Turbine Tech Drives Wind Into the Generation Mainstream

Issue 11 and Volume 112.

By Nancy Spring, Senior Editor

From the first megawatt-size wind turbine erected in the U.S. in 1941 to the sleek and powerful designs of today, technological advancements have steadily upgraded wind energy’s position in the generation mix so that now, it’s right in the mainstream.

The U.S. is the world’s largest wind power market. For the third straight year, the U.S. led the world in wind capacity additions. More than 27 percent of all new global wind power was installed in the U.S. during 2007. Analysts expect growth to continue, projecting more than $16 billion in new wind power investment for 2008.

AES Corp.’s 170 MW Buffalo Gap 3 wind farm expansion near Abilene, Texas. With 74 Siemens 2.3 MW wind turbines, total capacity of the farm is now 524 MW. Photo courtesy Siemens Energy. Click here to enlarge image

“There has been explosive growth in wind power installations in the U.S. since 2004,” said Matt Stanberry, senior consultant, renewable and distributed energy group, Navigant Consulting Inc. “Each year outstripped its predecessor and 2008 is continuing that trend.”

To supply the market for utility-scale installations, wind turbine manufacturers are working to improve efficiency and reliability and gearing-up for increased production. Stanberry said basic long-term trends include increased blade size, turbine height and nameplate capacity.

“Improvements in gearbox and blade design and weight are some areas being looked at to improve efficiency,” he said. Onshore wind is now a well-established industry, so the learning curve isn’t that steep in terms of the cost reductions that can be achieved. “At this point,” he said, “you’re primarily looking for steady improvement rather than a technology breakthrough for onshore turbines.”

New challenges confront the industry. Supply chain snags have struck the wind power industry with the same force other generation sectors have felt. Stanberry said he expects a shorter-term but significant rise in installed costs driven by increasing commodity prices, rising demand, a drive for higher margins and supply chain constraints. Market responses include consolidation of the big players, like Iberdrola’s purchase of PPM Energy, and vertical integration, such as Suzlon’s acquisition of gearbox manufacturer Hansen.

Manufacturers are trying to contain installation and maintenance costs, causing a shift in investment as manufacturers move into the U.S. Complete turbine assemblies, blades, nacelles and smaller components are now built in these plants.

Partners for Progress

Wind accounted for 30 percent of all new U.S. electricity generation constructed in 2007, surpassed only by natural gas. Installed capacity has now reached the 20,000 MW mark in the U.S. According to the American Wind Energy Association, 10,000 MW of capacity was added from 2006 to 2007, achieving in two years what had previously taken two decades. Even with those numbers, however, wind’s share of the U.S. electric supply is less than 2 percent.

The Department of Energy expects that share to grow tenfold and recently signed a memorandum of understanding with six leading wind turbine manufacturers to develop a shared strategy to promote wind energy. During the agreed-upon two-year collaboration with DOE, GE Energy, Siemens Energy, Vestas Wind Systems, Clipper Turbine Works, Suzlon Energy and Gamesa Corp. will address siting strategies, workforce concerns and the following advanced turbine technology issues:

  • Turbine reliability and operability research and development to create more reliable components, improve turbine capacity factors and reduce installation and operations and maintenance costs
  • Standards development for turbine certification and universal generator interconnection
  • Manufacturing advances in design, process automation and fabrication techniques to reduce product-to-product variability and premature failure while increasing the domestic manufacturing base.

AWEA’s executive director, Randall Swisher, appeared sanguine about the DOE’s goal. “Wind energy installations are well ahead of the curve for contributing 20 percent of the U.S. electric power supply by 2030 as envisioned by the U.S. Department of Energy,” he said in a press release.

Partnerships between government and industry have always played a key role in the growth of the U.S. wind power sector. Research at the National Renewable Energy Laboratory’s National Wind Technology Center (NWTC), a research facility managed by NREL for the DOE, has been instrumental in the development of multimegawatt wind turbines. (See sidebar on page 75.) In fact, much of the wind industry’s success today can be attributed to research conducted there.

