passive cooling + peltier?

knutinh · Post by **knutinh** » Tue Sep 21, 2004 4:26 am

It would seem that transport of heat from the cpu to fins is a major limiting factor, even when using heatpipes, meaning that the cpu may fry/shut off even when the fins are not overly hot?

What about using a peltier element between cpu and heatpipes to "convert the energy" to higher temperatures and letting the fins be hotter for improved convection?

regards
K

Post by **Rusty075** » Tue Sep 21, 2004 6:08 am

That's actually something I thought of while playing with the ActiveCool AC4G Thermoelectric cooler:

AC4G review wrote: In theory, TECs have a couple of distinct advantages over conventional HSFs:

1. They can cool the CPU to below ambient temperature.
2. They allow the heatsink to be at a higher temperature than the CPU.

For our purposes, the second characteristic is particularly important. With a conventional air cooler the heatsink is always cooler than the CPU. For a heatsink, the heat transferred is proportional to the difference between the heatsink temperature and the air temperature. All other things held constant, doubling the differential doubles the heat transfer. In other words, getting the heatsink hotter lets you move more heat (watts) with the same CFM (and noise), thus making the heatsink perform more efficiently.

Sounds great, more heat transfer, same noise.... but there's a catch. (Isn't there always?) TEC' are not perfect conductors of heat. For every watt that they move from the cold side to the hot side, they must also consume energy, which is also released on the hot side. The measurement for this is referred to as a TEC's coefficient of performance (CoP). The CoP is defined as: "the amount of heat energy being moved divided by the amount of supplied electrical power". For a typical TEC, the coefficient of performance is between 0.4 and 0.7. That means that to move 60 watts from the cold side you will be releasing between 85 and 150 watts from the hot side. That extra 25 to 90 watts works against the efficiency gains made by increasing the temperature of the heatsink, and there is also the issue of producing that wattage. It has to be produced by the PSU (with its accompanying efficiency heat loses), and then be removed from inside the case.

Once you factor in the efficiency loses at the TEC, and at the PSU, you generally end up with a net gain in heat and temps, and thus noise. Which is why pelt's have been aimed at people who don't particularly care about noise.

But I could see the potential for a system designed specifically for silent pc'ers: Take a fairly low power TEC (since there's no need to cool below ambient), attach a very efficient passive heatsink to it. (like the new NCU-2000), and use an external brick PSU to keep that heat out the case, and you may have something workable.

Might make for a fun little experiment.

sthayashi · Post by **sthayashi** » Tue Sep 21, 2004 7:03 am

Bah.... I wanna see a TEC integrated into a plate w/ heatpipes leading out to a BIG heatsink.

Post by **Rusty075** » Tue Sep 21, 2004 7:25 am

Something like a TEC-equipped Zalman TNN-500?

sthayashi · Post by **sthayashi** » Tue Sep 21, 2004 7:34 am

Rusty075 wrote:Something like a TEC-equipped Zalman TNN-500?

Yes, but I want it to cost LESS than my car.

Post by **Rusty075** » Tue Sep 21, 2004 7:35 am

picky, picky.

May I suggest the obvious solution:

Buy a more expensive car.

Straker · Post by **Straker** » Tue Sep 21, 2004 8:31 am

well, TECs themselves are relatively really really cheap, shame how hard they are to work with though.

mathias · Post by **mathias** » Tue Sep 21, 2004 6:21 pm

Straker wrote:well, TECs themselves are relatively really really cheap, shame how hard they are to work with though.

Really? Then shouldn't sticking a big one on the case work? Or how about on a water cooling radiator?

Straker · Post by **Straker** » Tue Sep 21, 2004 11:22 pm

probably not, just because they're relatively inexpensive doesn't make them awesome.

it's just sort of a tease because you can cool below ambient with something that takes up less space (by itself) than a normal heatsink or even a waterblock, and costs maybe 5% of a phase change setup, but then the problem is getting enough heat away when it's still so close to the CPU or whatever.
i don't think there really are any that are "big", mostly 4-6cm on a side, if you used low-power ones and covered a large area that'd probably get expensive.
the biggest problem imho is that most TECs can only tolerate temps of 100-120C max, so the gradient isn't actually that much larger than normal... if they could remain intact up to say 300C then yeah, you'd be able to use a fairly small heatsink/radiator on the hot side to get rid of a lot of heat.

silvervarg · Post by **silvervarg** » Tue Sep 21, 2004 11:40 pm

if they could remain intact up to say 300C then yeah, you'd be able to use a fairly small heatsink/radiator on the hot side to get rid of a lot of heat.

Would you be comfortable with a 300C heatsink in your computer?
Imagine what happens if a cable happens to touch the heatsink.
Or if you touch it with your hand by accident...
Not to mention the strange air-smell that you get when air is heated a lot.

Already at 100C you start to experience many of these problems.
I would say the loads of problems you get from peltiers make them useless in combination with passive cooling.
The best usage I have seen so far is in combination with water cooling, but that is to increase cooling power, not to decrease noise.

Another good common usage is in mobile refrigirator boxes. They have good use for the peltiers ability to cool below ambient, cheap cost and small size.