Page | 1
Page | 2

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.

Available electronically at http://www.osti.gov/bridge 

Available for a processing fee to U.S. Department of Energy

and its contractors, in paper, from:

U.S. Department of Energy

Office of Scientific and Technical Information

P.O. Box 62

Oak Ridge, TN 37831-0062

phone: 865.576.8401

fax: 865.576.5728

email: mailto:reports@adonis.osti.gov 

Available for sale to the public, in paper, from:

U.S. Department of Commerce

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22161

phone: 800.553.6847

fax: 703.605.6900

email: orders@ntis.fedworld.gov 

online ordering: http://www.ntis.gov/help/ordermethods.aspx 

Printed on paper containing at least 50% wastepaper, including 10% post consumer waste.
Page | 3

Executive Summary

This report presents key findings from the Department of Energy’s Advanced Drivetrain Workshop, held 

on June 29-30, 2010 in Broomfield, Colorado, to assess different advanced drivetrain technologies, their 

relative potential to improve the state-of-the-art in wind turbine drivetrains, and the scope of research 

and development needed for their commercialization in wind turbine applications. 

The Advanced Drivetrain Workshop evaluated next-generation technologies to identify those with 

potential to radically improve the reliability, performance, and capital cost of wind turbine drivetrains 

beyond the incremental improvements that might be expected for current state-of-the-art technologies. 

The workshop featured four separate discussion tracks, each focused on a broad category of drivetrain 

technologies: Superconducting Drivetrains; Advanced Permanent Magnet Generators; Continuously 

Variable Transmissions and Fluid Drive Systems; and Innovative and Non-Traditional Drivetrain 

Concepts. 

Workshop participants identified major research topics with the potential to improve on the state-ofthe-art in wind turbine drivetrains for each of the four broad drivetrain technology categories. The 

participants also listed the major barriers to the commercialization of these technologies and potential 

research and development pathways for overcoming those barriers. Those results are summarized in 

Table 1 below. 

Table 1. Benefits, Barriers, and Development Pathways for Advanced Drivetrain Technologies

Topic Benefits Barriers Pathways

Superconducting Drivetrains 

Superconducting 

Direct Drive 

Generators

Capital costs; 

reliability; energy 

capture; supply chain 

security

Technical; 

perception; 

scaling; IP

Sub-component reliability testing; 

demonstration project deployment; 

engage independent third parties

Next Generation 

Superconducting 

Technologies 

Energy capture Technical Laboratory testing

Permanent Magnet Generators

Advanced Magnetic 

Materials 

Capital costs; 

reliability; energy 

capture; safety and 

serviceability

Technical; 

commercial; 

perception; 

certification 

Materials science research; tradeoff 

analysis; cost/benefit analysis; 

International Electrotechnical 

Commission (IEC) standards 

development

System Design and 

Topology 

Capital costs; 

reliability; energy 

capture; safety and 

serviceability

Technical; 

commercial; 

certification; 

Laboratory research; cost/benefit 

analysis; IEC standards development

Power Electronics Capital costs; 

reliability; 

performance

Technical; 

perception; 

commercial; 

Laboratory research; workforce 

education and training cost/benefit 

analysis; 

Diagnostics and 

Maintenance 

Reliability; Reliability; 

operator 

training; 

certification 

Reliability research; workforce 

education and training; IEC standards 

development
Page | 4

Topic Benefits Barriers Pathways

Continuously Variable Transmissions

Frictional Contact 

Drives

Capital costs; 

reliability; energy 

capture

Technical; 

scalability; 

materials

Cost-benefit analysis; design and 

modeling tools; demonstration

projects; component development 

and testing

Fluid Drives Capital costs; 

reliability; energy 

capture; energy 

storage 

Technical; 

scalability; 

materials; 

perception

Cost-benefit analysis; design and 

modeling tools; component 

development and testing; 

demonstration projects 

Innovative Concepts

Uptower Direct 

Current (DC) 

Generator 

Reliability; grid 

benefits

Technical Feasibility studies; equipment survey

Ground-Level 

Generators 

Reliability Technical Feasibility studies

Rim-Drive Turbines Reliability Technical Feasibility studies

Tandem Generators Availability; 

scalability

Commercial Feasibility studies

Complete Up-Tower 

Gearbox Reparability

Reliability Technical Feasibility studies

Other Innovative 

Drivetrains

Capital costs; 

reliability

Technical; 

perception

Design studies

The technologies outlined above can be grouped into three broad categories based on their potential 

impact on the market for current and next-generation drivetrains. The first category includes conceptstage designs for next-generation wind turbines that offer potentially significant but still uncertain 

advantages over the state-of-the-art. The second category includes technologies that are currently being 

considered for incorporation into current wind turbine drivetrain designs. The third category includes

technologies that have potentially transformative application in next-generation turbine designs with 

large nameplate capacities (5+ MW), particularly in offshore wind turbines. The unique challenges facing 

each category will require different research and development strategies in order to successfully 

commercialize these technologies.
Page | 5

Table of Contents

Executive Summary....................................................................................................................................... 3 

List of Tables ................................................................................................................................................. 5 

List of Figures................................................................................................................................................ 5 

1.0 Introduction ...................................................................................................................................... 6 

2.0 Workshop Results............................................................................................................................. 9 

2.1 Superconducting Drivetrains ........................................................................................................ 9 

2.2 Permanent Magnet Generators.................................................................................................. 13

2.3 Continuously Variable Transmissions ......................................................................................... 19

2.4 Innovative Concepts................................................................................................................... 23

3.0 Summary ......................................................................................................................................... 27

4.0 References ...................................................................................................................................... 28

5.0 Appendix: Workshop Agenda ......................................................................................................... 29

List of Tables

Table 1. Benefits, Barriers, and Development Pathways for Advanced Drivetrain Technologies................ 3 

Table 2. Benefits of Superconducting Direct Drive Generators.................................................................. 11

Table 3. Barriers to Commercializing Superconducting Direct Drive Generators.......................................13

Table 4. Benefits of Permanent Magnet Technology Improvements......................................................... 15

Table 5. Barriers to Commercializing Permanent Magnet Technology Improvements .............................18

Table 6. Benefits of Continuously Variable Transmissions......................................................................... 21

Table 7. Barriers to Commercializing Continuously Variable Transmissions..............................................23

Table 8. Benefits of Innovative Drivetrain Concepts .................................................................................. 24

Table 9. Barriers to Commercializing Innovative Drivetrain Concepts....................................................... 26

List of Figures

Figure 1. Conventional gearbox drivetrain configuration ............................................................................ 6 

Figure 2. Cutaway of multi-stage gearbox .................................................................................................... 8 

Figure 3. Superconductor coils in vacuum-insulated rotor........................................................................... 9 

Figure 4. Benefits of permanent magnet technology improvements ........................................................ 14
Page | 6

1.0 Introduction

This report presents key findings from the Advanced Drivetrain Workshop, which was organized by the 

U.S. Department of Energy (DOE) Wind and Water Power Program to discuss the next generation of 

wind turbine drivetrain technologies. The workshop, held on June 29-30, 2010 in Broomfield, Colorado, 

convened experts from industry, academia, the federal government, and national laboratories. The 

purpose of the workshop was to assess different advanced drivetrain technologies, their relative 

potential to improve the state-of-the-art in wind turbine drivetrains, and the scope of research and 

development needed for their commercialization in wind turbine applications. The Wind and Water 

Power Program sponsored the workshop as part of its larger efforts to reduce the cost of energy 

produced by wind turbines. 