NREL’s research efforts led to the development of GE’s 1.5 MW machine. Today, more than 5,000 of the turbine units are installed worldwide. Clipper Windpower produced a prototype of its four-generator turbine there and NREL assisted Northern Power Systems in reviewing and analyzing its design for a 2 MW direct-drive wind turbine. NREL’s facilities and technical support continue to be critical components of the development of new wind turbine technology.

Propelling Turbine Technology

At the most basic level, wind turbine manufacturers want to increase capacity, improve reliability and enhance efficiency. They are making steady, incremental improvements in all three areas by increasing turbine size, exploring new materials and cutting manufacturing costs. Operations and maintenance costs appear to be declining, too, with the introduction of newer models.

Capacity factors have increased for projects installed in recent years, driven by a combination of technological advancements, higher hub heights and improved siting. According to the “Annual Report on U.S. Wind Power Installation, Cost, and Performance Trends: 2007,” released in May 2008 by the DOE’s Office of Energy Efficiency and Renewable Energy, the best wind resource areas can see capacity factors exceed 40 percent.

Click here to enlarge image

An August 2008 NREL report found the most likely scenario to be a sizeable increase in capacity factor with a modest drop in capital cost. In the Conference Paper, “Wind Energy Technology: Current Status and R&D Future,” the authors explained that the long-term drive to develop larger turbines stems from the desire to place rotors higher to take advantage of wind shear—wind speed increases with height. “This is the major reason that the capacity factor of wind turbines has increased over time,” the report said.

Besides growing taller, turbine rotor length has increased and with it the average size of today’s wind turbines. The once-dominant 1.5 MW to 2 MW class is being overtaken by 2.5 MW and 3 MW turbines. According to a report in the July/August 2008 issue of Renewable Energy World, in this class, rotor diameter sizes now reach 116 meters. The article said the first 1.5 MW class of wind turbines built in the mid-1950s featured rotor sizes of 60 meters to 66 meters. Today the 70-meter to 82.5-meter range is common. The first Nordex N80 model, for instance, used an 80-meter rotor in 2001, while the 2.5 MW N100 model, announced in 2007, is fitted with a 100-meter rotor, a 56 percent increase in rotor swept area.

High and low wind sites have different turbine requirements and wind turbine manufacturers have become adept at scaling their turbines by altering rotor diameter. Wind turbines generally start producing power at wind speeds of about 12 mph, reach maximum power output at 28 to 30 mph and stop power production and rotation at 60 mph. Gamesa offered four different 2 MW turbines with rotor diameters from 80 meters to 90 meters to meet those diverse needs, reported a 2007 study by Merrill Lynch.

Designing specifically for low wind areas is another important way to eke out more megawatts. “Some of the research is looking at how to improve wind turbine performance in low wind areas because lower wind regimes are a large untapped market opportunity,” said Navigant’s Stanberry.

Lighter weight designs can help in low wind areas by reducing cut-in speeds and low-speed wind aerodynamic efficiency can be optimized with custom airfoils.

According to the NREL report, the aerodynamic performance of a modern wind turbine has improved dramatically over the past 20 years. Because of the design of custom airfoils for wind turbines, the rotor system can capture about 80 percent of the theoretically possible energy in the flow stream. NREL noted that although rotor design methods have improved significantly, room still remains for improvement.

At some point, increasing size is no longer economic because the cost of building the larger turbine is greater than the energy output revenue. But NREL said studies have shown improved blade design has been able to push that tipping point farther off in the future by using material more efficiently to trim weight and cost. “If advanced research and development were to provide even better design methods, as well as new materials and manufacturing methods… then it would be possible to continue to innovate around this limit to size.”

Most rotor blades are built with advanced fiberglass composites and manufacturers are investing R&D dollars in materials like epoxy, vinyl and polyester resins. Their goal is to keep weight down—the blades of the REpower System LM 61.5P weigh almost 18 tons—and stiffen the blade. Designers are also working with lighter and stronger carbon fiber in highly stressed locations. But carbon fiber is expensive and can cost as much as 10 times the price of fiberglass.

