Peltier junctions (besides not being very efficient) also have the problem of thermal fatigue, and this occurs if they are used as heating/cooling elements, or the other way--as thermo-electric cells.
The temperature difference between the two sides causes stress in the semiconductor junctions that join the two sides. (the side plates are ceramic to minimize the expansion problems, but this still occurs) To maximize the lifetime of the cells, the best method is to "start" and "stop" them v-e-e-e-r-r-r-y slowly. You're not supposed to hook them straight to a power source with a regular switch; the ideal controller circuit setup takes at least an hour (or more) to switch them from zero% to 100%, and it should also go just as slow when powering them down.
Peltier junctions are a nifty concept, but they have a couple drawbacks if you are going to go building anything around them.
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