Figure 1. Conventional gearbox drivetrain configuration

The drivetrain of a wind turbine converts the low-speed, high-torque rotation of the turbine’s rotor 

(blades and hub assembly) into electrical energy. The most common drivetrain configuration (Figure 1)

consists of a low-speed shaft (also called a main shaft), a gearbox, a high-speed shaft, and a generator

(Hau 2006). The low-speed shaft connects to the rotor hub and is supported by the main frame (or 

bedplate) through one or two main bearings. As a result, the low-speed shaft rotates at a rotational 

speed of roughly 12-30 revolutions per minute (rpm) for current utility-scale wind turbines. A gearbox 

connected to the low-speed shaft steps up the rotational speed to 1,200-1,800 rpm via a set of gears in

two or three stages, typically consisting of planetary and parallel gear configurations (Hau 2006). A highspeed shaft then transmits mechanical power from the gearbox to a generator, which requires high 

rotational speeds (1,200-1,800 rpm) in order to economically generate electricity. Generators often use

power electronics to allow variable speed operation and to condition the electrical power output of the 

turbine so that it can be integrated into the power grid easily and efficiently. Some wind turbine 

configurations forgo a gearbox and instead generate electricity by coupling the low-frequency rotation 

of the main shaft directly to the generator. Such “direct-drive” configurations typically require large 
Page | 7

diameter generators and extensive power electronics to allow variable speed operation and to condition 

the electrical output for the grid. 

Drivetrains contribute to the total cost of energy produced by wind turbines in multiple ways. First, the 

drivetrain includes some of the most expensive components in a wind turbine, namely the gearbox and 

generator, which account for nearly one half of the turbine’s total capital cost (Fingersh, Hand and 

Laxson 2006). Heavier drivetrains also require more substantial towers to support their weight, further 

driving up the turbine’s overall capital equipment costs. Second, drivetrains represent a significant 

proportion of the total energy losses within a wind turbine, reducing electrical power output, and thus

revenue, of an individual turbine. Finally, conventional drivetrains are not meeting their expected 20-

year operating lifetimes primarily due to premature gearbox and bearings failures that necessitate 

turbine downtime as well as expensive and time-consuming repairs or replacements, often via the 

deployment of very large and expensive lifting cranes (Musial, Butterfield and McNiff 2007). In addition 

to the costs of replacement equipment, crane rental, and lost revenue, widespread gearbox failures can 

lead financiers to demand higher interest rates for project loans due to a higher perceived level of 

technology risk.

To help lower the cost of wind energy, the Wind and Water Power Program works to improve reliability, 

increase performance, and decrease capital costs for wind turbine drivetrains. The program funds the 

Gearbox Reliability Collaborative, a public-private partnership between national laboratories, 

universities, gearbox and component manufacturers, engineering consultants, and wind turbine 

manufacturers. The mission of the Gearbox Reliability Collaborative is to gather, analyze, and share data 

from operational and experimental gearboxes (Figure 2). The goal of this effort is to identify critical 

design parameters and root causes that are responsible for gearbox failures in the field. The program 

also funds the development of large dynamometers capable of testing next-generation wind turbine 

drivetrains – this functionality allows turbine manufacturers to validate and refine new designs before 

deploying them in the field. In some cases, the program has worked with specific companies to develop 

new wind turbine designs that incorporate advanced drivetrain technologies, such as multi-generator 

systems, gearbox component load mitigation, or permanent magnet replacements for wound 

electromagnets in the generator to reduce system size. Finally, the program funds laboratory research 

and development on a variety of topics with the potential to improve the reliability and performance of 

turbine drivetrains. Two key topics are advanced materials that improve the lubrication and wear 

resistance of drivetrains and drivetrain components, and the use of condition-based health monitoring

to reduce equipment failures through predictive maintenance. 
Page | 8

Figure 2. Cutaway of multi-stage gearbox

The Advanced Drivetrain Workshop evaluated next-generation technologies to identify those with

potential to radically improve the reliability, performance, and capital cost of wind turbine drivetrains 

beyond the incremental improvements that might be expected for current state-of-the-art technologies. 

The workshop began with introductory remarks covering DOE’s existing portfolio of drivetrain research 

and development projects, the current state of the art in drivetrain design configurations, and the 

purpose and structure of the meeting. The workshop participants then split into four groups to discuss 

separate technologies: Superconducting Drivetrains; Advanced Permanent Magnet Generators; 

Continuously Variable Transmissions and Fluid Drive Systems; and Innovative and Non-Traditional 

Drivetrain Concepts. Over a series of three sessions, the participants provided input on the current state 

of different advanced drivetrain technologies; described how these technologies could offer an 

improvement over the state-of-the-art; listed some of the current uses of these technologies and the 

barriers to commercialization in wind turbine applications; and outlined potential paths to 

commercialization of these technologies, including estimated development timelines and key areas of 

research and development. The Workshop concluded with a report-out session in which the four groups 

presented their key findings to the larger assembly. 
Page | 9

2.0 Workshop Results

The following sections describe the major technologies discussed in each of the four technology 

breakout sections, their potential benefits over the state-of-the-art, and possible pathways for their 

commercialization. 

2.1 Superconducting Drivetrains

This session discussed the potential application of superconducting materials (materials capable of

conducting electricity with near-zero direct-current (DC) electrical resistance) in wind turbine 

generators. Superconducting materials only exhibit their superconducting properties at very low 

temperatures and are classified by the critical temperature at which they operate. Low temperature 

superconducting materials typically require temperatures below 20 Kelvin (-253 Celsius), while high 

temperature superconducting materials operate between approximately 30 and 40 K. Superconducting 

materials and cooling systems are currently used in many non-wind applications, including Magnetic 

Resonance Imaging machines, particle accelerators, electrical transmission, and prototype ship 

propulsion systems. Superconducting generators generally feature superconducting materials in the 

rotor (Figure 3) and a conventional (non-superconducting) stator, although alternate topologies exist 

(including homopolar designs, “inside-out” designs in which the rotor encloses the stator, and designs 

combining permanent magnets and superconducting materials). Future superconducting technologies 

may make it practical to also use superconducting material in the generator stator; however, with 

current technologies, the alternating current inherent in the stator results in time-varying magnetic 

fields that incur substantial losses.

Figure 3. Superconductor coils in vacuum-insulated rotor
Page | 10

Superconducting generators have the potential to offer several cost and reliability improvements over 

conventional wind turbine drivetrains when scaled up to very high capacities (5 MW and larger) and 

when used in direct drive systems. Direct drive wind turbines do not require a gearbox, thereby 

eliminating a major source of reliability failures. Although superconducting generator technology can be 

used with a gearbox, this session focused on the application of superconducting generators in direct 

drive wind turbines due to perceived advantages in commercial and economic viability of such designs. 

