Performance
The science of wind acceleration around a structure, as well as the vortex shedding benefits of a shroud/diffuser, are well understood and tested. 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. 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. It is generally thought that since the amount of power produced by a wind turbine is proportional to the cube of the wind speed, any acceleration benefit is potentially statistically significant in the economics of wind. As noted though this is an inaccurate as it ignores the impact of the exit to area ratio and is therefore an apples to oranges comparison. In the case of a typical CWAT/DAWT the power result in perfect theoretical operation once adjust for the area of the shroud is actually the square of the velocity at the rotor. As the CWAT/DAWT diverges from theoretical function the power increase drops significantly according to the formula derived from mass conservation,
Power ratio DAWT to HAWT = (Athroat/Aintake)(vthroat/vfreestream)3
Power ratio DAWT to HAWT = (1/2.75)(27.5ms/10ms)3 = 7.56 increase
So for example a DAWT operating at theoretical function of 1.8 with a 2.75 area ratio per Flodesign,
Power ratio DAWT to HAWT = (1/2.75)(18ms/10ms)3 = 2.12 increase
For the highest claimed velocity increase in a DAWT of 1.6 x freestream
Power ratio DAWT to HAWT = (1/2.75)(16ms/10ms)3 = 1.48 increase
Not significant enough to offset the associated costs. 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.
The challenge has always been, and remains, installing, operating, and maintaining these structures for a cost that is less than the incremental value gained by their presence. 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 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. Based on its rotor:exit ratio and the published power performance this turbine appears to have a confirmed 2 times increase in power output over a HAWT of the diameter of the exit area. It is still unlikely that this machine can scale to larger ratings but based on published data the Enflo appears to be the best performing DAWT/CWAT yet built.
Read more about this topic: Compact Wind Acceleration Turbine
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