Drivetrains

J. Cotrell and T. Stehly

Technical Report

NREL/TP-5000-58909

September 2013
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

This report is available at no cost from the National Renewable Energy

Laboratory (NREL) at www.nrel.gov/publications.

Contract No. DE-AC36-08GO28308

National Renewable Energy Laboratory

15013 Denver West Parkway

Golden, CO 80401

303-275-3000 • www.nrel.gov

An Assessment of U.S. 

Manufacturing Capability for 

Next-Generation Wind Turbine 

Drivetrains 

J. Cotrell and T. Stehly

Prepared under Task No. WE11.0620

Technical Report

NREL/TP-5000-58909

September 2013
NOTICE

This report was prepared as an account of work sponsored by an agency of the United States government. 

Neither the United States government nor any agency thereof, nor any of their employees, makes any warranty, 

express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of 

any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately 

owned rights. Reference herein to any specific commercial product, process, or service by trade name, 

trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, 

or favoring by the United States government or any agency thereof. The views and opinions of authors 

expressed herein do not necessarily state or reflect those of the United States government or any agency thereof.

This report is available at no cost from the National Renewable Energy

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Cover Photos: (left to right) photo by Pat Corkery, NREL 16416, photo from SunEdison, NREL 17423, photo by Pat Corkery, NREL 

16560, photo by Dennis Schroeder, NREL 17613, photo by Dean Armstrong, NREL 17436, photo by Pat Corkery, NREL 17721.

 Printed on paper containing at least 50% wastepaper, including 10% post consumer waste.
4 

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Acknowledgments

The authors would like to thank the U.S. Department of Energy Office of Energy Efficiency and 

Renewable Energy, Wind and Water Power Technologies Office for making this project 

possible, with special thanks to Cash Fitzpatrick for his support and guidance. The authors would 

also like to acknowledge input from many wind industry members and laboratory colleagues 

who provided essential information and perspectives for this report.
5 

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Executive Summary

Project Objectives

Robust U.S. wind turbine manufacturing capabilities and supply chains are important for the 

country to reduce the cost of electricity generated from wind turbines; are necessary for the

invention and commercialization of new wind turbine technologies; and provide high-quality 

jobs. The development of advanced drivetrain technologies for wind turbine applications—such 

as medium-speed and direct-drive generators, silicon-carbide (SiC) insulated gate bipolar 

transistor (IGBT)-based power electronics, and high-torque-density speed increasersis 

advancing the state of the art for drivetrain design by producing higher capacity and operating 

reliability than conventional drivetrains. However, these advanced technologies require different 

manufacturing and supply chain capabilities that present both risks and opportunities for U.S. 

wind turbine manufacturers and the wind industry as a whole. 

The primary objective of this project was to assess how advanced drivetrain technologies and 

trends will affect the U.S. wind turbine manufacturing industry and its supply chains. The U.S. 

Department of Energy (DOE) and other industry participants will use the information from this 

study to identify domestic manufacturing gaps, barriers, and opportunities for developing U.S. 

wind turbine manufacturing capabilities and supply chains for next-generation drivetrain 

technologies. This report also includes recommendations to help prioritize technology areas for 

possible investments by public, private, or nonprofit entities, with the intention of reducing the 

levelized cost of wind-generated electricity.

Project Approach

National Renewable Energy Laboratory (NREL) researchers performed literature surveys, expert 

interviews, and manufacturing facility visits to obtain the information and industry perspectives 

needed to inform the study. NREL also drew upon previous experiences with DOE and industryfunded drivetrain projects to compile a list of technologies for consideration and evaluation. The 

compiled list contained approximately 80 different drivetrain technologies. NREL engineers then 

identified three key metrics to assess the technologies, and selected 13 of the highest scoring 

advanced drivetrain technologies to evaluate in more detail (see Figure ES1).

The project scope was further narrowed by eliminating several technologies that received a 

particularly low score in one or more of the three metrics. For example, superconducting 

generators were eliminated from the technologies list because of the particularly low 

technological maturity score. The remaining technology categories were then consolidated into 

three categories for additional study. 
6 

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Figure ES1. Out of 80 drivetrain technologies identified, NREL researchers 

reduced the list to 13 using three key metrics

Key Findings and Recommendations

Figure ES2 shows the three main drivetrain technology categories selected (generators, speed 

increasers, and electrical systems) and the authors’ overall qualitative scores in each key metric. 

The red and orange colors indicate a stronger rational for focusing investments in the designated 

technology area. The first metric, Potential Reduction for Levelized Cost of Energy (LCOE), 

describes the relative potential impact of a drivetrain technology to reduce the LCOE through 

performance and reliability improvements or capital cost reductions. The second metric, 

Manufacturing Disruption, is an indication of the amount of change in the drivetrain 

manufacturing and supply chain that would be caused by implementing the advanced 

technology. The third metric, Existing U.S. Manufacturing Activity and Research and 

Development (R&D), represents the existing U.S. manufacturing and R&D activity and supply 

chain capabilities for the technology. An example of this third metric can be illustrated by 

examining the scores for the technologies considered in Figure ES2. The authors found no 

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7 

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companies performing serial manufacturing of permanent-magnet (PM) or medium-voltage 

(MV) wind turbine generators in the United States (although several companies have the 

capabilities for this manufacturing), and limited PM generator R&D resulting in an assessment of 

little or no manufacturing or R&D activity (red). In contrast, domestic manufacturers reported 

that they have substantially more manufacturing capacity for large wind turbine gearboxes [up to 

roughly 9 gigawatts (GW) per year capacity for new wind turbine gearboxes], thereby indicating 

moderate manufacturing and R&D activity (yellow). The extent of manufacturing activity for 

full SiC switches was also deemed moderate because of the R&D activities conducted by other 

industries, such as transportation and defense. 

Figure ES2. A matrix of qualitative scores summarizes the study results and 

helps prioritize investments in the three drivetrain technology areas evaluated

Overall, the authors identified the medium-voltage and permanent-magnet generators drivetrain 

category as the highest priority for possible investments. This category is perceived to have 

moderate potential for LCOE reductions, and will disrupt current manufacturing activities by 

requiring new technologies, process capabilities, and manufacturing facilities. In addition, there 

is little or no manufacturing and R&D activity in the country for this category compared to the 

other technology categories.

According to the study results, high-torque speed increaser technologies (such as the use of 

journal bearings or flex pins in the planetary stages of gearboxes) is also a promising technology 

area because it has moderate potential for reducing the cost of energy. However, manufacturing 

Metric 1 Metric 2 Metric 3

Potential Reduction 

for LCOE

Manufacturing 

Disruption

Existing U.S. 

Manufacturing 

Activity and R&D

Recommended 

Priority for 

Drivetrain 

Investments

Generator: MV & 

PM Technologies First Priority

Speed Increaser: 

High Torque 

Technologies

Second Priority

Electrical Systems: 

Full SiC MV 

Switches

Second Priority

Legend

High Potential High Disruption Little or No Manf. Activity and R&D

Medium Potential Medium Disruption Low Manf. Activity and R&D

Low Potential Low Disruption Moderate Manf. Activity and R&D

No Potential No Disruption High Manf. Activity and R&D
8 

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disruption is expected to be small and the existing U.S research and development activity is 

larger than for medium- and low-speed permanent-magnet generators. As a result, additional 

investments in high-torque gearbox manufacturing are expected to affect the gearbox supply 

chain less than investments in medium-voltage and permanent-magnet generator technologies. 

