This article focuses on works of various engineering units, which are working to design large wind turbines that can extract more power with greater efficiency. One of the biggest recent developments in wind energy is that grid operators have been successful in finding ways to integrate copious quantities of variable, location-constrained energy resources at reasonable costs, without compromising the overall requirements of a safe and reliable electricity supply system. While most of the wind power development in the United States is in onshore wind farms, internationally some big projects are being built offshore. The economics—and politics—of offshore wind allow for extraordinarily large turbines. Researchers are exploring the aerodynamic design of blades, advanced approaches to control the turbulent flow around the blades, and lighter and structurally more resilient materials for blades, gearbox technology, and power transmission systems. The integration of advanced computational approaches with laboratory and field-scale experimentation is helping researchers understand the very complex interaction between turbulence in the atmosphere and the machine.
Wind turbines dotting the landscape may seem futuristic, but the roots of the technology are ancient. Early sailors were probably the first to exploit the power of the wind, and by around 100 A.D. a Greek mathematician and engineer named Heron devised a wind-driven wheel to power an organ in ancient Alexandria.
Soon windmills were being used across Europe, the Middle East, and Asia to process grain and to pump water. In the United States, 19th century sodbusters used wind-powered pumps to pull water from wells, helping to transform vast acres of semi-arid prairie into irrigated farmland. Today, the multi-bladed wind pump is an icon of the American heartland.
The basics of wind power haven’t changed much over the centuries. The force of the wind turns airfoils—originally sails, now blades—around a rotor, which is connected to the main shaft. The shaft then spins a generator to generate electricity.
“A wind turbine seems easy enough to build, but it's actually very complicated,” said Paul Veers, chief engineer at the National Renewable Energy Laboratory's National Wind Technology Center in Golden, Colo. “Engineers must integrate aerodynamics, structural dynamics, and material fatigue with gear boxes, electric generators, and other components, and connect it all to the grid.”
A lot of engineers, however, don’t have much experience with such large-scale projects. Wind turbines are the largest rotating structures in the world. The entire process—from design to installation to operation and maintenance—deals with very large components.
“Wind turbines are immense machines,” said Douglas Adams, professor of mechanical engineering at Purdue University in West Lafayette, Ind., and director of the Purdue Center for Systems Integrity. “Onshore, utility-scale wind turbines stand nearly 300 feet off the ground and have rotor diameters approaching 300 feet. It is fascinating that the operation of such immense machines depends critically on very small things—such as the lubricating layer 1-micron-thick between the gear teeth in the gear box.”
Wind farms are rising against the horizon in Arizona (opposite), Colorado (left), Oklahoma (near left), and elsewhere.
Photos: Iberdrola Renewables, Inc.(opposite); Jenny Hager
Photography (left); Todd Spink (near left).
Rotors have increased in size from about 150 feet in diameter 20 years ago to about 400 feet in diameter today, with towers over 300 feet in height.
A bigger turbine diameter means a larger area can be swept; a taller tower allows turbines to catch faster-blowing winds at greater distances from the ground. When combined, these trends enable the turbine to extract more power from the wind.
“As a result, utility-scale turbines can produce as much 5 MW of power today,” said Fotis Sotiropoulos, a civil engineer and director of St. Anthony Falls Laboratory and the EOLOS Wind Consortium at the University of Minnesota in Minneapolis. “This is the result of major advances in rotor aerodynamics, wind turbine controls, and materials, all of which enable lighter and structurally more reliable turbine designs that can operate safely and efficiently under high wind speeds.”
“We haven’t hit the barrier yet for how large these machines can be,” Veers said.
“The only restriction for size on land,” Veers added, “is the difficulty in transporting parts. Offshore, wind turbines continue to get larger because components can be brought in by barge.”
New Turbines Aren’t just Bigger—they are Getting Smarter, Too. Smart wind turbines are one of the most important developments in wind energy technology. With their embedded sensors and data processing algorithms, smart wind turbines can recognize when wind conditions, such as direction and speed, have changed, so the turbine control system can quickly adapt to those changes.
