North Carolina’s predominant utility is backing away from a long-held plan to double the size of its largest pumped storage hydropower plant — just as data centers and other voracious energy users threaten to stretch power supplies to their limit.

The reversal was tucked away in Duke Energy’s latest long-term blueprint, which was filed in October and will be evaluated and finalized by regulators this year. Clean energy advocates had expected to fight that blueprint on familiar fronts — from its inclusion of new gas-fired power plants to its complete lack of near-term wind energy — but they were surprised by the backpedaling on the Bad Creek storage facility, located just over the border in South Carolina.

“Duke put this forward as something they were going to do, and everybody agreed,” said David Neal, senior attorney at the Southern Environmental Law Center. ​“To take out the one thing that everybody agreed on, without any announcement, without any fanfare,” he said, ​“is just baffling to me.”

Pumped hydro is a uniquely useful form of carbon-free electricity. It’s available on demand and can dispatch power over a much longer period than a lithium-ion battery can. It’s also rare: Construction of new pumped hydro facilities in the U.S. has stagnated for decades.

Duke’s original Bad Creek expansion plan would have catapulted the company to become the nation’s leader in pumped hydro. Now, advocates fear its about-face will undermine the state’s zero-carbon law by opening the door for a fleet of new gas plants instead.

A massive natural battery

Hydropower is one of the oldest forms of electricity generation — and it’s how Duke Energy, then called the Catawba Power Company, got its start in the early 1900s. A trio of entrepreneurs, led by James B. Duke, built a series of dams and lakes along the Catawba River, fostering the growth of mills and other industries that helped diversify the region’s economy.

Pumped storage hydropower — like that at Bad Creek — is a related but different beast. Two bodies of water at different elevations are connected with reversible turbines, producing or storing electricity, depending on what the grid needs.

“Let’s say it’s a spring day, a sunny day, a lot of solar on the grid, but not a lot of demand. You just bring that water uphill,” to store in the upper reservoir, Neal explained. ​“When you’re in a peak period, you run the water back downhill to generate electricity. It’s a very efficient, clean way of having storage.”

Duke launched its first pumped storage project in 1975 after building a dam between what is now Lake Jocassee and Lake Keowee below it. On the South Carolina side of the Blue Ridge Mountains, the four reversible turbines are slated to operate for at least another two decades.

The Bad Creek complex followed in 1991. The upper reservoir sits at an elevation of 2,310 feet, and Lake Jocassee, more than 1,000 feet below, serves as the lower reservoir. They’re linked by an underground concrete tunnel and a four-turbine powerhouse, capable of supplying enough electricity to power 1 million homes.

Duke recently upgraded the existing Bad Creek facility, increasing its capacity to nearly 1.7 gigawatts. But the utility has also long envisioned drilling a new tunnel and adding another four-turbine powerhouse at the site, adding another roughly 1.8 gigawatts. Doing so would help the company zero out its carbon emissions by midcentury, as required by state law.

In 2022, the company offered four pathways to limit its pollution; all included the expansion, dubbed Bad Creek II, by 2034. The additional Bad Creek capacity was also cemented in a compromise Duke struck with stakeholders to help get its last carbon-reduction plan approved. Regulators on the North Carolina Utilities Commission blessed the deal, directing the company to pursue ​“all reasonable development activities” to put Bad Creek II in place.