peltier guide

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Peltier Guide

History and introduction

In 1821, J. T. Seebeck (1770-1831) discovered that dissimilar metals that are connected at two differentlocations (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 temperaturedifference between the junctions. This results in a small heat pump, later referred to as also known as athermo-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 thethermocouples; the semiconductors are heavily doped, which means that additional impurities are addedto either create an excess (N-type semiconductor), or a lack (P-type semiconductor) of free electrons.The thermocouples in TECs are made 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 Peltierelement 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, referredto as delta Tmax, is around 70C.

Does this mean that simply adding a Peltier element between heatsink and heat source will cause thetemperature of the cooled device to drop by 70C? 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 nottransport any heat - a situation that does not occur in real-life cooling solutions. The actual deltaT 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.


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Looking at the graph, you can see that, for example, if the Peltier element will have a delta T of55C 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 hasreached its maximum thermal transfer capability (Qmax). So, our example Peltier element cannottransport more than 40W. I admit that this graph is a bit oversimplified; in following parts of thePeltier 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 emitconsiderable amounts of heat (and thus use considerable amounts of electricity). So, theheatsink will have to dissipate substantially more heat than before, and will get much hotter. Wewill get into more detail about this issue in Part 3 of the Peltier Guide, where we analyze underwhich circumstances a Peltier element is useful, and under which conditions you are better offwith 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 Peltierelement.

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

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 temperaturedifference across the two sides is achieved, if the polarity is reversed on the wires - then the temperaturedifference is also reversed. The TEC is placed in between the CPU/GPU and the heatsink withappropriate thermal interface materials (thermal grease). So one thing we might note is that if thevoltage is applied in the wrong direction then the TEC will cool your heatsink and heat your CPU!


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Peltier elements come inpaddedand non-paddedversions. On non-padded Peltiers, the thermocouplesare visible from the side. On padded Peltier elements, you can only see the padding material (oftensilicon) from the side.

Problems related to Peltier cooling

As mentioned above, high power usage and high power dissipation are the biggest problems related toPeltier 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 aPeltier element and a heatsink is quite a challenge, and ready-made Peltier coolers are scarce andexpensive. 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 willbe expensive to run, due to its high power usage, and not very eco-friendly. The large power dissipationwill 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 disastrous that if a conventional cooling system fails. Even ifyour CPU has a thermal protection that will cause it to shut down if the temperature gets too high, thePeltier element may still kill it by continuing 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 componentsbelow ambient temperature using Peltier elements, condensation may occur, which is something you'lldefinitely want to avoid - water and electronics don't mix well. The exact temperature at whichcondensation occurs depends on ambient temperature and on air humidity; we will look at this in moredetail 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 maintenance.

Getting into the details

The second part of the Peltier Guide deals specifically with sizing Peltiers and heatsinks, to fit a givenapplication. Hopefully it will also show some of the problems in more detail, and help you judge about

merits and tradeoffs when using Peltiers.

The fact that you are still reading, and weren't scared off by the first part, show that you have a keeninterest in Peltier cooling. If you are more interested in general information about Peltier elements, or inin-depth information about the theory behind them, you should definitely have a look at Melcor's

excellent Thermoelectric Engineering Handbook.

This article here mainly focuses on how to apply Peltier cooling to PC processors or graphics chips; it isnot as general as the information you can find on the websites of Peltier manufacturers.

Do not supply power to a Peltier element without a heatsink, after a while it will overheat and the

connectors will melt.


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In the last part of the guide, you will find an Excel spreadsheet with VBA code that will help you withthe necessary calculations for designing a Peltier-based cooling system for your PC, but I ask you toplease read the article first before you download the spreadsheet.

Peltier Performance

One thing we must consider is that athermocouple will always be a thermocouple -and thus when you apply a voltage and get atemperature difference - you will also cause aback voltage created by the Seebeck effect. This

is very similar to the back EMF created within anelectric motor - and thus much like motors TECsshow a negative linear load dependent outputcurve. The other thing that happens when avoltage is applied across the TEC unit is that

current flows through the TEC. This causesinternal heating through I2R losses. This is a very

important fact because this imposes a lot moreheat on the heatsink to cool - we will get to that

later.

A performance curve fromTellurex is shown here at the left. This, by the way, is the same curve thatJoe over at overclockers.com shows in one of his articles.

