Trouble with the Cooling Power of Fans

Question

Thank you for taking some time out of your day to read my question, I have improved the specifics of my question:

I have a project in which I cool a very low power Peltier device (none of this question will regard any Peltier specifics) using a laptop fan keeping constant voltage and current to the Peltier. Peltier is 5v 1.5A, and I am using Artic Silver Thermal Paste on all mounting devices. Here is what is troubling me:

1) Using this fan, I get the cold side of the Peltier to 30 Fahrenheit: Some description of the item:

• The Heatpipes are 2mm thick (almost flat), the fan runs .45A 5V, the mounting pad for the Peltier (copper plate) fits the Peltier Entirely (30mmx30mm Peltier, same dimension fan).

2) Using this fan, I get the cold side of the Peltier to 43 Fahrenheit: Some description of the item:

• The Heatpipes are 3mm thick (have some thickness), the fan runs .21A 5V, the mounting pad for the Peltier (copper plate) does not fit the Peltier Entirely (30mmx30mm Peltier, 20mmx20mm copper pad, only fits the center). The end portion which the fan blows into has a quite thick aluminum sheet vent vs the last fan is less substantial aluminum vent.

3) Using this fan, I get the cold side of the Peltier to 45 Fahrenheit: Some description of the item:

• The Heatpipes are 1mm thick (paper flat), the fan runs .5A 5V, the mounting pad for the Peltier (copper plate) does not fit the Peltier Entirely (30mmx30mm Peltier, 20mmx20mm mounting pad, only fits the center of the Peltier).

Question: - Despite the double higher amperage of the last fan, but same "mount" size as the second one but smaller heat vent and heat pipes, the second fan cools better than the third fan.

So, using the first fan as a guide for proper mounting/size of vents and heatpipe analysis, what makes the second fan a better cooler than the last fan despite having less than 1/2 the current? What should I be looking at in a smaller fan (mainly concerned with the last two) that will allow it to cool more than what I currently have presented. What are the determining factors in cooling power, as seen current is not a factor as 1/2 the current cooled better? Heat pipes? Vent size? Mounting Size?

Answer 1

The power rating of the fan only tells you just that: the power rating of the fan. There is more to heat transport than how much power your fan uses (wastes). This includes:

• How much energy is wasted pushing the air through the fins
• How large the thermal resistance between the peltier and your cooler is
• How large the thermal resistance between the peltier mounting point and the heatsink fins is, including the non-linear behaviour of the heatpipe for the temperature at which you are working

These define how well you can get rid of the excess energy from your peltier element and what temperature it will see on its hot side.

A few notes on things you didn't ask:

• Using the cold side temperature to gauge how well the setup works is a bad idea. It depends on too much you haven't under control to be actually useful. A better way is to keep the cold side at a specific temperature (close to what you intend it to be used) and measure how much heat is transported at what power consumption of the peltier.
• The peltier's surface should be covered by the cooler. Leaving spots open will cause the peltier to form hotspots at these uncovered parts and slowly degrade its performance. If the mounting area is too small, consider adding a 3-5mm copper plate (or 10mm aluminium) of the size of the peltier to ensure that no hotspots will form.
• Please use proper SI units when discussing engineering topics. We do not use our hands and feets for measuring anymore.