Full SiC switch technology, which uses both a SiC switch and SiC diode, is also a promising 

area for investment although presently expensive to manufacture. The survey of subject matter 

experts indicated that full SiC switches have the highest potential for LCOE reduction. A leading 

factor that contributes to the technology being a lower priority recommendation than mediumvoltage and PM generators is that several other industries (especially transportation and defense) 

are already funding R&D in this area. The authors expect that further investments will have a 

smaller marginal benefit than the other technology areas because of diminishing returns on 

additional investments.

Table ES1 contains specific recommendations that will foster supply chain development and 

LCOE reductions in the three advanced drivetrain technology areas explored. In addition to these 

recommendations, NREL identified a list of key findings from the project that merit emphasis. 

These findings include the following: 

• A stable U.S. market for wind energy is necessary for drivetrain manufacturing and supply 

chain growth, but it may not be sufficient alone for this growth. Additional investments may 

be required to compete with the extensive investments in wind turbine drivetrain 

manufacturing capabilities in regions such as Europe and Asia made over the last decade.

• The U.S. wind energy supply chain and manufacturing capabilities are central to the health of 

the U.S. wind turbine innovation ecosystem. 

• Generally, drivetrain supply chain investments that incentivize smaller, risk-tolerant 

companies to collaborate with large, risk-adverse companies in technology development and 

commercialization offer one of the most promising paths to facilitating innovation and 

strengthening U.S. manufacturing capabilities and supply chains. 

• Supply chain and manufacturing status, projections, and interrelationships are dynamic and 

complex, thereby making generalizations difficult. Therefore, continued study, monitoring, 

and stakeholder input are recommended to best inform investment decisions over time.
9 

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at www.nrel.gov/publications.

Table ES1. Summary of Recommendations for Possible Investments in Advanced 

Wind Turbine Drivetrain Manufacturing and Supply Chains

Technology Recommendations

Generators: 

Medium Voltage 

and Permanent 

Magnet 

1. Fund advanced manufacturing process R&D and equipment investments for mediumspeed and direct-drive permanent-magnet generator technologies, such as automated, 

high-density coil-winding processes and automated magnet magnetizing and placement 

2. Continue to facilitate commercialization paths for generator innovations via competitive 

technology solicitations that target collaborations between universities, small companies, 

and global original equipment manufacturers (OEMs) and target OEM installation of 

advanced generator technologies in new wind turbines

3. Strengthen the generator industrial commons (the R&D, engineering, and manufacturing 

capabilities needed to turn inventions into commercial products) by: 

• Supporting the development of an institute for manufacturing innovation that focuses 

on permanent-magnet motors and generators 

• Issuing drivetrain technology development solicitations

• Supporting wind-related magnet R&D at universities and encouraging the development 

of generator design and analysis professionals through wind turbine manufacturing 

fellowships

• Performing and publishing more detailed U.S. wind turbine drivetrain competitiveness 

analysis.

Speed Increasers: 

High Torque 

1. Continue to facilitate commercialization paths for gearbox innovations via competitive 

technology solicitations that target collaborations between universities, small U.S. 

gearbox technology companies, global OEMs, and gearbox refurbishers

2. Strengthen the U.S. gearbox industry commons and innovation ecosystem by: 

• Issuing additional advanced drivetrain solicitations that target high-risk innovation

• Funding activities like the Gearbox Reliability Collaborative at higher power ratings

• Supporting university programs that develop gearbox design and analysis professionals

• Encouraging information exchange at events (e.g., a recurring drivetrain technology 

workshop)

3. Consider policies that increase or stabilize demand for gearboxes manufactured in the 

United States, such as requiring a minimum amount of domestic content for U.S.-installed 

wind turbines.

Electrical 

Systems: Full SiC 

Medium-Voltage

Switches

1. Facilitate commercialization paths for SiC technology innovations by incentivizing the 

demonstration of hybrid SiC power electronics in a wind turbine and funding 

collaborations between small U.S. SiC companies and global OEMs

2. Monitor full SiC and high-temperature SiC technologies to leverage opportunities and 

collaborate with other industries 

3. Provide funding to explore and document the power electronics supply chain in more 

detail, engage laboratory expertise in wind turbine power converter technologies, or 

develop the expertise, if necessary. 
10 

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at www.nrel.gov/publications.

Recommended Future Work

To the authors’ knowledge, this report is the first public study of its kind to assess the impact of 

advanced wind turbine drivetrain technologies on U.S. manufacturing and supply chains. 

Recommended work to be considered for the future that was not undertaken during the initial 

study includes: 

• Providing a more in-depth assessment of manufacturing and supply chain issues for power 

electronics

• Formally vetting the results of the initial study with industry stakeholders 

• Developing a strategic plan or vision for wind turbine manufacturing and supply chain 

activities in the United States.
11 

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at www.nrel.gov/publications.

Table of Contents

Acknowledgments ..................................................................................................................... 4 

Executive Summary.................................................................................................................... 5

1 Project Objectives................................................................................................................13

2 The Importance of U.S. Wind Turbine Manufacturing and Supply Chains .....................14

3 Identification and Selection of Drivetrain Technologies for Assessment ......................15

3.1 Identification and Evaluation of Advanced Drivetrain Technologies ........................15

3.2 Metrics for Analyzing the Drivetrain Technologies ...................................................17

3.3 Selection Method for Advanced Drivetrain Technologies ........................................18

4 Analysis of the Four General Drivetrain Categories.........................................................22

4.1 SiC and Medium-Voltage Power Electronics Analysis .............................................22

4.2 Rotor-Stretching Technologies Analysis ..................................................................24

4.3 Advanced Drivetrain Medium-Speed Gearbox Analysis ..........................................24

4.4 Advanced Drivetrain Permanent-Magnet Generator Analysis ................................. 30

5 Investment Prioritization and Summary of Recommendations .......................................34

6 Future Work ..........................................................................................................................37

References................................................................................................................................. 38

List of Figures

Figure ES1. Out of 80 drivetrain technologies identified, NREL researchers reduced the 

list to 13 using three key metrics.......................................................................................... 6

Figure ES2. A matrix of qualitative scores summarizes the study results and helps 

prioritize investments in the three drivetrain technology areas evaluated ...................... 7

Figure 1. Highest scoring drivetrain technologies resulting from the first iteration of 

quantitative ranking .............................................................................................................19

Figure 2. Highest scoring drivetrain technologies resulting from the second iteration of 

quantitative ranking .............................................................................................................20

Figure 3. A comparison of the approximate cost to manufacture and ship a 2-MW 

generator within Texas and from China and Europe to Texas ........................................31

Figure 4. The authors used a matrix of qualitative scores to summarize the results and 

help prioritize investments in the three drivetrain technology areas evaluated............34
12 

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List of Tables

Table ES1. Summary of Recommendations for Possible Investments in Advanced Wind 

Turbine Drivetrain Manufacturing and Supply Chains ....................................................... 9

Table 1. Two Categories of Advanced Drivetrain Technology Drivers: Primary and 

Additional..............................................................................................................................15

