[4] Other CWATs under development include the WindTamer from AristaPower, WindCube, Innowind (conceptual offshore application) and Enflo turbines.
The technologies mentioned above all use diffuser augmentation that is substantially similar to previous designs as the primary means of acceleration.
At the end of a decade of wind tunnel research and design funded by Grumman, NASA, and the DOE, it was determined that the DAWT system's economics were not sufficient to justify commercialization.
The attempt to commercialize the Vortec 7 in New Zealand from 1998 to 2002 proved it to be economically untenable when compared to the dominant HAWT (horizontal axis wind turbine) technology.
The first factor regards power increase and the method of comparison used by DAWT (and more recently CWAT) designers to determine whether the system is worth developing.
As Van Bussel of Delft (The Science of Making More Torque from Wind: Diffuser Experiments and Theory Revisited, G.J.W.
[citation needed] In the case of Optiwind (now defunct), there appears to be a growing body of evidence that they believe have solved for both the acceleration and economic challenges posed by CWAT/DAWT designs.
In addition, the ongoing operational and maintenance cost of the entire unit appears to be successfully addressed in this design.
[citation needed] It is absent the significant cost drivers of HAWT systems - large composite blades, gearbox, yaw motor, pitch control, lubrication, etc.
Ogin's MEWT (mixer-ejector wind turbine, another CWAT variation) is differentiated from previous DAWT's by using a lobed two stage diffuser (Grumman and Vortec machine were also two stage, but conical instead of lobed) to equalize the pressure over the exit area of the diffuser.
The theory is that creating a uniform pressure distribution with the lobes and the injection of external flow will prevent boundary layer separation in the diffuser thereby allowing the maximum acceleration through rotor.
Werle and Presz's (Flodesign's chief scientists) paper, AAIA technical note Ducted Water/Wind turbines revisited - 2007, details the theory behind their design.
Maximum acceleration detailed in their paper is 1.8× the ambient velocity from which they derive that 3 times more power is available at the rotor than for an unshrouded turbine.
Ogin's turbine based on released images and CAD's appears to be substantially similar to the Vortec and Grumman machines except for the lobed inner annulus.
From Bernoulli forward, science has substantially vetted these concepts and there is general academic consensus as to their veracity and their potential impact on wind power production.
DAWT's however have the classic boundary layer separation problem experienced by airfoils at a "stall" angle of attack.
[citation needed] This significantly reduces the acceleration achievable by a DAWT relative to the theoretical rate indicated by its exit to area ratio, per Flodesign paper mentioned above.
[citation needed] In the case of a typical CWAT/DAWT the power result in perfect theoretical operation once adjusted for the area of the shroud is actually the square of the velocity at the rotor.
[citation needed] As the CWAT/DAWT diverges from theoretical function the power increase drops significantly according to the formula derived from mass conservation,
[citation needed] The problem with optiwind is even more severe since the system only covers a fraction of the swept area available to a HAWT of the stack height.
Recent developments in material science, installation methodology and overall system integration have led to the far more realistic view that we are very close to this advent and the dawn of a new, highly sustainable class of wind turbine if the issues elucidated above can be dealt with which still remains highly questionable[citation needed] for the DAWT geometry.
Among the recent DAWT designs that appear to have a definitive positive power, if not cost, comparison to HAWTs is the Enflo turbine.
It is still unlikely that this machine can scale to larger ratings but based on their published data (not confirmed by third party testing) the Enflo appears to be the best performing DAWT/CWAT yet built.