“What you’re trying to do is put together something that balances the need for structural integrity and lighter weights,” said Stanberry.

With composite materials, blades can be configured with different bends and twists. These designs can help reduce fatigue loads but are quite complex and more testing is needed before they are considered optimized.

New blade airfoil shapes are also being designed to reduce cost. They are thicker where the blades need it most, according to the NREL report. “In general, thin flat structures like airfoils are very inefficient at carrying structural loads. The trick is to make a thick and structurally efficient blade airfoil shape that doesn’t give up much in aerodynamic performance.”

O & M Costs Coming Under Control

Even with increased size and output, the new wind turbines don’t necessarily cost more to operate. The DOE “Trends” report said that although tracking operations and maintenance costs is difficult, larger turbine projects with more sophisticated designs may have lower overall O&M costs when measured on a per-MWh basis. The NREL report said O&M costs have reportedly been as high as $0.3 to $0.5/kWh for wind farms with 1980s technology. For the latest generation of turbines, the figure has dropped below $0.1/kWh.

Stanberry said he has seen an increased emphasis on the use of predictive maintenance techniques. Monitoring procedures that are commonplace at traditional fossil power plants such as gearbox oil analysis and infrared photography to detect transformer faults are increasingly being used at utility-scale wind plants.

A service employee performs scheduled maintenance work on a Siemens 2.3 MW SWT-2.3-93 wind turbine. Photo courtesy Siemens Energy. Click here to enlarge image

Clipper Windpower’s turbines, for instance, are built with accelerometers for drivetrain vibration analysis and an oil particle counting device to track microscopic particles of metal in the oil.

“You don’t care what the count is, but you do if it changes,” said Robert Gates, Clipper Windpower senior vice president, commercial operations. “With condition-based monitoring, maintenance can be scheduled and parts ordered ahead of time.”

Stanberry said typical wind farm data are collected and analyzed then compared with fleet data for a variety of uses. “This can help to identify problem areas that are either specific to a turbine or common to all turbines of a particular model or those containing a particular component.”

Historically, the area with the greatest maintenance is on the high-speed end of the drivetrain, said Gates. On the Clipper Liberty turbines, the high speed gears on the drivetrain are designed to be easily removed and replaced with a cartridge.

Larger Turbines, More Challenges

As wind turbines grow in size, so do the challenges. Besides weight and cost, there are practical considerations that limit the size of wind turbines. One of the most difficult logistical problems is transporting these large systems.

“Imagine shipping these huge blades via semitrailer over the interstate highway system,” said Stanberry. “It requires extended semi beds and specific routes. No one has managed yet to successfully build and market a LEGO-type blade.” For a 100-turbine wind farm, hundreds of semitrailer trips may be required just to move the blades.

If the blades are made overseas, “not only are you talking semi transport, you’ve also got the container ship transport and the associated fuel charges,” he said. “That’s part of why we’ve seen the trend of manufacturers opening plants in the U.S.”

New tower configurations are being developed to allow for easier transportation and installation. Some of the new blade shapes could also make highway transport less problematic and on-site manufacturing and segmented blades offer possible solutions to the transportation issue.

The NREL report said crane requirements are another constraint limiting the size of the turbines. “Crane requirements are quite stringent because of the large nacelle mass in combination with the height of the lift and the required boom extension.” As the height of the lift to install the rotor and nacelle on the tower increases, the number of available cranes with the capability to make this lift becomes fairly limited, the report said.

“Cranes can cost between $200,000 and $300,000 and may take 18 separate 18-wheelers to deliver to the site,” said Gates. “And in this industry, you could wait a long time to get one.” The Clipper Liberty 2.5 MW has an onboard crane that can handle some aspects of turbine maintenance, including lowering the generators. The turbine is designed with four output shafts that drive four generators. “Each generator is smaller than one large generator,” said Gates. “The total output is the same but each one is lighter weight” making them manageable for the onboard crane.

