Diffuser Augmented Wind Turbine Theory–Some Comments

By Paul Gipe

In the past I’ve singled out what are variously called Diffuser Augmented Wind Turbines (DAWT) or Shrouded Rotors for derision.

Wind turbine engineers, manufacturers of commercial wind turbines, and renewable energy analysts throw up their hands in despair at the continuing re-invention of these devices and the rapidity with which the media, the unsuspecting public, and venture capitalists–who should know better–seize on them.

I am not an aerodynamicist or an engineer—of any stripe—I’ve simply recounted what others I respect, such as Heiner Dörner  have said about such inventions.

More importantly, in a long career in the wind industry I can report on what I know from experience: These machines have never performed as promised.

In my recent perusal of the literature, I came across some comments by Martin Hansen in his 2008 book on Aerodynamcis of Wind Turbines. In his Chapter 5 on Shrouded Rotors, Hansen says quite clearly, “It is possible to exceed the Betz limit.” He goes on to explain that “If the cross-section of the diffuser is shaped like an aerofoil, a lift force will be generated by the flow through he diffuser. . .”

So far, so good. As noted by Hansen and others, “the results are dependent on the actual diffuser geometry, in other words the amount of lift which can be generated by the diffuser.”

I’ll summarize, you can get speed up and hence a multiple for the power output of a shrouded rotor, relative to a conventional wind turbine, when designed right, and facing the wind squarely without any disruptions in the flow.

Few argue with this. Experimentally you can get multiples of 1.4 to 1.6 increase in power with a shrouded rotor, when everything is optimal.

Though there have been repeated attempts to develop such technology, “this still has to be demonstrated on a full-size machine,” says Hansen.

And worse, none have fared well commercially because the shroud or duct is large and materially expensive. This causes two, as yet insurmountable problems.

You have to pay for the shroud and it has so far it has always proven cheaper to simply extend the blade on a conventional wind turbine to make up for whatever multiplier effect the shrouded turbine provides.

The shroud or duct adds to the drag on the wind turbine, tower, and foundation requiring all to be much bigger and heavier than they would on an equivalent commercial wind turbines. Why? Because all wind turbines have to withstand high winds and storms. Ducted wind turbines are effectively big blobs of material stuck into the wind. Conventional wind turbines have long slender blades that typically can be feathered to ride out storms.

Are there ways around these limitations? Possibly. But no one has done it successfully yet. And people have been trying for at least three decades. That should tell inventors something.

Aerodynamcis of Wind Turbines, Second Edition, Martin Hansen, 2008, Earthscan, London, Chapter 5, Shrouded Rotors.