Wind turbine manufacturers have long believed it didn't matter which way their blades rotate. Consistency keeps things simple, so almost all turn clockwise. However, three engineers are challenging this idea, saying we could be producing a lot more clean power if some of our turbines turned the other way.
Dr Antonia Englberger of the German Aerospace Center agrees the direction of spin is irrelevant for a single wind turbine operating in splendid isolation. These days, however, most wind energy comes from large wind farms made up of dozens, or even hundreds, of machines. Placing the towers close together reduces the costs of powerlines and maintenance. However, it creates the risk the first set of blades to meet the oncoming wind will generate a wake that affects the power generated by the farms' other elements.
Wind farm designers put a lot of effort into analyzing prevailing wind directions to assess how to position an array of turbines relative to each other. Now, Englberger claims in Wind Energy Science that far more can be achieved by having many turbines rotating in the opposite direction to the one whose wake they are catching. The wake a leading turbine creates is called veering or backing, depending on how it impacts those behind it.
“A veering wind in combination with counterclockwise rotating blades would result in a power output increase of 11.5 percent for a downwind turbine in comparison to a clockwise rotating upwind turbine in the Northern Hemisphere,” Englberger and co-authors write.
Coriolis forces mean the same is true for backing winds in the Southern Hemisphere. The effect is stronger at night when the wind tends to be more stratified with height above the ground, increasing the longevity of wakes. Under optimum conditions, a trailing turbine spinning in the opposite direction to the leading one could see it produce 23 percent more power than it would if both turned the same way, the authors claim. After modeling a variety of wind conditions, they report that following a clockwise-rotating turbine with a counterclockwise one almost always results in more power than having both the same, at least in the Northern Hemisphere.
The paper was published in January but went almost unnoticed until this month when an article in Tagesspiegel drew attention to it. A flurry of interest means wind energy companies worldwide are probably frantically trying to work out if they've been drastically under-producing the potential from their farms or if there is an error in Englberger's modeling.
If the study is even close to right, its implications for the future of electricity could be enormous, even though 23 percent extra output is a ceiling, rather than a typical value. With no change to the leading turbine, the average across an entire wind farm would be smaller still. On the other hand, every technological improvement that lowers the cost of any renewable energy source increases the amount that gets built. The resulting economies of scale magnify the benefits, shifting the balance in the battle between fossil fuels and clean energy sources.