This is a great example of using a thermoelectric generator for a project. [Joohansson] made both a functional, and aesthetically beautiful fan using components from a computer.
Thermoelectric generators (TEGs for high temperatures, and cheaper TECs for lower temperatures) are also called peltier elements, which look like small square pieces of ceramic with two wires sticking out of them. If you supply power to it, one side will become hot, and the other cold. The TECs [Joohansson] is using want a temperature difference of 68C between either sides. They are typically used for cooling electronics and even some of those cheap mini-fridges will make use of one with a giant heat sink on the hot side.
In addition, they can be used as an electric generator, thanks to the seebeck effect. If you can create a temperature differential between the two sides, you can generate electricity. Using a CPU heatsink, cooler, and fan, [Joohansson] was able to power a small DC fan using only a candle. It’s a brilliant demonstration of the seebeck effect.