Thermoelectric generators are devices that convert heat into electricity.
This happens due to the ”Seebeck effect” where a temperature difference makes charged carriers in the metal to diffuse from the heated side to the cool side, in the same way that a gas expands when it gets hotter. The charged carriers moving to the cooler side leave behind their oppositely charged nuclei at the hottter side, giving rise to a thermoelectric voltage. They are solid state devices with no moving parts except for a user replacable fan.
Thermoelectric generators are used in a number of situations. Using waste heat is where thermoelectric generators come into their own. Where you are already creating heat from wood stoves or gas boilers anyway, the waste heat can be reused to generate electricity. There are some stove top generators available that typically generate 12v current.
This electricity is often developed at a nominal 12v DC as a convenience for those wishing to charge 12v batteries, for example in boats or caravans.
Companies are developing consumer-level applications to capture the energy commonly wasted during cooking. A handful of USB cooking pots have emerged, such as the Hatsuden Nabe thermoelectric cookpot, Stealth Power Systems,and the PowerPot.