PISGAH, Alabama — The Tennessee Valley Authority is turning to the sun for much of its future power with plans over the next two decades to buy or build solar farms capable of producing 10,000 megawatts of solar power generation.
But to help balance the electricity needed when the sun doesn't shine, TVA is also drilling underground to explore the options of a power storage system.
At two sites along the Tennessee River and Sand Mountain in Northeast Alabama, TVA is studying the feasibility of building pumped storage facilities similar to the one erected four decades ago atop and within Raccoon Mountain just west of Chattanooga.
Pumped storage facilities, which have already been built by electric utilities at 42 sites across the nation, act like a giant storage battery. By building a reservoir atop Sand Mountain, TVA could pump water from the Tennessee River for storage and then drain the water through hydroelectric turbines within the mountain to produce electricity when it is needed the most.
"This is a very mature and efficient way to store energy and it is an important way to keep the electric grid stable and reliable when other sources of power are not available," said Scottie Barrentine, TVA's project manager for the study. "When power demand increases or one of our units trips and goes offline, the pumped storage units can be started up and producing power in a matter of minutes to help meet any immediate need."
In an interview at one of the Jackson County sites where seismic and drilling tests are underway, Barrentine said any decision about whether and where to build another pumped storage facility is still a couple of years away, and actual operation of any such facility will likely take another decade or more to develop and construct before it produces any power.
But the idea is compelling enough that TVA is spending $11 million to evaluate potential pumped storage sites near the Rorex Creek in Pisgah, Alabama, and atop Sand Mountain near Flat Rock, Alabama, as well as the idea of expanding the Raccoon Mountain pumped storage plant.
TVA considering sites in Jackson County, Ala., for energy storage, backup power production
TVA projects a pumped storage facility with a 600-acre upper reservoir and four 400-megawatt reversible turbines and pumps could produce 1,600 megawatts of electricity — more than the output from any single one of TVA's nuclear reactors. The utility projects a new pumped storage facility similar to one on Raccoon Mountain should cost somewhere between $2 billion and $4 billion to build.
Pumped storage facilities take about 20% more power to fill the upper reservoir than what the water from upper lake generates in electricity when it is released for power production, Barrentine said. But the loss in energy efficiency is more than offset by the benefit in being able to store and then deploy the electricity during peak demand periods when it is needed most.
The Raccoon Mountain Pumped Storage plant, a mountaintop facility that opened in 1978, is TVA's biggest energy storage facility and works like a giant water battery. TVA fills the 528-acre reservoir atop Raccoon Mountain at night by pumping Tennessee River water upward through a vertical tunnel drilled through the mountain. It flows into the reservoir through a reinforced concrete water intake structure that sticks up 220 feet from the reservoir bottom — taller than the Statue of Liberty.
When TVA needs a lot of power in a hurry — for example on a 95-degree day when people crank up the air-conditioning — it pulls the reservoir's plug.
Sand Mountain appeal
TVA looked at more than 60 sites across the Tennessee Valley to locate another pumped storage facility, but Sand Mountain proved most attractive because of the presence of a mountain ridge near a major waterway (the Tennessee River) and the sandstone and limestone rock formations of the mountain to house both the upper reservoir and the power plant at the base of the mountain, Barrentine said.
For the past month, engineering crews from Stantec Engineering and other consultants have been drilling into Sand Mountain and conducting seismic tests in the area to determine whether a 100-foot-deep lake could be located atop the mountain and used as a storage reservoir to power the new pumped storage facility.
The two targeted sites TVA is now studying for another pumped storage plant are across the Tennessee River from two shuttered TVA power plant sites. The Pisgah site is across from TVA's abandoned Bellefonte nuclear power plant in Hollywood, Alabama, where there were surface coal mines in the 1950s, Barrentine said.
TVA crews and consultants are working during weekdays from leased space in the Moonlight Over Water wedding venue in Pigsah, which overlooks the twin cooling towers and abandoned Bellefonte nuclear plant that TVA quit building more than a decade ago.
Sixteen miles to the north, the proposed pumped storage facility site in Flat Rock, Alabama is across the river from TVA's former Widows Creek Fossil Plant, which was shut down in 2015.
The former TVA power plants still have transmission line access to help power and then dispatch the power that could be produced from a new pumped storage plant in the area, TVA spokesman Scott Fiedler said.
Before any decision is made to buy property or build a pumped storage facility in Jackson County, an environmental impact study will be conducted to assess the need for the facility, and any environmental issues that would have to be addressed, according to TVA.