National Renewable Energy Laboratory 

Innovation for Our Energy Future 

A national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

NREL is operated by Midwest Research Institute ● Battelle Contract No. DE-AC36-99-GO10337 

Advanced Wind Technology: New 

Challenges for a New Century 

R. Thresher and A. Laxson 

Presented at the European Wind Energy Conference 

Athens, Greece 

February 27–March 2, 2006 

Conference Paper 

NREL/CP-500-39537 

June 2006 
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Advanced Wind Technology 

New Challenges for a New Century

Robert Thresher, Alan Laxson,*

 National Renewable Energy Laboratory, 

1617 Cole Blvd, Golden, CO, 80401-3393, USA 

Summary 

Wind Energy demonstrated phenomenal growth and improvement in the later three decades of the 20th century. From 

small niche markets for pumping water and charging batteries, wind became an important supplier of grid tied 

electricity. These improvements saw modern commercial wind electrical generators grow from simple 50-kilowatt 

(kW) units to sophisticated multimegawatt (MW) machines capable of generating millions of kilowatt-hours (kWh) per 

year. The technical advances in wind energy in this time frame were not revolutionary. Scaling to larger size was the 

primary approach used to improve the cost effectiveness; and improved design methods to properly size turbines for 

their operating wind environment. Advances in generators, gearboxes, blade designs, blade materials, controls, and 

computers using improved design codes allowed machines to improve in performance and grow in size until the 

average modern wind turbine is now more than 1 MW in rating. But as with any maturing technology, most of the 

easier improvements have already been implemented. 

Now, at the beginning of the 21st century, wind energy is facing a new set of technical challenges to achieve cost 

effectiveness in the lower wind speed regimes located closer to large load centers, while avoiding transmission 

congestion points as much as possible. Success in lower wind regimes and mature turbine design methodologies will 

require that more technically challenging innovative designs be explored. The application of taller towers to take 

advantage of higher winds aloft to increase energy capture at the lower wind speed sites is necessitating new research 

into the turbulent environments at levels of 80 to 150 meter above the ground. At this elevation, the atmospheric 

boundary layer is much different than near the ground, and new simulation models are needed for this so called mixing 

layer above Great Plains sites. In addition, new turbine designs of much greater size are being considered for offshore 

deployment in shallow and even deep waters in both the United States and Europe, where the offshore design 

environment is quite different than for land-based turbines. 

Anticipated improvements in technology that will be seen in the next decade in response to these new challenges 

include: custom designed permanent-magnet (PM) generators; variable-speed power electronic power converters with 

improved performance characteristics and reliability; unique gearbox designs that are smaller and lighter; improved 

aerodynamic and structural dynamics codes that accurately predict unsteady loads and aerodynamic stall effects for 

lighter more flexible turbines under wide ranging atmospheric conditions; improved rotors that utilize aeroelastically 

tailored blades of advanced materials such as carbon-epoxy composites; advanced controls techniques that can decrease 

total machine loads without increasing machine cost; higher blade tip speeds with reduced blade chord and lower 

aeroacoustic emissions; novel new towers employing self erection or advanced composite materials; offshore machines 

supported on floating platforms; and improved marine electrical collection systems. Any one of these improvements 

would be considered a major technological advance, but the wind turbine of 2015 will need many, if not all of these 

innovations to become the low-cost electricity generator of the future. 

A Long Road Traveled 

Until the early 1970s, wind energy filled a small niche market, providing mechanical power for grinding grain and 

pumping water. With the exception of a small number of battery chargers and the rare experiments with larger 

electricity producing machines, the windmill of 1850, or even 1950, differed little from the device of 1200 AD, but the 

later half of the 20th century saw spectacular changes in the technology. Drag-based devices and simple lift-based 

designs gave way to experimentally designed and tested high-lift rotors, many with full span pitch control. Blades that 

had once been made of sail or sheet metal progressed through wood to advanced fiberglass composites with added 

carbon fibers for stiffness and strength. The DC alternator gave way to the induction generator that was grid 

synchronized, which is giving way to variable-speed designs dependent upon the high-speed switches of advanced 

power electronics. From simple inertial controls that feathered or furled a machine, designs moved to high-speed 

digital controls. A 50 kW machine, considered large in 1970, is now dwarfed by the 1.5 to 3.6 MW machines being 

 

* Employees of the Midwest Research Institute under Contract No. DE-AC36-99GO10337 with the U.S. Dept. of Energy 

have authored this work. The United States Government retains, and the publisher, by accepting the article for 

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license to publish or reproduce the published form of this work, or allow others to do so, for the United States Government 

purposes.

