NASHVILLE (AP) — The nation's largest public utility is looking at shutting down three of its five remaining coal-fired power plants, saying they are old and no longer practical. But despite President Joe Biden's goal of a carbon-pollution-free energy sector by 2035, the Tennessee Valley Authority, an independent federal agency, is considering replacing the lost megawatts from coal with another carbon-producing fuel — natural gas.

At a public hearing this week on the proposed closure of the Kingston Fossil Plant, TVA Senior Manager of Enterprise Planning Jane Elliott stressed the fact that gas provides reliability and flexibility as a fuel that can be called upon at any hour of any day. Solar generates energy only about 25% of the time, Elliott said, so "you have to add more solar to get the same amount of energy from gas."

Gas is also currently cheaper than solar, Elliott said, although prices are falling and solar should become cheaper towards the end of the decade.

Samantha Gross, director of the Brookings Institution's Energy Security and Climate Initiative, said reliability and flexibility are real considerations, but TVA already has lots of that with its current gas and hydroelectric plants. Any new gas plants will likely be around for decades, long past Biden's 2035 goal to decarbonize.

"That's important," Gross said of the goal. "We're fried if we don't do it."

Scientists have warned that failing to meet that target will only lead to more intense and more frequent extreme weather events, as well as droughts, floods and wildfires.

TVA's Kingston and Cumberland plants together produce around 3,900 megawatts of electricity. The utility is not looking to replace electricity lost from the shut down of its smaller Bull Run plant, but for the other two, the utility is studying three replacement alternatives. Two of them are different types of gas plants. The third option is for renewables — most likely solar — plus storage.

The utility already has plans to add 10,000 megawatts of solar power to its system by 2035, but that won't be a replacement for the coal plants. Utility spokesperson Scott Brooks said most of that will go to large industrial customers like Google that want to power their facilities with renewables.

Marilyn Brown is a professor of energy policy at Georgia Institute of Technology who served on the TVA board of directors from 2010-2017. She said what's missing from TVA's proposals is decreasing the need for new electrical generation altogether. That can be done through stronger investments in energy efficiency and demand response — which involves helping customers change their usage patterns to flatten peak demand periods.

Demand response can drop a load just as quickly as firing up a gas turbine to meet that load, Brown said. "Why not help people control their thermostats and appliances when generation is in short supply?" As an example, she said, studies have found you can cycle off air conditioning for 17 minutes in an hour without any noticeable difference.

One challenge is that TVA does not sell electricity directly to homes. Instead, that's done through 153 local power providers. But Brown said it's a challenge they could overcome. Going all-in on gas would be a backwards solution, but "the risks are low, and they know how to do it," Brown said. "The issue is getting the utility to move in a direction it's not as familiar with."

Meanwhile, critics say TVA already has failed to accurately weigh the environmental impacts of a separate proposal to add new gas turbines at its Paradise plant in Kentucky and Colbert plant in Alabama. TVA's draft environmental impact statement states these additions will not negatively affect greenhouse gas emissions or climate change because the utility is reducing emissions elsewhere in the system.

A group of seven environmental organizations has written to TVA, calling their analysis flawed and a violation of the National Environmental Policy Act. "If building new gas-fired power plants does not negatively impact climate change, nothing does," the letter states.

TVA President Jeff Lyash said earlier this year that the utility is on track to reduce greenhouse gas emissions by 80% by the year 2035, compared to 2005 levels. He said they will not be able to meet the 100% reduction goal without technological advances in energy storage, carbon capture and small modular nuclear reactors. The utility has its own aspirational goal of net zero emissions by 2050.

Any final decision on whether to shut down the coal plants and what to replace them with will have to be approved by TVA's board.