Q. I have heard that there are solid-state cooling devices. What are they and how do they work?
A. These are called "Peltier Junctions." Unlike conventional cooling devices, they do not use mechanical pumps and refrigerants. There are no moving parts. basically, certain materials are used to make a PN junction. The junction is forward biased. The current carries heat away from one side, which becomes cooler, and transfers it to the other side, which becomes warmer.
Peltier devices are not at all as energy-efficient as conventional mechanical cooling devices, and large amounts of current are needed to make them work.
They are also more expensive than mechanical systems, for the same amount of cooling power. But their reliability and lack of moving parts makes then useful in some applications. For example, they are used in nuclear submarines, where electrical power is abundant, and in fact that they make no sound is a great advantage!