It's not an optical illusion.

The newest wind turbines gracing the nation's countryside actually are turning more slowly than their older cousins.

The languid pace is the most visible consequence of new-generation wind turbines that are taller, have longer blades, capture more wind and produce more power.

The first Minnesota wind farm with towers reaching 100 meters, or 328 feet, is now under construction by Juwi Wind near Worthington in southwestern Minnesota. The towers will be about three-fourths the height of Foshay Tower (not counting its pinnacle) in Minneapolis.

Across the nation, the wind power industry is reaching higher into the atmosphere and adding bigger rotor blades to boost electrical output. Last year, nearly 5 percent of new U.S. wind turbines were 100 meters tall, and the push upward is expected to continue, according to the American Wind Energy Association.

"That trend has been underway for 30 years, and really there is no reason to expect it to stop," said Fort Felker, director of the National Wind Technology Center at the National Renewable Energy Lab in Boulder, Colo. "Generally you get stronger winds at higher elevations above the ground."

Most wind towers built in the last few years reached 80 meters, or 262 feet, at the blade hub. Older units can be half that height, with correspondingly smaller blades that capture less wind area even as they spin at a faster pace.

Mortenson Construction, based in Golden Valley, is a pioneer in this industry trend, building 100-meter towers at five wind farms in Texas, New York, Illinois and Iowa.

Taller towers in Minnesota

Juwi Wind, a German renewable energy company whose U.S. headquarters is in Boulder, is installing 15 turbines on 100-meter towers at a 3,000-acre wind farm in the Buffalo Ridge region northwest of Worthington. The project is expected to be finished this fall.

"We are farming more wind," said Aaron Peterson, a former state legislator who is now manager of community relations and regulatory affairs for Juwi, pronounced ''you-vee.''

Data from the Iowa Energy Center indicate that wind at a height of 100 meters flows 4 1/2 percent faster than at 80 meters, Peterson said. Because wind energy increases at a logarithmic rate to wind speed, the energy gain from the extra height should be about 14 percent, he said.

The 30 megawatts of power generated by the Community Wind South project will be sold to Xcel Energy Inc., the Minneapolis-based utility, under a 25-year purchase agreement. That's roughly the power used by 10,000 homes.

Juwi owns 95 percent of the approximately $55 million project. The remaining 5 percent stake is being offered to qualified landowners in or near the project site and transmission lines, Peterson said.

The construction is winding up as the wind power industry faces the expiration of a key incentive. The industry has lobbied Congress to reauthorize the production tax credit, but while the Dec. 31 deadline looms, many future wind power projects are on hold. Current projects need to be finished this year, or lose out.

Why so slow?

The paradox of bigger wind machines rotating more slowly than older models illustrates the complex science of wind speed and efficiency.

On the big, newer turbines, the blade tips are moving at nearly the same speed as the older, faster-rotating models -- about 160 miles per hour, experts said. If the long blades rotated much faster, experts said, the tips would go too fast, and be too loud.

The bigger blades' advantage is that they cover a greater area, collecting more wind flow.

"They are much more efficient because of the amount of energy they are capturing," said Mark Ahlstrom, CEO of WindLogics Inc., a St. Paul firm that assesses wind conditions for wind energy projects.

Taller towers allow turbines to have longer blades. But they also offer a separate, related benefit -- wind tends to be stronger and steadier higher above the ground.

"If you go up high enough," said Ahlstrom, "the winds are not influenced by the interaction with the Earth."

Ahlstrom said the extra height can boost not only the average wind speed, but the percentage of time that a turbine is producing power.

For wind farm developers, he added, the costs of building taller units, which require more concrete and steel, must be weighed against the long-term gain in output.

Wind power, like other forms of power generation, also faces increased competition from natural gas, whose price recently hit a 10-year low thanks to expanded U.S. drilling using innovative extraction techniques.

Even so, Peterson said he "cautiously optimistic" that wind power will grow increasingly efficient and competitive with fossil fuels.

"The technology continues to improve," he said. "Developers just don't quit and manufacturers are always tweaking, improving and competing."

David Shaffer • 612-673-7090