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By Doug Peeples

SGN News Editor

Wind farms showed record growth in 2012 with a 19% increase in capacity, says the Global Wind Energy Council. China has 38 offshore wind power projects underway with the expectation of producing capacity of 16,500 MW. Google is spending $200 million for a stake in a West Texas wind farm (Google really likes clean energy). And the American Wind Energy Association says U.S, utilities are climbing over themselves for wind power contracts, encouraged at least in part by the extension of the wind production tax credit and lower prices.

Americans really like their wind power, although they may not be terribly fond of forests of turbine towers near scenic locations. At the end of 2012, U.S. wind power capacity was recorded at 60,007 MW, second in the world behind China (75,564 MW, for the record).

It's easy to see why wind is so attractive. It's cleaner than traditional fossil fuels and while wind is intermittent it's not going to run out as an oil well can. So the idea that wind power is essentially limitless seems sensible.

But to turn a tired expression around, every silver lining has a cloud.

Do we really have all that capacity?

Harvard applied physics professor David Keith, who has a very long list of credentials, brings one of those clouds to the discussion. While he does not say current estimates of wind capacity at large-scale installations are definitely wrong, he suggests it, as a Phys.Org article put it. It's a pretty strong suggestion.

The problem, according to research by Keith and co-author Amanda Adams, previously a postdoctoral fellow with Keith, is that wind turbines leave a "wind shadow" behind them – it slows down the air because of the drag created by the turbine's rotating blades. The best solution is to find the right balance between filling the farm with as many turbines as possible and installing them far enough apart that the wind shadow effect is less. But as wind farm size increases, they begin to interact and wind patterns in the area become more important.

The research says capacity of mega-scale wind farms (upwards of 100 square km) may top out at 0.5 to 1 watt per square meter. Earlier estimates of 2 to 7 watts ignored the wind shadow effect.

It's tricky, but as Adams explained in the article, "One of the inherent challenges of wind energy is that as soon as you start to develop wind farms and harvest the resource, you change the resource, making it difficult to assess what's really available."

"If wind power's going to make a contribution to global energy requirements that's serious, 10 or 20 percent or more, then it really has to contribute on the scale of terrawatts in the next half century or less," Keith was quoted as saying.

And wind shadow isn't the only problem, he says. There's the matter of economics and geography too.

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"It's clear the theoretical upper limit to wind power is huge, if you don't care about the impacts of covering the whole world with wind turbines. What's not clear – and this is a topic for future research – is what the practical limit to wind power would be if you consider all of the real-world constraints. You'd have to assume that wind turbines need to be located relatively close to where people actually live and where there's a fairly constant wind supply, and that they have to deal with environmental constraints. You can't just put them everywhere."

No, Keith isn't anti-wind. "Our findings don't mean that we shouldn't pursue wind power – wind is much better for the environment that conventional coal – but these geophysical limits may be meaningful if we really want to scale wind energy up to supply a third, let's say, of our primary energy."

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