A Duke Energy plant in Charlotte leaks an incredibly potent greenhouse gas at the highest rate in the country — 6 percent — according to non-partisan environmental reporting organization Inside Climate News. Data from the Environmental Protection Agency shows that Duke Energy Power Delivery Carolinas leaked the equivalent of more than 300,000 pounds of carbon dioxide in 2021, in the form of sulfur hexafluoride.

“Sulfur Hexafluoride (SF6) is a gas used as an insulator in high voltage equipment like breakers and switch gears, and has been in use in Duke Energy’s service area since the 1970s,” Jeff Brooks, a spokesperson for Duke Energy, wrote in an email to the Caswell Messenger. “Gas-insulated breakers were a replacement for oil-filled breakers. Many switches are hermetically sealed. As one of the largest utilities in the nation, we maintain a large inventory of gas-filled equipment.”

SF6 is used in gas-filled circuit breakers and insulation for power distribution because of its incredibly stable molecular structure. However, the EPA also describes it as, “most potent greenhouse gas known to-date.”

“Over a 100-year period, SF6 is 22,800 times more effective at trapping infrared radiation than an equivalent amount of carbon dioxide (CO2),” the EPA states on their website. “SF6 is also a very stable chemical, with an atmospheric lifetime of 3,200 years. As the gas is emitted, it accumulates in the atmosphere in an essentially un-degraded state for many centuries. Thus, a relatively small amount of SF6 can have a significant impact on global climate change.”

For its part, Duke Energy says leaks can occur slowly, over a long period of time, and that numbers may be skewed based on the way that they are reported.

“The leak is reported in total at the time it is refilled, even though the gas may have been leaking slowly over several years,” Brooks wrote. “This reporting disparity can sometimes make it appear that the gas is leaking faster or at a larger rate than it actually is.”

However, EPA data shows that the Charlotte facility emitted the equivalent of 300,000 metric tons of carbon dioxide in 2021, up from around 239,000 metric tons in 2020 but down from 338,520 metric tons in 2019. Greenhouse gas emissions from less common gases are often measured in the amount of carbon dioxide that would have to be emitted to have the same effect on the atmosphere.

Duke Energy also says that SF6 from its power distribution network is just a fraction of their total emissions.

“While SF6 is much more potent than carbon dioxide as a greenhouse gas (GHG), it represents less than half of one percent (about 0.3 percent) of our scope 1 emissions, on a CO2 equivalent basis,” Brooks wrote. “For comparison, carbon dioxide emissions from Duke Energy’s generation fleet comprise more than 60 percent of our greenhouse gas inventory. We are working aggressively to shift toward cleaner energy options and significant carbon reductions in our operations.”

At the same time, the Charlotte plant does stand out among similar power distribution networks. The TVA Transmission System, which stretches from West Virginia to central Mississippi, has an SF6 capacity around 50,000 pounds larger than Duke’s Power Delivery Carolinas system — but emitted 74,619 metric tons in 2021 compared to Power Delivery Carolinas’ 300,936 metric tons.

Gas-based breakers and insulation are expected to stick around, at least for the time being, Brooks said, though he added that refinements to current designs can increase efficiency and new insulation are being tested.

“Newer gas-filled breaker designs are much more effective at containing insulating gas, so replacements of legacy technologies and grid equipment improvements will help to reduce SF6 emissions over the next few years,” he wrote. “We also continue to monitor developments of vacuum-insulated breakers as a potential replacement technology.”