1.1 BACKGROUND DATA

1.1.1 History

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).

French watchmaker, Jean Charles Athanase Peltier, discovered thermoelectric cooling effect, also known as Peltier cooling effect, in 1834. Peltier discovered that the passage of a current through a junction formed by two dissimilar conductors caused a temperature change. However, Peltier failed to understand this physics phenomenon, and his explanation was that the weak current doesn’t obey Ohm’s law. Peltier effect was made clear in 1838 by Emil Lenz, a member of the St. Petersburg Academy. Lenz demonstrated that water could be frozen when placed on a bismuth-antimony junction by passage of an electric current through the junction. He also observed that if the current was reversed the ice could be melted. In 1909 and 1911 another scientist Altenkirch derived the basic theory of thermoelectrics. His work pointed out that thermoelectric cooling materials needed to have high Seebeck coefficients, good electrical conductivity to minimize Joule heating, and low thermal conductivity to reduce heat transfer from junctions to junctions. Shortly after the development of practical semiconductors in 1950’s, bismuth telluride began to be the primary material used in the thermoelectric cooling

Thermoelectric cooler (TEC), or Peltier Cooler is a solid-state heat pump that uses the Peltier effect to move heat. The modern commercial TEC consists of a number of p- and n- type semiconductor couples connected electrically in series and thermally in parallel. These couples are sandwiched between two thermally conductive and electrically insulated substrates. The heat pumping direction can be altered by altering the polarity of the charging DC current. TEC schematic is illustrated in Figure 1.1. The typical materials used for constructing TEC are:

1. Substrate: aluminum oxide (Al2O3), aluminum nitride (AlN), or barium oxide (BaO)

2. Conductor: Copper

3. Thermoelectric semiconductor

i. n-type: bismuth-telluride-selenium (BiTeSe) compound

ii. p-type: bismuth-telluride-antimony (BiTeSb) compound

4. Assembled and joined by solder.

Fig 1.1: Thermoelectric cooler

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. The TEC can be made in different shapes and sizes, but most common shape is a square or a rectangular substrate device. The practical size of a single stage TEC ranges from 3 mm x 3 mm up to 60 mm x 60 mm. The size limitation of 60 mm x 60 mm is due to the thermal stress. This stress comes from thermal expansion deformations between the cold and the hot junctions of the TEC. To obtain a larger temperature difference, a multistage TEC can be build. The multistage TEC is usually in cascade shape and 6 stages are the maximum practical limit. 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.

1.1.2 Peltier basics

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. 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:

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.

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 (Qmax). 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.

Fig 1.2: Power transferred versus temperature change in peltier

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 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.

Fig 1.3: Padded TEC 40 X 40 mm Peltier element

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.

The more cooling you want from a peltier, the larger the difference between the amount of heat your CPU generates and the rated cooling power of the peltier. However, more powerful peltiers require substantially more cooling – air cooling can only get you so far and water cooling is the only sure route to super-cooling. For max cooling, be prepared to build you own rather than buy.

As peltiers become more common tools in the overclocking kit-bag, understanding the peltier’s inherent limitations and practical application techniques becomes more important. It is very easy to find commercial peltiers, it is less easy to find ones that work well and even less easy to find ones that super-cool. What follows are some practical guidelines in buying or building a peltier CPU cooler with emphasis on the practical rather than theoretical.

1.1.2.1 Limitations of Peltier element

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.

1.1.2.2 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.

1.1.3 Peltier Equations

For a thorough theoretical understanding of peltiers, use the links listed below. The following explanation covers the basics and is somewhat simplified – it not intended to be an exhaustive nor so rigorous an explanation as to be unreadable.

A peltier is an electronic package that consists of two sides – the hot side and cold side. By energizing the peltier, this package transfers heat from the cold side to the hot side. The hot side requires a heatsink to move the heat from the hot side to someplace else – if not, the cold side gets hot. The more efficient the removal of heat from the hot side, the colder the cold side will become. It is not uncommon to get a 65 C difference between the two sides under no load conditions.

Peltier cooling consists of three basic components (see diagram below):

1. The CPU, with heat output measured in watts;2. The Peltier, with cooling power measured in watts;3.  The Heatsink, either air cooled or water cooled, with efficiency measure as C/W.

Fig 1.4: Peltier Cooling

we have to get into some math to understand why some peltiers work well and some are downright dangerous to put into your system. As the diagram shows, the CPU and peltier generate a combined heat load, measured in watts, of 111 watts. Qc is the CPU’s heat load under

whatever load you specify – in this case a C366 running at 550 MHz. The peltier rating is given by the manufacturer and consists of 4 factors –Qmax, Vmax, Imax and Tmax.