Superconducting generators have less mass and require less volume than comparable permanentmagnet direct drive generators, particularly as wind turbine capacity ratings continue to increase (power 

output tends to scale with volume more quickly in superconducting generators than in conventional 

direct-drive generators). Lower generator mass and size reduces the capital cost of generator and 

reduces the loads on the tower, resulting in an overall reduction in turbine capital costs. Cost

advantages produced by such weight savings are most likely to be realized initially in large turbines 

(roughly 5+ MW). In addition to weight savings, superconducting generators have the potential to 

increase drivetrain reliability by allowing a much larger air gap tolerance between rotor and stator as 

compared to permanent-magnet direct drive designs. Conventional direct-drive generators must 

maintain an air gap between rotor and stator of only a few millimeters, making contact between rotor 

and stator much more difficult to avoid, and ultimately leading to increased weight due to increased 

structural support requirements (Mueller and McDonald 2008). It can be difficult to manufacture and 

operate conventional direct-drive generators with such tight tolerances. Superconducting direct-drive 

generators, with air gaps on the order of several centimeters, can be more easily designed to avoid this 

mode of failure. 

Superconducting direct-drive generators are especially suited for offshore wind applications, because 

there is an economic incentive to place as large of a turbine as possible on a single foundation to 

compensate for the high installation costs for offshore wind, and because these designs have the 

potential to offer greater reliability and require less maintenance than both geared and conventional 

direct-drive designs. 
Page | 11

Table 2 below lists major areas of interest in superconducting direct-drive generator research and 

development and the potential benefits that these areas of interest offer over the state-of-the-art in 

wind turbine drivetrains.

Table 2. Benefits of Superconducting Direct Drive Generators

Area of Interest Description Benefits

Low-Temperature and 

High-Temperature

Superconducting Direct 

Drive Generators

Superconducting direct-drive 

generators eliminate electrical 

resistance in rotor coils by supercooling them, allowing for extremely

strong magnetic flux and high torque 

densities without the need for adding 

material (larger coils or permanent 

magnets). As a result, superconducting 

generators tend to be less massive and 

much smaller in size than conventional 

direct drive generators, especially at 

generating capacities of 5 MW and 

higher. 

 Reduced Capital Costs: potentially 

significant drivetrain mass advantage 

begins around 5 MW and scales 

rapidly with generating capacity. The 

lighter construction and reduced 

material usage can result in a lower 

drivetrain cost, decreased tower 

loading, and easier installation 

logistics.

 Improved Reliability: larger air gap 

between rotor and stator leads to 

reliability and design benefits 

compared to permanent-magnet 

direct-drive designs. This may lead to 

reductions in unscheduled 

maintenance.

 Increased Energy Capture:

superconducting direct-drive designs 

promote scalability to taller and higher 

generating capacity turbines due to 

smaller and lighter nacelles.

 Supply Chain Security: 

Superconducting direct-drive 

generator designs use readily available 

materials to generate magnetic flux, 

thereby providing an alternative to the 

rare earth magnet materials, typically 

mined outside the U.S., used in other 

direct-drive designs.

Next Generation 

Superconducting 

Technologies 

Advances in superconducting 

technology that were discussed 

include: superconducting rotor AND

stator, linear superconducting 

generators, and synchronous 

generators with air cores and copper 

stators. 

 Increased Energy Capture: advances in 

superconducting technology will 

increase the efficiency of 

superconducting direct-drive systems; 

increased efficiency may result in 

higher magnetic flux densities, lower 

parasitic losses (currently <~1%), or 

further design optimization.

The major barriers to incorporating superconducting generators in wind turbine designs are in large part 

commercial rather than technical in nature. The perception of superconducting generators as overly 

complex, undeveloped, and high-risk technology is expected to increase the cost of financing projects 

using this technology. Perceived safety concerns regarding the use of super-cooled nitrogen or hydrogen 

may also hinder the adoption of this technology, despite its widespread use in common commercial 
Page | 12

applications such as Magnetic Resonance Imaging (MRI). The lack of a scaling reference or an operating 

prototype turbine incorporating superconducting direct-drive technology render it difficult to compare 

the costs and performance of this technology with current drivetrain technologies, though initial testing 

has been performed on multi-megawatt-scale superconducting motors. In addition to these commercial 

risks, superconducting generators face potential technical risks related to reliability and durability of 

superconducting materials, such as wire performance and durability degradation resulting from 

mechanical and thermal cycling. A further concern is cooling system fatigue and degradation resulting 

from thermal cycling. 

Due to the commercial barriers outlined above, a research and development program to commercialize 

superconducting generators in wind turbines might focus heavily on the demonstration of these 

technologies to establish their technical viability and cost and performance advantages. The 

demonstration of a large-scale prototype turbine with a superconducting generator, leveraging existing, 

appropriately-sized turbine components such as blades to speed deployment, at a pre-permitted landbased or offshore site could address many of the perception concerns. Existing DOE initiatives, such as 

the Large Blade and Large Drivetrain Testing Facilities and ongoing certification and standards 

development, could support the development of this prototype. Component and subcomponent 

reliability testing would be especially effective in addressing perceptions of high technology risk before a 

full scale prototype is tested in a turbine. 
Page | 13

Table 3 below lists major barriers to the commercialization of superconducting direct-drive generator 

technology and potential research and development pathways for overcoming those challenges.

Table 3. Barriers to Commercializing Superconducting Direct Drive Generators

Area of Interest Commercialization Barriers Development Path

Low-Temperature and 

High-Temperature

Superconducting Direct 

Drive Generators

 Technical Risk: cooling system 

reliability, fatigue caused by thermal 

cycling, and coolant containment 

are untested in a wind turbine 

environment. 

 Conduct sub-component reliability 

testing: accelerated reliability testing 

could utilize shaker tables to test 

operating cryo-systems and coil 

windings under representative wind 

turbine conditions. 

 Perception Risk: misperceptions 

regarding the “newness” of 

superconducting direct-drive

technology, system complexity, and 

safety uncertainty cause concerns 

about its implementation in wind 

turbine systems.

 Conduct reliability testing and deploy 

demonstration projects: accelerated 

reliability testing and deployment of 

operational demonstration 

superconducting direct-drive wind 

turbines would help alleviate 

misperceptions about this technology.

 Scaling Risk: project economics and 

manufacturing requirements for 10-

MW wind turbines have not been 

demonstrated. There are no 10-MW 

scale prototype test platforms

 Deploy demonstration projects: 

successfully deployed large wind 

turbines could establish their viability. 

 IP Risk: superconducting technology 

owners and developers may be 

hesitant to collaborate on turbine 

research, development, and testing.

 Engage independent third parties:

federal or state governments, 

universities, certification bodies, or 

other independent organizations could 

assist in data aggregation and 

benchmark development.

 Certification Risk: wind turbine 

standards have not been developed 

for direct-drive or superconducting 

technologies

 Establish IEC standards for 

superconducting direct-drive: support

working groups to establish standards 

language.

Next Generation 

Superconducting 

Technologies 

 Technical Risk: advanced 

superconducting technologies are 

still in the basic research phase.

 Conduct laboratory testing: continued 

research, development, and testing

will advance the Technology Readiness 

Level of advanced superconducting 

technologies. 

2.2 Permanent Magnet Generators

This session discussed advanced permanent-magnet generator topologies for wind turbine applications. 