Table 2. Advanced Drivetrain Taxonomy................................................................................16

Table 2. (Continued) .................................................................................................................17

Table 3. General Categorization of the Selected Nine Drivetrain Technologies for Analysis21

Table 4. Wind Turbine Power Electronics Switch Technology Characteristic Comparison 

for Conventional, Advanced Hybrid, and Full SiC Switch Technologies........................23

Table 5. Overview of Medium-Speed Gearbox Technology Drivers.....................................25

Table 6. Summary of Medium-Speed Gearbox Technology Challenges .............................26

Table 7. Status and Capabilities of the U.S. New Wind Turbine Gearbox Manufacturing 

Industry ................................................................................................................................. 27

Table 8. U.S.-Based Tier 1 Gearbox Industry Assessment ...................................................28

Table 9. Status and Capabilities of the U.S. Refurbishment/Repair Wind Turbine Gearbox 

Manufacturing Industry .......................................................................................................29

Table 10. Known Capabilities of Large, U.S.-Based, Utility-Scale Wind Turbine Generator 

Manufacturers.......................................................................................................................32

Table 11. Known Capabilities and Status of Small and Midsized U.S.-Based Utility-Scale 

Wind Turbine Generator Manufacturers ............................................................................33

Table 12. Summary of Recommendations for Possible Investments in Advanced Wind 

Turbine Drivetrain Manufacturing and Supply Chains .....................................................36
13 

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1 Project Objectives

The development of advanced drivetrain technologies for wind turbine applications—such as 

medium-speed and direct-drive generators, silicon-carbide (SiC) insulated gate bipolar transistor 

(IGBT)-based power electronics, and high torque-density speed increasersis advancing the 

state of the art for drivetrain design by producing higher capacity and operating reliability than 

conventional drivetrains. These drivetrain technologies require different manufacturing and 

supply chain requirements and processeswhich present both risks and opportunities for U.S. 

wind turbine manufacturers and supply chains.

The primary objective of this project was to assess how advanced drivetrain technologies and 

trends will impact the U.S. wind turbine manufacturing industry and its supply chains. This 

information can be used to identify domestic manufacturing gaps, barriers, and opportunities for 

developing U.S. wind manufacturing capabilities and supply chains for next-generation 

drivetrain technologies. This report includes recommendations to help prioritize technology areas 

for possible investments by public, private, or nonprofit entities. 
14 

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at www.nrel.gov/publications.

2 The Importance of U.S. Wind Turbine Manufacturing

and Supply Chains

A supply chain can be defined as the network created amongst different companies for 

producing, handling, and distributing a specific product. A robust supply chain is at the heart of a 

successful manufacturing industry, and is an important part of reducing the cost of electricity 

generated from wind turbines, inventing and commercializing new wind turbine technologies, 

and providing high-quality jobs. 

Supply chains also affect technological innovation. As of June 2011, manufacturing companies 

are responsible for about 70% of industrial research and development (R&D) in the United States

and employ 63% of domestic scientists and engineers [1]. Therefore, a decrease in domestic 

manufacturing could have a direct effect on the innovative capacity of the nation. Locating 

supply chains and manufacturing facilities near design centers encourages essential 

communication between production and design. This communication facilitates innovation by 

enabling the person-to-person contact that is required to transfer the often tacit knowledge 

underlying emerging technologies [2]. 
15 

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3 Identification and Selection of Drivetrain 

Technologies for Assessment

3.1 Identification and Evaluation of Advanced Drivetrain 

Technologies

A wide variety of advanced drivetrain technologies exists at a range of technology readiness 

levels. For this report, the National Renewable Energy Laboratory (NREL) drew upon previous 

experiences with the U.S. Department of Energy (DOE) and industry-funded drivetrain projects 

to compile a list of technologies for consideration and evaluation. The majority of ideas on the 

list stemmed from a brainstorming session of six NREL staff members. To improve the 

comprehensiveness of the list, the authors also reviewed literature, such as DOE’s Advanced 

Wind Turbine Drivetrain Concepts Workshop Report [3].

The compiled list contained approximately 80 different advanced drivetrain technologies and the 

factors (or drivers) that motivate the development of these technologies. The importance of each 

driver (see Table 1) was determined using a ranking system that is described in Section 3.3. 

Table 1. Two Categories of Advanced Drivetrain Technology Drivers: 

Primary and Additional 

The authors organized the numerous drivetrain technologies by creating a drivetrain technology 

taxonomy with four categories: speed increasers, generators, electrical systems, and other 

technologies. The taxonomy list of advanced drivetrain technologies for each of the categories is 

shown in Table 2. 

Primary 

Drivers

•Reliability 

•Torque density [kiloNewton meters 

(kNm)/kilograms (kg)]

•Material availability 

Additional 

Drivers

•Power rating

•Reparability 

•Capital cost

•Loads reduction

•Efficiency/performance

•Transportation and installation/removal costs

•Intellectual property issues

•Wind market size for power electronics
16 

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Table 2. Advanced Drivetrain Taxonomy

•Gearboxes

•Multiple output gearboxes (i.e., multiduored)

•Single- or double-stage gearboxes

•Four-stage gearboxes

•Lubrication improvements

•Torque density improvements

•Journal bearings

•Flex pins

•Multiple planets

•New steels

•Coatings and finishes

•Integrated drivetrains (i.e., V90)

•Spindle or torque tube configurations 

(i.e., Alstom)

•Alternative teeth geometries

•Hydraulic drives

•Chain drives

•Continuously variable transmissions

•Toroidal

•Belts 

•Chains

•Friction 

Speed Increaser Technologies

•Medium voltage 

•Insulation materials

•Dielectrics 

•Nano-technologies 

•New materials (chemistries)

•Alternative generator architectures

•Axial flux generators

•Air core generators

•Ring generators

•Sectional generators

•Permanent-magnet technologies

•New materials (nano, dysprosium 

replacement) 

•New processes

•Nonhomogeneous magnets

•Laminations

•Conventional laminations

•Powder core

•High-flux density and low core 

losses 

•Transverse flux generators

•Reluctance generators

•Superconducting generators

•Magnetic gearbox

•Vernier generator

•Variable pole generator (stamping 

generator)

•Homo-polar generator

•Variable-speed synchronous generator

Generator Technologies
17 

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Table 2. (Continued)

3.2 Metrics for Analyzing the Drivetrain Technologies

The number of advanced drivetrain technologies identified required down-selection to achieve a 

manageable scope. The authors used a spreadsheet-based ranking system to aid in the selection 

of technologies that would most affect the supply chain. Technologies that received low scores 

overall were eliminated from further consideration. The metrics used to evaluate the advanced 

drivetrain technologies are as follows. 

