THE HEATSINK GUIDE: Peltier Guide, Part 1

History and introduction

In 1821, J. T. Seebeck (1770-1831) discovered that dissimilar metals that are connected at two different locations (junctions) will develop a micro-voltage if the two junctions are held at different temperatures. This effect is known as the "Seebeck effect"; it is the basis for thermocouple thermometers.

In 1834, a scientist called Peltier discovered the inverse of the Seebeck effect, now known as the "Peltier effect": He found that if you take a thermocouple and apply a voltage, this causes a temperature difference between the junctions. This results in a small heat pump, later referred to as also known as a thermo-electric cooler (TEC).

Practical TECs use several thermocouples in series, which allows a substantial amount of heat transfer. A combination of the semiconductors Bismuth and Telluride is most commonly used for the thermocouples; the semiconductors are heavily doped, which means that additional impurities are added to either create an excess (N-type semiconductor), or a lack (P-type semiconductor) of free electrons. The thermocouples in TECs are made of of N-type and P-type semiconductor pieces bonded together.

Since peltier elements are active heat pumps, they can be used to cool components below ambient temperature - which is not possible using conventional cooling, or even heat pipes.

What is a peltier cooler?

A peltier cooler is a cooler that uses a peltier element (TEC). Peltier coolers consist of the peltier element itself, and a powerful heatsink/fan combination to cool the TEC.

Peltier basics

The typical maximum temperature difference between the hot side and the cold side of a TEC, referred to as delta Tmax, is around 70°C.

Does this mean that simply adding a peltier element between heatsink and heat source will cause the temperature of the cooled device to drop by 70°C? No, that would be too good to be true. Two important factors must be considered:

A quick look at typical peltier elements

Peltier element
Typical 40x40mm Peltier element
This is a "padded" TEC

Peltier elements come in various forms and shapes. Typically, they consist of a larger amount (e.g. 127) of thermocouples arranged in rectangular form, and packaged between two thin ceramic plates. Multi-stage modules, to reach higher delta T values, are also available, but less common.

The commercial TEC unit of interest for PC geeks is a single stage device, about 4 - 6 mm thick and somewhere from 15 to 40 mm on a side.

The TEC will have two wires coming out of it, if a voltage is applied to those wires, then a temperature difference across the two sides is achieved, if the polarity is reversed on the wires - then the temperature difference is also reversed. The TEC is placed in between the CPU/GPU and the heatsink with appropriate thermal interface materials (thermal grease). So one thing we might note is that if the voltage is applied in the wrong direction then the TEC will cool your heatsink and heat your CPU!

Peltier elements come in padded and non-padded versions. On non-padded peltiers, the thermocouples are visible from the side. On padded peltier elements, you can only see the padding material (often silicon) from the side.

Problems related to peltier cooling

As mentioned above, high power usage and high power dissipation are the biggest problems related to peltier cooling. In the days of first-generation Pentium CPUs, readymade peltier/heatsink combinations were widely available, which could be installed and used just like a regular heatsink.

For today's CPUs having a power dissipation of over 100W, building a Peltier CPU cooler using just a peltier element and a heatsink is quite a challenge, and ready-made peltier coolers are scarce and expensive. With such coolers, over 200W of heat may be dissipated inside the case. For modern CPUs, it is better to combine peltier elements with watercooling. In any case, the resulting cooling system will be expensive to run, due to its high power usage, and not very eco-friendly. The large power dissipation will require powerful (and thus loud) fans.

Also, keep in mind that if the cooling of the peltier element fails (e.g. fan failure or pump failure in case of watercooling), the results will be more disasterous that if a conventional cooling system fails. Even if your CPU has a thermal protection that will cause it to shut down if the temperature gets too high, the peltier element may still kill it by continueing to heat it up long after it has shut itself down.

Another problem related to peltier cooling is condensation. Since it is possible to cool components below ambient temperature using peltier elements, condensation may occur, which is something you'll definitely want to avoid - water and electronics don't mix well. The exact temperature at which condensation occurs depends on ambient temperature and on air humidity; we will look at this in more detail in part 3 of the Peltier Guide.

Advantage of peltier elements

After having focused on problems related to Peltier cooling, let's not forget about their biggest advantage: They allow cooling below ambient temperature, but unlike other cooling systems that allow this (vapor phase refrigeration), they are less expensive and more compact. Peltier elements are solid-state devices with no moving parts; they are extremely reliable and do not require any maintainance.

Go to part 2 of the Peltier Guide

Note: Parts of this article are based on writings by "Bo", a visitor of The Heatsink Guide, who wishes to remain anonymous.

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