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US research on the long-term viability of wind power has found that the cumulative effect of so-called “wind shadows” from turbines may significantly reduce the amount of wind available for future energy generation.
The research, published in the journal Environmental Research Letters, concluded that the generating capacity of large-scale windfarms may have been “overestimated” and that wind power may not be the unlimited resource it has come to be seen as.
According to David Keith, professor of applied physics at Harvard University, every wind turbine creates a wind shadow in which the air has been slowed down by drag on the blades.
Windfarms are normally built with sufficient space between the turbines to reduce the impact of these wind shadows.
However, atmospheric modelling shows that as windfarms grow larger they begin to interact and affect regional wind patterns, reducing the wind’s speed.
Keith believes the peak generating capacity of windfarms larger than 100km2 may therefore be between 0.5 and 1W/m2, considerably lower than previous estimates of between 2 and 7W/m2.
Keith said: “People have often thought there’s no upper bound for wind power. It is still one of the most scaleable renewables, but our research suggests that we will need to pay attention to its limits and climatic impacts if we try to scale it beyond a few terawatts.”
If we were to cover the entire earth with windfarms, he said, “the system could potentially generate enormous amounts of power, well in excess of 100TW, but at that point my guess, based on our climate modelling, is that the effect of that on global winds, and therefore on climate, would be severe – perhaps bigger than the impact of doubling CO2”.
He went on: “Our findings don’t mean that we shouldn’t pursue wind power – wind is much better for the environment than conventional coal – but these geophysical limits may be meaningful if we really want to scale wind power up to supply a third, let’s say, of our primary energy.
“The real punchline is that if you can’t get much more than half a watt out, and you accept that you can’t put them everywhere, then you may start to reach a limit that matters,” Keith added.
Study co-author Amanda Adams of the University of North Carolina at Charlotte said: “One of the inherent challenges of wind energy is that, as soon as you start to develop windfarms and harvest the resource, you change the resource, making it difficult to assess what’s really available.”