Thermoelectric systems produce electrical energy out of a heat-source.

Peltier Elements

• General article
• Design #1

Thermoelectric generators

• Made of aluminium and a pencil

From Wikipedia:

• 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.