Solar Towers
<Space.com article
Using a greenhouse to generate hot air is an interesting alternative to using parabolic mirrors to concentrate sunlight onto pipes filled with oil. I don't think it would be as efficient. That would mean a much larger construction site. I personally think that the invention of a really inexpensive, highly reflective coating would be the real breakthrough in the area of solar energy. Then we can build large solar concentrators at minimal cost. Trouble is, tin foil doesn't cut it, and as with large reflector telescopes, we know how expensive quality mirrors can be!