The curve shows heat pumped versus temperature difference achieved across the Peltier for 3 different

current inputs. I find this plot not particularly easy to read. The main problem I have is that theinformation is presented at constant current, whereas PC freaks are likely to have a constant voltagesource available. The other thing that is not shown is the power generation from the TEC itself - you canhowever glean this information from the voltage and current. I have rearranged the same informationinto another chart I find more usable.
http://www.tellurex.com/http://www.tellurex.com/http://www.tellurex.com/http://www.tellurex.com/


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This chart shows the same TEC as above - but only at 12 volts. The left-hand vertical axis is for bothtemperature difference (C) and also for total power to heatsink (watts). The right-hand vertical axis isfor current(amps). The first thing you'll notice is that the independent axis is power transferred (CPUpower). The next thing you might notice about this format is that you can immediately see the amountof power transferred to the heatsink as a function of the amount of power transmitted by the TEC. Forexample at 15 watts of heat transfer across the Peltier element another almost 30 watts of heat is addedby I

2R losses to make nearly 45 watts transferred to the heatsink. This illustrates that TEC applications

add a lot of "overhead" heat to the total system, as it was already pointed out in the first part of thisguide.

Variables that affect TEC performance

Another thing that is important to realize is that TECs are affected by voltage and temperature. I refer to

the Melcor site as a place to read up on how to calculate TEC performance based upon voltage, absolutetemperature of the TEC, the number of thermocouples in the TEC, and something described as thegeometric factor. I won't bother to rewrite all of those relationships .

You might be saying to yourself, "Dude- the manufacturer gives you thatinformation - why bother?". Well, the

fact is that TEC performance is very

sensitive to temperature; the curve youget from the manufacturer might be forconstant hot side temp of 50C - your hotside temperature might be a lot different,

your voltage might be different - andQMax and DTMax are all tied up in that.The plot shown to the right is for a TECwith QMax = 72 watts @ 20C hotside. Itshows the solved cold side temperaturedifference versus ambient using a 33watt load (CPU heat), and a heatsinkresistance of RHeatsink of 0.5 C/W (see theheatsink information section for detailsabout heatsink performancemeasurements).
http://www.heatsink-guide.com/heatsinkinfo.shtmlhttp://www.heatsink-guide.com/heatsinkinfo.shtmlhttp://www.heatsink-guide.com/heatsinkinfo.shtml


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The only variable is ambient temperature (ranging from 20C to 60C). This shows that your TECworks better when it is hotter, but moreover the total system performance changes by 8C relative toambient over the 40C span! The temperature is important because it affects the Seebeck coefficientelectrical resistivity of the thermocouples as well as the thermal conductivity of the substrate.

The voltage obviously is important because it affects the enforced temperature difference.

Two parameters we haven't looked at until now is the maximum allowed electrical current I max throughthe device (exceeding the current will damage the TEC), and the geometry factor G.

The number of thermocouples and the geometry factor help to describe the size of the device - morethermocouples means more pathways to pump heat - the geometry factor is not explained by Melcor.They offer the factor (G) for their devices - but that doesn't help when trying to calculate performance

for another manufacturer's TEC. One thing I did observe about G is that it is related to the density ofthermocouples per square area and it is also related to the thickness of the TEC. After looking at the

Melcor data I finally discovered that G = Imax/50. It is a perfect match for every Melcor TEC. When Iwent to make the above plot for the Tellurex TEC (using the Melcor relations), I had to play with G alittle to get the right curve - 3.9/50 = 0.078 , but I found that G = 0.084 was about right to match the

Tellurex chart.

This "empirical" determination of the geometry factor G is clearly a hack - but it is all I have - if anyoneknows the calculation of G more specifically please email me.

Melcor has a downloadable program called Aztecthat can handle all this for you automatically, but Ididn't like the choices of independent versus dependent variables they used. As it turns out I am tryingto calculate hot side temp and cold side temp - not continually guess at what they might be - but hey -Aztec works and it's free. The other obvious problem is that it is only for Melcor TECs (but other TECmanufacturers, such asKryoTherm, offer similar software for download as well).

Thus I went to the Melcor information page and spent some looking over their equations trying to come

up with a few quick rules of thumb. I finally realized that the sensible thing to do was to implement theirequations into a few custom Excel VBA functions. These functions are the basis for all of the plotsshown in this article - details follow later. One final note - I used the Melcor supplied values for theSeebeck coefficient, resistivity and thermal conductivity - all of this applies to Bismuth-Telluride TECsonly!