1
actively marketed and installed today. Many advances in design technology have contributed to these changes. 

Airfoils are tested in wind tunnels and are designed for insensitivity to surface roughness and dirt. Our knowledge of 

aero induced loads and the ability to incorporate this knowledge into finite element models and structural dynamics 

codes make the machine of today more robust but lighter on a relative basis than those of a decade ago. However, as 

with any maturing technology, many of the simple improvements have been tapped out. 

Challenges for a Maturing Technology 

At the beginning of the 21st century, wind energy is facing a new set of challenges. Many of the most desirable high 

wind speed sites have been developed, and those sites that are still available face severe difficulty gaining access to 

declining transmission capacity. To continue its phenomenal growth, wind turbine designs must evolve to make lower 

wind speed sites more attractive. It has been shown that Class 4 sites with wind speeds of 7.25 m/s represent almost 20 

times the developable wind resource as Class 5 and 6 sites with wind speeds of 7.5 to 8.8 m/s. Class 4 sites on average 

are also much closer to many loads centers, averaging 100 miles versus 500 miles for Class 6 sites. Making wind 

energy more cost effective in the Class 4 sites will drastically improve wind’s future opportunities. To accomplish this 

and continue wind energy growth, machine designs must overcome a number of barriers. 

Machine growth – Over the past 20 

years, average wind turbine ratings 

have grown almost linearly (Figure 

1), with current commercial 

machines rated at 1.5 MWs. Each 

group of wind turbine designers has 

predicted that the machines of today 

are as large as they will ever be. 

However, each new generation of 

wind turbines (roughly every five 

years) has grown along the linear 

curve and has achieved reductions in 

life cycle cost of energy. The 

primary argument for limits in size is 

based on the “square-cube law.” 

Roughly stated this says “as a wind 

turbine rotor goes up in size, its 

power increases as the square of the 

rotor, while its mass increases as the 

cube.” Engineers have successfully 

overcome this barrier by innovation 

in design and materials. Studies have 

shown that in recent years, blade 

mass has been scaling at roughly an 

exponent of 2.5 versus the expected 3 [1]. The desire to increase the size of machines is also driven by the deployment 

of wind in off-shore environments. The cost of installing marine foundations, being significantly larger than onshore, 

demands that the maximum energy capture be achieved from each foundation, thus leading to larger rotor diameters. 

But how long can such im

Figure 1. Wind turbine growth. 

provements continue and not reach a ceiling of size and mass? 

Land transportation represents yet another potential limiting factor for wind turbine growth. Cost-effective 

transportation can be achieved by remaining within standard over-the-road trailer dimensions of 4.1 m high by 2.6 m 

wide. Weights should remain under 80,000 lbs GVW corresponding to a cargo weight of about 42,000 lbs. Loads that 

exceed 4.83 m in height will trigger expensive utility and police assistance. These dimension limits have the most 

impact on the base diameter of wind turbine towers. Rail transportation is even more dimensionally limited. Overall 

widths should remain within 3.4 m while heights are limited to 4.0 m. Limitations are driven by tunnel and overpass 

widths and heights. Transportation weights are less of an issue in rail transportation with GVW of 360,000 lbs possible 

[2]. 

