Peltier devices, also known as thermoelectric (TE) modules, are small
solid-state devices that function as heat pumps. A "typical" unit is a few
millimeters thick by a few millimeters to a few centimeters square. It is
a sandwich formed by two ceramic plates with an array of small Bismuth
Telluride cubes ("couples") in between. When a DC current is applied heat
is moved from one side of the device to the other - where it must be removed
with a heatsink. The "cold" side is commonly used to cool an electronic
device such as a microprocessor or a photodetector. If the current is
reversed the device makes an excellent heater.
As with any device, TE modules work best when applied properly. They are not
meant to serve as room air conditioners. They are best suited to smaller
cooling applications, although they are used in applications as large as
portable picnic-type coolers. They can be stacked to achieve lower
temperatures, although reaching cryogenic temperatures would require great care.
They are not very "efficient" and can draw amps of power. This disadvantage is
more than offset by the advantages of no moving parts, no Freon refrigerant, no
noise, no vibration, very small size, long life, capability of precision
temperature control, etc.
Taken from http://www.peltier-info.com/info.html