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New for November 2007! TECs use the Peltier effect to move heat. The Peltier diode has the property of moving heat from one side to the other, when a DC current is applied to it.
The opposite of the Peltier effect is the Seebeck effect. In the Seebeck effect, a temperature differential across the diode can be used to create a voltage, and supply power. Don't get excited about this, the efficiency is very poor.
TECs are fabricated in modules, where a large number of Peltier diodes are configured in series. We borrowed this diagram from another corner of the world wide web, hope they don't mind...
Here's an application that most people are familiar with. TECs are used to build cheap beer coolers, they have no moving parts. They are advertised to cool up to 40 degrees F below ambient. We recommend that you don't buy one of these, they are far less efficiency than a "real" refrigerator, and an occasional purchase of a bag of ice makes more sense (and colder beverages) if you need portability. If you happen to leave this type of cooler in your car, and your car supplies power to the cigar lighter even without the key, you will soon be looking for jumper cables, as they draw 5 amps continuously. TECs are NOT efficient cooling devices. Notice you don't see the "Energy Star" sticker on the cooler shown below...
TECs do have their place in microwave electronics. One application is temperature control of an oscillator. It is easy to add heat to drive up temperature, but to facilitate a good closed-loop system you need a means of removing heat (and reducing temperature). Often there is no room for anything but a TEC.
More to come!