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# Heatsinks and Peltier Chips-WWWWWUUUUTTT? Answered

I need a peltier chip for a project and I'm a little confused how they work. I'm pretty sure I got it right, but not positive. Can someone tell me if this is right? The hot side of a peltier chip is attached to a heatsink with "grease" (can someone tell me where to get that?) then a fan blows the hot air away from the peltier chip from the side and another fan blows across the side of the peltier chip to its intended destination for cooling. Is this right?

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These peltier chips are an example of something called a heat pump,
http://en.wikipedia.org/wiki/Refrigerator
http://en.wikipedia.org/wiki/Heat_pump
also called a refrigerator, but the name "heat pump" is more descriptive, because that's what a heat pump does:  it moves heat.  What's even more remarkable is that a heat pump moves heat in a way that heat doesn't normally move:  it moves heat from a place that's cold to a place that's hot.

The cost of this "dark magic", moving heat from a cold place to a hot place, is the machine that does it must be supplied with sacrificial work, which is a sort of thermodynamically "pure" form of energy.  In the case of these Peltier modules, the work is electrical work, the product of current and voltage.

(Basically you just connect the wires going into this thing to a current source;  e.g. a car battery, and then magically one side of it gets hot, and the other side gets cold.)

But where does the electrical work go?  The first law says everything goes somewhere.  Without explaining exactly how or why, I claim that the supplied electrical work gets turned into heat, and that heat gets delivered unto the hot side of your heat pump.  That's why I included one of those diagrams with the wide arrows.  The width of each arrow represents energy flow. If the picture is drawn correctly, then it visually shows QH=QC+W
(Image source: http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/heatpump.html)

And that's part of the reason why you need a big heat sink on the hot side. There's a lot of heat to get rid of: the heat pumped out of the cold reservoir, and the work put into the system (turned to heat).

Another good reason for a heat sink is that there is some maximum temperature above which the Peltier device will be damaged, if the user/designer is foolish enough to let the device get this hot.  Temperature is, roughly speaking, a measure of the concentration of heat.  Let too much heat build up in one place, and  the result is high temperature.  Hence the desire for a heat sink, to help keep the heat moving.

And that's the goal here.  Moving heat is what a heat pump is all about.

Heat sink grease, aka thermal grease,
http://en.wikipedia.org/wiki/Thermal_grease
is essentially silicone grease mixed with other materials(particles of metal oxides, or metals) to improve its thermal conductivity.  The idea is to use this stuff to fill in the small air gaps which will inevitably be present between the chip and the heat sink.  This is because air gaps are a very poor conductor of heat.  Even small air gaps can impede heat flow. So you fill the gaps in with thermal grease.

The cheapest source I have ever found for thermal grease was one of the Hong Kong retailers.  I found him on eBay.  Note that this was for ordinary, run-of-the-mill, thermal grease, the kind made with ZnO, or MgO, definitely not the fancy stuff with real powdered silver.  They also sell small tubes of this stuff at Radio Shack(r), or at least they used to.

Will what work? Do you mean the cheap thermal grease, as compared to the pricey kind? Yeah it will work. I mean that's what the pros use, and to clarify what I mean by that I mean for example a factory that makes 12V Peltier-based beer cooler fridges (to keep your beer cold while you drive).  They use the cheapest thermal grease they can find, and so should you.

The only people who use the expensive thermal grease are the overclockers, kids trying to get more clock cycles per unit time from the processor chips in their PCs, at the expense of more necessary heating and cooling.

Worth noting is the fact that microprocessor chips often come with a on-chip temperature sensor. That way, with the appropriate software, one can actually get some feedback and see if the expensive thermal grease is having an effect, i.e. is the chip any colder than it was using the cheap thermal grease?  With the on-chip sensor you can actually see a temperature improvement as small as 1 degree, 2 degrees C, something like that.

In summary the expensive thermal grease is for performance freaks and demanding applications.

BTW, Peltier chips typically do not come with a on-chip temperature sensor(s), although it would be neat if they did.  Basically, for whatever it is you're building,  you'll have to supply the thermometers, if you want to.

I'm not sure if any of the people building things with Peltier modules are obsessing over temperature.  Usually its sufficient to just slap a few heat sinks on it, and then be conservative with the supplied electrical power; i.e don't exceed the recommended voltage or current rating.

Here's a search for instructables using the word "peltier". This should provide a few examples of people actually building things with Peltier modules.
https://www.instructables.com/tag/?sort=none&limit%3Atype%3Aid=on&q=peltier

No, no, no. What I mean is: Will the design I explained work for cooling a laptop or possibly a human being?

A Peltier chip can be thought of as a heat pump; it pulls heat from one side and pushes it to the other side. A "heat sink" just provides a larger surface area for heat transfer, and can be useful on either side of the chip. Heat sink grease improves the transfer of heat between chip and heat sink; you can get this anywhere that sells electronic components and/or heat sinks.