July 2002

Wind Power for Pennies

Continued from page 1

By Peter Fairley

Steady as She Blows

Wind turbines are like giant fans run in reverse. Instead of motor-driven blades that push the air, they use airfoils that catch the wind and crank a generator that pumps out electricity. Many of today's turbines are mammoth machines with three-bladed rotors that span 80 meters-20 meters longer than the wingspan of a Boeing 747. And therein lies the technology challenge. The enormous size is needed if commercial wind turbines are to compete economically because power production rises exponentially with blade length. But these vast structures must be rugged enough to endure gales and extreme turbulence.

In the 1970s and '80s, U.S. wind energy pioneers made the first serious efforts at fighting these forces with lightweight, flexible machines. Several startups installed thousands of such wind turbines; most were literally torn apart or disabled by gusts. Taking lightweight experimentation to the extreme, General Electric and Boeing built much larger prototypes-behemoths with 80-, 90- and even 100-meter-long blades. These also proved prone to breakdown; in some cases their blades bent back and actually struck the towers.

All told, U.S. companies and the Department of Energy spent hundreds of millions of dollars on these failed experiments in the 1980s and early 1990s. "The American model has always gravitated toward the light and the sophisticated and things that didn't work," says James Manwell, a mechanical engineer who leads the University of Massachusetts's renewable-energy research laboratory in Amherst, MA.

Into these technology doldrums sailed researchers from Denmark's Ris National Laboratory and Danish companies like Vestas Wind Systems. During the past two decades they perfected a heavy-duty version of the wind turbine-and it has become the Microsoft Windows of the wind power industry. Today, this Danish design accounts for virtually all of the electricity generated by wind worldwide. Perhaps reflecting national inclinations, these sturdy Danish designs had little of the aerodynamic flash of the earlier U.S. wind turbines; they were simply braced against the wind with heavier, thicker steel and composite materials. They were tough, rugged-and they worked.

What's more, in recent years, power electronics-digital silicon switches that massage the flow of electricity from the machine-further improved the basic design. Previously, the turbine's rotor was held to a constant rate of rotation so its alternating-current output would be in sync with the power grid; the new devices maintain the synchronization while allowing the rotor to freely speed up and slow down with the wind. "If you get a gust, the rotor can accelerate instead of just sitting there and receiving the brute force of the wind," says Manwell.

Mastering such strains enabled the Danish design to grow larger and larger. Whereas in the early 1980s a typical commercial machine had a blade span of 12.5 meters and could produce 50 kilowatts-enough for about a dozen homes-today's biggest blades stretch 80 meters and crank out two megawatts; a single machine can power more than 500 homes.

The newest challenge facing the Danish design is finding ways for it to weather the corrosive and punishing offshore environment, where months can pass before a mechanic can safely board and fix a turbine. Vestas, for one, is equipping its turbines with sensors on each of their components to detect wear and tear, and backup systems to take over in the case of, say, a failure in the power electronics.

Vestas's approach goes to the test this summer, as Denmark's power supplier begins installing 80 Vestas machines in shallow water 14 to 20 kilometers off the Danish coastline. It will be the world's biggest offshore wind farm, powering as many as 150,000 Danish households.