One of the most prevalent problems with the huge multimegawatt wind turbines is gearbox reliability.

“The gearbox is the primary focus for maintenance,” said Stanberry. “That’s where we see the challenges for maintenance crews and breakdowns in the machine.”

Steve Blankinship reported on gearbox problems in “Keeping Wind Turbines Spinning,” Power Engineering, August 2008. “Blades on larger machines produce massive torque loads through the typical three-stage gearbox used in these big turbines,” wrote Blankinship. “In an attempt to meet the increased torque requirement, manufacturers have developed huge, costly ring gears and bearings.”

Because of design failures and fleetwide gearbox maintenance issues, the NREL report said it has become standard practice to perform extensive dynamometer testing of new gearbox configurations for durability and reliability. Wind turbine drivetrain technology is likely poised to evolve significantly in the next few years.

Technology Differentiators

Wind turbine designs vary with each manufacturer in distinct and meaningful ways. A survey of the big names in U.S. wind turbine companies reveals some of the latest developments in wind turbine technology.

GE Energy: GE Energy supplies more wind turbines to the U.S. market than any other manufacturer. GE has had success with its 1.5 MW turbine series and the company continues to invest in technology to improve reliability and efficiency.

GE’s new 2.5xl units feature increased rotor size, a double bearing main shaft to minimize gearbox thrust, a permanent magnet-type synchronous generator and cyclic rotor blade pitching. GE said that integrating the converter and transformer down tower, rather than the nacelle, ensures that vibration loads do not affect the power electronics reliability.

The converter cooling system has been designed to minimize moving parts for reliability and features passive coolers that use the same wind that powers the turbine. The 2.5 MW wind turbine can generate reactive power even when the wind is not blowing, reducing the need for additional voltage-ampere-reactive generating equipment. The nacelle and tower dimensions of the 2.5xl allow for transportation and installation procedures comparable to GE’s standard 1.5 MW turbines.

GE’s 3.6 MW series, a larger version of the 1.5 MW design, was built for high-speed wind sites, with a rotor diameter of 104 meters and a swept area of 8,495 square meters. GE said the 3.6 MW machine is well-suited for offshore applications. It uses a distributed drivetrain design where all nacelle components are joined on a common structure to enhance durability. The generator and gearbox are supported by elastomeric elements to minimize noise emissions.

Vestas: Vestas ranks among the leading wind power turbine producers in the world, with more than 35,500 of its machines operating in 60 countries. The company is expanding its manufacturing presence in the U.S.

Vestas said keeping turbine weight down is a priority. To reduce weight, Vestas uses materials such as lightweight carbon fiber in blades, strengthens the tower with a stronger type of steel and uses magnets that reduce the overall amount of steel required.

Vestas produces a 1.65 MW turbine and two 2.0 MW turbines, available with an 80- or 90-meter rotor. Vestas introduced its 3.0 MW turbine, the V90, in 2002, which the company said weighs less than the V80 2.0 MW turbine despite the larger rotor and generator. The V90 has a lighter, stronger tower and a nacelle design that produces more power with less weight. The blade structure, different from previous blade technologies, incorporates new materials and a revised blade profile design.

Clipper Windpower: Clipper’s stated design mission is to go beyond “just scaling up,” which the company said pushed conventional wind turbine design at multimegawatt proportions to its limits, “resulting in increasing component failures and rising unscheduled maintenance costs.” Clipper’s goal with its Liberty turbine series is to increase crew safety benefits, advance preventive maintenance and provide economical installation advantages.

Clipper Windpower’s 330,000 sq. ft. manufacturing facility in Cedar Rapids, Iowa. Photo courtesy Clipper Windpower. Click here to enlarge image

The Liberty turbine is designed for weight savings, so it requires only limited crane capacity. The largest turbines in the Liberty 2.5 MW series are the C-99, with a rotor diameter of 99 meters, and its sister turbine, the C-96. The Liberty turbine is variable speed, with what the company called a “unique” distributed powertrain. The drivetrain has four load paths to reduce the load on the gear teeth and four permanent magnet generators. If one generator is out for repair, the others will still run. Maintenance can be done with the onboard crane and the Liberty series has several condition-based monitoring features.

