The Tennessee Valley Authority announced that it plans to retire eight coal-fired generators. - 

Last week, the Tennessee Valley Authority announced that it plans to retire eight coal-fired generators. The TVA says more closures could come, and this announcement follows the closures of older coal-fired plants by other electric utilities. So, to what extent is coal actually fading away as an energy source?   

Vlad Dorjets is an economist at the United States Energy Information Administration. When utilities retire power plants -- or make plans to -- they file a report with the agency, and he crunches the numbers. He says that of the 1,400 coal-fired generators that existed in 2008, almost 400 will be gone by 2017. 

But his models that look past 2017 don’t show the trend continuing very far. Most of the generators going away now are old -- built before 1960. "Basically everything that's going to retire, will retire in the next couple years," he says. "And then you've got a pretty steady fleet." 

Even so, by 2020 he estimates that generating capacity from coal will be about 15 percent lower than it was in 2008. That's about half the reduction the Sierra Club would like to see, but it's still a big chunk. 

So, what’s taking the place of that coal? "Shale gas, and renewables to a lesser extent," says Marty Rosenberg editor-in-chief of Energy Biz Magazine.  

He thinks shale gas looks like a reliable source for some time to come. But, he says, there's no such thing as a sure thing. "You say it’s a hundred-year supply for sure -- then, sure enough in two, three years, something will happen."

Follow Dan Weissmann at @danweissmann