Pumped storage plant ready for summer after repairs that shut Raccoon Mountain facility for two years

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photo John McCormick, the TVA senior vice president of river operations, points out problems in the underground facility at the Raccoon Mountain Pumped-Storage Station in this file photo.

Chattanoogans should be able to crank up the air conditioning this summer with no worries of crashing the power grid.

That's because the Tennessee Valley Authority's Raccoon Mountain Pumped Storage Plant is back in business after being shut down two years for some $90 million in repairs.

"For air quality, it's a good thing," said Robert Colby, director of the Chattanooga-Hamilton County Air Pollution Control Bureau. "The cleanest power comes from solar and hydroelectric. The more hydroelectric, the better."

The plant, which was completed in 1978 and looks like the setting for a James Bond movie, is a power source that TVA relies on when electrical demand is high -- including on scorching summer days when air conditioners are set to full blast.

"It's a hydroelectric battery, essentially," TVA spokesman Jim Hopson said. "It's stored potential energy."

It works like this: TVA fills the 528-acre reservoir atop Raccoon Mountain at night by pumping Tennessee River water uphill. Doing so burns electricity that TVA has to spare, since its nuclear power plants run at full tilt around the clock and demand is down when people are asleep.

When TVA needs a lot of power in a hurry, it pulls the reservoir's plug. Water comes blasting down a vertical tunnel drilled through the mountain and spins turbines that power four generators.

TVA shut the Raccoon Mountain plant down in March 2012 as a precaution because of cracks in the rotors that spin inside the generators' magnetic field, creating electricity. Because similar rotor cracks caused a catastrophic failure in 2009 at a pumped storage plant in Austria, TVA opted to replace and repair all the rotors.

"It was a case of preventative maintenance," Hopson said.

Fixing the rotors cost $55 million. Since the plant was down anyway, TVA did other work, such as installing a 500,000-pound power transformer last fall. All told, the repairs cost about $90 million, Hopson said. The work is maintenance, he said, and won't cause a rate increase for the utilities that buy power from TVA. The plant came entirely back on line in April.

Everything at Raccoon Mountain is on a massive scale: the towerlike intake structure that fills and empties the reservoir is taller than the Statue of Liberty; the powerhouse is carved into solid rock about 1,000 feet deep, and visitors used to ride an elevator 106 stories down into the mountain to see the plant's workings.

Those public elevator rides are a thing of the past, now. While a renovated visitors center still offers exhibits, photos and videos explaining how the facility works, the elevator was closed as a security measure after the Sept. 11, 2001, terrorist attacks.

"It was related to the enhanced security put in place after 9/11," Hopson said.

Contact staff writer Tim Omarzu at tomarzu@timesfreepress.com or twitter.com/TimOmarzu or 423-757-6651.