Mountaintop Marvel

In the early 1960s, Valley was growing rapidly, and so was demand for electricity. More families were investing in things like air conditioning, dishwashers and televisions for the first time, putting pressure on the grid. TVA needed to find a way to respond to demand that was not only rising but also fluctuating—sometimes wildly.

Although demand for electricity was highest during the day, TVA’s generators produced electricity at a constant rate, night and day. And so engineers set about finding a way to store energy so that they could call on it when needed—at times when dishwashers, air conditioners and televisions were all running at peak.

One way of doing this was to pump additional water behind a dam and hold its potential energy there until needed. In the 1950s, TVA had experimented with this idea on a small scale at Hiwassee Dam in North Carolina. There it employed an energy-generating turbine that was run in reverse during low-demand hours to pump water below the dam into the upper reservoir for release during high-demand hours. And it worked, on a modest scale.

But what if it could be bigger, TVA engineers wondered—much, much bigger. What if water could be pumped not just to the other side of a dam, but to a reservoir on top of a mountain? And what if the water could drop down a 1,000 feet to a powerhouse far beneath the earth?

It sounded crazy—lawmakers groused about it as “a waste of taxpayer money” and one even likened it to “a Rube Goldberg invention”—but these dreams laid the groundwork for what would become Raccoon Mountain Pumped-Storage Plant, just outside Chattanooga, Tennessee.

Studies of the site indicated that the work would be difficult—but not impossible. A small lake would need to be hollowed out at the top of the mountain. A 1,000-foot tunnel would need to be drilled into the mountain. Four great turbines would need to be installed within the mountain itself in a subterranean pumphouse, generating electricity from water plummeting down to the drive generators in the mountain's underground power plant and then released into the Nickajack Reservoir. Then, in times of low demand, the turbines would reverse—pumping water back up to the top of the mountain at a rate of 7 million gallons a minute.

Throughout the 1960s, TVA engineers fine-tuned the idea—and in 1970, the TVA Board of Directors gave the project its blessing. More than 1,000 workers excavated 10 million cubic yards of earth to build the lake at the top of the mountain, and built 12,000 feet of subterranean tunnels, carving a space the size of a football field out of solid limestone. The limestone fill was used to build the 8,500-foot-long dam—the largest rock-fill dam ever built by TVA.

When the space was ready, four giant and powerful Allis-Chalmers pump turbines were installed underground—an engineering marvel that in 1974 won the site a prestigious award for outstanding engineering from the National Society of Professional Engineers. The project was finished in 1979, and its performance not only met but exceeded expectations, allowing TVA tremendous flexibility to balance load on its grid.

Today, the facility continues to do what it was meant to do: balance load quickly and evenly. With its more than 1,600 megawatts of capacity, the Raccoon Mountain Pumped-Storage Plant generates 14 times more power than nearby Chickamauga Dam. This one-time pipedream is now a major factor in the efficient, reliable operation of the entire TVA power system—a true success story.