I'm guessing that a very powerful windmill can be created by harnessing naturally occurring thermals along mountain ridges or above valleys.
1. Use solar energy to create lots of heat.
2. Position a windmill with specially designed airplane-like blades to ride the upward-moving thermal on one side and then ride the downward-moving cold air on the other.
3. A special ductwork can be created to draw in cold air on one side and blow out hot air on the other for a man-made thermal.
without compression the speed of air I believe would be poor this is why they used steam engines a steam engine would be much more efficient or a sterling engine which is already being used today with some start ups ...
Nice thought just already in practice in the best form known currently does not mean there cannot be improvements though cause we are far from the top output theoretically possible but they are already twice as efficient as solar panels if I recall correct and much cheaper to build as well.
Great idea: the solar power tower link;
http://en.wikipedia.org/wiki/Solar_updraft_tower