Zds Project Log

Introduction

In computer enthusiast circles thermo-electric Cooling Modules, or TEC modules, or Peltier elements and best known for their use in keeping overclocked monster CPUs cool, or getting below the ambient without compressor equipment. In extreme overclocking TECs are usually asked to pump as much heat as they can and are operated near 80% of the maximum wattage they can move. They have however a lot more uses than that, and in this article I am diving into completely other part of their range of usage.

The TECs have an important property I am going to take advantage of: the less you ask them to do, the more they can do per given amount of power. This means that if you have TEC rated for moving 172 watts of power at maximum (Qmax 172W) and you ask it to move 150 watts, you need to throw in a lot more than the 150W to get it done. But if you ask it to move 30W, it might need only 10 watts of power to do it, thus having Coefficient of Performance of 3.0: moving 3 watts per watt consumed.

In theory this should mean that if your TECs are clearly more powerful than the component to be cooled and you run them seriously undervolted, you should get the component kept cooler than without TEC, without changing the other parts of the cooling system.

Test setup

To validate this theory I first used Kryotesc, TEC system calculation software provided by Kryotherm, one of the leading TEC manufacturers. By varying parameters I ended up having considering overrating Qmax 7-8 times being a good compromise for efficient cooling.

So I acquired a DRIFT-0,8 TEC module from Coolputer. It’s rated for Qmax 172W and 24.6V. This means I can seriously undervolt it using just the pre-existing ATX power lines, +5V and +3.3V. Here’s the test equipment in group portrait:

The other components are 1ohm/100W power resistor, el cheapo two-sensor thermometer, 50×50mm piece of 3mm aluminium flat, passive heatsink used to cool Pentium Pro and old Enermax ATX power supply. Part of the test but not in the picture are two small active coolers.

The power resistor was connected to +5V line, which theoretically should make it produce 25W of heat. In practise the regulation of the power supply and resistance of the wires lowered this figure quite a lot.

I did not use any thermal compound, so to get comparable results without TEC I used the aluminium flat in place of it:

The thermometer sensor was taped to the resistor and we were good to go.

Three sets of configurations were tested: Just the resistor with heatsink and aluminium flat for comparison, and heatsink, resistor and TEC running at 3.3V and 5V (nominal).

On the batch of runs I tried using the passive heatsink (originally used to cool my Pentium Pro 180 CPU), but while it showed expected results, it was way too prone to changes in the environment. It was hard to get it to stabilize even to within a whole degree of Celcius.

The next cooler to try was an old tiny aluminium heatsink with a 40mm fan, possibly from some 486dx100 CPU. I reasoned that despite of the small size it should easily beat the all-passive heatsink, but the smell of overheated electronics soon signalled I was terribly wrong. The fan seemed to have worn out and the cooler had lost even the smallish cooling power it had once had.

After letting the parts to cool down a bit I brought in what I think is fairly new chipset stock cooler. With aluminium flat the setup looks like this:

And with TEC:

Caveats

In the end the results I got are to be considered preliminary, as while measuring the setup and running tests I found several weaknesses in my testing methodology:

1) The inherent (and pre-known) weakness is that as the resistor is not insulated in any way, inevitably fair amount of heat moves to air without going through our test heatsink assembly.

2) The test PSU, old 230W Enermax ATX one, seemed to have exceptionally bad voltage regulation. It is rated for 20A for 5V line, but by no means I could run the resistor and the TEC module from it without voltage dropping to 4V or below. This means the heat load was smaller than hoped for and that the results between different setups are not that comparable, as the TEC load affects also the heat generated by the resistor.

3) The low resistance of the resistor itself (1 ohm) combined with bad voltage regulation means that there’s no way to measure the heat generated precisely, as even the wires from the PSU cause resistance in the same order of magnitude. Even putting the measuring equipment to the loop affects the system noticeably, so margin of error for the results is quite high. In the final run I hooked the resistor and the TEC module to the ATX motherboard connector, not the molex ones, to get a bit more stable voltages.

4) The thermometer used in these tests is cheap and is pretty certainly not calibrated. However I have all reasons to believe it is consistent, ie. given the same real temperature it shows always the same reading. And as temperature delta and differences between deltas in different setups are the relevant things, I don’t think this weakness really skews the results.

5) The experiments should be done in a space with more constant airflow and temperature conditions.

Results

With all these disclaimers, here are the numbers. Ambient temperature was between 24 and 28 degrees during testing.

Setup	Measured thermal load	TEC power usage	Temperature delta	C/W	CoP max.	CoP est.
ALU flat	4.6V * 4.15A = 19.1W	n/a	27.8°C	1.46	n/a	n/a
TEC @5V	4.65V * 4.1A = 19.1W	4.94V * 1.54A = 7.6W	21.2°C	1.11	2.51	2.01
TEC @3.3V	4.67V * 4.07A = 19.0W	3.31V * 1.11A = 3.7W	18.8°C	0.99	5.14	4.11

First of all, the C/W numbers for TEC setups are there just to get some idea about the cooling performance between these three setups. You can *not* extrapolate these results linearly. Second, the CoP numbers are just estimates, as it is not known how large portion of the heat gets transferred to air without ever going through the TEC. The first CoP value sets the upper limit by assuming that all the heat would go through the TEC and the second, more realistic but more vague number calculates the CoP assuming 80% of the heat goes through the TEC.

As you can see, at least these results show the same pattern the simulator does: both TEC setups beat the reference aluminium flat and TEC running at 3.3V does better job than the same TEC running at 5V. This is due to better CoP, so less extra heat is produced and thus heatsink has to do less work.

What this mean in practise is that you can get your chip run cooler *with the same heatsink and fan* if you employ TEC properly. You can extrapolate the rating of the TEC(s) needed by multiplying all the parts with the same multiplier: if you for example double the heat load, you need to double the rating of the TECs and double the efficiency of the heatsink to keep the temperature the same. All other means of linear extrapolation are doomed due to non-linear behaviour of the TECs.

Also in practical applications you want some intelligent circuitry to run the TEC(s), so that they do not accidentally put the chip below the dew point and thus risk condensation occurring. One way to do this would be to harness a microcontroller to monitor the temperature and PWM the TEC to keep the cold side at the same temperature regardless of the load. As TECs need quite a lot of power and they do not like PWMing, you need to chain some hi-power transistor to keep the microcontroller circuitry from frying and hi-capacity capacitor in between to even out the voltage. Needless to say, if you are not familiar with these concepts, ask someone who is to help.

Further work

First of all, the TEC and the temperature source should definitely be fed from different PSUs to rule out the possibility of the TEC load affecting the thermal load. Second, the thermal load should be better insulated so that we’d have clearer idea about what exactly is the thermal load we are coping with.

Third, it would be interesting to hook up two TEC modules electrically in series and physically both in parallel and in series and see how that compares to a single one. Simulations with Kryotherm software indicate the electrically series physically parallel configuration should yield even better CoP, but it’d be nice to get that validated myself.