For another TEC flavour we would need to adapt those values - but Bismuth-Telluride is the onlymaterial commonly used for TECs that are suitable for temperature ranges common in electronicscooling.

System Integration of TEC with CPU and Heatsink

We should know the following: CPU (or graphics chip) power output, heatsink thermal resistance, TECparameters, ambient temperature. That is all we need.

If we don't know the CPU or GPU power output - then we look on the web, manufactures publish it. Avery good page where you can find processor electrical specifications of all common CPUs is ChrisHare's Processor Electrical Specification page.

Getting your heatsink's thermal resistance could be tricky - some manufacturers specify the thermalresistance of their heatsinks, but the values are often not very precise, or "optimized" for marketingpurposes. Check out theheatsink information pagefor more information on thermal resistance, and howto calculate it. You must know your TEC parameters, at the very least Q

Maxand hopefully more.
http://www.melcor.com/software.htmlhttp://www.melcor.com/software.htmlhttp://www.kryotherm.ru/http://www.kryotherm.ru/http://www.kryotherm.ru/http://www.heatsink-guide.com/heatsinkinfo.shtmlhttp://www.heatsink-guide.com/heatsinkinfo.shtmlhttp://www.heatsink-guide.com/heatsinkinfo.shtmlhttp://www.heatsink-guide.com/heatsinkinfo.shtmlhttp://www.kryotherm.ru/http://www.melcor.com/software.html


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Lets suppose we have the Tellurex TEC curve from above. Also suppose our CPU is at 15 watts (keepin mind that the power usage of current CPUs is much higher!), our heatsink has RHeatsink = 0.5 C/Wand ambient temperature is 25C.

Here is the simplest method:

Interpolate along the TEC curve to the CPU output (15 watts) and find that DT = 45C. Look at the total

power output and see that it is about 43 watts to the heatsink.

43 watts*0.5C/W = 21.5C.

Thus the heatsink will be 25 + 21.5 = 46.5C.

The TEC is enforcing a 45C difference and thus the cold-side temp of the TEC will be

46.5 - 45 = 1.5C.

That's pretty cold - good stuff for an overclocker; but you might encounter problems with condensation.

See the last part of the Peltier Guide for details about condensation problems.

Let us now look at a less favorable example: Suppose you have a poor quality heatsink; suppose RHeatsinkis 1.5 C/W - then your heatsink will be 65C over ambient 65+25 = 90C and then your coldside temp

would be 45C. What if you didn't use the TEC? Then your CPU would be 15watts * 1.5C/W = 22.5Cover ambient or at 47.5C. In that case it is probably is not worth using the TEC because you aredumping 30 extra watts into your case and drawing 3 amps off of your power supply.

In the next part, we analyze in more detail in which situations a Peltier element will help cooling, and inwhich situations it won't.

Examining the influence of different parametersLet us now look at the relationships of RHeatsink , QCPU and Qmax in a more general sense. Below, you see

a few contour plots. The first thing to consider is: Given a powerful TEC ( Q max = 75 watts), what kindof CPU temperatures can be expected based upon varying heat load (QCPU) and varying RHeatsink? The

following two plots are based upon performance for the TEC at 12V.


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This plot shows cold side temp of the TEC for various heat loads and heatsink capabilities. So forexample we can see that at QCPU= 33 and RHeatsink= 0.475, the cold side temp is about 28C, or 3C overambient - not too bad. What about the fact that the TEC is adding heat? We must make sure that we are

progressing not regressing! For that I took the same information and compared it to the temperature thatwould have been achieved at the same CPU load and with the same heatsink but without the TEC. Isubtracted the calculated temperature with TEC from the calculated temperature withoutTEC. Thus, inthe next plot, I am showing the advantage of using the TEC. When the numbers are positive, the TECdoes improve cooling; when they are negative, the TEC is a disadvantage.

Again, what is plotted here is the advantage of using the TEC versus not using one - for example fromthe conditions before (QCPU= 33 and RHeatsink = 0.475) we see that the advantage is about 12.5C. The

TEC coldside temp was 28C; using the heatsink alone, we would have reached a temperaturedifference of 33 watts * 0.475C/W = 15.7C. Add that to 25C = 40.7C, which is 12.5C hotter thanthe solution with the TEC. I think this kind of plot can be really interesting - it shows us how quickly itcan go bad if we use either an undersized TEC or an undersized heatsink.


