(Credit: Ed Stoddard/Reuters)

A basic problem for wind turbines is that the wind often dies down. As a result, they produce far less electricity than if the wind blew constantly, at full speed.

A good wind machine, therefore, may harvest just 30 percent of its maximum potential energy. By contrast, a nuclear reactor with a similar energy rating might reach 90 percent of its maximum potential, because it is running virtually nonstop.

One major turbine manufacturer, Siemens Energy, is trying to increase the proportion of potential energy that the wind harvests — by making the blades longer. The new machines, by Siemens, all use the common three-blade design. But a new Siemens turbine has a rotor diameter of about 330 feet, rather than one with a 305-foot diameter.

The “swept area” — the circle of air from which the machine captures energy — is therefore about 18 percent bigger.



When the wind blows strongly the longer blades make no difference, because the size of the turbine’s generator is unchanged. At moderate speeds, the output goes up. At 17 miles an hour, for example, Siemens says that the larger blades produce about 7.5 percent more electricity than those with smaller blades. They do not spin faster, but they extract more energy from the passing air.

This approach may be especially good in spots where the wind is relatively weak, according to Michael B. Revak, vice president of sales at Siemens. But it has its limits. One problem is that as the blades get longer, it gets harder to move them from factory to hilltop. Longer blades are also more expensive to manufacture.

There are already two of the new machines in Washington state, near the Columbia River Gorge. Another is in Denmark, and two more are scheduled for use by Minnesota Power over the next few years.