These designs use permanent magnets composed of rare-earth mineral alloys in place of induction 

generators composed of wound electromagnets that require electricity to operate. This session 

discussed direct-drive designs (a class of drivetrain that does not require a gearbox to step up the 

rotational speed of the turbine rotor to the high rotational speeds desired for electricity generation) as 

well as the incorporation of permanent magnet generators (Figure 4) in conventional geared drivetrains. 

Permanent-magnet generators have been commercialized for wind turbine applications, both in geared 

and in direct-drive topologies, although opportunities for technology improvements still exist. 
Page | 14

Figure 4. Cutaway view of single-stage permanent magnet drivetrain

Permanent-magnet generators have the potential to offer several cost and reliability benefits over 

conventional induction generators using wound electromagnets. First, permanent magnet generators

have increased power densities over conventional induction generators using wound rotor or squirrel 

cage designs; permanent-magnet generators thus weigh less and require less space than comparable 

induction generators. Because permanent-magnet generators do not require electricity to induce a 

magnetic field in the rotor, they offer performance improvements (lower parasitic energy losses) over 

induction generators or wound field synchronous generators. In addition, permanent-magnet 

generators do not require generator brushes and rotor winding insulation, eliminating these 

components as potential sources of equipment failure. Direct-drive designs with permanent-magnet 

generators offer additional benefits, most notably the complete elimination of the gearbox from the 

drivetrain. The gearbox is one of the most expensive components of a wind turbine, requires the most 

scheduled maintenance and very expensive repairs, and is responsible for a large portion of the 

turbine’s energy losses. 

Although direct-drive permanent-magnet generator designs overcome these gearbox issues, they face a 

separate set of challenges. To generate power from low- speed, high-torque rotational motion, these 

topologies require large-diameter generators, which increase the size and weight of the drivetrain and 

use increasing amounts of expensive rare earth magnetic material. Furthermore, these topologies must 

maintain a very small air gap between rotor and stator (on the order of millimeters) to obtain higher flux 

densities. This leads to design, manufacturing, and operational challenges in handling the complex loads 

from the rotor as the generators increase in size. Finally, because direct-drive turbines operate as 

synchronous generators, they require extensive power electronics to allow variable speed operation and 

to synchronize their output with the frequency of the power grid. 
Page | 15

Table 4 below lists major areas of interest in permanent magnet generator research and development 

and the potential benefits that these areas of interest offer over the state-of-the-art in wind turbine 

drivetrains. 

Table 4. Benefits of Permanent Magnet Technology Improvements

Area of Interest Description Benefits

Advanced Magnetic 

Materials

Improved magnetic materials could 

lead to increased energy densities, 

which would result in smaller 

generators and, consequently, lower 

weight drivetrains. Materials 

improvements include bulk material 

property enhancement, nanostructured magnetic materials, and 

“flexible” magnets that can be 

magnetized or demagnetized postinstallation.

 Reduced Capital Costs: lower weight 

permanent magnets would result in 

lower weight generators, reducing uptower weight. 

 Improved Reliability: relaxed air-gap 

constraints would decrease the 

incidence of rotor/stator impact, 

decreasing maintenance costs.

 Increased Energy Capture: lower 

nacelle weights may make it cost 

effective to deploy taller towers with 

larger rotors. 

 Safety and Serviceability: magnets 

that are safer to handle during 

installation and maintenance would 

reduce the potential for accidents.

System Design and 

Topology

Possible system design and topology 

improvements include:

• Radial flux (geared or direct drive), 

transverse flux, Vernier designs, 

magnet gearing and coreless 

armatures; 

• Advanced air gap design to 

improve tolerances and reduce 

strikes; 

• Designs for in-situ maintenance 

(handling), such as individually 

replaceable coils or modularized 

designs.

 Reduced Capital Costs: advanced 

topologies could result in smaller 

generators, reducing up-tower weight.

 Improved Reliability: better designed 

and tolerance air-gaps would decrease 

the incidence of rotor/stator impact, 

decreasing maintenance costs.

 Increased Energy Capture: lower 

nacelle weights could make it cost 

effective to deploy taller towers with 

larger rotors. 

 Safety and Serviceability: systems that 

are designed for installation and 

maintenance would reduce the 

potential for magnet related accidents.

Power Electronics New concepts for power converters 

include advanced controls, better 

controlled voltage levels, improved

switching, and advanced architectures. 

Control and protection topologies for 

permanent magnet generators could 

influence demagnetization,

overvoltage due to loss of load, short 

circuit prevention, and unbalanced

load prevention)

 Reduced Capital Costs: technology 

advances may lead to lower cost 

power electronics.

 Improved Reliability: improved power 

electronics operations and controls 

would enable wind turbines to avoid 

reliability problems currently seen in 

PM generators.

 Improved Performance: losses due to 

power electronics may be decreased.
Page | 16

The session identified several areas of research and development that could lead to improved 

permanent-magnet generators, including magnetic materials research; development of new generator 

machine designs; improved power electronics characteristics and power converter architecture; and 

development of diagnostic methods. The specific sub-topics, benefits, and challenges of each of these 

research areas are described below.

Magnetic materials research could improve permanent magnets by improving their energy density, 

creating nano-structured magnets, or by creating materials that can be magnetized or demagnetized 

post-installation to simplify assembly and maintenance processes. These advances could result in size 

and weight reductions over current magnets, particularly important for large-diameter direct-drive 

generators. Development of such magnets would require basic materials science breakthroughs, and the 

high cost of such materials and lack of market demand could hinder commercialization in wind turbine 

applications.

Several opportunities exist for new permanent-magnet generator machine designs. Transverse flux 

permanent magnet topologies and Vernier machine designs could yield increased permanent-magnet 

generator power densities, resulting in smaller size and lower weight direct drive generators. 

Commercialization could be hindered by a lack of sufficient manufacturing capabilities and a lack of 

investor and project operator experience with these technologies. Mitigating the risk of air gap collapse 

would improve turbine performance and reliability but may cause capital cost increases; the need for 

and value of such a design have not been quantified. Coreless armature winding designs could improve 

reliability by removing steel from the generator coils (reducing corrosion risk), could reduce overall 

generator weight, and could provide better voltage regulation, but these designs would result in largerdiameter generators that increase assembly, transportation, and installation complexity. 

Standards There is a market need for PMG 

standards development, particularly 

pertaining to testing and 

measurement. 

 Improved Reliability: Standards 

typically lead to more reliable systems 

or components; this would be true for 

PMGs as well.

 Reduced Capital Costs: Future 

financing costs would decrease as 

systems become more reliable and can 

be certified to a standard. 

Diagnostics and 

Maintenance

Signature analysis utilizing smart 

sensors allow for advanced diagnostics 

and predictive maintenance.

 Improved Reliability: Operations and 

maintenance costs decrease as a result 

of well-scheduled maintenance that 

minimizes failures and turbine 

downtime.

Operating Environment As wind turbine move offshore (marine 

environment) and/or are located in 

more extreme operating environments 

(extreme temperatures, etc.), new 

materials and design considerations 

will be required to ensure successful 

long-term operations. 