Erection – Increases in machine size have several effects on turbine erection. Larger rotor diameters and the desire to 

take advantage of wind shear by increasing tower heights to place rotors in higher velocity winds are all driving hub 

heights higher. Average turbine hub heights are now 65 meters, but a 2.5 to 3.5 MW turbine on the Great Plains will 

demand a hub height of 80 to 100 meters with hub heights increasing for larger machines. Also, as hub masses and 

tower heights increase, tower diameters must increase, as well as tower thickness to withstand increased bending and 

buckling loads. Both effects can increase erection costs. But, assuming a tower can be segmented and erected in 

sections, the nacelle mass appears to be the limiting factor in turbine erection. Projected masses for nacelles in the 2.5 

2
to 3.5 MW range run from 295,000 to 430,000 lbs. Lifting this mass requires a crane with a much greater lifting 

capacity than expected because of the extreme nacelle height. (It must be remembered that crane ratings are based on 

lifts with little or no boom extension. As the boom is extended both up and out, the lift rating decreases.) Crane costs 

for placing a 2.5 MW turbine on a 110 m tower are expected to run $40,000 to $50,000. While crane costs for erecting 

a 5 MW turbine on a 156 m tower would run as high as $138,000. Reductions in tower height and in turbine tower 

head mass would significantly reduce these costs. These masses and costs are based on machine scaling projections 

and spreading the mobilization and demobilization costs over a 50 MW wind farm. Other limiting factors are that 

cranes with this lifting capacity are rare, are difficult to transport, require large crews, and they have very large 

mobilization and de-mobilization costs. In addition, moving cranes between turbine erection sites is also extremely 

costly and time consuming [2]. 

The Low Level Jet – 

Another factor that will 

effect the design of turbines 

of the future, particularly on 

the Great Plains, is the low 

level (or nocturnal) jet. The 

nocturnal jet is a poorly 

understood phenomenon 

that occurs at nighttime as 

cooling allows the winds to 

stabilize into parallel layers 

flowing at different 

velocities. 

Kelvin-Helmholtz (K-H) 

instabilities can form at the 

boundaries between these 

parallel layers (Figure 2). 

These instabilities can 

rapidly grow into largescale coherent vortices that 

can reach the ground and 

cause gust loads on turbine 

rotors. While the nocturnal 

jet can occur anywhere, it is 

most prevalent over areas of 

the Great Plains; regions 

that represent some of the 

best Class 4 wind 

resources. While the low 

level jet will not limit wind 

growth, it is clear that wind 

turbine designers must clearly understand the implications of the extreme coherent gusts as they place ever larger rotors 

on taller towers. 

K-H Waves

kM 

Figure 2. Kelvin-Helmholtz (K-H) instabilities can form at the boundaries 

between these parallel layers

kM 

 

Grid Interconnection – Concerns exist that the intermittent nature of wind power could adversely affect grid stability 

limiting wind penetration. Wind turbines do not follow a predictable pattern of power generation, except in rare 

locations driven strongly by local weather patterns and land forms, such as in some of the passes of California. The 

requirements of power plant operators deal with three different time scales: regulation – being seconds to minutes; load 

following – being tens of minutes to hours; and, scheduling – being daily (Figure 3). The timing associated with wind 

power generation has been difficult to predict until recently. The fear that the output of large wind power plants will 

plummet to zero in a matter of minutes or seconds has hampered a number of major developments. But recent research 

is shedding a brighter light on the ability of wind power plants to provide a more grid friendly connection. 

3
Time (hour of day)

048 12 16 20 24

System Load (MW)

seconds to minutes

Regulation

tens of minutes to hours

Load

Following

day

Scheduling

Figure 3. The requirements of power plant operators deal with three 

different time scales: regulation – being seconds to minutes; load following 

– being tens of minutes to hours; and, scheduling – being daily. 

Offshore – Recent years have seen a boom in offshore deployments in the Baltic and North Seas. Although these 

developments have been primarily driven by the dearth of new onshore development sites, significant wind resources 

have been identified in offshore regions of both Europe and the Americas. While the marine environments would seem 

similar, it is believed that weather extremes along the northern Atlantic coast may present additional challenges for 

developers. An improved understanding of wind and wave loading and bottom conditions in these novel environments 

will be critical for future development. The challenging marine environment will also make routine access to machines 

less than routine. 

Technologies for Future Success

The myriad of challenges facing wind turbine designers will demand a wide range of technical responses. Innovation 

in design approaches, materials, machine configuration, operational parameters, controls, and power conversion 

techniques will all have to be explored to achieve the projected future cost reductions. Many concepts are already on 

the drawing board or under development. 

