The biggest problem with Solar, wind etc. is indeterminacy, or ,"what do we do when the source stops?"
If the sun goes down, or the wind calms, these systems use batteries to keep the power flowing until it starts up again. Well, Batteries are expensive, and cause a lot of pollution, right? That's where human ingenuity comes in.
Presenting the Liquid Battery: http://www.ted.com/talks/donald_sadoway_the_missing_link_to_renewab...
Edit: I tried embedding the video, but it would never show up. Idk.