DIY Thermoelectric Generator
Thermoelectric systems produce electrical energy out of a heat-source.
Video with a lot of basic experiments. Main conclusion: Stirling are way more efficient (they can rise up to 40% instead of 1% for Peltier modules.)
Used to be made of bimetallic junctions.
Are now made of highly doped semiconductors, from bismuth telluride, or lead telluride, or calcium manganese oxide.
Electromechanical systems including turbines, such as Stirling engines, are more efficient.