•Power electronics

•Medium voltage (>1,000 volts)

•Full power conversion

•Silicon carbide (SiC) switches

•Silicon (Si)-SiC hybrid

•Full SiC

•Full SiC high temperature

•Power factor correction system

•Grid tie-in 

•Converter architecture

•Matrix converters

•Multilevel converters

•Resonant converters

•Transformer 

•Location uptower

•High-frequency transformers

•Superconducting transformers

•Collection system

•DC coupling of turbines

•AC 60 Hertz (Hz)

•AC variable frequency

•Flexible AC transmission system 

(FACTS) 

•Static VAR compensations

•Statcom

•Series compensations

Electrical Systems

•Alternative load path configurations

•Load control

• Mechanical load control

• Advanced load control

• Passive load control

• Active torque control using power 

electronics 

•Condition monitoring

•Self-erecting tower and nacelle 

technologies 

•Storage (short term and long term)

Other Technologies
18 

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Potential Reduction for Levelized Cost of Energy 

This first metric describes the potential impact of an individual drivetrain technology (relative to 

the other technologies listed) to reduce the overall levelized cost of energy (LCOE). 

Technologies with a low potential impact received low scores. 

Technology Maturity

Technologies that are less mature (a longer time period until commercialization) generally entail 

more risk. The expected value of the rate of return for investments in such technologies and the 

likelihood of commercialization are less. As a result, less mature technologies received a lower 

score for this metric.

Manufacturing Disruption

The third metric is a measure of the amount of change in manufacturing machinery, 

infrastructure, or supply chain if commercialized. Technologies that have more potential to 

disrupt domestic manufacturing activities received higher scores.

3.3 Selection Method for Advanced Drivetrain Technologies

3.3.1 First Iteration of Technology Down-Selection

The authors developed a numerical ranking system by assigning a numeric value to each of three 

metrics applied to the drivetrain technologies and then selecting the highest scored drivetrain 

technologies for further assessment. Roughly 20% of the highest scoring drivetrain technologies 

were selected to move forward with the analysis. The selected technologies and their scores are 

listed in Figure 1. 
19 

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Figure 1. Highest scoring drivetrain technologies resulting from the first iteration 

of quantitative ranking

3.3.2 Second Iteration of Technology Down-Selection

After the first iteration of technology down-selection, the number of technologies for 

consideration was still too large for the project scope; therefore, researchers implemented a 

second process for down-selecting the technologies. This process involved a more detailed 

investigation of each of the technologies shown in Figure 1 and was completed by evaluating the 

individual scores for each metric. If the score for any of the three metrics was a relatively low 

value, the technology was not considered for further analysis. One exception to this method was 

full SiC high-temperature switches, in which a low rating of technology maturity was not 

eliminated because it had the highest cumulative score relative to the other technologies. The 

technologies remaining after this second process of elimination are listed in Figure 2. 

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Quantitative Comparison of Top 20% Ranked Drivetrain Technologies

Potential Reduction for LCOE Technology Maturity Manufacturing Disruption
20 

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Figure 2. Highest scoring drivetrain technologies resulting from the second 

iteration of quantitative ranking

The selected technologies in Figure 2 were further condensed into four categories: permanentmagnet medium-voltage generators, medium-speed gearboxes, SiC power electronics, and rotorstretching technologies. Table 3 lists the final general categorization of the technologies. 

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Quantitative Comparison of Drivetrain Technologies for Analysis

Potential reduction for LCOE Technology Maturity Manufacturing Disruption
21 

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Table 3. General Categorization of the Selected Nine Drivetrain Technologies for 

Analysis

•Electrical systems: Full SiC switches

•Electrical systems: Full SiC high-temperature switches

SiC Medium-Voltage Power 

Electronics

•Speed increaser: Spindle or torque tube configurations

•Other technologies: Passive load controls

Rotor-Stretching Technologies

•Speed increaser: Single- or double-stage gearboxes

•Speed increaser: Torque density improvements

Medium-Speed Gearboxes

•Generator: Medium voltage

•Generator: Permanent-magnet technologies

•Electrical systems: Medium voltage (>1,000 volts) 

Permanent-Magnet Medium-Voltage 

Generators
22 

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4 Analysis of the Four General Drivetrain Categories

4.1 SiC and Medium-Voltage Power Electronics Analysis

Wind turbine power electronic switches can be categorized into three different 

categoriesconventional, hybrid, and full SiC power electronicsdepending on the 

composition of the switch and the junction diode. Conventional insulated-gate bipolar transistor 

(IGBT)-based power electronic switches use silicon (Si) switch and junction diodes, and are used 

in wind turbine electrical systems today.

Hybrid IGBT switches use a Si switch and SiC diode (Si-SiC), in which the advanced SiC 

junction barrier schottky (JBS) diode takes the place of the silicon-based planar positive doped 

and negative doped (p-n) junction diodes. Hybrid SiC, a type of wide-band gap power 

electronics, is available today in production ranges of tens to hundreds for engineering 

applications in research and development.

A third category of power electronicsreferred to as full SiC power switchesuses both a SiC 

switch and SiC diode. Full SiC power switches can be subdivided into low-temperature and 

high-temperature SiC. Low-temperature SiC is approximately 5 to 10 years away from 

commercialization and the time frame for high-temperature SiC commercialization is unknown. 

Switching losses dominate the efficiency of the inverter. SiC technologies provide efficiency 

increases by substituting the SiC junction barrier schottky diodes in place of Si-based planar p-n 

junction diodes. The SiC diode is a majority carrier device and exhibits minimal or no reverse 

recovery charge. By eliminating the reverse recovery charge from the diode, two loss 

mechanisms are significantly reduced. First, the turn-off energy associated with the diode is 

essentially eliminated. Equally important, the reverse recovery current, which must be carried by 

the commutating IGBT, is also essentially eliminated. The substitution of the SiC diode for the 

silicon p-n diode reduces the total loss of the converter by roughly 50% [4]. The performance of 

the JBS diode offers an ideal low- and medium-voltage alternative to conventional silicon p-n 

diodes because of minimal reverse recovery charge, positive temperature coefficient of forward 

voltage drop, low reverse leakage at elevated temperature, and robust reliability [5]. 

In general, SiC switches have many potential benefits for the next-generation wind turbine 

electrical power systems, including enabling higher power electronic and generator voltages, 

improving efficiency, and reducing the mass of the filters (“the magnetics”) used to compensate 

for harmonics caused by slow switching frequencies. The result is a substantial increase in

electrical power efficiencies because of the higher voltage and a reduction in electrical 

component mass and size (due to increased switching frequency). 

NREL provided approximate estimates for the possible mass reduction and converter size 

reduction for each switch technology compared to the conventional Si switch technology shown 

in Table 4. The NREL subject matter experts ranked SiC switch technology advancements with

very high scores, but high-temperature full SiC switches as having the highest overall score. In 

addition, NREL expects that the commercialization of this technology will substantially impact 

manufacturing and supply chains because of the new manufacturing processes and knowledge 

required to manufacture and package the switches.
23 

This report is available at no cost from the 

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at www.nrel.gov/publications.