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How about TEC voltage? Again plotted against CPU power, same TEC ( QMax = 75 watts) but choosingRHeatsink= 0.5 C/W.

This is quite interesting because it shows two things: First of all there is an optimum voltage - in thiscase about 10.5 - 11.5 volts, depending upon the power. Second, it shows that you can get away with amuch lower voltage and still do some serious cooling. This may be important in the case that your

power supply is putting out less than 12 volts due to the extra load of the TEC. Of course, under thesecircumstances one should consider an alternate power source for the TEC.

The last thing I'd like to show is a plot that illustrates how big the TEC and heatsink should be, based

upon the power load.

I've normalized the CPU power on the TEC Qmax as thus: Normalized Power = QCPU/Qmax.I also normalized the CPU power on RHeatsink: Normalized Cooling = QCPU*RHeatsink. The purpose of thisis that the plot directly tells how big the TEC and heatsink should be based upon the power requirement.The temperature shown is the advantage over using the same heatsink without the TEC.


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Coincidentally, the values along the horizontal axis also happen to be the temperature rise over ambientthat the heatsink working alone (without TEC) would produce.

The temperature contours are again the advantage of using the TEC versus using the same load and

heatsink without the TEC. Thus at 0 is break even - positive numbers (C) indicate that the TEC ishelpful, negative numbers indicate that the TEC is heating the CPU. So we can immediately see that if

the heatsink alone cannot keep the CPU less than 26C over ambient - then under no circumstances willit ever get better when you add a TEC. Furthermore, you should add a 5-10C penalty for the fact that

the extra heat inside your case caused by the TEC will work to your disadvantage - thus I'd say that youreally need a heatsink (or watercooler) that will keep your CPU no more than 15C -18C above ambientfor it to be suitable for applying a Peltier. Now - bear in mind that this is the CPU temp measured at theinterface with the heatsink - if you are measuring the temperature of the CPU from the CPU's internaltemperature diode, then you may see a higher temperatures than at the heatsink interface.

If the CPU load is about equal or greater than the TEC QMax - then even the very best heatsink in the

world will not justify using the TEC.

Finally, we can see that the optimum size of the TEC is such that the CPU power is 1/3 - 1/2 of QMax.

If you have a really good cooler, such as a powerful water cooler, then get a TEC that has QMax = 3times the CPU heatload. If the heatsink is marginal (15C-18C above ambient without TEC) then go forQMax = 2 times the CPU heat load.

Excel Spreadsheet

As mentioned before, you can download an excel spreadsheet with the functions I used to create theabove plots. As with any computation tool - check to see if the results make sense. Remember that your

outputs are only as good as your inputs (garbage in = garbage out). All theory comes from Melcor.Basically there are two user defined functions that may be useful for someone engineering their ownPeltier cooled assembly. One function (Peltier) will calculate the heat transferred across the TEC - thisis really only a support function for (cputemp) which will calculate the coldside temp directly. Thefunctions require that you know how many thermocouples are in your TEC and you must also come up

with something known as the "Geometry factor". As I mentioned above I found from the Melcor data


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that G = Imax/50 - but the acid test is to recreate a manufacturer's supplied curve at the specifiedtemperature.

After you open up the spreadsheet there is a page sort of akin to a user's manual - and another page tochart the TEC performance. You can access these two functions by inserting a user defined function. Idid not protect the VBA stuff so feel free to hack it - but this is my material - please give credit where itis due.

Condensation

As mentioned before, cooling below ambient temperature may result in condensation problems, whichis something you'll definitely want to avoid. Using paddedPeltier elements will prevent condensationinside the TEC, but it won't protect you from condensation on cooled components.

