I am working on a wood pellet stove/stirling engine combo. The goal is to be able to heat the greenhouse and generate enough electricity to run the auger and fans for the stove as well as the water pumps and other misc. electrical. The stirling engine is an external combustion engine, so it just sits on top of the pellet stove using the heat differential (stove/ambient air) to run. I have been working with a mechanical engineer and he is confident we can make it work.

The other thought is a steam engine but I am not keen on having a high pressure boiler around. This will be my fallback if the other becomes to difficult to produce.

Both have been in use for hundreds of years, and will run on any combustible fuel.

Also there is plenty of info. on how to make your own pellets from wood dust or any other bio-mass.