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Wind power is growing faster than ever. The latest numbers show that almost half the new sources of electricity added to the U.S. power grid last year were wind farms. And that's led several scientists to wonder: Is the sky the limit? But it turns out there's a problem. It's actually possible to have so many turbines that they start to lose power by stealing each other's wind. NPR's Christopher Joyce reports on scientists now trying to find that saturation point.

CHRISTOPHER JOYCE, BYLINE: There's a thing about wind that sailors call dirty air. If you're sailing directly downwind of another sailboat, you'll slow down. That's because the lead boat creates a turbulent vortex behind it as the wind spills off its sails. Dirty air means less power. The blades on a wind turbine make dirty air, too, so engineers space them far apart. But wind developers want to build over 100,000 turbines in the U.S. alone. Would that spread dirty air far and wide?

DAVID KEITH: Very large wind farms, we can now see long footprints that extend, in some cases, tens and tens of kilometers downstream where you have slower moving wind.

JOYCE: That's Harvard physicist David Keith. He's one of several scientists who've designed computer simulations to see what might happen at huge wind farms.

KEITH: If we're going to scale wind power up to supply a significant fraction of the global energy demand, say 10 percent of global energy demand as we get towards midcentury, then these effects begin to matter. Exactly how much they matter, we still don't know.

JOYCE: The answer has become a kind of puzzle for a group of atmospheric scientists. Just what, they ask, is the saturation point for wind power when the wind is so dirty that there's no point in building one more turbine? So far, that point is hypothetical. To get to saturation, you'd need huge wind farms, thousands of square miles of turbines. With 45,000 big turbines now in the U.S., that's not a problem.

Atmospheric scientist Mark Jacobson at Stanford University says turbines would have to get up into the many millions before you lose a serious amount of wind, and people are thinking how to get around the problem.

MARK JACOBSON: We found that by spreading out the wind farms themselves, then you reduce the impact of having low energy when you just have one huge wind farm with lots of turbines.

JOYCE: Elizabeth Salerno at the industry's American Wind Energy Association says wind developers are making sure they're not going to dilute the wind. Doing that would just lose money.

ELIZABETH SALERNO: Our developers spend a lot of time with experts and atmospheric scientists to ensure that they are sighting each individual wind turbine and the entire project in a location and a layout that's going to optimize their result.

JOYCE: But as you build more and more wind farms, spreading them out could present complications. You need to have transmission lines reasonably close by. As you get more wind farms, more people are likely to complain about their view, and lots of places aren't windy enough to build anyway. Even with those limits, though, Salerno says there's plenty of wind to go around.

SALERNO: We have enough wind resource in the U.S. on shore, on land to do 10 times over our power production today.

JOYCE: That's 10 times all the power produced now in the U.S. by coal, nuclear power, hydro, everything. Salerno says no one expects wind power to come anywhere close to that. For one thing, the nation's electricity grid runs more reliably if utilities can draw on different kinds of energy that can back each other up. At the moment, wind provides about three-and-a-half percent of all U.S. electricity.

The wind association says a reasonable goal is to raise that to 20 percent of our electricity needs by 2030. She says that would mean building maybe another 75,000 wind turbines and not building them all in one place. Christopher Joyce, NPR News.

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