Thermoelectric Cooling
From TPU Reference
Thermo-electric cooling (TEC, peltier) is a cooling system that takes advantage of the peltier effect to cool.
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Peltier Effect
When electricity is passed through two different semiconductors (one N-type and one P-type), 2 peltier junctions are formed. The current drives heat from one junction to the other, cooling one and heating the other. By placing a large number of these junctions in parallel, a TEC is formed. One side of a TEC will get hot and the other cold when a current is applied.
TECs in Practice
TECs have a few ratings to look for: Q, wattage; I, amperage; temperature delta; and V, voltage. Q is the maximum amount of heat a TEC can pump, measured in watts. An 80 watt QMax TEC will transfer 80 watts from the cold side to the hot side, regardless of the conditions it is in. If there is a load of 0 watts on the cold side, then the temperature of the cold side will be the temperature of the hot side minus the maximum temperature delta. As the load on the cold side increases, the temperature delta decreases in a linear relationship. When the thermal load on the cold side is equal to the QMax of the TEC, the temperature delta will be zero. Any higher thermal load will result in thermal runaway, which typically ends in a fire. The actual power drawn by a TEC is much higher than it's QMax. I, amperage, times V, voltage, will yield the actual power draw, which the QMax wattage will necessarily be lower than. Since TECs use raw current to provide a temperature differential, they are exceedingly inefficient. A TEC is only around 5-10% efficient compared to an ideal refrigerator (for comparison, compressor-based systems are typically 40-60% efficient).
From here, the hot side of the TEC must be cooled. The heat on the hot side will be approximately the drawn wattage plus the wattage of whatever is underneath the peltier. As such, thermal loads presented by TECs are typically very large, especially since the TEC should be able to pump around twice the wattage of the device it's cooling to get a decent cooling effect. Final wattage loads can be upwards of 300 watts; loads of 500 watts are not uncommon. As such, air cooling a TEC is entirely out of the question; no commercially availible air cooler will be able to pump heat away fast enough to avoid a catastrophic meltdown, which will end in fire. As such, air cooling TECs is extremely dangerous! TEC systems essentially mandate Water cooling; bong coolers are especially well-suited to the task, since they are very effective at cooling large thermal loads. Waterblock design also must be different from usual, since TECs present a large hot surface area (ranging from 40x40mm to 62x62mm). Common TEC waterblocks can cool a large surface area, such as the Swiftech MCW-600x (which can only cool TECs through modification), the MCW-500x series, the Apogee (internally identical to the MCW-500x), Danger Den Maze 4/5, and similar. More "modern" blocks, such as the Storm, are very poorly suited for TEC cooling, as they're designed to cool a smaller area (such as a CPU die).
In addition, between the TEC and CPU, there must be a cold plate to spread the heat from a small heat source (CPU die) to a large cooling surface (TEC cold side, 40x40mm-62x62mm). Cold plates are typically between 5 and 10mm thick pieces of copper.
Undervolting
To increase efficiency, TEC units are often undervolted (that is, run at a voltage lower than VMax). Since there is a point of diminishing returns in regards to voltage and Q, lower voltages can actually cool lower than higher ones, due to limitations of cooling the hot side.
Safety precautions
- TECs have absolute limits to their pumping capacity; if these limits are exceeded, thermal runaway will occur, which can easily lead to fires.
- TECs draw large amounts of power. Electricity is very dangerous and can easily kill people. TECs should not be run off of system power supplies, as the electrical loads (greater than 10 amps of 12vdc) will overload an ATX power supply's 12 volt rail, which can cause component failure or other serious damage.
- Cooling systems must be engaged before TECs are powered on. TECs can take a lot more heat than the components they are on; if not cooled, they can easily melt acrylic water block tops, to say nothing of burning CPUs, motherboards, and houses.
- Power-carrying wires must be adequately thick; if not, the high current load can cause them to heat up, catch on fire, melt plastic, etc.
Advantages
- Sub-ambient cooling.
- Solid-state device generates no additional noise and is relatively compact.
- Relatively easy to use, if the user is cautious.
Disadvantages
- Condensation can occur.
- Highest power draw of any cooling system (save maybe cascades, though they can cool a heck of a lot more); extremely inefficient.
- A large number of safety concerns.
See also
TECs, TEC blocks, etc.