The architecture of the electrical system has been improved, said Clipper’s Robert Gates, senior vice president, commercial operations, with variable speed constant frequency operation to control and eliminate excess power the larger rotors develop from wind gusts. “With electronics today, you can limit how much energy the generators will take out of the drive train and limit these gust-induced high loads,” he said. The 2.5 MW machines use solid state switching devices. When a wind gust comes, the Clipper turbine takes the generator output with its variable frequency, converts it to direct current and sends it down the tower. “At the bottom is a converter that takes the direct current and chops it up into 60 cycles,” Gates said. With this improvement over earlier designs, there is less chance the generator will fail because there is no current going into the armature where arcs could occur and cause pitting.

Siemens Energy: Siemens’ SWT-2.3-93 wind turbines have a capacity of 2.3 MW, with a 93-meter rotor diameter. The rotor is a three-blade cantilevered construction, mounted upwind of the tower. The power output is controlled by pitch regulation.

A new 101-meter rotor for the SWT-2.3 wind turbine is one of the company’s latest accomplishments, said Oliver Loenker, an energy sector spokesperson for Siemens AG. Compared with the standard 93-meter rotor for this turbine type, he said the new rotor has an almost 10 percent higher energy yield without any corresponding increase in structural loading. A 2.3 MW pilot model is being tested at the NWTC.

Siemens optimizes turbine performance in various ways. Blade aerodynamic improvements are based on a combination of computerized fluid dynamics calculations, wind tunnel experiments and field measurements, said Loenker.

Siemens’ blade technology is a proprietary concept where blades are cast in one piece, with no glue joints. Siemens said it offers two types of power limitation: stall regulation and pitch regulation. Both methods are based on the continuous adjustment of the pitch setting of the blades relative to the hub. Each blade has its own hydraulic actuator unit with position feedback, ensuring continuous stable operation.

The SWT-3.6-107 wind turbine is the latest model in the product range, suitable for onshore and offshore applications. Siemens is constructing its first U.S.-based wind technology R&D center in Boulder, Colo.

Gamesa: Gamesa gained market entry to the U.S. several years ago, installing 25 of its G-87 turbines in the Kumeyaay reservation in San Diego, Calif. In 2008, Iberdrola Renovables installed 36 G87 2 MW wind turbines in Illinois and 10 G83 2 MW wind turbines in Iowa.

Gamesa’s latest product range includes three turbine models, all rated at 2.0 MW. Gamesa said differentiating features include predictive maintenance and noise control systems.

Gamesa blades are manufactured using technology that the company said allows for lighter blades with enhanced resistance specifications to provide better dynamic performance and fewer loads passed on to the other components.

Gamesa’s wind turbines are equipped with a variable pitch technology that allows for feathering the angle of the blades for each wind speed. Gamesa said this system maximizes wind energy and reduces torque on the device, allowing the full blade to be used as an aerodynamic brake.

Gamesa’s wind turbine generators operate on the basis of variable rotation speed to provide stable generation at the grid frequency. The company equips its machines with doubly fed wound rotors. The supply frequency to the rotor is altered depending on the speed of mechanical rotation.

Mitsubishi: Mitsubishi wind turbine innovations include lightweight blades with full-span pitch control and advanced noise reduction technology.

The Klondike III project in Wasco, Ore., is the North American test site for the Mitsubishi 2.4 MW MWT92 turbine, a 92-meter rotor wind turbine manufactured in Japan. The highest point above the ground from the foundation to the tip of a rotor blade is 116 meters. The turbine is designed for ease of transport and assembly, said the company. The “smart yaw” safety feature rotates the machine in a reverse configuration so it acts in a weather-vane pattern during high wind speeds and changing wind directions. The company said this lightens the turbine wind load so it can cope with wind speeds as high as 156 miles an hour. At the low end, the turbine generates electricity in winds as low as six miles an hour.

