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Ducted & Shrouded Wind Turbines (DAWTs)

October 12, 2009
Paul Gipe

Ducted or Augmented Turbines--Enflo

Enflo diffuser-augmented turbine at the Husum Wind exhibition in 2005.


The following is adapted from Wind Energy Basics: A Guide to Home- and Community-scale Wind Energy Systems published by Chelsea Green in 2009. All rights reserved.

Ducted or shrouded wind turbines have been invented and reinvented throughout the 20th century. One such inventor was Dew Oliver, who installed his "blunderbuss," or ducted turbine, in California's San Gorgonio Pass in the 1920s. Historian Robert Righter notes that though Oliver claimed the turbine worked, that didn't prevent Oliver from being eventually convicted of fraud. (There should be a lesson in that.)

While many inventors opt for squirrel-cage rotors as a sure-fire means for extracting more energy from the wind than conventional wind turbines, others, such as Dew Oliver, opt for wind turbines encased in shrouds or ducts. As with squirrel-cage rotors and their curved deflectors, shrouded turbines are easy to visualize. Much like giant funnels, the shrouds concentrate or augment the flow across the wind turbine's rotor.

However, ducted turbines are often wrapped in mysterious, technical babble, such as diffuser augmentation, that is beyond the ken of most observers. Even supposedly sophisticated engineers have been snared by what at first appears to be a startling new technology "overlooked" for decades by everyone else.

Do augmented turbines work? Yes, of course. Cup anemometers work too, but we don't use them to produce electricity. Why? Because cup anemometers can't compete with modern, high-speed turbines. The same is true with ducted turbines. They have been tried--time and again--and found wanting.

Diffuser-augmented turbines can achieve conversion efficiencies much higher than conventional wind turbines with rotors of the same diameter. Consequently, ducted turbines periodically reappear in both the trade and professional press as a "promising" new technology--often in conjunction with a jump in the price of oil.

What is missed, however, is that a ducted turbine uses, well, a duct, and it is this big duct, or shroud, that increases the area intercepted by the ducted wind turbine relative to that of a conventional wind turbine of the same rotor diameter. And someone has to pay for that big shroud around the rotor, and pay to turn the whole assembly into the wind.

One outspoken critic of diffuser augmentation is professor Heiner Dörner at the University of Stuttgart's Institute of Aircraft Design. Sure, Dörner says, wind tunnel tests show you that this can double the wind speed across the rotor. But for this to happen, the wind must flow directly into the concentrator, a condition found only in a wind tunnel. Rarely would such conditions exist in the real world, where such a turbine would operate. Yes, he says, the complete ducted assembly can turn to face changes in wind direction, but it can rarely follow the wind accurately enough. Thus, the concentrating effect will be difficult, if not impossible, to achieve in operation.

This is one of the results seen by the few ducted turbines that have ever been built. Engineers first noted in their reports that yes, they were able to augment the flow (boost efficiency) as promised in the sales brochures; the engineers go on to say, however, that for various reasons--there are always extenuating circumstances--the concentrating effect was much less than they had anticipated. Moreover, there were other problems as well. The shroud was more expensive and difficult to construct than they first thought, and turning the ungainly turbine into the wind was more difficult, and so on.

In the fall of 2005 a Swiss company was displaying a "new" ducted turbine, the Enflo Windtec 0071, at the big wind extravaganza in Husum, Germany. The turbine is small and well suited for an indoor display. The rotor is only 0.71 meters in diameter. The shroud, which surrounds the rotor, is 0.87 meters in diameter. With the shroud included, the Enflo 0071 is about the size of an Ampair 100. For comparison, most turbines at the Husum trade show are so big they build the tents around the nacelles.

However, unlike the Ampair 100, a conventional micro turbine rated at 100 watts, the Enflo 0071 is rated at 500 watts.

First, let's only consider the area of the wind intercepted by the shroud, about 0.6 m2, not the smaller area of the rotor inside the shroud. If a turbine of this sized performed like other turbines in this class, it would be capable of about 120 watts, not 500 watts. By this criterion, the turbine claims it can produce five times more power than other wind turbines of its size. Another way to look at this is the rotor loading at rated power claimed by the manufacturer. The rotor loading for this turbines is 840 W/m² of shroud intercept area compared with the typical 250-300 W/m².

Second, let's compare the manufacturer's claimed performance to the Betz limit. The manufacturer claims that the Enflo 0071 will convert 70% of the energy in the wind at its rated power. Betz argued that the maximum we can extract from the wind is 59%. Even with the shroud included, the Enflo 0071 exceeds the Betz limit.

How then does the Enflo ducted turbine compare to other turbines? It claims, and these remain just claims, exceeds by a wide margin the performance--sometimes also claimed but often measured--of other turbines in its size class. At least the claimed performance of the Enflo 0071 falls well short of that by Mag-Wind. (MagWind claimed that its turbine would capture twice the actual power in the wind.)

Can the Enflo 0071 do what it claims? The manufacturer's claims are not so far from that theoretically possible, that it's conceivable. Is it likely? No. The odds are long that this turbine can come even close to delivering on its promise of outsize performance. The burden of proof is always on the manufacturer to prove that its turbine can do what it claims, more so in a case like this where the claims are at the theoretical limits.

Over the years augmented and ducted turbines have never produced the amount of energy promised at the cost promised. They have never fulfilled their often highly touted claims. Modern wind turbines, for all their limitations, reliably deliver quantities of electricity at increasingly competitive costs. In short, conventional wind turbines work, and they work better than the few ducted turbines that have been tested.

What lessons can you take away from this? As with most things in life, don't believe everything you read. There is a deep-seated desire in all of us to believe we can get something for nothing. In wind energy this trait manifests itself in our willingness to suspend critical judgment, to ask "Does this really work, and if it does where are the results?" Even the technically trained sometimes fall for the latest fad and get swept up in the "madness of crowds". In wind energy, it's Caveat emptor. It's "let the buyer beware."

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