JUN 09

The French aerospace lab, ONERA, has released information regarding their ongoing studies on the blades of wind turbines, with the objective being to make more effective and bigger turbines possible. Their expertise with helicopter rotors and blades position them ideally to undertake a thorough analysis of air flow running over the turbines.

With the monsterous size of many of the new turbines going up - as much as 6 MW - the wind industry is meeting some big challenges. A typical 5 MW unit can weigh between 400-500 tons, while each blade alone is about 20 tons. Being that the blades are generally built from fiberglass mixed with polyester resins or epoxy, they are fairly stiff and the stresses on them can be enormous, fatiguing and degrading them more quickly than we would like. ONERA believes that making the blades more supple, bendable, would relieve some of these stresses and even make the turbine more efficient.

Through specially designed helicopter modeling software, which they modified to deal with wind turbine specs, they calculated the characteristics of the blade composites to measure the response in elasticity. It also permitted them to see the wake of the blades. The main difficulty was to model the limiting layer, a very thin layer of air which makes contact with the blades and does what they call "unhooking", failing to transfer a large part of its energy to the blade.

"There are constant unhookings of the limiting layer in wind turbines. When the blades start, it's because of the sections closest to the rotor. Once the wind picks up, these sections don't work any more because the air is unhooking, and it's the ends of the blades that are doing the work. Thus, only 30-40% of the blade is working to transfer energy during normal operation," says one of the researchers.

Pitched blades, which represent 95% of the current market, help increase the efficiency over fixed blades, but there's still a lot of room for improvement.

Perfecting more supple blades that can bend will do some of the mechanical work themselves while unworking the deformations caused by the wind. Tests continue to find the optimum levels, but hopefully in a few years we'll see turbines that are 30% more efficient.

Comments (20)

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wrong direction?
written by james, June 09, 2008

I wonder why the fascination with bigger, regarding wind turbines. As the article points out, size just seems to make the whole process more problematic. Ecogeek's own article on Selsam's multi-blade design seems a better direction (see superturbine.net) since it eliminates many of the problems outlined in the article. I've been wondering if Selsam's design would start to gain attention from big energy producers, but as yet they seem to be under the radar.
Another good idea that hasn't seemed to catch attention is the so-called Floating Wind Turbines (also in Ecogeek). Also smaller, more easily mass produced and set up, and with a greater production of wind energy.
These giant turbines seem like the corn ethanol of the wind industry to me. Not the best idea out there...yet we seem drawn that direction by some kind of inertia or influence?

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5 MW surely
written by johno, June 09, 2008

A 5 GW turbine would be more powerful than most of the hydroelectric dams in the world. How did that slip past the author? twice.

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about the 5 gigs...
written by heidi, June 09, 2008

2.5 MW is pushing it for the industry, pretty sure a 5 GW turbine would eat Tokyo for dinner...

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Why Large Turbines are Better
written by Tyler, June 10, 2008

Larger turbines are inherently more efficient and cost effective. The obstacles to creating even more efficient large turbine blades will lead to a dramatic increase in the productive power of wind turbines. Small turbines are simply not cost effective at a utility scale which is where we need the production.

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1.21 Gigawatts of electricity!
written by Brian, June 10, 2008

Enough to send almost 4 Deloreans forward in time. Cool.


(might want to change that to MW)

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unhooking
written by matt, June 10, 2008

May I suggest "stall" for what you translated to "unhooking" (I imagine the french word was "décrochage").
Btw it's obviously 5MW, I'm not sure the biggest nuclear power plants reach 5GW.

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Limiting layer?
written by matthews, June 10, 2008

"The main difficulty was to model the limiting layer" and by that you mean boundary layer, right? Seriously, babelfish?

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test
written by surajit, June 10, 2008

nice post

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Micro Wind
written by Patrick Thomas, June 10, 2008

Why don't we copy nature more on this?

What about 100 watt windmills on every third telephone pole?

Or string the poles together with a design like this?
http://www.speakerfactory.net/wind_old.htm

Look at a tree? Thousands of tiny leaves making up the whole but also protecting itself from extremes.

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LOL
written by Robert Kepner, June 10, 2008

eat tokyo for dinner... thats godzillas job. noob.

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Why no mr. sunshine
written by David G., June 10, 2008

how come you don't see solar cells placed on the bodies of these things .... lots of real estate just sitting there empty. Wonder what the power output could be if they combine the tech.

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...
written by jake, June 10, 2008

currently working on rollings hills wind farm in wyoming...all turbines here are 2mw (178 of them)..in regards to the solar question, the collection of energy makes it infeasible to put one on top of the other...