Suzlon: Suzlon was ranked as the fifth leading wind turbine supplier in the world in 2007 by the BTM Consult ApS World Market Update 2007. Suzlon’s latest megawatt-series S82 1.5 MW wind turbine incorporates features like “micro pitch” technology, a high-performance gearbox and a yaw system. Suzlon’s largest wind turbine, the S88, is 2.1 MW.

Nordex: The Nordex product range includes one of the largest series of wind power systems in the world, the N80, N90 and N100. The N100 is a 2.5 MW wind turbine configured for low and moderate wind conditions. It has a rotor diameter of about 100 meters and is available on 100-meter steel tube towers. Nordex said the N100 offers low maintenance with easy access because of an integrated crane installed in the interior.

Several other manufacturers are worth watching as the U.S. market continues to expand.

The first installation of Accinoa Energy North America’s 1.5 MW turbines in the U.S. went online in July at the 180 MW Tatanka Wind Farm in North and South Dakota. The company’s latest model is the AW-3000, a 3 MW turbine that moves the company into the multimegawatt market. The company offers three rotor diameter sizes for the AW-3000: 100, 109 and 116 meters. To reduce gearbox loads, the company said it has fitted the main shaft onto a “double frame.”

Enercon is producing the E-82, a 3 MW machine that reportedly has an unusual level of high performance because of its rotor blade design.

Alstom is also working on a 3 MW class turbine, which could be in production by 2009. The ECO100 has a rotor diameter of 100.8 meters.

DeWind’s series of wind energy turbines includes a new 2 MW D8.2 model with a hydrodynamic torque converter developed by Voith AG. It is used in combination with a synchronous AC generator with high voltage output that is connected directly to the grid without the use of power conversion electronics.

Vensys entered the wind energy manufacturing scene in 2006, installing three prototypes of its 1.2 MW turbine in Canada. The 2.5 MW direct-drive Vensys 90 has a 90-meter rotor and the Vensys 100, a 100-meter rotor. The company developed a new generator for these turbines to address construction cost and weight issues.

Supply Chain Snag Strategies

AWEA has tracked increased investment in U.S. wind turbine component manufacturing facilities for the last year and a half, reporting that at least 41 facilities have been announced, opened or expanded. According to the DOE’s 2007 “Trends” report, GE Energy remains dominant, but other companies are making advances.

Vestas Blades America announced in August it had selected Pueblo, Colo., for what it calls the “the world’s largest windmill tower plant.” The new plant is scheduled to produce 1,000 towers a year. Vestas also operates a blade factory in Windsor, Colo., with capacity to build more than 2,000 blades a year. The company plans to build a second blade factory and a nacelle plant in the state. In all, the company reportedly is making a $1 billion investment in the U.S.

Clipper Windpower has a factory in Cedar Rapids, Iowa, where it expects to produce more than 300 turbines in 2008. DeWind D8.2 turbines are now being assembled at TECO Westinghouse Motor Co., in Texas. Siemens operates a wind turbine blade factory in Fort Madison, Iowa. Nordex plans to spend $100 million to establish its own U.S. production facilities for wind turbines and rotor blades; the company hopes to have 2.5 MW wind turbines produced in the U.S. as early as 2009. Suzlon manufactures rotor blades and nose cones in Pipestone, Minn. Acciona has a turbine assembly plant in West Branch, Iowa.

In the U.S., many wind turbine manufacturers are sold out of capacity through 2010.

“Wind turbines today are very hard to come by,” said Jason Fredette, director of investor and media relations at American Superconductor Corporation (AMSC). “You could wait many months or even years. The industry needs new suppliers of wind turbines to alleviate that.”

AMSC subsidiary Windtec licenses its wind turbine technology to manufacturers.Among them is Korean company Hyundai Heavy Industries, which announced such a deal in October. Hyundai already has a presence in the U.S. power equipment market and wants to break into the wind turbine market quickly. HHI licensed a 1.65 MW and a 2 MW tubine from Windtec and hopes to be ready to ship to the U.S. at the end of 2009.