Whether condensation occurs, depends on the temperature of the cooled object, on the ambienttemperature, and on the air humidity. Here is a table that will give you an indication whether you arerisking condensation problems:

Air humidityAir

temperature30% 35% 40% 45% 50% 55% 60% 65% 70% 75% 80% 85% 90% 95%

30 C 10,5 12,9 14,9 16,8 18,4 20,0 21,4 22,7 23,9 25,1 26,2 27,2 28,2 29,1

29 C 9,7 12,0 14,0 15,9 17,5 19,0 20,4 21,7 23,0 24,1 25,2 26,2 27,2 28,1

28 C 8,8 11,1 13,1 15,0 16,6 18,1 19,5 20,8 22,0 23,2 24,2 25,2 26,2 27,1

27 C 8,0 10,2 12,2 14,1 15,7 17,2 18,6 19,9 21,1 22,2 23,3 24,3 25,2 26,1

26 C 7,1 9,4 11,4 13,2 14,8 16,3 17,6 18,9 20,1 21,2 22,3 23,3 24,2 25,1

25 C 6,2 8,5 10,5 12,2 13,9 15,3 16,7 18,0 19,1 20,3 21,3 22,3 23,2 24,1

24 C 5,4 7,6 9,6 11,3 12,9 14,4 15,8 17,0 18,2 19,3 20,3 21,3 22,3 23,1

23 C 4,5 6,7 8,7 10,4 12,0 13,5 14,8 16,1 17,2 18,3 19,4 20,3 21,3 22,2

22 C 3,6 5,9 7,8 9,5 11,1 12,5 13,9 15,1 16,3 17,4 18,4 19,4 20,3 21,2

21 C 2,8 5,0 6,9 8,6 10,2 11,6 12,9 14,2 15,3 16,4 17,4 18,4 19,3 20,2

20 C 1,9 4,1 6,0 7,7 9,3 10,7 12,0 13,2 14,4 15,4 16,4 17,4 18,3 19,2

19 C 1,0 3,2 5,1 6,8 8,3 9,8 11,1 12,3 13,4 14,5 15,5 16,4 17,3 18,2

18 C 0,2 2,3 4,2 5,9 7,4 8,8 10,1 11,3 12,5 13,5 14,5 15,4 16,3 17,2

17 C -0,6 1,4 3,3 5,0 6,5 7,9 9,2 10,4 11,5 12,5 13,5 14,5 15,3 16,2

16 C -1,4 0,5 2,4 4,1 5,6 7,0 8,2 9,4 10,5 11,6 12,6 13,5 14,4 15,2

15 C -2,2 -0,3 1,5 3,2 4,7 6,1 7,3 8,5 9,6 10,6 11,6 12,5 13,4 14,2

14 C -2,9 -1,0 0,6 2,3 3,7 5,1 6,4 7,5 8,6 9,6 10,6 11,5 12,4 13,2

13 C -3,7 -1,9 -0,1 1,3 2,8 4,2 5,5 6,6 7,7 8,7 9,6 10,5 11,4 12,2

12 C -4,5 -2,6 -1,0 0,4 1,9 3,2 4,5 5,7 6,7 7,7 8,7 9,6 10,4 11,2

11 C -5,2 -3,4 -1,8 -0,4 1,0 2,3 3,5 4,7 5,8 6,7 7,7 8,6 9,4 10,2

10 C -6,0 -4,2 -2,6 -1,2 0,1 1,4 2,6 3,7 4,8 5,8 6,7 7,6 8,4 9,2

All values are in C.

Example for using this table: Ambient temperature=20C, air humidity=65%. Result: Condensationwill occur at a surface temperature (CPU, Peltier cooler) of13.2C

Condensation problems can be avoided by properly insulating the cooled components. The bettersolution is to use a temperature control for your TEC, to avoid temperatures that are so low that

condensation becomes a problem. A simple circuitry, like the onepresented here on The Heatsink Guideis useful for controlling fan speed, but not for controlling Peltier elements, due to their high powerusage. Pulse-width modulation can be used for controlling Peltier power; this is a rather complex issue,and beyond the scope of this guide.
http://www.heatsink-guide.com/control.shtmlhttp://www.heatsink-guide.com/control.shtmlhttp://www.heatsink-guide.com/control.shtmlhttp://www.heatsink-guide.com/control.shtml


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Summary

In conclusion, TECs are solid state heat pumping devices that can reduce component (CPU)temperatures, but they require some forethought to apply. If the TEC is misapplied, then the unit mayactually heat your CPU rather than cool it. The most important thing is that the heatsink and the TECmust be properly sized to suit the heat load. The heatsink must be at least good enough that it will keepthe CPU only 15-18C above ambient without the TEC. The TEC must have a maximum heat transfer

capability about 2 - 3 times more than the amount of heat that the CPU puts out.

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