February 25, 2013 – “People have often thought there’s no upper bound for wind power… that it’s one of the most scalable power sources,” said Harvard applied physicist David Keith. After all, gusts and breezes don’t seem likely to run out on a global scale in the way oil wells might run dry. Yet the latest research in mesoscale atmospheric modelling suggests the generating capacity of large-scale wind farms has been overestimated.

Each wind turbine creates behind it a ‘wind shadow’ in which the air has been slowed down by drag on the turbine’s blades. The ideal wind farm strikes a balance, packing as many turbines onto the land as possible while spacing them enough to reduce the impact of these wind shadows. But, as wind farms grow larger, they start to interact, and the regional-scale wind patterns matter more.

Keith’s research has shown the generating capacity of very large wind power installations (larger than 100 km²) may peak at between 0.5 and 1 W/m². Previous estimates, which ignored the turbines’ slowing effect on the wind, had put that figure at between 2 and 7 W/m². In short, we may not have access to as much wind power as scientists thought.

“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,” said co-author Amanda S. Adams.

“If wind power’s going to make a contribution to global energy requirements that’s serious—10 or 20% or more—then it really has to contribute on the scale of terawatts in the next half-century or less,” said Keith.

If we were to cover the entire Earth with wind farms, he noted, “the system could potentially generate enormous amounts of power—well in excess of 100 terawatts—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”.

Keith adds that their findings don’t mean that we shouldn’t pursue wind power, “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”.

“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,” added Keith. “You can’t just put them everywhere.”

The research was funded by the Natural Sciences and Engineering Research Council of Canada.