While we’re on the subject of wind energy, I really had to scratch my head at the selection of FloDesign to receive $8.3M in ARPA-E funding. New ducted turbine designs are a perennial theme in wind turbine design, a subset of the larger class of revolutionary new turbine designs that look really cool in CAD visualizations but never seem to make it to prime time.
I think the basic dynamic here is that the conventional wind turbine (by which I mean, three slender blades cantilevered radially from a horizontal axis which is aligned parallel to the wind direction) just “looks wrong” to a substantial subset of the general public. The blades themselves rarely cover over 5% of the “swept area” of the turbine, the circle defined by the path of the blade tips as they rotate. It’s understandable that the average person looking at a turbine of that design would think that “most of the wind will just slip through between the blades”. And it’s not a coincidence that just about every “alternative” turbine design you are likely to see has much greater “solidity” (the ratio of rotor frontal area to rotor swept area) than the conventional design.
The counterintuitive thing about conventional turbines is that from the standpoint of the wind, the turbine “looks” very nearly like a solid disk, rather than three slender blades. That’s because the blades are spinning fast relative to the speed of the wind, typically around six times faster than the speed of the wind at the blade tips. Circumference is around six times radius, and so in the time that a “hunk” of wind as long as the blade radius passes by the turbine, it must contend with not one but three blades passing through each element of the swept area. This causes the wind to slow down in a way that is not apparent to the eye. And slowing down the wind is exactly what must be done to extract the maximum energy possible from the incoming stream of air – to a speed one-third that of the free stream, as it turns out. (You can’t extract all the energy from the incoming stream, as it would of necessity stop dead in its tracks, a condition inconsistent with continuous operation.)
So, the performance of conventional turbines is not readily grasped on sight, and in this disturbed mental soil the seed of many a crackpot scheme is planted. It doesn’t help that the above-mentioned limitation is known formally by the name of its discoverer, ie. the Betz Limit, with a value of 59.3% to be exact. Mention of a “fundamental limit” to anything is like catnip to the imagination of the kind of person who is sure that the conventional design must be grossly inefficient, and this has been the genesis of a lot of hot air and wasted effort.
In the specific case of duct-augmented wind turbines (to which the FloDesign turbine belongs), it is in fact theoretically possible to extract more energy with a given size of rotor if you surround that rotor with a duct. But the diameter of the rotor is not the same as the diameter of the wind turbine – the duct is typically substantially larger in diameter than the rotor. This is the sleight of hand that’s usually employed by DAWT fans – talk about how much more energy you can extract from the same size rotor, and hope nobody will notice the giant donut thing you’ve stuck around that rotor. The key goal in designing a wind turbine is not how much energy can be extracted from a given size rotor, but rather how much energy can be extracted per dollar invested.
As a thought experiment, look at the artist’s renditions of the FloDesign machine and imagine how much more swept area you would intercept if you took all the material in the two artfully crenulated ducts, and instead formed that material into three long thin blades. The brilliance of the conventional design is that it covers a relatively large swept area with a relatively minimal use of engineering materials. The ducted turbine folks throw this advantage out of the window in building their great swodding donuts, chasing after an ancillary goal (getting more power out of a rotor) and losing sight of the ultimate goal (lower-cost clean energy).
There is a related, more subtle advantage to the conventional design as compared to the various DAWT concepts ( for another example, check out http://www.optiwind.com/optiwind150.html) that relates to survival in extreme conditions. The discussion above about how a conventional turbine looks to the wind more or less like a solid disk only applies when it is spinning. In hurricane conditions, the drag force acting on the structure can be decreased by an order of magnitude by simply stopping (or dramatically slowing) the rotor. This basic strategy does not apply to large blobby assemblies atop towers; while at best they may be relatively streamlined in one direction, it cannot be assumed that this orientation can be maintained in extremis, for instance when the grid goes down (large machines use active pitch drive). If you are following the gist of this discussion, it will not surprise you to learn that no ducted machine has made it into significant production to the point where extreme weather survivability could even become an issue.
I don’t intend here to directly accuse the FloDesign folks of crackpotism (for all I know they are credible in their own field of aerospace), or of intentionally defrauding their investors (who are explicitly looking for risky propositions), but it is amazing to me how often a serious team with serious money will assemble around a concept that doesn’t pass the sniff test. I saw the same thing when I was briefly involved as a consultant in the world of algae biofuels. And it’s amazing to me that the US DOE concluded that this DAWT rehash was one of the 37 most promising new energy concepts out of 10s of thousands of applications.
Web article: http://www.alternative-energy-news.info/wind-turbine-concept-jet-engines/
artist’s conception and description: http://alfin2100.blogspot.com/2008/02/renewable-energy-high-tech-wind-and.html That blog links to the following “careful analysis”, which as far as I can tell was written by someone with no knowledge whatsoever of the fundamentals of wind turbines – “fact check” miyarse: http://newenergyandfuel.com/http:/newenergyandfuel/com/2008/02/04/fact-check-on-the-new-wind-turbine-design/
That page has some material (which I can’t find on any page associated with FloDesign) comparing their theoretical performance of a “12′ diameter” turbine of their design with that of 12 and 24′ ARE conventional wind turbines; their claimed performance is equivalent to that of the larger ARE machine; perhaps not coincidentally, the overall diameter of their design appears to be about 2X the diameter of their rotor.
Tech Review article: the comments (shunted off into the TR forum) indicate a readership with a bit more sense.
Here’s industry expert Paul Gipe on ducted turbines
Here’s another clunky-looking attempt: Marquiss

