Green Power Switch News
Vol. 5, No. 1 Winter / Spring 2005
Wind site expansion complete
Southwest breezes are blowing more green energy our way with the inauguration on December 9 of TVA’s expanded wind farm, one of the largest commercial wind installations in the Southeast. Wind energy is now a major source of power supply for the Green Power Switch program.
The 27-megawatt facility, built by Invenergy Wind LLC, is located on Buffalo Mountain near Oak Ridge, Tennessee. TVA will purchase the turbines’ output under a 20-year, $60 million power-purchase agreement.
The wind farm started small in October 2000 with three turbines on Buffalo Mountain. That facility generated two megawatts of electricity, or enough to power 360 homes. With the addition of 15 larger turbines, the total capacity has increased to 29 megawatts, or enough for about 3,780 homes.
The new turbines are huge: they rise 262 feet from the base to the hub, and the blades are 135 feet long! The turbines are spaced about 2.6 rotor diameters apart on the west edge of a two-mile ridgeline, facing southwest. That’s the predominant wind direction on Buffalo Mountain.
The turbines rotate at about 15 revolutions per minute, says Rick Carson, TVA’s Renewable Operations Manager. “The speed is selected to control blade-tip speed. The smaller blades on the original three turbines rotate at 28 revolutions per minute.” Those turbines produce 690-volt power, which is stepped up to 13 kilovolts (kV) by transformers at the base of each tower.
“The new turbines,” says Carson, “have the transformers in the nacelles—the containers on top of the towers that house the gearbox and generator. That’s where the power is stepped up to 35 kV before being stepped up again to 161 kV at the substation located on the mountain near the turbines.”
The capacity factor for the original turbines has averaged 23 percent over the past couple of years, Carson says. “The new turbines are expected to have a capacity factor of 28 percent because the towers are 49 feet taller. The low capacity factor is related to the availability of the wind resource in the Southeast.” (Capacity factor is the ratio of electricity generated to the amount of energy that could have been generated at continuous, full-power operation during the same period.)
Carson says the blades start spinning at wind speeds of two to three miles per hour. “Energy is generated when the wind speeds reach nine to 10 miles per hour,” he adds. “The turbines generate at their rated capacity with winds of about 25 miles per hour and automatically shut down with winds of 55 miles per hour or more. The turbines produce some amount of energy about 70 percent of the time.”
The expanded site is staffed, unlike the old one, and that allows the turbines to be available 98 percent of the time.
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— Jim West