Wind Turbine Rotors – The conversion of aerodynamic powe

of wind turbine operation. The ability to withstand 

changing aerodynamic loads and effectively extract power 

from the wind over a 20-year lifespan makes wind turbines 

cost effective. As machines grow larger and larger, rotors 

must increase accordingly. A 1.5 MW turbine has a rotor 

of approximately 70 meters in diameter. A 3 MW rotor is 

roughly 99 meters in diameter. While the blade length only 

increases by 41%, the blade root flap bending moment can 

increase by levels approaching 160%, depending upon 

design [3]. These load increases demand significant 

innovation in design. Several approaches have already 

been developed or are being tested to help either alleviate 

these load levels or create load resistant designs. Carbon 

fibers have been incorporated into wind turbine blades for 

some years to provide localized strength. But the future 

will see lighter stiffer blades incorporating large volumes of 

carbon fabrics presenting options for both tolerating higher 

blade loads as well as reducing overall machine loads by 

reducing rotor weight. Blade planforms are being 

developed that incorporate unique inboard airfoils using 

truncate airfoil shapes (Figure 4). These planforms provide 

increased thickness to chord ratios, providing a greater 

structural cross section to react to outboard loads. 

r to mechanical torque is perhaps the most unique aspect 

Figure 4. Blade planforms are being developed that 

incorporate unique inboard airfoils using 

truncate airfoil shapes 

4
Active designs that can reduce loads are also being explored. The 

concept of aeroelastic coupling, the change in shape of the blade as 

it bends under load, offers possibilities for loads alleviation. Flappitch, also called bend-twist, coupling allows the outer portion of the 

blade to twist as it bends (Figure 5). This is accomplished by 

designing the internal structure of the blade, or orienting the 

fiberglass and carbon plys within the layups, in such a way as to 

make the blade flex in pitch as it bends. This twisting therefore 

changes the angle of attack over much of the blade. If properly 

designed, this change in angle of attack will reduce the lift and 

therefore reduce loads. Such designs offer a method of almost 

instantaneously tailoring blade profiles to alleviate loads without 

placing additional demands upon blade pitch systems. The designs 

must be developed, tested, and optimized so as not to adversely 

impact energy production. 

Figure 5. Flap-pitch, also called bendtwist, coupling allows the outer portion 

of the blade to twist as it bends.

Concepts such as on-site manufacturing and segmented 

blades are also being explored to help reduce transportation 

costs. It may be possible to segment plan molds and move 

them into temporary buildings close to the site of a major 

wind installation so that the blades can be made close to 

the site and transported only a few miles. 

Novel Drivetrains: Converting torque to electrical power 

has historically been achieved using a speed increasing 

gearbox and an induction generator. Many current 

megawatt-scale machines depend on a three-stage gearbox 

consisting of varying arrangements of planetary and 

parallel shaft gearing. Generators are either squirrel cage 

induction or wound-rotor induction, with some newer 

machines using the doubly-fed induction design for 

variable speed. But as machines continue to grow in size, 

additional stages and ever larger gearboxes are required. 

Figure 6. Unique drivetrain designs under development 

to reduce size and cost. 

Several unique designs are under development to reduce the impact of this growth (Figure 6). By 2000, Enercon had 

developed a megawatt-scale direct-drive generator with a wound-rotor salient pole design with 80 magnetic poles. 

However, wound-rotor direct-drive generators for growing megawatt-scale machines are 10 meters and more in 

diameter, making there transport over land problematical unless techniques such as segmented rotor designs are 

employed. The decrease in cost 

significantly effect the cost of 

future generator designs. 

Neodymium-Iron-Boron, the 

permanent magnet (PM) material 

of choice, has a density close to 

steel with a magnetic flux density 

close to that of an electromagnet. 

However, this magnetic flux 

density is achieved without the 

additional mass, size, and cost of 

the copper windings required for 

an electromagnet. This allows the 

creation of smaller generators. A 

preliminary design for a 1.5 MW 

direct-drive generator using rare 

earth PMs has been studied under 

the WindPACT project at the U.S. 

Department of Energy’s National 

Renewable Energy Laboratory 

(NREL). This design uses 56 

poles and is only 4 meters (m) 

in diameter, versus the 10 m for 

and increase in availability of rare earth permanent magnets is expected to 

Figure 7. Single-stage drivetrain uses low-speed generator. 