 Improved Reliability: Extreme 

operating conditions will have a 

negative effect on component and 

system life-time. Technology 

improvements with this in mind may 

lead to wind turbines that remain 

reliable regardless of location. 
Page | 17

Permanent-magnet generator designs that allow in-situ maintenance (without the use of cranes) could 

lead to increased reliability and availability, decreased operations and maintenance costs, and reduced 

project risk. However, the need for such designs has not been quantified, and commercial users may not 

recognize the value of such designs. 

Research on the transportation, installation, and support structures of permanent magnet generators 

could lead to reductions in capital equipment costs, logistics costs, and overall system weight, but 

certain designs, such as segmentation requiring field assembly, may reduce system efficiency and impact 

the rest of the system. A related issue is the ongoing competition between modularized and integrated 

design approaches to permanent magnet direct-drive turbines. The benefits and risks of developing new 

integrated generator designs that could reduce system weight, cost, and maintenance need to be 

evaluated in comparison to modular designs that allow turbine manufacturers to retain the expertise of 

and share risks with dedicated generator manufacturers. Magnetic gearing has the potential to reduce 

or remove many of the reliability problems associated with conventional gearboxes, but the extra 

permanent-magnet materials costs, lower power densities, and uncertain efficiency gains of such 

gearing need to be evaluated in order to determine the potential for commercialization. Finally, 

development of ruggedized permanent magnet generator designs for cold-weather or marine conditions 

could increase performance, reliability, and equipment lifetime for turbines operating in these 

environments, but commercialization is hindered by the lack of relevant codes and standards, the 

increase costs of implementing such upgrades, materials limitations, and uncertain market demand.

Workshop participants identified two major areas of research to improve power electronics for 

permanent-magnet generators: new power converter concepts, and improved control and protection 

topology. New power converter concepts could focus on controls, voltage levels, switching, and 

converter architecture. Advances in these areas could yield increased efficiency, reliability, and 

availability, but successful commercialization would have to first overcome the technical limitations of 

semiconductors, the lack of experience with high-voltage systems on the part of wind turbine designers 

and operators, and the overall lack of operational experience of these systems. Control and protection 

topology research could focus on demagnetization, overvoltage, short circuit prevention and 

management, and unbalanced operation. This research could yield improved performance and energy 

capture and would require software (rather than hardware) modification. The major barrier to 

commercialization is the cost of developing commercial products from existing technical papers and 

results. 

Development of diagnostic methods for permanent-magnet generators might feature signature analysis 

including smart sensors to improve turbine reliability and availability. While deployment of these 

sensors would increase upfront capital costs, the improvements to reliability, scheduled maintenance, 

and performance should outweigh these costs. Possible technical barriers include electromagnetic 

interference with sensors from the permanent magnets, the potential for faulty information, and need 

for operator training. Lack of industry codes and standards related to sensors and diagnostic methods 

and the higher upfront costs could limit commercial deployment.
Page | 18

Table 5 below lists major barriers to the commercialization of permanent magnet generator 

technologies and potential research and development pathways for overcoming those challenges.

Table 5. Barriers to Commercializing Permanent Magnet Technology Improvements

Area of Interest Commercialization Barriers Development Path

Advanced Magnetic 

Materials

 Technical Risk: basic materials 

science breakthroughs are required. 

 Conduct materials science research: 

basic research into novel, low-cost 

magnetic materials is needed. 

 Commercial Risk: high cost of 

permanent magnet materials may 

increase capital costs.

 Develop tradeoff analysis: advanced

magnet costs may be offset by uptower weight savings.

 Perception Risk: current lack of 

market demand. 

 Develop cost/benefit analysis: 

definitive studies showcasing costs 

and benefits of advanced permanent 

magnet materials, plus the 

development of an offshore wind 

industry, would drive demand.

 Certification Risk: wind turbine 

standards have not been developed 

for advanced permanent magnet 

materials.

 Establish IEC standards: develop 

working groups to establish standards 

language for permanent magnet 

direct-drive.

System Design and 

Topology

 Technical Risks: manufacturing 

process breakthroughs are required 

to affordably implement advanced 

designs and topologies.

 Conduct laboratory research: basic 

research into novel permanent 

magnet topologies may allow for their 

eventual deployment in the field.

 Commercial Risk: the need for such 

designs has not been quantified; 

therefore the effect on overall costs 

is unknown. 

 Develop cost/benefit analysis: 

definitive studies showcasing costs 

and benefits of advanced permanent 

magnet topologies would begin to 

overcome this area of risk.

 Certification Risk: wind turbine 

standards have not been developed 

for advanced PMDD technologies.

 Establish IEC standards: develop 

working groups to establish standards 

language.

Power Electronics  Technical Risk: limitations of 

existing semiconductors need to be 

overcome. 

 Conduct laboratory research: basic 

research into advanced Power 

Electronics may allow for their 

eventual deployment in the field.

 Perception Risk: lack of wind 

industry experience with highvoltage equipment.

 Increase education and training: 

education and training programs will 

increase high-voltage experience. 

 Commercial Risk: the cost of 

developing commercial products

and lack of operational experience

may hamper future power 

electronics development.

 Develop cost/benefit analysis: 

definitive studies showcasing costs 

and benefits of advanced power 

electronics may lead to a compelling 

business case for further 

development.

 Increase education and training: 

education and training programs will 

increase experience.
Page | 19

Improved Diagnostics 

and Maintenance

 Reliability Risk: electromagnetic 

interference may increase the

potential for faulty information.

 Conduct research: laboratory and field 

research may help overcome 

electromagnetic interference issues.

 Operator Training Risk: operators 

without proper training may add to 

permanent magnet-related 

operations & maintenance costs.

 Increase education and training: 

Education and training programs will 

increase operations & maintenance 

reliability.

 Certification Risk: wind turbine 

standards have not been developed 

for advanced permanent magnet

technologies.

 Establish IEC standards: develop 

working groups to establish standards 

language for permanent magnet 

diagnostics and maintenance.

2.3 Continuously Variable Transmissions

This session discussed the use of continuously variable transmissions, including fluid drive systems, in 

wind turbine applications. Continuously variable transmissions can shift smoothly between an infinite 

number of effective gear ratios within a range, as opposed to conventional geared systems that offer a 

limited number of fixed gear ratios (Cotrell 2004). In wind turbine applications, these transmissions can 

vary their gear ratio seamlessly to ensure that the low-speed rotation of the turbine’s rotor, which 

changes with the incoming wind speed, is always stepped up to the exact high-speed rotation required 

for electricity generation. These transmissions allow wind turbines to operate at higher aerodynamic 

efficiencies over a larger range of wind speeds. Conventional wind turbines manage variable-speed 

operation through the use of power electronics, whereas continuously variable transmissions manage 

variable-speed operation mechanically. Continuously variable transmissions can also mitigate rotor 

loads by allowing the rotor to accelerate during wind gusts; this load mitigation reduces required rotor 

and drivetrain structural strength and therefore cost.

The session examined two major categories of continuously variable transmissions: designs that use 

frictional contact between components to vary the effective gear ratio, and hydraulic designs that use a 

fluid to vary the ratio. Frictional designs include rolling traction systems; belt or chain drives; variable 

diameter gears; and differential planetary drives. Rolling traction systems vary the degree or angle of 

physical contact between transmission components (usually spheres, torroids, discs, or cones) to change 

the effective gear ratio. Belt- or chain-drive systems feature two variable diameter pulleys connected by 

a belt or chain; varying the diameter of the input and output pulley(s) changes the gear ratio. Variablediameter gears can change their diameter while engaged to alter the gear ratio. Differential epicyclical

drives, also called differential planetary drives, control the effective gear ratio by rotating the annulus 

(outer ring) of a planetary gearbox.