Qmax is the maximum heat load the peltier can transfer from the cold side to the hot side assuming it is running 100% efficient. Vmax is the most efficient voltage, Imax the most efficient current (amps) and delta Tmax (dT) its most efficient temperature difference between the hot side and cold side, under no load. Each peltier model has its own rating and is listed by peltier manufacturers. Let’s assume we have a peltier which has the capability to move 50 watts of heat from the cold side to the hot side. This requires 78 watts of power @ 14 volts, 5.6 amps and 65C dT (i.e. it can maintain a 65C temperature difference between the hot and cold sides if there is no load on the cold side).

We can represent these relationships as follows:

Th = Tamb + (C/W)(Qh)Qh = Pin + QcdT = Th – Tc

Th = Peltier hot side temperatureTamb = Ambient temperature inside your system’s caseC/W = Heatsink efficiency (Degrees C/watts)Qh = Total heatsink load (watts)Pin = Peltier power input (watts)Qc = CPU heat load (watts)dT = Total temperature difference between the peltier’s cold and hot side.Tc = Peltier cold side temperature

Th, heatsink temperature, is one of the most critical element controlling the cooling performance of a peltier system. Let’s assume the case temperature is 25C, Pin is 67 watts (NOTE: the peltier is rated at 78 watts at 14 volts – at 12 volts, it is 67 watts (Amps x Volts)), Qc is 33 watts and the heatsink’s efficiency is 0.5 C/W. Substituting all terms into the Th equation, we get:

Th = Tamb + (C/W)(Pin + Qc)Th = 25 C + (0.5 C/W)(67 + 33 Watts)Th = 25C + 50C = 75C

This means under the conditions outlined above, the heatsink’s temperature will be 75C (167F) – pretty damn hot. It gets this hot because it does not have the ability to get rid of the total heat load generated (the CPU AND the peltier) as well as we might like. This is critical because all peltiers have “Performance Curves” that indicate what the expected cold side temp will be under different heatsink temperatures, voltages and amps input. Reading these values off the performance curves then gives you the lowest cold side temperature achievable within our system’s parameters.

The “Performance Curve” for one particular model peltier is shown below. The three critical factors to use in understanding what this particular peltier can do for you are Heatsink Temperature (Th), Voltage (Vmax) and Heat Pumped (watts). Assuming the system can supply 12 volts at 2.7 amps (32 watts to the peltier), Th of 65C for a heat load of 15 watts, then this particular peltier will give you a cold side temp of 20C (65C – 45C = 20C).

Fig 1.5: Peltier performance curves

These “Performance Curves” should be available from the manufacturer for varying heatsink temperature levels – obviously, the lower the heatsink temperature, the lower the cold side temperature. If you want maximum CPU cooling, you have to use the best possible heatsink to lose the most heat possible that’s generated by the CPU and peltier combination. For example, another model peltier is rated at three Th’s as follows:

Th = 27C, Cold Side Temp = 8.9CTh = 35C, Cold Side Temp = 12.2CTh = 50C, Cold Side Temp = 22.2C

Basically there’s no such thing as a free lunch – if you want to maximize CPU cooling, you have to use the biggest peltier you can find. The bigger the peltier, the more total watts you have to cool. The more you need to cool, the more heat is dumped into your system raising its ambient temperature. Higher ambient temp leads to more heat load leads to less cooling, and if the heatsink can’t take the load, it gets even hotter – so the cycle begins.

If you want maximum CPU cooling with a peltier, forget air-cooled solutions – it is virtually impossible to build considering the heat loads generated. Water-cooling is the answer, and dumping heat outside the case is a better solution than closed loop systems residing inside the case. It’s simple – water is a lot denser than air and hence considerably more efficient at absorbing heat than air.

If you buy a commercial peltier, then understand what the ratings mean by plugging numbers into the equations above. Even if you don’t have all the variables, if you are interested in buying a commercial peltier that is rated at 32 watts to use on a CPU that generates 40 watts, the peltier will not cool; it will probably wind up generating more heat to the CPU than a good air-cooled heatsink. The “Peltier from Hell” is a good example of a commercial peltier that is downright dangerous because it is undersized relative to the heat load and is unable to handle the heat load.

Even if you find a commercial peltier that is rated at double the CPU’s heat load, if it is mounted on a crap heatsink, it will not give you the performance you would expect simply based on the peltier’s rated wattage. Therefor the other factor which you normally do not find in commercial peltier heatsink ratings is heatsink efficiency (C/W). For rough sizing, use 0.5 C/W. If you know C/W and the peltier’s rated watts, you can plug numbers into the equation to at least get some idea of what to expect, assuming you can maintain certain ambient temps and assuming something about the peltier’s “Performance Curve” under the stated conditions.ie, do not expect too much from commercial air-cooled peltiers. The cooling task requires balancing a host of factors within an economic budget, and typically the tradeoffs compromise cooling performance. We have not discussed the condensation issue – it is a real challenge and is why you will not see too many super-cooling “off-the-shelf” units on the market.