THE HEATSINK GUIDE: Peltier Guide, Part 1

History and introduction

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?

Peltier basics

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:

• The specified maximum value of delta T only occurs when the peltier element does not transport any heat - a situation that does not occur in real-life cooling solutions. The actual delta T is a linear function of the power transferred through the thermal element, with negative slope. An example of such a function, for one particular TEC, is illustrated in the following graph, which is a stripped-down version of the graph found in part 2 of the Peltier Guide.
  

  Looking at the graph, you can see that, for example, if the peltier element will have a delta T of 55°C if it has to move 10W of power (in the form of heat). You will also see that at one point - at 40 Watts in the case of this example - delta T becomes zero. This occurs when the TEC has reached its maximum thermal transfer capability (Q_max). So, our example peltier element cannot transport more than 40W. I admit that this graph is a bit oversimplified; in following parts of the Peltier Guide we will get into more detail.

• Imagine that you are cooling a CPU with a power usage of 35W, using a conventional heatsink. Will the temperature drop if you add our example peltier element between CPU and heatsink? No. For a simple reason: In addition to transporting heat, peltier elements also emit considerable amounts of heat (and thus use considerable amounts of electricity). So, the heatsink will have to dissipate substantially more heat than before, and will get much hotter. We will get into more detail about this issue in Part 3 of the Peltier Guide, where we analyze under which circumstances a peltier element is useful, and under which conditions you are better off with just a conventional heatsink.

  Peltier elements have very low efficiency. They will consume more power than they transport! Actual peltier elements may consume twice as much energy (in the form of electricity) as they transport (in the form of heat). So, if you are using a peltier element, the heatsink it is used with must be much more powerful than a heatsink used for cooling a heat source without peltier element.

  Do not confuse the maximum amount of power a peltier element can transport with the maximum amount of power usage of the peltier element. Some retailers sell "80W peltier element", without stating what this value actually means. This is misleading - what you want is a high transport capability, but a low power consumption.

  To help you decide what kind of peltier element you need for an overclocked CPU, you can find instructions for estimating power usage of overclocked CPUs here on The Heatsink Guide.

A quick look at typical peltier elements

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

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

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.