5
a wound rotor design [4]. This machine has been built and is undergoing testing a the National Wind Technology 

Center. 

A hybrid of the direct-drive approach that offers promise for future large-scale designs is the single-stage drive using a 

low-speed generator (Figure 7). The single-stage drive consists of a single-stage gearbox with a speed increase range 

from 6 to 1 to as high as 10 to 1. This allows the use of a low-speed generator that is significantly smaller than a 

comparable direct-drive design. The WindPACT drivetrain project has developed a prototype for such a drivetrain. 

This design uses a single-stage planetary drive operating at a gearbox ratio of 9.16:1. This gearbox drives a 190 RPM, 

72-pole, PM generator. This approach, which reduces the diameter of the 1.5 MW generator to 2 m, [4] was fabricated 

and is awaiting testing on the 2 MW dynamometer at NREL’s National Wind Technology Center. 

Another approach that offers promise for reduced size, weight, and cost is the distributed drivetrain. This concept is 

based on splitting the drive path from the rotor to drive several parallel generators. Studies have shown that by 

distributing the rotor torque on the bull gear over a number of parallel secondary pinions, a significant size and weight 

reduction is achieved. Clipper Windpower developed a 2.5 MW prototype that incorporates this approach in 2005 that 

was tested at a site near Medicine Bow, Wyoming (Figure 8) and is currently being marketed. 

Power Conversion and Conditioning: The growing desire to allow variable-speed operation and produce high quality 

power is being met by a new generation of power electronics and circuit designs. The advent of medium voltage/high 

amperage power electronics with high-speed switching rates, such as insolated gate bipolar transistors (IGBTs) has 

allowed a revolution in the operation of wind turbines. However, wind turbine power converter designs present a 

unique problem to circuit designers. Because of the profile of wind velocities at most wind sites, wind turbines operate 

far below their power ratings (30% to 40%) a majority of the time. This means that generators and power converters 

that are designed for normal industrial applications and achieve their highest performance at rated power are inefficient 

for wind turbine operation. New designs are being explored that aim to achieve their highest performance at below 

rated power or have a relatively flat performance curve. Other designs are being explored that employ novel circuit 

designs to reduce the number of switches, such as matrix converters. Yet other designs will explore the potential 

advantages of novel switch materials, such as silicon carbide that may allow higher voltages, currents, and operating 

emperatures in smaller packages. 

Figure 8. Clipper Wind Power

distributed drivetrain. 

's new 2.5-MW turbine incorporates an innovative 

t

6
Another major factor that is beginning to impact design approaches is the need for extremely high reliability for 

offshore designs. Power converters, computer controls and other power components must be extremely robust or 

incorporate redundancy to reduce nuisance faults that are much more difficult to deal with on remote offshore 

installations. 

Controls: Advanced control methodologies, as applied to wind turbines, are designed primarily to reduce the cost of 

energy (COE) in one of two ways: increase energy capture or reduce mechanical loads. To increase energy capture, 

researchers are exploring a technique called "model referenced adaptive control." This is a control technique in which 

the controller constantly adapts itself in an attempt to get the real plant (the turbine) to behave like a plant model. In 

this way, the turbine controller can adapt itself to perform optimally despite rotor variability, blade erosion and sitespecific parameters. In an attempt to reduce turbine mechanical loading, a technique called "full-state feedback" with 

"disturbance accommodating control" (DAC) and periodic control has been implemented. In such a system, many 

turbine states are fed back through a control loop and a DAC controller with time-varying gains to decide what controls 

should be activated. A DAC controller has the advantage of allowing disturbances (such as turbulent eddys) to pass 

through the system while tightly controlling parameters of interest, such as rotor speed and blade loads. Employing 

time-varying gains that are synchronized to the azimuthal position of each blade allows the advantage of taking in to 

account turbine parameters that vary during a rotor cycle such as yaw inertia and gravity loads. It is believed that such 

a system will show significant reduction in turbine rotor, shaft, and tower loads. 