Table 4. Wind Turbine Power Electronics Switch Technology Characteristic 

Comparison for Conventional, Advanced Hybrid, and Full SiC Switch 

Technologies

Switch 

Technology

Voltage 

(V)

Switching 

Frequency 

[kilohertz 

(kHz)] 

Magnetics (Filters) 

Mass Reduction 

(%)

Converter Size 

Reduction (%)

Technology 

Readiness

Si 1,700 3 - - Conventional 

Technology

Si-SiC (hybrid) 6,000 3 53 - Prototype

Full SiC LowTemperature10,000 20 87 33 5−10 years

Full SiC HighTemperature10,000 20 87 67 Undetermined

4.1.1 Specific Recommendations for SiC Switch and Medium-Voltage Power 

Electronics

The recommendations for SiC switches and medium-voltage power electronics are: 

• Facilitate commercialization paths for SiC technology innovations by incentivizing the 

demonstration of hybrid SiC power electronics in a wind turbine and funding collaborations 

between small U.S. SiC companies and global original equipment manufacturers (OEMs) 

• Monitor full SiC and high-temperature SiC technologies to leverage opportunities and 

collaborations with other industries

• Provide funding to explore and document the power electronics supply chain in more detail 

and engage laboratory expertise in wind turbine power converter technologies or develop the 

expertise, if necessary.

Full SiC switch technology has many benefits for power electronics for wind power generation. 

However, this technology is at a low technology readiness level. Incentivizing collaborations 

between smaller U.S. SiC switch companies and global OEMs with competitive technology 

development and demonstration solicitations will help establish commercialization paths 

between smaller, more risk-tolerant companies, and larger companies.

The use of SiC switch technology will benefit a variety of industries. For example, the 

automotive industry is interested in using this technology in electric vehicles for similar reasons 

that benefit wind turbines (e.g., power quality and small size), and because it could eliminate a 

separate coolant loop for the power electronics in hybrid vehicles. The wind industry may be 

able to take advantage of this broader interest. In addition, DOE’s recently announced financial 

opportunity for an Innovation Institute on Wide Bandgap Semiconductors could provide avenues 

for collaboration. The authors recommend monitoring these opportunities, especially for full SiC 

low- and high-temperature technologies. Furthermore, the power electronic manufacturing and 

supply chain should be explored more fully beyond the cursory survey performed in this study. 
24 

This report is available at no cost from the 

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at www.nrel.gov/publications.

4.2 Rotor-Stretching Technologies Analysis

Rotor-stretching technologies, such as advanced controls and passive load reduction techniques 

(e.g., flap-twist coupled blades), are not specific elements of the drivetrain. However, these 

technologies do affect the drivetrain by decreasing the amount of nontorque loads that are

transferred to the drivetrain. As a result, larger rotors can be used on the wind turbine that impart 

larger torque loads to the drivetrain. Because these loads are important design parameters for 

drivetrains, the NREL subject matter experts scored rotor-stretching technologies as having a 

significant indirect impact on wind turbine drivetrains. Accordingly, these technologies were 

considered for comprehensiveness and consistency. However, the authors believe that rotorstretching technologies will have little direct impact on the drivetrain supply chain. For this 

reason, the authors recommend focusing drivetrain supply chain investments on drivetrain 

component technologies to increase the torque density of the wind turbine drivetrain rather than 

on the rotor-stretching technologies.

4.3 Advanced Drivetrain Medium-Speed Gearbox Analysis

4.3.1 Medium-Speed Gearbox Technology Drivers

Although emerging direct-drive technologies, such as permanent-magnet and superconducting 

machines, may have the potential to reduce LCOE, the geared, medium-speed drivetrain solution 

avoids some of the risks associated with newer direct-drive technologies. An overview of the 

medium-speed technology drivers that motivate the use of medium-speed gearboxes are listed in

Table 5. 
25 

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Table 5. Overview of Medium-Speed Gearbox Technology Drivers

Medium-Speed Gearbox 

Technology Drivers Driver Explanation

Torque Density

A medium-speed drivetrain configuration is projected to have a 22% 

increase in torque density compared to a 5-megawatt (MW) highspeed gearbox configuration [4].

Reliability 

Gearboxes are a primary cause of wind turbine downtime and 

operation and maintenance expenses. Medium-speed gearboxes have 

fewer components than high-speed gearboxes, thereby leading to 

potential increases in reliability.

Material Availability

Medium-speed-geared drivetrains have a substantial reduction in rare 

earth elements compared to permanent-magnet direct-drive 

drivetrains.

Efficiency A medium-speed gearbox typically has a 1%−2% greater efficiency 

than a high-speed gearbox. 

4.3.2 Primary Technical Challenges and Medium-Speed Gearbox Technologies 

For medium-speed gearboxes (and large wind turbine gearboxes in general), there are three 

primary technical challenges: 1) reliability of bearings, 2) manufacturability of the large diameter 

ring and bull gears, and 3) ability to handle the higher torque loads generated by larger turbines. 

A summary of these challenges are shown in Table 6. To help mitigate these technology 

challenges, researchers are investigating advanced next-generation drivetrain technologies, such 

as gearbox flex pins, journal bearings, and premium steels. 
26 

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at www.nrel.gov/publications.

Table 6. Summary of Medium-Speed Gearbox Technology Challenges

Medium-Speed Gearbox 

Technology Challenges Challenge Description

Reliability 

There are a high number of failures associated with gearbox bearings. 

Failures in the gearbox are generally not repairable uptower and 

require special equipment to complete the repair.

Manufacturability

Medium-speed gearboxes typically have a large diameter ring gear 

(inside diameter tooth form) or bull gear (outside diameter tooth 

form), and are difficult to manufacture. These components are on the 

order of 2 to 3 meters and require extremely tight machining 

High Torque Density

As wind turbine power ratings continue to increase, gearbox torque 

requirements will continue to increase exponentially, as rotor tip 

speed is constrained. These large gear elements are heavy and 

difficult to manufacture. 

4.3.3 Capabilities and Status of the U.S. Gearbox Industry

NREL performed an assessment of the capabilities and status of the U.S. gearbox industry using 

the results of a literature review and by conducting industry interviews. Table 7 lists wind turbine 

gearbox manufacturing companies in the United States and their capacity to manufacture new 

gearboxes. Table 9 provides information on the gearbox refurbishment companies. 

4.3.3.1 New Gearbox Industry

According to the American Wind Energy Association, the United States installed 6.8 gigawatts 

(GW) of wind power in 2011 [6]. Assuming that 90% of wind turbines installed in the United 

States utilize a gearbox in the drivetrain, an approximate demand for new domestic wind turbine 

gearbox manufacturing is more than 6 GW per year. Accumulation of theoretical annual capacity 

for the U.S.-based gearbox manufacturers reflects a manufacturing and/or assembly capacity of 

9.5 GW annually. These results indicate ample U.S. gearbox manufacturing or assembly capacity 

(tier 1 suppliers), with the ability to support the majority of new wind plants installed 

domestically. However, the price of new gearboxes has dropped over the past few years by 

nearly 40%. In addition, large gearbox manufacturers are building new capacity in Brazil in 

order to meet domestic content requirements. There also appears to be no remaining independent 

U.S. suppliers providing gears and other components to the gearbox manufacturers and 

assemblers (tier 2 suppliers). Tier 2 suppliers cited two reasons for exiting the wind industry: 1) 

wind turbine market uncertainty, and 2) loss of their primary domestic customers (e.g., Clipper 

Windpower). 
27 

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Table 7. Status and Capabilities of the U.S. New Wind Turbine Gearbox 

Manufacturing Industry

Note: Gearbox manufacturers that have recently stopped producing new gearboxes for the wind industry 

are indicated by text that has been stricken through.

U.S.-Based New 

Gearbox 

Manufacturer

Tier 

Classification 

U.S. 