Hydraulic systems include hydrodynamic drives and hydrostatic drives. Hydrodynamic drives use the 

inertia of the fluid to transmit power, while hydrostatic drives use only static pressure. Hydrodynamic 

drives feature an impeller on the input shaft and a turbine on the output shaft; the impeller and turbine

are coupled via a hydraulic fluid so that torque can be transmitted. This concept is also called a hydraulic 

torque converter. Hydrostatic drives convert mechanical rotation into fluid flow using a pump and then 

back into mechanical motion using a hydraulic motor; the variable speed management is achieved by 

changing the displacement of the pump or motor.
Page | 20

Continuously variable transmissions may offer an improvement over current wind turbine designs that 

use constant-ratio transmissions. These transmissions could allow turbines to operate at a higher 

aerodynamic efficiency over a larger range of wind speeds than do conventional designs. This 

performance improvement would increase the amount of energy produced by the turbine, ultimately 

decreasing the cost of energy. These systems also have the ability to absorb rotor torque from sudden 

wind gusts, thus alleviating fatigue loading and improving overall turbine reliability. Finally, continuously 

variable transmissions reduce the need for power electronics for power conditioning and variable-speed 

operations purposes (see Camm et al. for a detailed description of doubly-fed induction generators in 

variable-speed wind turbines). A reduced role for power electronics could lower overall turbine capital 

cost, reduce power losses due to power electronics inefficiencies, and improve reliability by eliminating 

the need for maintenance-prone slip rings (Cotrell 2004). Finally, some hydrostatic drive configurations 

offer the possibility of energy storage through hydraulic fluid accumulators, potentially smoothing the 

integration of wind turbine energy output into the electrical grid.

Table 6 below lists major areas of interest in continually variable transmission research and 

development and the potential benefits that these areas of interest offer over the state-of-the-art in 

wind turbine drivetrains.
Page | 21

Table 6. Benefits of Continuously Variable Transmissions

The session identified a number of technical and commercial challenges to the commercialization of 

continuously variable transmissions for wind turbine applications. First, these technologies may 

introduce new cost and reliability challenges because their application in wind turbines is currently

untested. Incorporating new components and systems into the turbine could increase operations and 

maintenance costs and introduce new avenues of reliability failure. Second, the scalability of these 

components and materials has not been established. Certain design approaches, such as rolling contact 

designs and belt drives, may not scale to the high torque produced by megawatt-scale wind turbine 

rotors. Third, materials constraints may limit the commercialization of these technologies; hydraulic 

drive systems may need research and development of environmentally benign fluids, while certain 

frictional contact drive designs could require new high-friction materials in order to effectively transfer 

power between components. Finally, the perception of continuously variable transmissions as complex 

and undeveloped technologies may limit the commercial adoption of these technologies.

Area of Interest Description Benefits

Frictional Contact 

Drives

These designs change the physical 

contact between components to vary 

the effective gear ratio. Physical 

contact designs include rolling traction 

systems; belt or chain drives; variable 

diameter gears; and differential 

planetary drives.

 Reduced Capital Costs: these systems 

reduce the need for power electronics.

 Improved Reliability: these systems 

can absorb rotor torque from sudden 

gusts, thereby alleviating fatigue 

loading. These designs could also 

improve reliability by eliminating 

generator slip rings.

 Increased Energy Capture: these 

designs increase turbine energy 

production by allowing turbines to 

operate at a higher aerodynamic 

efficiency over a larger range of wind 

speeds.

Fluid Drives These designs use pressurized hydraulic 

fluid to vary the effective gear ratio. 

Designs include hydrostatic and 

hydrodynamic (or hydraulic torque 

converter) designs. 

 Reduced Capital Costs: these systems 

reduce the need for power electronics.

 Improved Reliability: these systems 

can absorb rotor torque from sudden 

gusts, thereby alleviating fatigue 

loading. These designs could also 

improve reliability by eliminating 

generator slip rings.

 Increased Energy Capture: these 

designs increase turbine energy 

production by allowing turbines to 

operate at a higher aerodynamic 

efficiency over a larger range of wind 

speeds.

 Energy Storage: hydrostatic drives 

offer the possibility of energy storage 

by means of hydraulic fluid 

accumulators.
Page | 22

A research and development effort to facilitate the commercialization of continuously variable 

transmissions would focus on analyzing, proving, and demonstrating the benefits of the respective 

designs and technologies within this larger category. The overall gains in turbine performance, 

reliability, and cost need to be modeled and analyzed for different continuously variable transmission 

designs and in comparison to conventional variable-speed turbine designs. Part of this effort could 

include the development of specialized design, modeling, and analysis tools. Engineering work to prove 

and improve the reliability of continuously variable transmission components and materials in wind 

turbine applications is also needed. Finally, the demonstration of different continuously variable 

transmission designs in turbine test platforms could help establish the costs, benefits, and reliability 

impacts of these technologies. 

Table 7 below lists major barriers to the commercialization of continuously variable transmission 

technologies and potential research and development pathways for overcoming those challenges.
Page | 23

Table 7. Barriers to Commercializing Continuously Variable Transmissions

Area of Interest Commercialization Barriers Development Path

Frictional Contact 

Drives

 Technical Risk: these technologies 

may introduce new cost and 

reliability challenges or increase 

operations and maintenance costs.

 Undertake cost-benefit analysis: 

model and analyze the gains in turbine 

performance, reliability, and cost over 

conventional designs.

 Design and modeling tools: develop 

tools for designing, modeling, and 

analyzing continuously variable 

transmission turbine concepts.

 Scalability Risk: scalability of these 

components and materials has not 

been established for high-torque 

applications. 

 Demonstration: deploy continuouslyvariable transmissions in turbine test 

platforms to establish benefits.

 Materials Risk: could require new 

high-friction materials in order to 

effectively transfer power between 

components.

 Component development and testing: 

optimize components, materials, and 

designs through testing.

Fluid Drives  Technical Risk: may introduce new 

cost and reliability challenges or 

increase operations and 

maintenance; fluid cleanliness 

requirements.

 Undertake cost-benefit analysis: 

model and analyze the gains in turbine 

performance, reliability, and cost over 

conventional designs.

 Design and modeling tools: develop 

tools for designing, modeling, and 

analyzing continuously variable 

transmission turbine concepts.

 Scalability Risk: scalability of these 

components and materials has not 

been established for high-torque 

applications. 

 Demonstration: deploy continuouslyvariable transmissions in turbine test 

platforms to establish benefits.

 Materials Risk: may need research 

and development of environmental 

fluids.

 Component development and testing: 

optimize components, materials, and 

designs through testing.

 Perception Risk: these technologies 

could be perceived as complex and 

undeveloped, limiting commercial 

adoption.

 Demonstration: deploy continuouslyvariable transmissions in turbine test 

platforms to establish benefits.

