Swedish wood combustion technology has pinnacled with the floating sauna (refer pictures - sorry for size, working on how to upload bigger ones). Instructions for use: 

1. Gather a heat engineer or two, preferably one you feel comfortable naked with (if this is too big an ask, just find a friend and work out the technical issues as you go).
2. Ensure a good supply of dry firewood, say moisture content less than 25%
3. Stoke up wood stove to ensure temperature inside floating sauna at least 45 degrees C.
4. Cast floating sauna off from shore (ensure you have a small outboard or at least a paddle, otherwise you will need a good supply of wood to stay warm until the wind returns to whence you came).
5. Strip. Take care not to enjoy the 'naked' thing too much - after all, this is primarily in the interests of wood energy research. Swedes have a different attitude to this stuff than us of uptight Anglo-Saxon heritage. They are nature bunnies and dance around maypoles for real, not some pansy morris dancing sort of stuff.
6. Enjoy sauna. When too hot, dive into icy lake. Repeat for as long as you want.
7. When ready,  come back to shore for pickled herrings and akvavit (alcohol 40% by volume).

WARNING: Ensure the akvavit is consumed after the above, not before. Optional for teetotallers if you must but the nudity is mandatory - true woody types are heavily into nature. Are you game enough to take the real woody biomass inititation? P.S. Renewable energy is more fun than energy efficiency. Throw another log on the fire and pass the pickled herring.........