An Israeli company is building a new, efficient system in Arizona, which is said to be more efficient than any previous major system with the exception of hydro-electric. Apparently a patent existed as recently as 1975.
http://scitechdaily.com/gigantic-downdraft-energy-tower-planned-near-us-mexico-border/ (http://scitechdaily.com/gigantic-downdraft-energy-tower-planned-near-us-mexico-border/)
Incidentally, I may have played a role in uncovering the patent when I e-mailed an Israeli intellectual around 1999.
Anyway, it's an interesting story, but the major upshot is that it's a highly clean, highly reliable source of energy that can be built 'anywhere where hot air rises.'
I think developers will do well to consider the application to large skyscraper type structures, which encounter a similar natural cooling-and-heat-rising phenomenon within their air-conditioning systems. This system appears to be truly remarkable.