“This adaptive capability is a game-changer because it allows the turbine to maximize the power it produces, while simultaneously ensuring the reliability of the turbine is maintained,” Adams said. “In other words, smart wind turbines are a key technology for minimizing the cost of wind energy. When wind energy costs come down, more wind farms will go up.
“Historically, wind turbines have been designed to operate optimally in specific wind conditions,” Adams added. “Therefore, smart wind turbines should help expand the operating envelope of wind turbines so that they operate optimally, even when wind conditions vary.”
The U.S. Department of Energy's wind research focuses on integrated, systems-level optimization of the entire wind plant. Researchers are interested in understanding the multi-scale physics that impact the performance and reliability of wind plants, ranging from mesoscale atmospheric flow to the microscale flow over the blade surface.
“Research has shown that the performance of the downstream wind turbines is significantly degraded when operating in the wakes of the upstream turbines,” said Shreyas Ananthan, wind program aerodynamicist for the Department of Energy in Washington, D.C. “Understanding the evolution of the wind turbine wakes within the wind plant, and its dependence on the atmospheric stability and turbulence, are keys to improving performance and reducing the cost of energy in the coming years.”
Wind turbines on a wind farm actually interact with one another through these “wake effects,” which occur when wind passes through the rotor of a wind turbine and passes downstream. The wake consists of a volume of lower velocity air that creates fluctuations downstream which then encounter the next rotors. Detailed computer modeling shows how turbulence in the atmosphere and complex topography affect the energy-capture ability of wind turbines and impact structural loads on the turbine blades.
“With computer models we actually see the wakes and how they affect the downstream turbines,” Veers said. “By aligning rotors with the wind in specific ways, curved wakes can be created that can be steered away from downwind turbines.”
Another “smart” improvement is passive loadshedding capability. Dynamic controls allow each blade to pitch at its own ideal angle, independent of the other blades, depending on the location of the wind shear. “This can reduce load by 20 to 25 percent,” said Veers.
Load shedding is also enhanced by feed-forward controls using light detection and ranging, or LIDAR. Such systems shoot laser beams in front of the turbines; the light reflects off particles carried in the wind, allowing calculation of both velocity and changes in velocity before the wind gets to the turbine. The LIDAR data provide advanced information that allows for the most efficient blade positioning.
The University of Minnesota's EOLOS wind energy research consortium uses innovative computational approaches to study offshore wind farms—especially how turbines respond to the combined effect of turbulence induced by wind and ocean waves.
“Experiments in our wind tunnels are helping us understand how the relative arrangement of wind turbines impacts the power production of wind farms,” said Sotiropoulos. “We have found, for example, that staggering the turbines can be more efficient than aligning them in rows. We have also learned that using variable size turbines, and mixing together larger and smaller turbines, can be quite beneficial for optimizing wind farm energy extraction.”
The Economics—and Politics—of Offshore Wind Allow for Extraordinarily Large Turbines. Those with Rated Power in the 10 mw to 20 mw Range and Rotor Diameters as Great as 500 Feet are now being Developed.
Computational Might is also being Employed to Study how to Make the Turbine Blades Larger and more Efficient. Todd Griffith, offshore wind technical lead for the Wind and Water Power Technologies Department at Sandia National Laboratories in Albuquerque is investigating potential technology barriers for future large blades. Recent work includes study of carbon materials for large blades and an associated manufacturing cost analysis. Researchers are also investigating the impact of flatback (or thick) airfoils on the weight and performance of very large blades.
“Research aimed at improving operations and maintenance processes is another active area of research at Sandia,” Griffith said. “We are developing a roadmap for structural health and prognostics management applied to wind plants, as this technology has great potential to reduce operational and maintenance costs and increase energy capture. We are also developing tools for evaluating the techno-economic feasibility of enhanced blade sensing and smart load management.”
Purdue University is collaborating with Sandia National Laboratory to develop these sensing strategies for wind turbine blades, as well as the data processing algorithms that work along with these sensors to reduce the cost of wind energy. For instance, inertial sensors are mounted inside the blade cavity to detect small amounts of aerodynamic imbalance due to phenomena such as pitch error in the blade setting, soling of the blade, or ice accretion.