Electrical Interconnection – Spatial Variations in 

Wind. While it is impossible to absolutely control the 

availability of wind generated electricity, research and 

development activities are now rapidly improving our 

understanding of wind energy’s reliability, 

predictability, and how well it can be integrated into a 

utility distribution grid. In the past, the fear existed 

that large wind farms would have their power output 

drop to zero in a matter of seconds requiring utilities 

to have spinning reserves on line equal to the wind 

farm capacity. Studies now show that the large 

variations in the power output of wind farms occur on 

the order of hours, fitting well into existing utility 

procedures for dealing with load following. As Figure 

9 shows, while the farm output did drop to zero and 

rise from zero to full power output, these changes 

occurred relatively smoothly over 1 to 4 hours, with 

intermittent peaks and valleys exhibiting much of the 

same spatial variance. The primary reason for this 

smoothing effect is the large scale of wind farms 

(their distribution over a wide geographic area). 

While the wind gusts and slows at any given 

turbine, its effects on other turbines in the wind 

farm vary differently causing the output of the total wind farm to smooth significantly. Figure 9 also shows the output 

of different wind farms within the same general area but up to 100 miles distant display the same general trends, though 

time shifted indicating the movements of weather patterns between the different wind farms. These studies performed 

by NREL are continuing with additional wind farms being instrumented and evaluated to show the overall leveling of 

variations over more widely dispersed wind farms and wind turbines within the same farm. 

Figure 9. Changes in power output occur over 1 – 4 

hours rather than seconds. 

Other research is evaluating the ability to forecast wind output in advance based on short-term weather prediction 

modeling. While such forecasting will not be able to tell a utility operator what the exact wind farm output will be 

hours or perhaps up to a day in advance, they are expected to reliably indicate available power generation from locally 

grid connected wind farms within the planning horizon of the utility. 

Innovative Towers: The cost impact of extremely large cranes and the transport premiums for large tower sections is 

driving the exploration of novel tower design approaches. Several concepts are under development or being proposed 

that would eliminate the need for cranes for very high, heavy lifts. One concept is the telescoping or self erecting 

tower. This concept allows assembly of the nacelle and rotor at close to ground level and then utilizes hydraulics to 

jack the tower and nacelle to its operating height. Other self erecting designs are looking at lifting dollies or tower 

climbing cranes that use tower mounted tracks to lift the nacelle and rotor to the top of the tower. These concepts have 

the added advantage of being able to bring the nacelle to the ground for major overhauls that would otherwise require 

expensive cranes.

7
Construction materials for towers are also 

under study. A study recently completed for 

NREL [5], examined the trade-offs for 

concrete, steel tower hybrids, (Figure 10) 

looking at the possible cost reductions that 

may be realized by limiting the size of steel 

tower sections. 

Composites are an additional material under 

study for wind turbine towers. Small 

Business Innovation Research projects are 

exploring several advanced tower concepts 

using composite materials and novel 

structures, both of which may lend 

themselves to onsite manufacturing and ease 

of transport. 

The Ocean Realm: A recent study shows 

that the United States has the second highest 

offshore wind resource outside of the 

European Union [6]. European experience 

has already demonstrated a range of 

methods for deploying wind energy in 

shallow waters (< 30 meters) using driven 

piers or tripod foundations. To optimize the 

cost expenditures for these foundations the largest rotor diameters and ratings possible must be considered. GE 

Windpower has developed a 3.6 MW turbine for offshore deployment, and models of this machine are being installed 

in European waters. The challenge for the U.S. shallow water environments appears to be a lack of well documented 

knowledge about the potential wind, wave, and ice loadings in desirable sites. Preliminary studies show that 

significantly greater resources can be accessed if turbines can be deployed on floating platforms in relatively deep 

water (up to 200 meters). These installations will challenge designers to develop inexpensive floating or tension leg 

platforms (Figure 11) with affordable and reliable anchoring systems. Although the United States does not currently 

have any offshore developments, several projects are in the permitting process. 

Figure 10. Concrete/steel hybrid towers may reduce 

construction costs by limiting the steel required for towers. 

Additional challenges will face electrical designers who will have to design cost-effective high voltage power 

collection systems, including offshore substations. However, experience in the offshore oil and gas industry would 

indicate that the technical challenges can be overcome. 

