Mankind has made big efforts to concentrate the wind for increasing the natural windvelocity or to enlarge the power ratio. Windtunnel investigations have delivered some hopefull result, but this was never proofed in reality, up today.
Since 1996 we have a "new" old, huge, new-fangled saurian, a device erected in New Zealand, look to the following adresses:
http://www.vortecenergy.co.nz/intro.htm
or to the picture-chips.

Some data: blade diameter 7,3 m; height 22 m; rated power 1,2 MW (at ? m/s wind). The plant can double the ambient wind by suction effects, has said the company.
But as usuall in such cases, the concept hasn't prooved its efficiency, and nevertheless new machines are presented to the surprised shareholders: The Maxi Vortec and the Mini Vortec, to be erected in the first quarter of 1999?
Who will loose money is the question?
For example: Take the wonderfull inner rotor of the Maxi Vortec, as an excellent slender free running turbine, and take all material (and energy) of the huge shround and tubular strut and build, say 5 or 6 or more further free running turbines from it. And than, please, compare the output. . . . .
Actually, the Vortec Energy World Power Company is now going offshore, to earn more money from new shareholders. The sketch shows a 5 MW machine with a V66 blade

Ducted fans, shrouded turbines, diffusers, three names for the same device but the same physical effect. You need a length for the well shaped tube, 7 to 10 times the diameter of the rotor. The intake well rounded, the diffuser flared to the end smoothly. The slot at the rotor tip should reach a value of zero concerning the losses, and the wind, absolutely homogenous, must go absolutely straight into the tube.
These conditions are not existing in a natural wind.
Think at a 70 m tube for a 10 m diameter windrotor. What happens if the winddirection control fails? Terrible drag forces occur, perpendicular to the tube-axis. And what's about the fluctuating winddirection in a fraction of a second? If the control of the huge tube is not able to follow absolutely exact a new winddirection, then a separation of flow will occur at the inner wall of the intake. The result is a collapse of the needed homogenous flow for a concentrating effect.
The question is: Is it possible to double the windvelocity for example from 10 to 20 m/s? Experiments in windtunnels with excellent artificial conditions, and with threefold rindflaps (Ozer Igra,1976) have shown a concentrating effect of roughly 1.8.
To shorten the shroud you can try to work with slots or flaps to control the inner boundary layer or to bring more energy from the outer part of the windstream into the shroud.
The future of this experiment will become thrilling. The efforts in New Zealand will give an answer to the wind fan company: Is there an concentrating effect with this device in the atmospheric complete turbulent boundary layer? The windenergy science has to live with a complete turbulent boundary layer above all kinds of terrain, offered by the natural wind to our earth, a layer 300 m thick, at least, depending on the terrain structure. . . .and we can't reach the jetstream unfortunately with devices being economically.

Perhaps the wind fan company will get the answer to a lot of further very important questions:
Energy amount for this concentrating device, energy pay back factor, maintenance costs, behaviour of the wind direction control mechanism, lifetime a.s.o.
Personally I mean to this experiment in New Zealand: a better way to use windenergy economically with hardware-machinery is:
take all the material for a vortex or concentrating device and build up one HAWT (or even more), this means enlarge the rotor and try to reach the biggest area cut out of the windstream and slowing down the windvelocity to 1/3 by the rotor. This is the old Betz postulate (1926). In this case, please have a sincere look to the least material amount possible for the rotor and all subsystems.
If you want to double the windvelocity, if this is possible at all by a diffuser vortex system, you can get the same effect with a 2.8 fold diameter.
For example, precisely, build instead a 10 m HAWT better a 28 m plant, the effect is the same, you get then the 8-fold power output naturally for the same windvelocity and the same powercoefficient.
Look to the following references or click the above installed pictures:

Igra, O. "Shrouds for Aerogenerateurs", AIAA Journal, Vol. 14, No.10,
October 1976, p. 1481-1483

R.A. Oman, K.M. Foreman and B.L. Gilbert, "Investigation of Diffuser-Augmented Wind Turbines", PartII-Technical Report, ERDA, Report C00-2616-2, Jan. 1977.

Some more references:

Fletcher, H.S.
"Experimental Investigations of Lift, Drag and Pitching Moment of Five Annular Airfoils"
NASA TN 4117, 1957

Glauert, H.
"The Elements of Airfoil and Airsrew Theory"
Cambridge University Press, Cambridge, 1959

Kogan, A. and Nissim, E.
"Shrouded Aerogenerator Design Study, Two-Dimensional Shroud Performance"
Bulletin of the Research Council of Israel, Vol. 11c, 1962, pp.67-88

Kogan, A. and Nissim, E.
"Shrouded Aerogenerator Design Study, Two-Dimensional Shroud Performance"
Proceedings of the Fifth Israel Annual Conference on Aviation and Astronautics, Israel, 1963

Kogan, A. and Seginer, A.
"Shrouded Aerogenerator Design Study II, Axisymmetrical Shroud Performance"
Department of Aeronautical Engineering, Technion, T.A.E. Report 32, 1963,
also
Kogan, A. and Seginer, A.
"Final Report on Shroud Design"
Department of Aeronautical Engineering, Technion, T.A.E. Report 32A, 1963

Oman, R.A. and Foreman K.M.
"Cost Effective Diffuser Augmentation of Wind Turbine Power Generators".
Grumman Aerospace Corporation, Bethpage, New York 11714,
Second Workshop on Wind Energy Conversion Systems, Washington, D.C., June 9-11, 1975

Igra, O.
"Design and Performance of a Turbine Suitable for an Aerogenerator"
Energy Conversion, Vol. 15, 1976

Igra, O. and Schulgasser, K.
"Design and Construction of a Pilot Plant for a Shrouded Wind Turbine"
Proceedings of the Second International Symposium on Wind Energy Systems, F-1,
Amsterdam NL, Oct. 1978

O. Igra
Research and Development for Shrouded Wind Turbines
European Wind Energy Conference, Hamburg, 22.-26. October 1984, Hamburg,
Proceedings, Pages 236-245

If you want some more informations about the most economiest wind utilizing system these days, then click

Nowadays in architecture some groups make earnest efforts to concentrate the natural wind by buildings.
They propose to make the shape of new tal buildings in such a manner that these structures act as huge parts of a diffuser.

One group you will find by using:
http://www.eru.rl.ac.uk/web.htm.
This is an ENERGY RESEARCH UNIT at the RUTHERFORD APPLETON LABORATORY at Chilton, UK,
together with members of the University of Stuttgart, Germany, mail to:
joerg.hieber@po.uni-stuttgart.de

Meanwhile two new companies make some efforts in concentrating windssystems again,

the one in France:
http://www.cita-wind.com/index.htm,

the other in Germany:
http://www.kbe.de/frame.htm
click on: Branchen and then at Windkraftindustrie.

Time will show if these companies can beat the physics.

doerner@ifb.uni-stuttgart.de Since 20th of mai 1997