“Sources of imbalance like these can substantially reduce the power produced by the turbine and degrade the reliability of the blades and driveline,” said Adams. “If a turbine is not available, it cannot produce power, which drives up the cost of energy. By detecting aerodynamic or mass imbalances, we can adapt the operation of the turbine to ensure that it remains productive.”
Also, if sensors indicate that the blade has been damaged, the turbine can be operated to produce less power so that it does not over-exert itself, leading to a major blade failure.
Safety, efficiency, and power production can intersect in other ways. One of the biggest recent developments in wind energy is that grid operators have been successful in finding ways to integrate large quantities of variable, location-constrained energy resources at reasonable costs, without compromising the overall requirements of a safe and reliable electricity supply system.
This balance has been accomplished through new wind turbines with more grid-friendly electrical characteristics, better wind forecasting techniques, advanced grid-wide communications and control, and regulatory changes to eliminate uncertainty in technical requirements.
“Turbine designs for onshore applications appear to have reached some practical limits for size—around 3 to 3.5 MW for power rating and 300- to 400-foot rotor diameters,” said Steve Williams, senior engineer for S&C Electric Co. in Milwaukee. S&C Electric works with many renewable plant owners to facilitate interconnection and control of wind plants.
“We are also seeing more applications of energy storage systems to integrate wind and solar energy sources, for both large, centralized power systems and microgrids,” Williams said.
Energy storage helps maximize wind plant output and mitigate inherent intermittency issues in order to make renewables a more stable and reliable source. This makes it more profitable for project owners and much easier for utilities to manage.
“These solutions make wind more viable and attractive, which is especially important as the U.S. seeks energy independence and cleaner sources of power,” Williams said. “Many states have renewable portfolio standards and rigorous goals to meet.”
While most of the Wind Power Development in the United States is in Onshore Wind Farms, Internationally Some Mam-Moth Projects are being Built Offshore. For instance, an offshore wind farm with over 2 GW capacity is currently being developed off the coast of South Korea.
“I believe the wind energy industry will increasingly look at offshore opportunities because of the desirable aspects of the offshore wind resource and that wind energy technology will increasingly involve global research collaborations,” Adams said. “As new technologies continue to reduce the cost of manufacturing, installing, and maintaining turbines, I think we’ll see more onshore wind energy installations in North America.”
The economics—and politics—of offshore wind allow for extraordinarily large turbines. Turbines with rated power in the 10 MW to 20 MW range and rotor diameters as great as 500 feet are now being developed.
“Larger and taller turbines are critical for helping the industry achieve economies of scale, making wind farms cheaper to install and maintain since fewer turbines are required,” Sotiropoulos said.
Bigger turbines also mean engineers must deal with bigger mechanical stresses.
“I recently read that one blade designer was moving away from the traditional spar design, as they increased machine size due to inherent limitations of spars,” Williams said. “Other designers are looking to improve reliability by eliminating the gearbox, and instead using a high-pole-count, low-speed electrical machine directly connected to the turbine shaft. Another example is reducing tower mass by incorporating active damping control systems. All of these improvements point to modern analysis and optimization techniques, instead of brute-force solutions.”
Researchers are also exploring the aerodynamic design of blades, advanced approaches to control the turbulent flow around the blades, and lighter and structurally more resilient materials for blades, gear box technology, and power transmission systems. The integration of advanced computational approaches with laboratory and field-scale experimentation is helping researchers understand the very complex interaction between turbulence in the atmosphere and the machine.
“Such knowledge, which was not available to the engineers who designed the current generation wind turbines, will revolutionize wind turbine design,” Sotiropoulos said. “Most importantly, this knowledge and advanced research tools will enable engineers to tackle in the near future the problem of wind plant scale optimization, something that has never been possible in the past.”
According to Douglas Adams, professor of mechanical engineering at Purdue University, wind farms are taking a heavy toll on bats. Bat mortality rates are as high as 50 bats per turbine per year.
“This significant ecological impact has been successfully addressed by wind farm operators through the process of curtailment—choosing not to operate the turbine during the peak bat hours/season,” Adams said.
That does, however, decrease revenues from the wind farm, so operators are trying to find new ways to solve the bat mortality problem. One of the technological solutions being considered is using the blade as a loudspeaker to produce an acoustic fog that will repel bats.