Wind turbines may also change in this unique environment. Some architectural aspects driven by proximity to human 

habitation will be removed. Aero-acoustics may cease to be a major design driver allowing even higher tips speeds, 

lower chord, and reduction in loads. The three-bladed upwind machine may eventually give way to the two-bladed 

downwind, more flexible machine, as more and more machines are deployed afloat and better understanding of the 

offshore environment is obtained. 

8
Figure 11. As wind technologies move from onshore to deepwater, new technologies, such as 

floating platforms, will need to be developed. 

Summary 

From a distance, the wind turbine of 2015 will not look much different from the large-scale commercial machine of 

today. The difference in size will be most evident change, with rotor diameters and tower heights dwarfing older 

machines. But besides the obvious difference in size, machines of the past and machines of 2015 will differ drastically 

in ways unseen; in the materials of construction, the means of manufacturer, the design of controls and power 

conversion, advanced smaller and lighter generators, internal blade structure, and many other improvements yet to be 

quantified. But even upon dissection, the improved techniques, knowledge, and tools used to design these machines 

will be undetectable except in the lower cost of energy, higher reliability, and reduced cost. 

References: 

[1] Griffin, D. A. (2001). WindPACT Turbine Design Scaling Studies Technical Area 1 -- Composite Blades for 80- to 

120-Meter Rotor; 21 March 2000 - 15 March 2001. 44 pp.; NREL Report No. SR-500-29492. 

[2] Smith, K. (2001). WindPACT Turbine Design Scaling Studies Technical Area 2: Turbine, Rotor and Blade 

Logistics; 27 March 2000 - 31 December 2000. 224 pp.; NREL Report No. SR-500-29439. 

[3] Malcolm, D. J.; Hansen, A. C. (2002). WindPACT Turbine Rotor Design Study: June 2000--June 2002. 82 pp.; 

NREL Report No. SR-500-32495. 

[4] Poore, R.; Lettenmaier, T. (2003). Alternative Design Study Report: WindPACT Advanced Wind Turbine Drive 

Train Designs Study; November 1, 2000 -- February 28, 2002. 556 pp.; NREL Report No. SR-500-33196. 

[5] LaNier, M. W. (2005). LWST Phase I Project Conceptual Design Study: Evaluation of Design and Construction 

Approaches for Economical Hybrid Steel/Concrete Wind Turbine Towers; June 28, 2002 -- July 31, 2004. 698 pp.; 

NREL Report No. SR-500-36777. 

 [6] Siegfreidsen, S; Lehnhoff, M; Prehn, A; “Offshore Wind Energy Potential outside the European Union” aerodyn 

Engineering GmbH. 

9
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NREL/CP-500-39537 

5e. TASK NUMBER

WER6.7502 

6. AUTHOR(S)

R. Thresher and A. Laxson 

5f. WORK UNIT NUMBER

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

National Renewable Energy Laboratory 

1617 Cole Blvd. 

Golden, CO 80401-3393 

8. PERFORMING ORGANIZATION 

REPORT NUMBER

NREL/CP-500-39537 

10. SPONSOR/MONITOR'S ACRONYM(S)

NREL 

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

11. SPONSORING/MONITORING 

AGENCY REPORT NUMBER

12. DISTRIBUTION AVAILABILITY STATEMENT

National Technical Information Service 

U.S. Department of Commerce 

5285 Port Royal Road 

Springfield, VA 22161 

13. SUPPLEMENTARY NOTES

14. ABSTRACT (Maximum 200 Words)

This paper describes the growth and advances of as well as the challenges faced by the wind energy industry in 

2006. 

15. SUBJECT TERMS

wind energy; low wind speed technologies; cost of energy; 

16. SECURITY CLASSIFICATION OF: 19a. NAME OF RESPONSIBLE PERSON 

a. REPORT 

Unclassified 

b. ABSTRACT 

Unclassified

c. THIS PAGE 

Unclassified

17. LIMITATION 

OF ABSTRACT

UL 

18. NUMBER 

OF PAGES 

19b. TELEPHONE NUMBER (Include area code)

Standard Form 298 (Rev. 8/98) 

Prescribed by ANSI Std. Z39.18