The Mountaintop Marvel

Scorned at the outset as a scheme worthy of Rube Goldberg, TVA’s pumped-storage generating plant inside Raccoon Mountain became one of the engineering wonders of the Tennessee Valley.

Steep Raccoon Mountain, located along the eastern fringe of the rocky Cumberland Plateau, has always been a remote place, sparsely settled and hard to get to even though it’s only six miles west of Chattanooga, Tennessee, as the crow flies. Downstream from the city, even the mighty Tennessee River takes a sharp hairpin turn northward to avoid this big bulk of limestone and pine.

Unchanging as the mountain looks, it’s not the same as it was 30 years ago. On top of it is something that didn’t exist when Cherokee hunters stalked bear and wildcat there in times past: a man-made lake holding about 60 million cubic yards of water.

And deep below the lake, hundreds of feet down in the dark heart of Raccoon Mountain, are elevators, lighted tunnels, and huge pieces of heavy machinery. It’s not the underworld home of the spirit people of Cherokee legend, nor the lair of some crazed James Bond villain. It’s TVA’s Raccoon Mountain Pumped Storage Plant, whose unassuming name belies the fact that it’s one of the engineering marvels of the 1970s. There is nothing else like it in the Valley.

TVA started speculating about the project as early as 1961, seeking a way to deal with the Valley’s burgeoning demand for power. Almost everyone had electricity, and many people were buying air conditioners and dishwashers and televisions for the first time. It was soon apparent that demand would not only continue to increase but also fluctuate wildly.

Although the use of electricity was typically much greater in the daytime than at night, most generators produced power at a constant rate. TVA engineers searched for a way to store energy so that they could call on it when demand rose. One way of doing this was to pump additional water into the reservoir behind a dam and hold its potential energy there until it was needed.

During the 1950s, TVA had experimented with the idea on a smaller 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. On that relatively modest scale, the idea seemed to work.

The Hiwassee experiment inspired TVA’s engineers to dream bigger. What if water could be pumped not just to the top of a dam, but to a reservoir on top of a mountain? And what if a long drainpipe could funnel the water down, say, 1,000 feet to a powerhouse far below?

Most agencies would have dismissed the idea, calling it wild-eyed and impractical. Indeed, when the Kennedy administration suggested a feasibility study of pumped-storage systems in 1961, the response was underwhelming. “A waste of the taxpayer’s money,” groused Representative James Haley, a member of the president’s own party. Walter Rogers, chairman of the House Irrigation and Reclamation Subcommittee and himself a sponsor of the study, termed the idea “a Rube Goldberg invention.”

But TVA found funding and put its engineers to work. Careful studies of the Raccoon Mountain site suggested that such a project would be very difficult, but not impossible. A small lake would be hollowed out on the mountaintop and sealed by a huge 8,500-foot-long dam. In a subterranean pumphouse, four great turbines would generate electricity from the plummeting water. Then, during times of low power demand, they’d be shifted into reverse and would pump the water back up to the top of the mountain at a rate of seven million gallons a minute.

During times of high demand for electricity, water is released from the Raccoon Mountain reservoir to generate power. When demand is low the generating turbines are reversed to pump water back to the top of the mountain.

Throughout the 1960s TVA engineers fine-tuned the idea, using models built at the agency’s lab in Norris, Tennessee. Finally, in February 1970, the TVA Board gave the go-ahead. Construction began that summer.

More than 1,000 workers labored at Raccoon Mountain during the next several years, excavating 10 million cubic yards of earth to build the lake, digging 12,000 feet of subterranean tunnels, carving a space the size of a football field out of solid limestone, and installing four huge Allis-Chalmers pump turbines. Over 10 million cubic yards of fill was used to construct the mountaintop dam; it’s the largest rock-fill dam ever built by TVA.

In 1974, the National Society of Professional Engineers named the work at Raccoon Mountain one of America’s most outstanding engineering projects. When completed, the plant would be the largest water-storage facility of its kind in the world.

The project took longer than expected. Originally scheduled for completion in 1973, it wasn’t finished until 1979, at a cost of just over 300 million dollars. But it worked well beyond all expectations. The storage reservoir gives the Raccoon Mountain plant a tremendous amount of flexibility to balance the load and supply on the TVA system. The power used for pumping provides needed load during low-demand hours while providing power during peak-demand hours. The plant is also able to change power output very rapidly, thus matching load and supply as well as providing a back-up power source throughout the day.

With its 1.6 million kilowatts of capacity, the Raccoon Mountain Pumped Storage Plant generates 14 times more power than nearby Chickamauga Dam, and it is a major factor in the efficient, reliable operation of the entire TVA power system.

It has done that every day for almost a quarter of a century and, in the wake of the plant’s current modernization and upgrade in capacity, it will continue to do it for years to come.