Manufacturing 

Facility Location

Product Size 

(MW) 

Annual Facility Capacity

(MW/Year)

Winergy Drive 

Systems Corporation

Tier 1 Elgin, Illinois Up to 6.5 600 MW in the United 

States [7] 

ZF Tier 1 Gainesville, 

Georgia 

Up to 6.2

(2 U.S.)

2,000 MW in the United 

States [8] 

(12,000 MW worldwide)

Brevini Tier 1 Muncie, Indiana Up to 4.5 

MW 

(depending 

on geometry)

4,500 MW in the United 

States [9] 

Moventas Tier 1 Portland, 

Oregon

Up to 2.0 Can assemble up to 2,400 

MW in the United States

[10] 

Clipper Windpower OEM/Tier 1 Cedar Rapids, 

Iowa

Up to 2.5 1,000 MW (in 2006) [11]

GE Drivetrain 

Technologies (a 

division of GE 

Transportation)

Tier 1 Erie, 

Pennsylvania

Up to 2.5 Estimated 2,000 MW

Brad Foote Tier 2 Cicero, Illinois Up to 5.8 2,000 MW

The authors expect domestic gearbox manufacturers/assemblers to have the highest probability 

of remaining engaged in the U.S. wind industry relative to manufacturers of other drivetrain 

components. Table 8 summarizes the rational for this assessment.
28 

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Table 8. U.S.-Based Tier 1 Gearbox Industry Assessment

U.S. Tier 1 Gearbox Industry 

Assessment 

U.S. Tier 1 Gearbox Industry Assessment Details

Ample Existing Manufacturing 

Capability 

The U.S. tier 1 gearbox suppliers have a theoretic capability to supply 

gearboxes for 9.5 GW of installed wind power. This leaves 2.7 GW of 

potential additional capacity when considering the 6.8 GW of total 

installed wind power in 2011. Gearbox prices have dropped and new 

capacity is being constructed in countries such as Brazil to meet 

domestic content requirements.

Access to Europe’s Industry 

Commons

The U.S.-based wind turbine gearbox industry has access to the wellestablished industry commons in Europe that provide cost-effective 

gearbox manufacturing and shipping. 

Access to Capital for Necessary 

Expansion

Most tier 1 gearbox suppliers are in a position for additional capital if 

manufacturing facility and tooling expansion is necessary. 

Barriers to Exit Many gearbox manufacturing companies have large stranded U.S. 

manufacturing assets or “trapped capacity.”

Domestic Logistical Advantages

A U.S.-based gearbox manufacturing facility mitigates the strict OEM 

gearbox shipping requirements (e.g., gearbox rotation while being 

shipped) and shipping costs (i.e., specialty storage containers on boat 

or rail). 

4.3.3.2 Refurbishment/Repair Gearbox Industry

Another important market to consider in the wind industry is the gearbox refurbishment/repair 

market. The knowledge and experience contained in the gearbox refurbishment/repair industry is 

a sizeable component of the gearbox industry commons. Additionally, the industry players in this 

market have the potential to expand into tier 1 suppliers or assemblers, as well as contribute to 

gearbox innovations and reliability improvements.

Gearboxes have a history of being one of the most problematic drivetrain components and 

typically require repairs before the end of their design life. This repair time frame, often between 

5−7 years of operation, creates a large and generally consistent refurbish/repair market for 

gearboxes. In fact, several gearbox tier 1 suppliers are capitalizing on this. The demand for 

gearbox refurbishment is relatively stable compared to new gearbox demand. A summary of the 

status and capabilities of the U.S. refurbishment/repair market is shown in Table 9. 
29 

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Table 9. Status and Capabilities of the U.S. Refurbishment/Repair Wind Turbine 

Gearbox Manufacturing Industry

U.S.-Based Gearbox 

Refurbishment/Repair 

Company

Wind Industry Work 

Description 

U.S. Gearbox 

Facility 

Location

Facility Capabilities and 

Capacity

Eickhoff • Gearbox inspection, repair, 

rebuilds, and overhauls

Pittsburgh, 

Pennsylvania

• Up to 2.5 MW gearboxes 

[12]

• Throughput capacity 

unknown

Gearbox Express • Remanufactured gearboxes

and down-tower services

Mukwonago, 

Wisconsin

• 45,000 sq ft facility

• 3.1 MW test stand

• Throughput capacity 

unknown [13]

Moventas • Overhaul and maintenance Portland, Oregon • Assemble 100 to 150 

gearboxes annually

• Turbines up to 2 MW [14] 

Winergy • Workshop service, repairs,

and upgrades

Elgin, Illinois -

Broadwind Energy 

Maintenance Service (EMS)

• Manufacture gearbox parts 

as needed for 

repairs/remanufacturing

Abilene, Texas • 500 MW of annual 

throughput capacity

• 3 MW full-load gearbox 

testing [15]

Bosch Rexroth Hydraulics • Gearbox repairs Lake Zurich, 

Illinois

• Repair capabilities of 100 

gearboxes annually

• Repair up to 3 MW gearbox

[16] 

ZF Services • Remanufacturing and parts 

distribution

• Complete repair of gearbox 

and main shaft

• Engineering upgrades and 

uptower services

Vernon Hills, 

Illinois

• 2.5-MW load test bench

• 200 annual gearbox repairs

[17] 

• 30-ton crane capacity

The Gear Works • Repair work Seattle, 

Washington

• Repair up to 1.5-MW 

gearbox

• Currently repair 100 

gearboxes/year

David Brown [16] • Gearbox service Kentucky [18] -

NextEra Generation Repair 

Services (GRS) [19] 

• Overhaul and repair

• Parts and supplies storage 

facility

Story City, Iowa -
30 

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at www.nrel.gov/publications.

4.3.4 Specific Recommendations for Medium-Speed Gearbox Supply Chain

NREL has developed the following specific recommendations to support a more robust U.S. 

advanced gearbox supply chain: 

• Continue to facilitate commercialization paths for gearbox innovations via competitive 

technology solicitations that target collaborations between universities, small U.S. gearbox 

technology companies, global OEMs, and gearbox refurbishers. 

• Strengthen the U.S. gearbox industry commons and innovation ecosystem by: 

o Supporting university programs that develop gearbox design and analysis 

professionals 

o Issuing additional advanced drivetrain solicitations

o Funding activities like the Gearbox Reliability Collaborative at higher power 

ratings 

o Encouraging information exchange at events such as a recurring drivetrain 

technology workshop

• Consider policies that increase or stabilize demand for gearboxes manufactured in the United 

States, such as requiring a minimum amount of domestic content for U.S.-installed wind 

turbines. 

4.4 Advanced Drivetrain Permanent-Magnet Generator Analysis

During this study, NREL researchers focused on two types of generator technologies: mediumspeed permanent-magnet and direct-drive permanent-magnet. 

4.4.1 Wind Turbine Generator Technology Trends

As the wind turbine sizes and ratings for offshore and land-based machines continue to grow, the 

torque and power requirements for wind turbine drivetrains increase. Medium-voltage systems

are attractive because of the decrease in material and mass that are a result of reductions in 

electrical current. Medium-voltage generators use less copper in the windings for a given power. 

