Well this is fairly simple so I guess there's a flaw in my 'plan' somewhere , but hey here goes. I'm putting this in babbling and not general science because it's just a bit of banter; if this is a credible method of generating power I'll eat my hat!
Ok, current solar cells are only about 25% efficient, however instead of using sunlight to directly generate electrical current why not use it as a heat source?
Of course sunlight is normally low grade heat, but why can't giant lens systems be used to focus the sunlight on a smaller area? Lenses are just ground class and they could be pretty crude, and the only energy losses would be from sunlight reflecting off the surface of whatever they're heating. If you heated a fairly volatile liquid with a low reflectivity (um...water with black dye? ok maybe something more refined than that) then you would only need to heat up the water to just a bit over boiling point to have a theoretical maximum efficiency of >25% (if your cold reservoir was the sea or something cold like that), and considering how tightly you could focus the sunlight the efficiency could be bumped to well over that of a conventional solar cell, and all that using ground glass and a simple heat engine.
So where's the flaw? why hasn't it been done? would the yeilds be too low to bother with the whole thing? Is making giant lenses harder than I think?