“We’re bringing those manufacturers into the market very, very quickly with proven wind turbine designs, and Hyundai is the latest to do that,” said Fredette.

Order backlogs, of course, are one of those problems a company likes to have. But serious supply chain problems dogging other generation sectors have started to hit the wind industry.

Some analysts say supply chain management is the biggest challenge facing wind turbine manufacturers today. Researchers for the Merrill Lynch study found “no spare capacity in the supply chain.” Some components, like gear boxes, are especially difficult to source.

This year, an Emerging Energy Research study analyzing global wind turbine markets and strategies found intense competition among wind turbine suppliers and their component providers to keep up with demand. According to EER, it will be a seller’s market for wind turbines in the short term, while the industry builds out for a new phase of stable, global growth, stimulating the market to surpass $56 billion by 2015.

According to Timothy Poor, AMSC vice president of global sales and business development, the company leverages its position in the supply chain for its licensees.

“We coordinate the supply chain and assist with setting up manufacturing and assembly,” he said. “As part of the licensing arrangement, we supply the core electrical components. We provide full service, soup-to-nuts, turnkey support to companies that want to become wind turbine manufacturers.”

Collaboration can also be an effective supply chain strategy. Siemens and E.ON entered what the companies call a “new era of collaboration” when they signed a large wind power deal in September. Siemens will deliver 500 2.3 MW wind turbines for E.ON’s projects in the U.S. and Europe, the largest single wind power deal in Siemens’ history. The machines will feature various combinations of tower heights and rotor diameters. Through the collaborative process, the companies hope to improve turbine efficiency and reduce maintenance costs.

Rapid scale-up has already caused some manufacturing issues. The DOE 2007 “Trend” report found that several companies have experienced problems. Suzlon, for instance, had blade issues but moved to resolve the problem with a retrofit program the company announced in March. Blade cracking issues discovered during the operation of some of its S88 turbines in the U.S. will be addressed by structurally strengthening 1,251 blades. Suzlon will maintain a rolling stock of temporary replacement blades to minimize the downtime for operational turbines. The company said it expects the program to be completed over a six-month period and estimated the total cost at $25 million.

Designs for the Future

Can we expect to see revolutionary design modifications in the near future? Siemen’s Loenker doesn’t think so.

“The wind industry does not have many examples of truly revolutionary design modifications,” said Loenker. “Most ideas that may appear novel have actually been invented decades ago, sometimes as early as the 1800s.”

Loenker said Siemens does expect design modifications of major importance for turbines in the near to medium-range future, including new blades for the large turbine types based on the aerodynamics of the new Siemens 2.3 MW 101 meter rotor; the introduction of direct drive (gearless) technology for large offshore wind turbines; new tower concepts; and new offshore foundation concepts.

According to the NREL report, while no big technology breakthrough is around the corner, the cumulative effect of evolutionary and incremental technical steps could lead to a 30 percent to 40 percent improvement in the cost-effectiveness of wind technology over the next two decades.

Many analysts are looking offshore for the next major technological advancements.

“One area for a potential technology breakthrough is offshore wind,” said Navigant’s Stanberry. “That’s an area where manufacturers are developing some really innovative designs.”

Interest is growing in the application of high temperature superconductive (HTS) machines to improve efficiency.

“These systems can be used to drive down maintenance costs because they eliminate the need for reduction gearing,” said Stanberry. “We’re seeing R&D investment in these types of systems with the focus being reducing or eliminating gearbox maintenance costs.”

AMSC’s Fredette said the company kicked off a program a year ago with TECO-Westinghouse Motor Co. to develop technologies needed for a high temperature superconductor wind generator. He said work is progressing on a 10 MW offshore wind turbine.

“Inside each turbine is a direct drive generator made with superconductor wire,” said Fredette. “You can shrink the generator way down in size and weight. The superconductor technology is definitely on the horizon and we think it will be successful in the future.”