They need more insulation, but the increase in insulation thickness is less than the decrease in 

copper size. The result is a lower mass generator overall, which is especially important for 

direct-drive generators and, to a lesser degree, medium-speed generators. In addition, the highervoltage ratings of the generator help facilitate nacelle-mounted power electronics and 

transformers because of associated mass reductions. In addition, smaller and lighter power cables

can be run down the tower. 

Although this report focuses on the benefits, manufacturing challenges, and supply chain 

opportunities for medium-speed and direct-drive generators, an important trend in land-based 

wind turbine generators is a return to the use of doubly-fed induction generations (DFIG). This 

renewed interest and use of DFIG machines will potentially delay the development and 

deployment of permanent-magnet generators for land-based and possibly offshore machines. 

Evidence of this trend includes GE’s announcement that it will be returning to DFIG generators 

for its latest generation of wind turbines. Additionally, the Spanish generator manufacturer 

Ingeteam invested $21 million (including facility equipment) in a U.S. generator manufacturing 

facility equipped to primarily produce DFIG generators [20]. 
31 

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at www.nrel.gov/publications.

DFIG technology is less capital-intensive than permanent-magnet generator technology because

it only requires a partial power converter with 20%−35% of rated capacity [21]. However, the 

DFIG technology for the GE 2.X series turbines was abandoned roughly 10 years ago, with the 

expectation that DFIG generators in the 1.5 MW series would not be able to meet future grid 

connection codes. Recently, substantial progress has been made with DFIG power electronics 

technologies that allow DFIGs to meet the latest grid codes. Additionally, GE cites the benefit 

that DFIGs enable a single generator model for both 50 hertz (Hz) and 60 Hz wind markets. 

Another factor encouraging the return of DFIG technology is the price volatility of permanentmagnet materials. The price variability and supply assurance for the rare earth materials 

neodymium and dysprosium introduce cost uncertainties for permanent-magnet generators.

4.4.2 Primary Wind Turbine Generator Manufacturing Challenges

Generator manufacturing is inherently a high-volume, low-margin business that requires a large 

amount of capital to be competitive. For example, Europe, particularly Spain, was viewed as the 

manufacturing market leader of wind turbine generators. Their position is, in part, due to 

extensive generator manufacturing investments that were provided during the formative years of 

the wind industry market. These early investments offered “first mover” advantages in the 

generator market that led to the development of a supply chain that supports a global wind 

market. One data point collected from a wind turbine generator manufacturer indicates that DFIG 

generators can be produced and shipped from Europe to the United States at less than half the 

cost of manufacturing the generator domestically. However, the reasons for Europe’s apparent 

advantage are not completely clear.

Figure 3. A comparison of the approximate cost to manufacture and ship a 2-MW 

generator within Texas and from China and Europe to Texas

Another challenge for the U.S. generator supply chain described by industry is the nonrecurring 

engineering costs associated with generators. Typically, each wind turbine OEM has different 

drivetrain system requirements that make investments difficult, and are further exacerbated by 

the variability in demand for wind turbines and changing generator designs. 

0

50

100

150

200

250

US China Europe

Approximate cost to manufacture and 

ship 2 MW generator ($ USD) 
32 

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at www.nrel.gov/publications.

4.4.3 Capabilities and Status of the U.S. Wind Turbine Generator Industry

NREL investigated the capabilities and status of the U.S. generator industry using available 

literature, generator manufacturing facility site visits to Ingeteam, Danotek, and Swiger Coil 

Systems, and interviews with four wind turbine OEMs and assemblers. Table 10 and Table 11

summarize the U.S. generator manufacturing companies and capabilities based on facility size. 

Table 10. Known Capabilities of Large, U.S.-Based, Utility-Scale Wind Turbine 

Generator Manufacturers

U.S.-Based Large 

Generator 

Manufacturer

Tier 

Classification

Manufacturing 

Facility Location

Product 

Size (MW)

Facility Capabilities 

and Capacities

Ingeteam Tier 1 Milwaukee, 

Wisconsin 

Up to 8 3,000 generators per 

year 

MV and PM 

manufacturing 

capabilities 

TECO Westinghouse Tier 1 Round Rock, Texas Up to 3 Currently not 

producing wind turbine 

generators, but has 

capability 

Of the two large U.S.-based generator manufacturing facilities visited, only the tier 1 generator 

supplier, Ingeteam, reported manufacturing wind turbine generators. The other, TECO 

Westinghouse, reported having only the capability and capacity to manufacture wind turbine 

generators. 
33 

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at www.nrel.gov/publications.

Table 11. Known Capabilities and Status of Small and Midsized U.S.-Based UtilityScale Wind Turbine Generator Manufacturers

Note: Companies that are no longer producing generators for the wind industry are represented by text that has been 

stricken through.

U.S.-Based 

Small/ Midsized 

Generator 

Manufacturer

Tier 

Classification 

Wind Industry 

Work 

Description 

Manufacturing 

Facility Location

Product 

Size (MW)

Facility 

Capabilities 

and Capacity

Northern Power 

Systems

OEM/Tier 1 Developer of 

direct-drive 

permanentmagnet wind 

turbines

Barre, Vermont 2.3 -

Danotek Motion 

Technologies

Tier 1 Customer specific 

generator designs

Canton, Michigan Power 

outputs up 

to 10.0

4,000 

generators per 

year [22]

Boulder Wind

Power

Tier 1 Development of 

axial gap, air 

core, permanentmagnet directdrive generators

Louisville, 

Colorado 

3.0 

(expected in 

2013-2014 

timeframe)

Unknown (in 

product testing 

phase)

Swiger Coil 

Systems

Tier 2 Generator coil 

repairs and 

complete wound 

generators for 

OEMs and tier 1 

suppliers

Cleveland, Ohio - -

4.4.4 Specific Recommendations for the U.S. Permanent-Magnet Generator 

Manufacturing and Supply Chain

To support the growth and success of the U.S. manufacturing and supply chain for the wind 

turbine generator industry, NREL researchers developed the following recommendations: 

• Fund advanced manufacturing process R&D and equipment investments for medium-speed 

and direct-drive permanent-magnet generator technologiesespecially targeting automated 

high-density coil-winding processes 

• Continue to facilitate commercialization paths for generator innovations via competitive 

technology solicitations that target collaborations between universities, small companies, and 

global OEMs 

• Strengthen the generator industry commons by: 

o Supporting the development of an institute for manufacturing innovation that focuses on 

permanent-magnet motors and generators 

o Issuing drivetrain technology development solicitations 

o Supporting wind-related magnet R&D at universities and encouraging generator design 

and analysis professionals using wind turbine manufacturing fellowships 

o Performing and publishing more detailed U.S. wind turbine drivetrain competitiveness 

analysis. 
34 

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 

at www.nrel.gov/publications.

5 Investment Prioritization and Summary of 

Recommendations

This section provides recommendations for prioritizing technology areas for possible 

investments by public, private, or nonprofit entities that will reduce the levelized cost of windgenerated electricity, foster opportunities to invent and commercialize new wind turbine 

technologies, and provide high-quality jobs in the United States. Figure 4 shows a summary of 

the analyzed technologies, selection metrics, and prioritization of the study results. The red and 

orange colors indicate stronger rational for focusing investments in the designated technology 

area that will achieve these objectives.

