What are the techno-commercial aspects of Diffuser Augmented Wind Turbine?
I think these are debatable. The commercial viability of ducted wind turbines will depend on the cost of other energy both economically and environmentally. Technically we know ducted wind turbines should be capable of higher efficiencies but obviously at higher manufacturing costs. But the manufacturing costs of the turbine alone are a small percentage, less than 15% of overall cost. See Innovation in Wind Turbine Design, P.Jamieson. Once the industry wakes up and accepts that Betz's Limit is not valid, new technologies will evolve that extract thermal energy as well as kinetic energy from the wind in cost effective manners. Ducted wind turbines will eventually become the norm.
In the late 70s when the topic of ducted wind turbines was being discussed it was felt that instead of putting material on diffusers a little extra could be put on blades. At that time some designers were claiming even a tip-wane may give better advantages.
If one performs a control volume analysis of an isolated ducted wind turbine, one easily sees that the maximal increase in the power coefficient is linear with the "acceleration" (in fact flow concentration) provided by the duct. And this is only the case if the diffuser following the turbine does not separate. Which is very difficult to obtain.
Many of such concepts have been advocated by (inappropriately) applying Betz using the accelerated velocity, giving thereby predicting a power increase proportional to the cube of the acceleration factor. This is not correct, and can be easily checked: one would subtract more energy than available in the flow.
Theoretically, this can be circumvented by mixing the flow coming out of the turbine with a (faster) surrounding flow in order to reenergize it before returning to the ambient conditions. But this means the duct needs to be much larger than the turbine. This occurs for instance the case in natural flow concentrations (eg. in mountainous regions, between buildings, etc.).
In 1970's this topic was heavily discussed , It was applied to ship propeller, well that improved propeller thrust. Well for propeller type wind turbinet the duct may add force on the pillar , and may caused the tower to swy in the wind.
Ducted wind turbines are not worth making. The extra cost and complexity of the duct, its wind loading and danger of the blades hitting the duct as it wobbles and flexes all outweigh the cost of getting the same increase in power by using longer blades.
The exception would be circumstances where there is another good reason for a duct, e.g. to prevent bird or bat strike, and then only when there is no practical alternative to a wind turbine.
But, we are reaching the point where the square-cube law is becoming significant. (The concept that although the power extracted by the wind turbine is proportional to the square of the blade length; the volume, weight and cost are proportional to the cube of its length.) I think the technology is still worth pursuing and as demand for distributed energy sources increase so will the cost effectiveness of ducted turbines.
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