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13 June 2009
Author: Giorgos Lazaridis
The Peltier Thermo-Element

An example calculation

Suppose that we want to cool down our old Pentium 4 423 @ 1.4G. This had cooling needs reaching up to 72 Watts.

From the first characteristic curve we find that we will draw about 6 amperes (6.05A) and that a temperature difference of about 5 degrees.

Then we go to the second diagram. Following the line of the 6.05A, we see that for 5 degrees we need to have supply 10.8 Volts to the Peltier.

Cooling a peltier
NEVER POWER A PELTIER WITHOUT A COOLING METHOD

A Peltier thermo element mounted on the surface of a heat-sink with a cooling fan

That is a rule if you want your TEC to go on working. The Peltiers generate a lot of heat on the hot surface, far more than the heat they dissipate. And that is because the TEC itself draws a lot of current that generates itself heat due to I2xR heat losses.

The most common cooling method for TECs is the air cooling. The hot plate of the TEC is mounted (with heat transfer paste always) on the surface of a heat-sink that carries a cooling fan. The heat sink must be calculated in a way that it will be able to draw all this heat power that comes of the TEC's hot plate.

As you will find out later on this article, air cooling is sometimes inefficient. You should consider building a project with water cooling if you would like to have maximum cooling results from a TEC, as a heat sink to handle the heat power generated under heavy conditions would be almost impossible to be built.

How to calculate the power that the TEC cooler must have

It is sometimes difficult for people to follow mathematical forms and types, but it is always necessary to do for the sake of efficiency.

At first, let's give some symbols to our terms:

• TAMB = Ambient temperature
• QC = Heat load (in watts) that we need to cool
• TH = Peltier hot plate temperature
• QΗ = Total heat sink load (watts)
• C/W = Heat sink efficiency (Degrees C/watts)
• PE = Peltier electrical power input (watts)
• V = Voltage that the Peltier is powered with (Volts)
• I = Current that the Peltier draws from our power source (Amperes)

Now to connect all them together!

TH = TAMB + (C/W)(QH)

QH = PE + QC

PE = V x I

And what have we come up with? Suppose that we have a Peltier that is powered with 10.8 Volts, draws current 6.05 Amperes and pumps 72 Watts of power. This system is placed in a box that has inside 25 degrees. What can we do with the above formulas is to calculate the appropriate heatsink power needed in order to cool efficiently our peltier.

The QH parameter is the one that we are currently interested at. This is the total thermal load that needs to be sinked from the heat sink attached to the TEC's hot plate. The formula is:

QH = PE + QC

PE is the electrical power drawn by the TEC, that would be VxI:

PE = 10.8 x 6.05 = 65.34 Watt

QH = PE + QC => QH = 65.34 + 72 = 137.34 Watt

So, we need to place a heat-sink to the hot plate of the Peltier that will be able to sink 137.34 Watts of thermal power.

How to calculate the temperature of the hot plate of the TEC during operation

Another very important parameter that needs to be taken into account is the temperature of the hot plate of the TEC that needs to be cooled. This parameter is the TH. Let's calculate it for our P4 cooler!

TH = TAMB + (C/W)(QH) => TH = 25 + 0.55 x 137.34 = 100.5°C
A typical heatsink efficiency is about 0.55 C/W

Now you understand the reason for this calculation. The hot plate will reach up to 100 degrees during operation. The TEC must be cooled efficiently otherwise it will be destroyed. The temperature will be able to boil water!

Peltiers are a cheap and easy way to achieve sub-ambient temperatures for any reason. They are solid state and minimal in dimensions, and this gives them also the advantage of portability, something that other systems are incapable of. Also, they are solid state with no moving parts inside. This makes them extremely reliable and needs of no maintenance at all.

TECs comes with a series of disadvantages that makes them sometimes completely inappropriate for cooling parts. TECs are extremely inefficient. They draw a lot of current to operate and generate too much heat. If a part is cooled with normal heat sink methods and a Peltier is added to boost this operation, then the heat sink that needs to be mounted on the TEC sometimes must be double in power than the original heat sink that was attached to the part! This makes also the TECs a little bit of ecology enemies.

Considerations

Cooling with TECs is not always an easy task. If a CPU with temperature load of 100W needs to be cooled, then mounting a Peltier on it and try to cool it is not always the right thing to do. Peltiers should be used combined with water cooling. This makes the system even more reliable and more controllable.

When you apply a voltage to a TEC and get a temperature difference you should have in mind that you will also cause a back voltage created. This is caused by the Seebeck effect and is similar to the back EMF caused by electric motors. It is possible therefore to get a negative linear load output curve.

A Peltier cooler should have controllable power supply or current flow. The power provided to the TEC should ne in conjunction to the thermal load and the ambient temperature. This would increase the efficiency of the TEC. You can use PWM to control the power provided to the TEC

Always have in mind that the more cooling you want from a peltier, the bigger the difference between the amount of heat your CPU generates and the rated cooling power of the peltier. This is because the efficiency of the TECs is getting even lower as the temperature pumping demands are increased. In such situations it is good to have as rule that "The bigger the peltier the better the result". However, bigger peltiers will require much more cooling. Sometimes air cooling will be not enough. Water cooling is the only solution if you want extreme sub ambient temperatures to be achieved.

Be aware also that underrated TECs applied to CPUs for cooling may not only cool the CPU but heat it even more. There are some small TECs 25-30 watts around in the market. Forget about them. You need brute power, even if you use just a part of it.

The same will happen if you do not have a good cooling system for the TEC...

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