I've made a few interesting discoveries. Firstly, Robert Van de Graaff patented the idea of a fluid-based machine in the 1930s. Here's the patent:
https://docs.google.com/viewer?url=p.../US1991236.pdf

A semi-successful machine was built using hexane as the fluid in the 1970s. This is a New Scientist article which describes the design. It contains some interesting observations. I am now wondering if a more suitable liquid for a hobby generator could be found. It proves at least that the concept can function:
https://books.google.de/books?id=2sH...ge &q&f=false

Someone mentioned the Kelvin water dropper. I was considering if it might be possible to build a Kelvin water dropper with pressurised liquid and some kind of deliberately unstable flow through a special nozzle, or a rotor which cuts the liquid flow. This paper describes something similar on a very small scale:
http://pubs.rsc.org/en/content/artic...0a?page=search

Chris