[Al__Dante]

One thing to bear in mind is that there are theoretical Limits to windmill efficiency: http://en.wikipedia.org/wiki/Betz's_law

In short, we can't extract more than about half of the available wind energy, no matter what the design.

This is because if all kinetic energy is extracted, the air would no longer be moving behind the windmill, so it would stop turning. Betz's law is the optimum compromise between extracting energy and sufficient airflow to keep the windmill rotating.

[caminante]

Wow! The concepts underlying Betz's Law are intuitive, though, still too clever for me!

I found the economic implications interesting. Given a Betz Limit of ~59% wind energy extraction, "utility-scale wind turbines" ...

  A) can only capture 80% of the peak (Betz Limit)
  B) achieve 25-60% of their potential output (constant wind) on an annual basis

Combining this with the NREL maps shared by animats below, you can make SWAG's for energy production in locations.

[JoeAltmaier]

I'm concerned about all the math being per-area, which makes bigger windmills seem inevitable. But surely its per-dollar that determines commercialization potential. 1000 small windmills could be the better choice if they could be made cheaper per watt after all.

[jacquesm]

Wind energy economics favor: fewer blades (though the three bladed design has some mechanical properties that make them the default choice), larger swept area, horizontal axis turbine (even though you need to aim those at the wind nobody has been able to produce a very large VAT that survived for even a short amount of time), direct drive (because gearboxes are very much prone to failure).

1000 small windmills would not be commercially competitive with a single large machine, would not extract as much power from the same amount of wind and would not be as safe. Small machines have to make a lot more RPMs so are inherently higher maintenance (bearing wear), harder to operate when connected to the grid and so on.

I'd love it if that weren't the case (which is why I built my own 5 meter/16' machine) but ultimately wind power will be dominated by several MW behemoths.