How about a solar distiller? I remember seeing one in an old copy of CPN
from the days before it was printed on glossy paper. It was made by
putting a black plastic tray (I believe it was a photographer's
developing tray, or some such thing) into an aquarium which was covered
by a piece of clear glass or plastic. After sitting in the sun for a
day, just open it up and pour the condensed water in the aquarium into
your reservoir. This, of course, makes one dependant on sunny days for
one's water supple, but it's powered by the cheapest energy supply.
I imagine more complex as well as larger scale systems could be
devised, and may already exist. So, what do y'all think.
Ben