Yep, several ways to use heat to produce electricity! Seebeck effect (TEG, Thermo-Electric-Generator, which most of these used), Stirling engine, and in mass enough quantity the good 'ole steam engine (rankine cycle)
The moral of the story for a lot of these demos is that a heat engine only works when there's a temperature difference. Getting the "cold side" cold enough accounts for much of the material and engineering you see here... water cooled TEGs piped to radiators, a big hot-water storage tank and outside radiators consuming the heat from the Stirling engine, etc.