2.4 Innovative Concepts

This session discussed a variety of novel concepts, design approaches, components, and technologies 

that could be deployed in wind turbine drivetrains. The major concepts discussed were uptower direct 

current generators; innovative drivetrain topologies; ground-level generators; traction- or rim-drive 

turbine designs; tandem generators; and complete uptower gearbox reparability. These technologies 

are at different stages of readiness, from concept studies to field deployment in wind turbines, and each 

offers its own benefits over the state-of-the-art as well as commercialization challenges. Session 

participants also discussed belt drives, variable diameter gears, hydrodynamic torque converters, 
Page | 24

magnetic gears, magnetic bearings, hydrostatic bearings, fluid drives, and the use of advanced materials 

in drivetrains. To avoid duplication, the findings of the Innovative Concepts session with respect to these 

technologies have been incorporated in the preceding sections 2.1 through 2.3. Finally, although not 

strictly an ‘innovative’ concept, session participants identified research and development to overcome 

the gearbox failures afflicting current turbine designs as a crucial effort.

Table 8 below lists major areas of interest in innovative drivetrain concept research and development 

and the potential benefits that these areas of interest offer over the state-of-the-art in wind turbine 

drivetrains. 

Table 8. Benefits of Innovative Drivetrain Concepts

Area of Interest Description Benefits

Uptower Direct Current 

(DC) Generator 

This concept would incorporate DC 

generators into turbine nacelles, 

feeding into a plant-wide DC collection 

system. 

 Improved Reliability: reduced need 

for transistors for AC-DC-AC 

conversion.

 Grid Benefits: energy storage 

potential; improved fault ride-through 

capabilities.

Ground-Level Generators This design would place the generator 

at the base of the tower; the main 

shaft in the nacelle would connect to a 

vertical driveshaft with a bevel gear.

 Improved Reliability: generator at 

ground level may be easier to service 

and replace.

Rim-Drive Turbines An outer rim would enclose the 

turbine’s rotor; generators embedded 

along that rim would produce power.

 Improved Reliability: potentially 

eliminates the need for a gearbox.

Tandem Generators These designs would stack multiple 

permanent-magnet direct drive 

generators in tandem along the same 

horizontal-axis driveshaft.

 Improved Availability: turbine can 

continue to produce power if one 

generator is taken out of service.

 Scalability: concept may scale well to 

larger generating capacities.

Complete Uptower 

Gearbox Reparability

Development of gearboxes and 

drivetrains that can be repaired in situ 

without requiring external cranes

 Improved Reliability: repairs can be 

completed more quickly without 

requiring cranes or wholesale gearbox 

replacement.

Other Potential 

Innovative Drivetrain 

Topologies 

This concept refers to the wholesale 

reevaluation of turbine drivetrain 

layouts.

 Reduced Capital Costs: potential for 

improvement, but needs to be 

demonstrated.

 Improved Reliability: potential for 

improvement, but needs to be 

demonstrated.

Uptower direct current (DC) generators would enable a wind power plant-wide DC collection system, 

designed around the DC bus. Such a system could increase reliability by reducing the number of 

transistors for AC to DC to AC conversion. The system could also improve fault ride-through capabilities, 

increase transmission efficiency, especially for offshore wind systems, and provide opportunities for 

energy storage through capacitors and batteries. The major technical barriers to such a system are the 

design of the collection system and the need for extensive design work on control systems and power 

electronics. A potential research and development approach might include system design and cost 
Page | 25

studies to determine the potential benefits and costs of a plant-wide DC collection system and to review 

the available high-voltage DC hardware needed to build such as system.

Ground-level generator configurations would reduce weight in the turbine nacelle by placing the 

generator at the base of the turbine, potentially with gearbox stages to step up the driveshaft’s 

rotational speed. These designs would incorporate a bevel gear in the nacelle to connect the horizontal 

low-speed shaft to a vertical driveshaft running the length of the tower. This layout would reduce 

nacelle weight, thereby reducing the structural requirements of the tower, and would allow for easier 

maintenance and replacement of the generator. However, such a layout would be mechanically 

complex; specific technical challenges include maintaining alignment in the vertical driveshaft, 

transferring high torque rotation through right-angle gearing, and developing suitable components such 

as a light-weight vertical shaft. Initial research efforts could focus on analyzing the feasibility, costs, and 

benefits of such as system.

Traction- or rim-drive turbine designs represent a radical departure from current common drivetrain 

configurations. As the name implies, these designs enclose the turbine’s rotor within an outer rim; 

generators embedded along that rim would produce power as the rotor moves. Similar designs have 

been developed by the marine and hydrokinetic energy industry to produce power from tides and ocean 

currents. While such designs would reduce or eliminate the need for a gearbox, they would be 

technically challenging to design and construct. Research is needed to determine the technical feasibility 

of the concept and any potential benefits over existing technology.

Stacking multiple permanent-magnet direct drive generators in tandem along the same horizontal-axis 

driveshaft could improve overall turbine reliability and availability by creating redundancy. If one of the 

generators were to fail, the turbine could continue to produce power (albeit at a lower output) while 

that unit was repaired. Scalability of this concept is another major benefit, especially for offshore wind 

applications. However, adding components, weight, and expensive permanent-magnet materials to the 

turbine could drive up capital costs. Feasibility studies would be needed to evaluate the trade-offs 

between added capital costs and improved reliability and reparability. 

Innovative drivetrain topology concepts refer to the wholesale reevaluation of turbine drivetrain layouts 

to see if entirely new component configurations may improve on the current state-of-the-art by 

reducing weight and costs and improving reliability. Such work involves substantial technology risk, since 

these new configurations have not been tested and could create new problems. Research and 

development of these concepts would likely take the form of extensive concept studies to develop new 

design configurations. Should new configurations appear to offer improvements to the state-of-the-art, 

research efforts might expand to include design validation, optimization, and testing. 

Complete uptower gearbox reparability refers to the development of gearboxes and drivetrains that can 

be maintained and repaired in situ without requiring external cranes to hoist equipment. Such a gearbox 

would improve turbine availability and reduce maintenance and replacement costs by eliminating the 

need for expensive crane rentals. Such designs would likely require additional facilities uptower to lay 

out parts, built-in handling fixtures and tools, and extensive condition monitoring systems to determine 
Page | 26

which particular part or component has failed. Feasibility studies would be needed to demonstrate the 

cost-competitiveness of such concepts with costs of replacing or rebuilding conventional gearboxes. 

Table 9 below lists major barriers to the commercialization of innovative drivetrain concepts and 

potential research and development pathways for overcoming those challenges.

Table 9. Barriers to Commercializing Innovative Drivetrain Concepts

Area of Interest Commercialization Barriers Development Path

Up-tower Direct Current 

(DC) Generator 

 Technical Risk: design of the DC 

collection system; need to maintain 

variable-speed control on a DC 

generator. 

 Feasibility Studies: system design and 

cost studies would determine the 

potential benefits and costs of a plantwide DC collection system.

 Equipment Survey: review the 

available high-voltage DC hardware 

needed to build system.

Ground-Level Generators  Technical Risk: maintaining vertical 

driveshaft; transferring high torque 

rotation through right-angle 

gearing; component development. 

 Feasibility Studies: analyze the 

feasibility, costs, and benefits of such 

as system.

Rim-Drive Turbines  Technical Risk: extremely 

technically challenging to design and 

construct.

