The Betz proof is good for comparing one wind use design to another, but it’s irrelevant where building a turbine on a given site for energy is concerned. For me, it all comes down to dollars spent per wattage produced.
Thus, the paradox that wind energy is more cost-effective in my area (windy much of the time) than solar energy and far more cost-effective than propane or electricity (propane racket and no nearby grid power).
Feast your eyes on the information behind the link below, if you’re really analytically interested in wind energy and not subjectively attached to another pecuniary/political interest. Although we also use PV supplies (good during the few warmer months), the following is the most efficient (dollars/watts) in my area. And BTW, due to cost-saving measures by the commercially built wind turbine manufacturers (magnets, coils and hubs too weak), commercial turbines burn up here.
http://www.otherpower.com/otherpower_wind.shtml
I can build a 10-footer, including tower, permits and electronics for ~ $1200 to get an average of about 450 watts (about 700 tops from minute-averages) in a typical winter wind here. ...a 17-footer costing me about $5000 to build for about 1.5 kilowatts (about 3k tops from minute-averages) in typical winter winds. And those are rated much more conservatively and honestly than the machines built by the corporates.
For the mostly un-windy and un-sunny areas of the eastern lowlands...start burning fecal and other rotting matter for alternative energy, if you like. But don’t try to further regulate against the building of what works in windy and sunny parts of the West.
Its not feasible and you know it