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Boundary Layer
written by Tbyrd, June 10, 2008

I think it was obvious that he was talking about the boundary layer when he said limiting layer. Its also obvious some of this is translated from french so the literal translation probably just got infused into the article.

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Whale Fins - Knobbed Wings
written by Kevin Rice, June 10, 2008

I read an article recently noting that wings could be made much more efficient through water by adopting the shape of whale tail fins. These have knobs on them at intervals. Apparently this affects the fluid dynamics to make them more efficient. I'm wondering if this could be made to work in air turbines... Google for WhalePower to find reference to company doing this for wind turbines. It'd be interesting to see anyone else, especially a large corp. or government, verify this company's results.

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Wrong direction 2
written by Fred, June 10, 2008

current turbine designs gather around 50% of the wind energy available.
THERE IS A MAXIMUM amount of energy that can be extracted from a fluid (wind) that maximum is 59%. After extracting 60% (or more) from a fluid stream the fluid "stalls". It no longer has enough energy to continue on its way. This stalled fluid then blocks the inflow of "new" fluid - effectively stopping the extraction of energy from the stream.

As Wind turbines are already extracting around 50% of the available energy - the last few % are not really worth it until we (people) increase the efficiency of :
- transmission
- generation ( the generators themselves not the blades)

an increase of a few % in either of these areas will return FAR more power than a similar increase in blade efficiency.

Oh - and btw - a 30% increase in blade efficiency is not possible - you would be extracting 65% of the wind energy (see above).

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Better lift, Less Drag
written by Todd A, June 11, 2008

I caught an article about this in MIT's Emerging Technologies e-mag: The bumps on the leading edge of a humpback whale's fins help increase the lift and the acceptable angle of attack for the water as it goes across the whale's fin. This works just the same for wings and propellers. I hope the french aeronautics industry catches on to this design improvement.

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One Kilowatt Per Inch Of Blade
written by Tony Chessick, June 11, 2008

Blade increments speeding through the air at 100 - 150 mph tangentially can pick up one kilowatt per inch of blade length of each blade. This is what is found from the centers out to the tips of the blades of the big turbines. This high conversion rate can even be said to be a discovery, a result of the single minded approach of making blades longer. Also, the blades are least expensive there where the power conversion is the greatest (and more expensive nearer the blade roots where the power found is less). Some might conclude that a different rotor axis might allow *all* of the blade length to act like only these tips and be highly efficient thereby (but for only a portion of their rotor circuits) - one kilowatt per inch along the entire blade length of thin, narrow, and inexpensive blades. Surely it is guessed by now this refers to vertical axis (but along with some better answers to making it work).

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Director, Wind Department
written by Matt Tritt, June 11, 2008

Just a few comments on some of the other comments. :)

The larger the turbine, the lower the cost of generated electricity; this is why they just get bigger all the time.

If someone with deep enough pockets is willing to back Doug Selsam's conceptual designs to the point of actually making a large output Beta unit, we would all benefit from the conclusion - regardless of the results.

Wind turbines do not achieve efficiencies of 50% - no way, no-how. This comes way too close to Betz' limit of 59.3% maximum. Efficiencies in the low 30's are considered the best there is these days, including actual raw energy to electricity to the grid.

Huge numbers of tiny, 100 Watt, turbines would be hugely inefficient, impossible to maintain way too expensive per installed Watt to bother with. On the other hand, giant turbines (in the 1.5 mW class and up) aren't exactly what one would call a major enhancement for the viewshed (unless it happens to be in the middle of Iowa. :-))

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...
written by MIT student, January 09, 2009

The bumps on a whale's tail trip the flow from laminar to turbulent so the flow is less likely to detach at high angles of attack (or you could say that the stall angle increases). While turbulent flow is draggier than laminar, it is much less draggy than separated flow, which is why this is advantageous for whale's tails.

This fact is well know by aerodynamicists, and "trip strips" are often used on aircraft to increase their stall angle. However, as mention in the article, many turbine blades now days are variable pitched, to keep the angle of attack low throughout the blade's operating speeds. This negates the need for a trip strip, since the separation problem is removed.

I would also recommend against saying the flow "stalls" after the 59.3% limit. Stall means that an airfoil loses it's effective lift. It's a loose term, but generally it means that the flow over the wings has separated and an aircraft loses enough lift that it begins to fall and becomes generally uncontrollable. More specifically, it's the point where increasing angle of attack results in a decrease in lift instead of an increase. Read more if you're interested http://en.wikipedia.org/wiki/Stall_(flight)

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...
written by wedding dresses, October 13, 2009

that making the blades more supple, bendable, would relieve some of these stresses and even make the turbine more efficient.

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