 Feasibility Studies: analyze the 

feasibility, costs, and benefits of such 

as system.

Tandem Generators  Commercial Risk: adding 

components, weight, and 

permanent-magnets could drive up 

capital costs.

 Feasibility Studies: evaluate the tradeoffs between added capital costs and 

improved reparability.

Complete Up-tower 

Gearbox Reparability

 Technical Risk: may introduce new 

reliability challenges; would require 

additional up-tower facilities; 

requires extensive condition-based 

health monitoring systems.

 Feasibility Studies: analyze costcompetitiveness of concepts versus 

costs of replacing conventional 

gearboxes.

Other Potential 

Innovative Drivetrain 

Topologies 

 Technical Risk: new configurations 

have not been tested and could 

create new cost & reliability 

challenges.

 Perception Risk: designs have not 

been tested, developed, or 

deployed.

 Design Studies: extensive concept 

studies to develop new design 

configurations.
Page | 27

3.0 Summary

Participants at the Advanced Drivetrains Workshop discussed a wide range of technologies, system 

design concepts, components, and strategies that offer the potential to improve the state-of-the-art in 

wind turbine drivetrains. These areas of interest can be grouped into three broad categories based on 

their potential impact on the market for current and next-generation drivetrains. 

The first area of interest includes concept-stage designs for next-generation wind turbines; these 

concepts offer potentially significant but still uncertain advantages over the state-of-the-art. These 

technologies require analysis to evaluate and quantify the benefits they offer over conventional 

drivetrains, as well as feasibility studies to determine their overall technical viability. In many cases, 

these technologies were already studied extensively in the 1970s and 1980s and were determined to be 

not as commercially viable as the drivetrains that ended up in today’s modern wind turbine designs, 

which essentially won out during the 1990’s as the optimum configuration. However, given advances 

over the past thirty years in fields such as materials science, fluid dynamics, and computational design 

tools, there may be opportunities that present themselves upon reexamining innovative drivetrain 

concepts from the past, as well as more recent designs and ideas.

The second area of interest includes technologies that are currently being explored by wind turbine 

manufacturers and third parties for the current generation of wind turbine drivetrain designs. These 

technologies, some of which have been successfully commercialized in aerospace, automotive, marine 

propulsion, and industrial machinery applications, require applied research and development to 

optimize designs, test components in wind turbine applications, and address potential reliability and 

scalability issues. Novel designs in this category may lead to incremental improvements in wind turbine 

reliability, performance, or capital costs.

The third area of interest includes technologies that have potentially transformative application in nextgeneration turbine designs with large nameplate capacities (5+ MW), particularly in offshore wind 

turbines. Many of these technologies have been successfully commercialized in other industries and in 

different applications, such as magnetic resonance imaging (MRI), naval vessels, and civil transportation. 

These technologies require applied research, development, and demonstration programs to facilitate 

their operational deployment, and research programs in this area should have a multi-year perspective 

on the requirements of future wind turbines. 
Page | 28

4.0 References

Cotrell, J. Motion Technologies CRADA CRD-03-130: Assessing the Potential of a Mechanical Continuously 

Variable Transmission. Technical Report, Golden: National Renewable Energy Laboratory, 2004.

E.H. Camm, M. R. Behnke, O. Bolado, M. Bollen, M. Bradt, C. Brooks, W. Dilling,M. Edds, W. J. Hejdak, D. 

Houseman, S. Klein, F. Li, J. Li, P. Maibach, T. Nicolai, J. Patiño, S. V. Pasupulati,N. Samaan, S. Saylors, T. 

Siebert, T. Smith, M. Starke, R. Walli. “Characteristics of Wind Turbine Generators for Wind Power 

Plants.” IEEE Power & Energy Society General Meeting. Calgary: IEEE, 2009. 5.

Fingersh, L., M. Hand, and A. Laxson. Wind Turbine Design Cost and Scaling Model. Technical report, 

Golden: National Renewable Energy Laboratory, 2006.

Hau, E. Wind Turbines: Fundamentals, Technologies, Application, Economics. 2nd Edition. New York: 

Springer, 2006.

Mueller, M., and A. McDonald. “A lightweight low speed permanent magnet electrical generator for 

direct-drive wind turbines.” Proceedings of the 2008 European Wind Energy Conference & Exhibition .

Brussels: European Wind Energy Conference, 2008. 19.

Musial, W., S. Butterfield, and B. McNiff. Improving Wind Turbine Gearbox Reliability. Conference paper, 

Golden: National Renewable Energy Laboratory, 2007.

Poore, R., and T. Lettenmaier. Alternative Design Study Report: WindPACT Advanced Wind Turbine Drive 

Train Designs. Golden: National Renewable Energy Laboratory, 2003.

Ragheb, A., and M. Ragheb. “Wind Turbine Gearbox Technologies.” Proceedings of the 1st International 

Nuclear and Renewable Energy Conference (INREC10). Amman, Jordan, 2010. 8.
Page | 29

5.0 Appendix: Workshop Agenda

DAY ONE: June 29, 2010

11:00 am Registration and lunch

12:00 pm Opening remarks and overview of DOE Wind Program

Jacques Beaudry-Losique, Program Manager for DOE Wind and Water Power Program

12:10 pm DOE’s drivetrain research and development activities

Mark Higgins, Turbine R&D Team Leader for DOE Wind and Water Power Program

12:20 pm ARPA-E’s perspective on innovative drivetrain development

Mark Hartney, Program Director, ARPA-E 

12:35 pm Wind turbine drivetrain state-of-the-art

Dr. Fort Felker, Director, National Wind Technology Center

12:55 pm Breakout session instructions

Mark Higgins

1:00 pm Breakout sessions – technology status & benefits

Four working groups: Superconducting Drivetrains, Advanced Permanent Magnet

Topologies, Continuously Variable Transmissions, and Innovative and Non-Traditional 

Drivetrain Concepts.

1. What is the current state of this technology?

2. How does this technology offer an improvement over the state-of-the-art?

3:00 pm BREAK

3:30 pm Breakout sessions – barriers to commercialization

(Same working groups as before)

1. What are the barriers to the commercialization of this technology in wind turbine 

applications? 

5:00 pm ADJOURN
Page | 30

DAY TWO: June 30, 2010

8:00 am Breakfast

8:30 am Day One recap and instructions

Mark Higgins

8:45 am Breakout sessions – path to commercialization

(Same working groups as before)

1. What is the best way to commercialize this technology for wind turbine

applications? 

2. How would an R&D effort address the barriers to commercialization?

3. What key technical milestones or breakthroughs would be needed?

4. Describe a realistic timeline for commercialization.

5. How much would a successful R&D effort cost? 

6. What might constitute appropriate levels of government, industry, and academic 

involvement?

7. How can we measure the success of any DOE-sponsored projects in this area?

11:15 am BREAK

11:30 am Breakout sessions – consolidating results

Each working group will read through their notes with the facilitator to ensure accuracy, 

and will prepare their key finding to present to the larger workshop audience.

12:30 pm LUNCH

1:00 pm Presentation of breakout session findings

Each group has 20 minutes to present their major findings, including Q&A.

2:40 pm Conclusion and wrap-up

Mark Higgins

3:00 pm ADJOURN
Page | 31