But first, a brief explanation of what a exactly what a peltier is: A peltier uses a TEC (thermo-electric cooler) to act like a "heatpump" to draw heat away from the object it is cooling. They work by applying a voltage across a thermocouple, which causes a temperature difference. More information on peltier coolers can be found here. By using a powerful enough peltier cooler, you can achieve below-ambient temperatures on the object you are cooling.
However, there are a few drawbacks. First, peltier coolers are fairly expensive. CoolIT's cheapest cooler to offer peltiers retails for about $160. Second, they require quite a bit of power. A single peltier cooler used by CoolIT can draw up to 15watts. With their high-end cooler using 6 peltiers, the cooler needs more than 80watts of power. Thats almost as much power draw as the CPU they are cooling. Third, peltiers actually add heat to your system. That 15watts of power has to go somewhere, and it is converted into heat. The exciting thing about the CoolIT coolers is that by putting the peltiers right by the exhaust fan for the system, that additional heat does not affect your computer hardware, because it is immediately vented out the back of the computer.
Lastly, I need to caution that using peltier coolers can actually be dangerous for your computer hardware. Any time you are working with below-ambient temperatures, you have to worry about condensation -- just like a glass of cold water on a hot day. When we left the two coolers featuring peltiers running on a bench, the coolers accumulated condensation after about 20 minutes. While the heat from a CPU would prevent this from happening (the heat would not allow these coolers to operate at such a low temperature), a system that does not POST properly can potentially allow condensation to happen. When a system does not POST, the CPU is usually not being given very much voltage, which means it will not be providing any heat. This would allow the cooler to eventually get below the ambient temperature. After a few minutes, condensation can form on the cooler and drip onto your components, causing sparks to fly!
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