Evapotrons are devices used at Burning Man to evaporate grey water quickly and efficiently (and they look pretty damn cool!). A tray is filled with grey water and a drum runs over it which is covered by a dark absorbent material. This drum is constantly driven round by the wind. The absorbed water evaporates from the dark material at a much faster rate than if it were just sitting in the trough.
The website explains this process more thoroughly as well as providing information about the history and development of evapotrons over the years.
WeWantToLearn.net 
Hello Antony, thanks for your post, where can I see how it works? Is there a clear drawing somewhere?
Hey, heres a youtube video of one in action
Brilliant ! Thanks Antony, much clearer.