Maybe this is such a generator you have in mind.

A team around Larry Kostiuk used millions

of tiny tubes to generate enough current to

light a LED.

http://www.newscientist.com/article/dn4291-tiny-tubes-squeeze-electricity-from-water.html

http://iopscience.iop.org/0960-1317/13/6/320

http://www.theglobeandmail.com/technology/science/two-alberta-researchers-shed-light-on-electric-idea/article1047470/

http://preview-de.innovations-report.de/html/berichte/energie_elektrotechnik/bericht-22393.html

Regards,

Joachim

It is a kind of streaming potential; to extract some energy you must coordinate sedimentation as told in the above publications. Vicktor Shauberger in the past developed a machine able to recover some potential from falling water drops creating sparks. To have a better understanding of the phenomena you need an Electrokinetic book-