A CRP idea

Can wind turbines make use of the Contra-Rotating Principle the same way that ships propellers do? Andy Greaves asked the question and Dave Wilson went in search of the answer.

The wind shows us how close to the edge we are. – Joan Didion.

A few weeks ago, we ran a story on e4engineering about the Azipod, a downstream ship propeller unit that has been developed by ABB. When installed aft of a mechanically driven main propeller on a ship, the Azipod unit increases the propulsion efficiency of the ship by 10-15%. It works on a Contra-Rotating Principle (CRP), absorbing rotational energy from the main propeller.

Over a quick cigarette one afternoon, I was discussing this rather clever piece of engineering with Andy Greaves, our e4engineering UK Sales Executive. He asked me if the principle could also be used on wind turbines to boost their efficiency too.

Well, I must say, I was rather intrigued by Andy’s question, and, in true journalistic fashion, I set out to find him an answer.

Søren Jes Plagborg, the Product Manager at Vestas, a Danish outfit that should know a thing or two about wind turbines (because they make lots of them), pointed me in the direction of a chap called Betz.

Betz came up with a law that actually describes the maximum value for the power extracted from the wind. Turns out that it’s actually 16/27 of the total power in the wind or 0.59. If you are interested, you can read about it all <a href=’http://www.windpower.dk/stat/betzpro.htm’>here</a>.

Unfortunately, it’s that 0.59 figure that might signal the end for the use of an Azipod-like device in a wind turbine. Because, as Mr. Plagborg told us, since a modern wind turbine has an aerodynamic efficiency of approximately 0.46, an additional turbine would simply not be beneficial enough in terms of cost.

For a second opinion, we asked Dr. David Wood, an Assistant Professor at Newcastle University, what he thought of the idea. Unfortunately, he didn’t really seem too enamoured by it either. ‘Counter-rotation is unlikely to be used for wind turbines because the amount of rotation in the air leaving the blades is low, when compared to propellers and helicopter rotors,’ he said.

Additionally, of course, an extra rotor is likely to increase the load on the rest of the construction, resulting in yet more additional costs on the tower and foundation.

So, Andy has his answer. It seems highly unlikely that any sort of downstream propeller is to be seen on a wind power machine any time soon.

If only this were the end of the story, dear reader. But it isn’t. Having studied Betz’ law over lunch, Andy now wants to know if anyone has figured out a way to make the air denser in the proximity to a turbine. Because, according to Betz, the density of the air is proportional to the power output from the device.

Why doesn’t he go back to selling some advertising and leave me alone?