The Tennessee Valley Authority sharply accelerated a shift away from coal as an energy source on Thursday, saying it would shut down eight electricity-generating units that together will burn nearly a fifth of its coal this year.

The closings are part of a long-term strategy, also announced Thursday, for the authority to generate 20 percent of its electricity from coal, instead of the current 38 percent. It also plans to increase the use of renewable energy sources like solar and hydropower to 20 percent, from the current 15.7 percent.

The authority’s chief executive, Bill Johnson, said experts were studying whether more coal-fired plants should be shut down later.

“These were difficult recommendations to make, as they directly impact our employees and communities,” he said in a release announcing the shutdowns. “But the plan is what’s best in terms of its positive impact on T.V.A.’s rates, debt and the environment, and it will bring the greatest benefit to the people of the valley.”