spoiler alert, they're still electric

they use thermoelectric devices, also known at TEC or Peltier (pell-tee-ayy). they're flat square plates. give it electric current and it moves heat from one side to the other. reverse the electric current and the direction it moves the heat also reverses. but like most things electrical, they work as the opposite type of device. (meaning led makes light but can technically be used to detect light, and speakers make sound but can technically be used as a crude microphone, and motors make motion from electricity, but if you spin them they can generate electricity, etc). if you force heat to move through a peltier, from one side to the other, it will generate electrical current. heat moves from hotter to colder. so they just put a heatsink and fan on the "cold" side, and the "hot" side attaches to the stove. the heat moves through the peltier and into the heatsink. even without the fan, a heatsink will still remove heat. that eventually is enough to move decent amount of heat, to where it starts to generate enough to power the little efficient fan. once the fan starts spinning, it ADDS to the effect, as it helps the heatsink on the cold side remove heat even more quickly, so the faster you move heat through the think, the more juice it generates. that's how they work

before you start thinking of million dollar world saving ideas related to energy, these things are horrendously inefficient. silent dorm room coolers use these. (the ones that make noise are normal compressor and refrigerant based, but the ones that are dead silent use peltiers. well, silent other than maybe the sound of a fan). yes they move heat, if you want to use them as a cooler, but they're like 65% efficient or something, so to move 10 watts worth of heat through the peltier, it will output the 10 watts you move on the hot side, as well as an addition 4.5 watts in efficiency losses, so you have to dissipate 14.5 watts to move 10 watts. that's really bad compared to compressor heat pumps. compressors for example, can do something like move 50 watts of heat while using only 10 watts of electrical power. this is why you don't see peltiers used often for large scale things. they've been used in spacecraft where they are contantly radiating heat out of the crafts, and doing it through a peltier is a free way of generating extra power. they're used in some specific industrial and medical devices too, but usually in specific situations where it needs to be small, simple, silent, etc. they're also limited on how cold the cold side can get. they have a rating of the temperature difference they can create between the hot and cold side. if you stack two of them in series, they call that a cascade. but due to the inefficiency i mentioned, the first stage is usually tiny, and the second stage much larger. if a peltier can pump 10w of heat, but outputs 14.5 watts due to only being 65% efficient, the second peltier has to pump 14.5 watts, and it will also have 45% in losses added to that, so it needs to be larger than the first peltier. if one peltier can make a 40 degree temp difference between the hot and cold side, by cooling the hot side of the first stage with a second peltier, you can get a lower temperature. it a room is 70F and your peltier can get the cold side 30F colder than the hot side, your cold side will hit 40F. but that's not quite freezing, so if you want it to get below freezing, you'd size an even larger peltier to cool the hot side, so it can move the heat from the first stage, as well as the extra heat generated from efficiency related losses, but if it can also create a difference of 30F, then it cools the hot side of the main peltier to 40F, so now it can get another 30F below that, so now it can get to 10F. you can get even colder by stacking layers of these, but 65% efficiency is pretty bad, so this turns into an upside down pyramid of wasted energy pretty quickly and scales out of reason, especially when you consider the cost of aluminum and copper for heatsinks. ok enough rambling, i figured some people who have never heard of these devices might immediately get ideas for things you could do with them, so i thought going into a deep dive ramble might be interesting for some people. if you're into electronics hobby, you can get peltiers on amazon for around $5 per. but don't expect some world changing device. they're neat, but are hindered by their inefficiency. they still have their uses, but they're limited