Visitors to Duke Energy’s Bad Creek Hydro Station need to enter a sloping tunnel to visit the power plant, which is hidden inside a mountain between two manmade lakes.

The tunnel opens into a cavern a little longer than a football field. Noise from a massive air conditioner system fills the room, necessary to keep the lines carrying power generated here from overheating. Pipes snake across the cavern’s walls and ceilings, often pressed up against exposed blue granite.

Preston Pierce, Duke’s hydro general manager, believes facilities like Bad Creek will play a vital role as the company tries to keep its service reliable while shifting toward technologies like solar and wind that don’t always generate power.

“We consume it when it’s of no value, and we release when it’s at its highest value,” said Pierce, who likens the system to a giant battery.

Duke recently finished a five-year, roughly $200 million project to overhaul the four 75-foot tall turbines at Bad Creek. The project, which increased the plant’s generating capacity, was the first significant maintenance at the facility since its 1991 opening.

Those turbines are the heart of the facility. When demand is low, they use excess energy to suck water from Lake Jocassee into the upper reservoir.

Then when demand is high, Duke opens a 36-foot drain in the upper reservoir. The water falls straight down an 850-foot shaft, which elbows and declines another 350 feet before reaching the turbines.

The water runs through the turbines, its force spinning them and generating electricity as it flows into Lake Jocassee. When it is generating power, the turbine spins in the opposite direction as when it is pumping. The turbines push water up the same shaft it came down.

“We’re just moving water back and forth,” Pierce said.

Duke controls the whole system from Charlotte, including another plant, the Jocassee Pumped Storage Generating Station in the lake’s southeastern corner.

The company draws Bad Creek’s power from its grid in North and South Carolina, providing energy to customers in both states.

Bad Creek can generate 1,680 megawatts of electricity, enough to power about 1.3 million homes. Its reservoir can provide that energy for 24 hours.

“I think it’s the only proven long-term storage technology we have,” Pierce said.

Duke must renew the facility’s Federal Energy Regulatory Commission license by 2027.

The company is exploring building a second powerhouse with four additional turbines at Bad Creek, doubling the amount of energy available. Duke could finish the project by 2033. That project would need federal approval, as well as approval from both North and South Carolina utility regulators.

Duke would build the new station effectively parallel to the existing one, moving water between the same upper reservoir and Lake Jocassee. The company is optimistic that will make environmental permitting significantly easier than constructing a whole new set of reservoirs.

Company officials aren’t yet saying what the project could cost, but some estimates put it around $5 billion — about five times the $1 billion it cost to hollow out the mountain and build the entire facility in the 1980s. The Inflation Reduction Act includes incentives that could provide Duke tax credits worth up to 30% of its investment.

While adding a pump station would double the amount of available power at Bad Creek, it would also cut in half the amount of time that energy is available, to about 12 hours. On the flip side, Pierce said, it would also cut in half the amount of time it takes to fill the upper reservoir, from about 30 hours when it is at its lowest to about 15 hours.

“We’ve got both reservoirs already in place, the geology in Upstate South Carolina is perfect for it, so we certainly think it’s a good option. But it’s one we’re continuing to evaluate,” said Ben Williamson, a Duke spokesman.

If Duke were to try to achieve the same storage with traditional batteries, Williamson said, it would need to cover about 1,200 acres to achieve the same power. And batteries on the market right now have capacity for about four hours. So to match the 12 hours of storage offered by a second Bad Creek powerhouse, Duke would need to build 3,600 acres of batteries.

“As far as long-duration storage right now, batteries just aren’t there yet. They can’t provide the amount of storage capability that pump storage can,” Williamson said.