Offshore machines face a host of difficult challenges, including getting maintenance crews to the turbines and withstanding wave action and storms. While research on floating platform machines shows promise, Stanberry said commercial viability remains a ways off. “You’re still talking about placing structures on the sea floor, which has its own challenges.” He said some floating offshore rigs are being developed based on oil and gas rigs.

Offshore turbines are already larger than onshore machines and work is underway to push offshore turbine technology even farther. According to the British Wind Energy Association, Clipper Windpower is involved in a project with the National and Renewable Energy Centre (NaREC) in Northumberland to develop a 7.5 MW offshore turbine. Stanberry said Clipper is developing a 10 MW offshore turbine.

A potentially more radical design on the drawing board at NaREC is named the Aerogenerator. Promising to break the “propeller on a stick” mold, its V-shaped rotor is stacked with wing-like attachments swivelled to the vertical that sits upright on a horizontal platform. The aerogenerator is designed for offshore installations and is intended to generate up to 9 MW.

A little on the wild, revolutionary side is a wind turbine blade design that claims to be based on a fundamental advance in fluid dynamics. Tubercle technology is named after the bumps on the leading edge of the humpback whale’s flippers. According to WhalePower’s website, the company is building on the science by adding precisely formed versions of those bumps to the leading edges of the blades or rotors. In wind and water tunnel studies, WhalePower said tubercle blades easily overcome fluid dynamic “limitations” that were once considered unavoidable laws by engineers, technicians and scientists, which will make wind turbines more efficient and more reliable.

HTS systems and revolutionary blade designs are one thing, but laying transmission cables on the sea floor is another.

“Undersea transmission cables run in the millions of dollars per mile,” said Stanberry. “But wind speeds are much greater offshore than onshore and that leads to improved capacity factors. While there may be a number of real challenges, real potential exists there for technological breakthroughs.”

WIND research facility tests latest technologies

The National Renewable Energy Laboratory’s (NREL’s) National Wind Technology Center (NWTC), south of Boulder, Colo., is the nation’s premier wind energy technology research facility. Built in 1993, the center provides an environment for developing and testing advanced wind energy technologies.

A 45-meter wind turbine blade undergoes testing at the NWTC in Colorado. Photo courtesy NREL. Click here to enlarge image

Wind turbine research conducted at NREL assists U.S. industry in developing cost-effective, high performance wind turbine technology that will compete in global energy markets. NREL researchers work with industry partners that are selected through competitive solicitations and share in the cost of research and development projects. NREL’s wind energy research efforts led to the development of the GE 1.5 MW wind turbine. Today, more than 5,000 units of the turbine are installed worldwide. As a result of NREL’s research and accelerated testing capabilities, Clipper Windpower produced a prototype of its four-generator turbine in 2006. The design split the drive path from the rotor to drive several parallel generators, reducing size and weight.

Responding to the trend for taller and more flexible utility-scale wind turbines, NREL researchers are investigating ways to mitigate system fatigue by gaining better control of component interaction and movement. Studies conducted at NWTC include blade pitching and testing new components such as twist-coupled blades and advanced devices such as micro-tabs. NREL said its researchers are developing innovative hub control strategies to mitigate unwanted aerodynamic loads at the rotor hub and investigating ways to improve design codes.

As wind turbines become lighter and more flexible to reduce manufacturing costs, control mechanisms are needed that stop high winds from damaging the turbine but don’t interfere with capturing the maximum amount of energy from the wind. NWTC researchers are working with control mechanisms and computer codes to help wind turbines shed some loads in extreme or turbulent winds.

To accommodate the new longer blade designs, NREL has installed a larger blade test stand capable of testing blades up to 50 meters long and developed a hydraulic resonance blade test system.

At NREL’s 2.5 MW dynamometer test facilities, industry can conduct a range of system tests that cannot be duplicated in the field, including gearbox fatigue, wind turbine control simulations, transient operation, and generator and power system component efficiency and performance tests. As one of the only facilities of its kind in the world, the 2.5 MW dynamometer conducts full-system tests and identifies critical integration issues before field deployment.—NS