"Wind power is still one of the most scalable 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," said David Keith of Harvard University.
Along with Amanda Adams of the University of North Carolina, Keith used a parameterization of the atmospheric effects of wind-turbine arrays developed for use in a mesoscale model, in this case the Weather Research and Forecasting Advanced Research model (WRF-ARW). By running simulations for turbine arrays 2.7, 30 or 270 × 103 km2 in size, the pair found that wind-power production would saturate at around 0.5 to 1 W m2 as turbine density increases.
In a development that Keith finds "surprising and interesting" in a field that has seen "quite vigorous disagreements", these numbers correlate well with the most recent estimates of wind power that use global climate models. "It's one of the things I'm really happy about," Keith told environmentalresearchweb.
Earlier estimates of global wind-power capacity, which ranged from 56 to 400 TW, simply added up the best estimates of local wind. "At this point even though there are ways in which we and the [Mark] Jacobson group disagree, we both agree that you can't do that – you have to take account of these distant effects," said Keith. "Infinitesimal things happen a long way away. You can see footprints halfway around the globe from a 10 TW or 5 TW wind-turbine array."
By ignoring the effects of wind-turbine drag on local winds, these earlier estimates assumed that 2–7 W m2 of wind power can be produced over large areas. But Adams and Keith's model indicates that, for wind farms larger than about 100 km2, wind-power production is limited to roughly 0.5 W m2.
If that's right then...it's still true that the upper limit of wind power if you're willing to cover the entire world with wind turbines is very large, say 100 TW, so bigger than matters in policy terms," said Keith. "But if you had to do something more realistic, where you select where you're going to put wind turbines based on being relatively close to human energy demand, and you had to find relatively good wind resources and not have a big environmental impact, then you're down to a much smaller number." And that might be the point at which the constraint of half a watt per square metre becomes important.