Figure 4. The authors used a matrix of qualitative scores to summarize the results 

and help prioritize investments in the three drivetrain technology areas evaluated

The authors identified medium-voltage and permanent-magnet generators as the highest priority 

drivetrain category for possible investments. In general, such investments are expected to enable 

relatively high reductions in LCOE, but they require new technologies, processes, and 

manufacturing facilities to produce, and there is relatively little U.S. manufacturing activity, 

capabilities, and R&D occurring in this area. Overall, public investments in this technology area 

are expected to have the most impact in positioning U.S. manufacturers for new manufacturing 

opportunities. 

Full SiC switch technologies are also promising areas for investments. The survey of NREL 

subject matter experts indicated that full SiC switches have the highest potential for LCOE 

Potential Reduction 

for LCOE

Manufacturing 

Disruption

Existing U.S. 

Manufacturing 

Activity and R&D

Recommended 

Priority for 

Drivetrain 

Investments

Generator: MV & 

PM Technologies First Priority

Speed Increaser: 

High Torque 

Technologies

Second Priority

Electrical Systems: 

Full SiC MV 

Switches

Second Priority

Legend

High Potential High Disruption Little or No Manf. Activity and R&D

Medium Potential Medium Disruption Low Manf. Activity and R&D

Low Potential Low Disruption Moderate Manf. Activity and R&D

No Potential No Disruption High Manf. Activity and R&D
35 

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at www.nrel.gov/publications.

reduction. A leading factor that contributes to the technology being a lower priority 

recommendation than medium-voltage and permanent-magnet generators is that several other 

industries (especially transportation and defense) are already funding R&D in this area. The 

authors expect that further investments will have less marginal benefit than the other technology 

areas because of the diminishing returns on additional investments. 

High-torque speed increaser technologies (such as the use of journal bearings in the planetary 

stages of gearboxes) have good potential for reducing cost of energy. However, the 

manufacturing disruption is expected to be small and the relative amount of existing wind turbine 

gearbox manufacturing investments and activities in the United States is large compared to 

medium- and low-speed permanent-magnet generators. For these reasons, the marginal 

improvements and U.S. supply chain opportunities resulting from investments in high-torque 

gearbox manufacturing are expected to be smaller than for medium- and high-speed generators.

Table 12 contains specific recommendations that will foster supply chain development and 

LCOE reductions in the three advanced drivetrain technology areas explored. In addition to these 

recommendations, NREL researchers identified the following list of key findings from the 

project: 

• A stable U.S. market for wind energy is necessary for domestic wind turbine drivetrain 

manufacturing and supply chain growth, but may not be sufficient alone for this growth. 

Additional investments may be required to compete with the extensive investments in wind 

turbine drivetrain manufacturing capabilities in regions such as Europe and Asia. 

• Wind energy supply chain and manufacturing capabilities are central to the health of the U.S. 

wind turbine innovation ecosystem. 

• Generally, DOE drivetrain supply chain investments that incentivize smaller, risk-tolerant 

companies to collaborate with large, risk-adverse companies in technology development and 

commercialization are one of the most promising paths to facilitating LCOE reductions and 

strengthening U.S. manufacturing capabilities and supply chains. 

• Supply chain and manufacturing status, projections, and interrelationships are dynamic and 

complex, thereby making generalizations difficult. Continued study, monitoring, and 

stakeholder input is recommended to best inform investment decisions over time.
36 

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Table 12. Summary of Recommendations for Possible Investments in Advanced 

Wind Turbine Drivetrain Manufacturing and Supply Chains

Technology Recommendations

Generator: 

Medium Voltage 

and Permanent 

Magnet 

1. Fund advanced manufacturing process R&D and equipment investments for mediumspeed and direct-drive permanent-magnet generator technologiesespecially targeting

automated high-density coil-winding processes

2. Continue to facilitate commercialization paths for generator innovations via competitive 

technology solicitations that target collaborations between universities, small companies, 

and global OEMs and target OEM installation of advanced generator technologies in new 

wind turbines

3. Strengthen the generator industry commons by: 

• Supporting the development of an institute for manufacturing innovation that 

focuses on permanent-magnet motors and generators 

• Issuing drivetrain technology development solicitations

• Supporting wind-related magnet R&D at universities and encouraging the 

development of generator design and analysis professionals through wind turbine 

manufacturing fellowships

• Performing and publishing more detailed U.S. wind turbine drivetrain 

competitiveness analysis

Speed Increaser: 

High Torque

1. Continue to facilitate commercialization paths for gearbox innovations via competitive 

technology solicitations that target collaborations between universities, small U.S. 

gearbox technology companies, global OEMs, and gearbox refurbishers

2. Strengthen the U.S. gearbox industry commons and innovation ecosystem by: 

• Supporting university programs that develop gearbox design and analysis 

professionals

• Issuing additional advanced drivetrain solicitations

• Funding activities like the Gearbox Reliability Collaborative at higher power ratings

• Encouraging information exchange at events, such as a recurring drivetrain 

technology workshop

3. Consider policies that increase or stabilize demand for gearboxes manufactured in the 

United States, such as requiring a minimum amount of domestic content for U.S.-installed 

wind turbines.

Electrical 

Systems: 

Full SiC MV 

Switches

1. Facilitate commercialization paths for SiC technology innovations by incentivizing the 

demonstration of hybrid SiC power electronics in a wind turbine and funding 

collaborations between small U.S. SiC companies and global OEMs

2. Monitor full-SiC and high temperature SiC technologies for leveraged opportunities and 

collaborations with other industries

3. Provide funding to explore and document the power electronics supply chain in more 

detail and engage laboratory expertise in wind turbine power converter technologies or 

develop the expertise if necessary. 
37 

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 

at www.nrel.gov/publications.

6 Future Work

To the authors’ knowledge, this study is the first of its kind to assess the impact of advanced 

wind turbine drivetrain technologies on U.S. manufacturing and supply chains. Recommended 

work to be considered for the future that was not undertaken during the initial study includes the 

following. 

Provide a More In-Depth Assessment of Manufacturing and Supply 

Chain Issues for Power Electronics

Of the four advanced drivetrain categories analyzedSiC power electronics, rotor-stretching 

technologies, medium-voltage permanent-magnet generators, and medium-speed

gearboxesSiC was explored the least. A better understanding of these supply chain issues for 

power electronics focusing on SiC is warranted. Assistance from generator and power electronics 

industry experts and consultants should also be considered.

Formally Vet Project Results with Industry Stakeholders

The results of this project relied primarily on the experience, perspectives, and interpretations of 

industry information by the authors. A formal vetting process by stakeholders and industry 

experts will help ensure that the findings and recommendations are consistent across the wind 

industry.

Develop a Strategic Plan or Vision for Wind Turbine Manufacturing 

and Supply Chain Activities in the United States

Supply chain and manufacturing status, projections, and interrelationships are dynamic and 

complex, thereby making generalizations difficult. Therefore, a long-term strategic plan will help 

provide the coordination and consistency necessary to address the gaps in the U.S. supply chain 

and position the country’s wind industry to take advantage of manufacturing opportunities. 

Continued study, monitoring, and stakeholder input is recommended to best inform the strategic 

plan and investment decisions over time.
38 

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 

at www.nrel.gov/publications.

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