The Thermoelectric effect (TE) comes in two variants:
- The Seebeck effect makes it possible to turn a heat flow (based on a temperature difference) into electric power.
- The peltier effect does the reverse: it turns an electric current into a temperature difference across the two sides of the device.
The fundamental mechanism is a p-n-transition. So you have two different semiconducting materials, which means that the electrons are on different energy levels on both sides. When the electrons move from one side to the other, they have to absorb energy from the environment to get on the higher energy level themselves (p->n transition), or they give off energy (n->p transition), thus cooling or heating the environment.
With this technology, it is possible to build solid-state heat pumps that generate a temperature difference from an electric current with no moving parts! (i don’t know about efficiency or cost)
Peltier devices are what was usually used in car mounted “fridges” years ago. Not sure now. They’re not very efficient, so they’re only used when heat pumps aren’t practical.
I bought a beer fridge a few years ago that was a Peltier device and it came with a wall plug and a car adaptor, they’re still around. It worked but if you left it running for a week or so it would freeze the cans. It was lightweight and did the job and I knew the downsides when I bought it.