Enviromission's solar tower is a simple idea taken to gigantic proportions. The sun beats down on a large covered greenhouse area at the bottom, warming the air underneath it. Hot air wants to rise, so there's a central point for it to rush towards and escape; the tower in the middle. And there's a bunch of turbines at the base of the tower that generate electricity from that natural updraft.
A few dozen solar towers the same size as this project could completely replace the existing, primarily coal-based, energy supply for Los Angeles' 3.7 million-odd residents.