Inspired by the way palm trees move in high winds, a group of researchers at the University of Virginia and Sandia National Laboratory are developing an extremely long wind turbine blade that could make it possible to construct 50-megawatt turbines—far beyond the power of today’s, which tend to produce just two megawatts. The blades, designed under a program funded by the U.S. Department of Energy’s ARPA-E program, would be 200 meters long, 2.5 times the length of the longest blades commercially available today.
In recent years the wind power industry has moved toward longer and longer blades, driven by simple economies of scale: the larger the diameter of the rotor (the circular area swept by the turbine blades), the more power a single wind tower can produce. If the blades can be made and the tower erected cheaply, the cost of electricity goes down as blades get longer.
The Sandia superblades are based on concepts developed by Eric Loth, a professor of mechanical and aerospace engineering at the University of Virginia. They would have a series of joints along their length that would enable them to fold in response to the strength of the wind. Situated downwind of the tower (in contrast to conventional blades, which are upwind), the blades would clinch like a raptor’s talons in extreme wind conditions to lessen the forces on the turbine. In optimum conditions they would extend to their full length. They’re particularly well suited for offshore installations in hurricane-prone areas, and they would make it possible to produce power economically in regions where winds are light.
“What we’re proposing to do is very far beyond current designs and very high risk,” says Todd Griffith, the technical lead for Sandia’s Offshore Wind Energy Program.
The longest blades available today are 80 meters and built in a single segment. Blade lengths are constrained, for now, by logistics and transportation challenges: it’s hard to use trucks to move something dozens of meters long and several meters in diameter. Denmark-based Vestas recently began shipping 62-meter blades in the U.S. The first customer is Duke Energy, which will begin building a new wind farm using the large blades in Oklahoma later this year. The blades will be shipped to ports along Texas’s Gulf Coast, transported by rail to Oklahoma, and trucked to the site. Such an installation could not happen in more congested parts of the country.
Segmented blades, which allow for easier transportation and on-site assembly, are not new; European manufacturers including Gamesa and Enercon now offer them, but they have not been widely deployed yet. Wind-farm builders are wary of adding potential structural weaknesses to long, rigid blades subjected to high stresses, says Bruce Peacock, vice president for engineering and construction in Duke’s renewable-energy division: “Small structural flaws can lead to catastrophic failures.”
Even if the blades get to the site, erecting wind towers beyond 120 meters in height presents challenges of its own, because cranes aren’t tall enough. Manufacturers such as Terex and Manitowoc are developing specialized cranes for very tall wind towers, but it’s not clear that they’ll ever be high enough to put up a tower with a 400-meter rotor. Novel solutions are emerging. Keystone Tower Systems, a startup based in northern Colorado, has developed a mobile factory for shaping sheets of steel into a wrapped spiral design on site, eliminating the problem of shipping large towers by truck. There are also designs on the drawing table for so-called “climbing cranes” that essentially spider up the tower as it rises.
The prospect of new technologies and exotic blade materials that are lighter and stronger means the drive to supersize wind turbines is unlikely to die down. Gigantic offshore farms like the London Array are likely to become more common in the future, and blade size and tower height don’t face the same limitations at sea as they do on land. It will take a decade or more for the Sandia design to find its way to the marketplace, if it ever does, but even more futuristic concepts for extendable blades that telescope outward under favorable conditions are in the works.
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