Contra-rotating wind turbine more efficient, much quieter. The future of wind power?

My Clean Break column today takes a look at a wind turbine design out of the University of Ottawa. It’s a twin contra-rotating blade system, meaning there are two sets of blades each spinning in opposite directions. The engineers who designed it say that wind-tunnel tests prove it is more efficient and — an added bonus — much quieter because the vibrations from each blade system help cancel out each other. Read the full column below:

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Tyler Hamilton

There was a time not so long ago that seeing a single wind turbine spinning in the distance was a novel experience for most people.

Not so much any more. There are now hundreds of wind turbines scattered across the province, representing 1,700 megawatts of wind capacity in Ontario alone — or just over a third of all capacity in Canada.

Hundreds more are in the queue waiting to be installed. Most are large turbines, about 2 megawatts or more in size, and between 80 and 100 metres high. All look pretty much the same: a big tall white tower with three spinning blades attached by rotor to a massive nacelle, which houses the generator and gearbox.

But as researchers continue to improve the efficiency of wind power generation it’s quite possible that the turbine of tomorrow will look dramatically different.

There are proposals for two-bladed turbines. There are vertical-axis turbines that look and spin like egg beaters. One company called FloDesign has a turbine that looks like a jet engine on an airplane, while Toronto-based WhalePower wants future turbine blades to resemble humpback whale flippers.

Over at the University of Ottawa, a group of students and professors who dub themselves the “Green Engineer” have come up with their own creative approach — a wind turbine with two sets of blades each spinning in opposite directions.

They call it the contra-rotating small wind energy converter. Wind tunnel tests on a prototype have shown that the design is up to 40 per cent more efficient and far less noisy than a conventional single-rotor system.

The benefits of having contra-rotating blades are well known. In fact, the design has existed for more than a century and is widely used, for example, in propeller systems of submarine torpedoes. The concept is also used in airplane and boat propulsion systems, not to mention those remote-controlled toy helicopters you can fly inside your house.

Riadh Habash, professor of technology and engineering at the University of Ottawa, says his team decided three years to apply the same approach to wind turbines and are encouraged so far with the results — so much so that they’re busy building a second prototype that will be mounted next summer atop a building on the Ottawa U campus.

Why is having two blade systems spinning in opposite directions more efficient?

When the wind blows into a conventional three-bladed, single-rotor wind turbine less than 40 per cent of its energy is converted into electricity. The rest escapes, much of it in the air wake that’s created behind the blades. That wake spins in the opposite direction (i.e. counter-clockwise) to those blades.

If a second rotor with another set of blades is right behind the first rotor, and if it is designed to also spin counter-clockwise, it can capture energy from that wake. The end result is a turbine system that harnesses much more energy from the initial flow of wind.

Experiments to date also suggest that a turbine with such a design can operate at lower wind speeds, allowing it to tap into a broader range of wind resources.

Habash says an added, but just as important, benefit is that the design is also quieter. “We have observed that when you have two sets of blades that are contra-rotating they achieve a kind of vibration cancellation. There is a clear reduction in vibration.”

This is good news, as one of the biggest issues hindering the deployment of wind energy — particularly in Ontario, for some reason — is concerns related to noise and inaudible vibrations reportedly experienced by nearby residents.

The Green Engineers and their industry partners, including Ottawa-based TRIAS Innovations, have some other tricks up their sleeves. All of them are aimed at producing a superior wind turbine with all parts — blades, generator, power electronics, drive train and tower — manufactured in Canada.

They’re even chatting with WhalePower about incorporating its whale-inspired blade design, further adding to turbine efficiency and noise reduction.

“Our target market for now is small wind turbines,” says Habash, who leads the team. “We are aiming at 10 kilowatts. But in the future that could go up to 100 kilowatts, and if we can prove the concept it could then be applied to much larger turbines.”

The project has been funded by the university, Ontario Power Authority, Ontario Centres of Excellence and Natural Sciences and Engineering Research Council of Canada.

Habash hopes to be able to demonstrate the second machine in a number of locations. One would be part of a combined wind and energy storage project in an aboriginal community. “We have some investors who are very interested in using this for community power,” he says.

It’s still early days, but it’s an example of how wind power design could evolve over the coming years, based on innovation coming directly out of Ontario.

Check out this YouTube video from the University of Ottawa.

Tyler Hamilton, author of Mad Like Tesla, writes weekly about green energy and clean technologies. Contact him at tyler@cleanbreak.ca

5 thoughts on “Contra-rotating wind turbine more efficient, much quieter. The future of wind power?”

  1. Any news about wind turbines efficiency not mentionning Betz’law ( http://en.wikipedia.org/wiki/Betz'_law ) looks suspicious to me. Modern wind turbines are already very close to this limit of 59% at their optimal wind speed, especially in wind tunnels that fool Betz’law a bit.

    The efficiency of 40% currently attained is fairly good considering the wind speed erratic variations, mechanical frictions and generator leaks. I frankly doubt a contra rotating system can bring 40% more : 40% x 1.4 = 56%, which means reaching Betz’ law at all wind speeds, a result that would bring propellers back on all airplanes within months, by the way…

    FYI, there is a swiss company http://eotheme.com/ that works on pretty much the same stuff

  2. Dr Goulu – Betz limit applies to a single rotor plane and defines the amount of mass that escapes the blades as the streamtube expands forward of the machine. With is more complex forward, multi-plane, and multiple wake flow it is hard to predict whether the Betz limit is strictly applicable. That said watching the video of the smoke test they did it looks like a surprisingly normal betz streamtube expansion forward of and around the machine so you are likely right about the claims being somewhat overoptimistic.

    Jim Takchess

    The betz limit is not the key parameter with regard to shrouded machines. Almost any shrouded machine will exceed the betz limit at the rotor. Punch a hole in a wall and you will probably exceed the betz limit if you put a rotor in the hole. The question is whether you can exceed the betz limit with regard to the outer area of the shroud. having had a look at Flodesign’s machine on deer island their rotor to shroud area is around 2.75. They have a claimed power increase of around 2.5-3 at the rotor. That translates to an acceleration of roughly 1.4 – 1.6 and a power increase ratio of between 1:1 (e.g. no increase) or 1:1.25 over a normal wind turbine of their shroud dimension. This does not exceed the betz limit with regard to the machine as a whole.

    You can exceed the limit if you can come within about 75% of the theoretical rate of acceleration. They do not.

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