Passive Thermal Resistance
Quote:
Originally Posted by
Jorlen
I have considered fans. But I need to point out the keeping noise down is more important.
There is a rather large copper heatplate between the heatlanes on the heatsink and the hotside of the TEC. The plate is warmer then the heatsink by a noticable margin. Which to me suggests that thermalresistance between heatsink and TECs is far from optimal.
I knew this when I decided to include the hotplate. It comes with the heatsink. It made things a lot easier when coming up with a way to assemble everything. And the peltier is 40mm while the heatsink is about 60mm, which mean I would use only 2/3 of the small heatpipes in the heatlane without at heatplate in between.
Thermal resistance of heatsinks using passive convection goes down as they get warmer. I dont know if its obvious, but at constant airflow, energy flow between air and heatsink is in linear relation to temperature difference of the air and heatsink. But since the airflow increases with the higher temperature of the heatsink the C/W goes down. With this in mind I am still hoping to cool well without fans.
There are so many materials for the heat to transfer through in my setup:
CPU, thermalcompound, heatspreader, TC, Waterblock, water, waterblock, TC, peltier, TC, heatplate, TC, heatsink(which is complex as well, since it heatpipe-based), air.
Something about this is not optimal. ;)
Jorlen -
Brilliant work - I especially like the idea of using the fan controllers on the MB to control the TEC. If you need any help on that, glad to provide it - I have designed a lot of big power chopper control circuits and can help with device selection etc. With high speed PCM it's a good idea to add a low pass filter on the output - any old burned up PSU has all the parts you need for any power PCM designs at these levels.
I know the Heatlane Zen is designed for passive, but for others reading the thread, heatsinks designed for forced air don't do well passive - the fins are usually too close to each other to promote air flow from the convection transfer. Forced air sinks often have spacing of 1/8 inch or less, while passive usually have 1/4 or 3/8 spacing with longer fins.
I'm not a fan of the Heatlane Zen, despite its cool looks. I played around with one a few years ago and was disappointed with performance. My testing was done with a controlled heat source at 40W, 50W and 60W in 25C ambient. Even with a 40W load, the base was at almost 70C. In comparison, a big typhoon with the same load held the base under 30C, and a Melcor HX5-301 holds it to 28C. The Heatlane Zen is big and hard to mount, and passive performance is mediocre - about 1 C/W. But I was able to get better cooling with a little airflow - I mounted a Vantec Stealth 120 to blow through it, with the fan at 9V turning under 1000 RPM, which was virtually silent. That airflow took the performance to .55 C/W.
I'm pretty sure you are using 12708 TECs. At 5V, you should see about 2.1A per TEC, with a 25C differential. That puts you at a CoP of 1.5, about as good as you can do with a 25C differential, which is all those sinks will do without active airflow. That means you are pumping about 15W per TEC, and a total heat load of about 25W.
Going to 6V probably takes you to the 2.45A level, and now you are burning almost 15W per TEC. You will move 20W per TEC, but efficiency has fallen to 1.36 CoP. Your hot side is probably around 60C - I'm surprised you got any improvement in cold side at all.
Just adding minimal airflow to those sinks should take them to under .6 C/W, which with 5V on the TEC takes your hot side from probably around 45C to more like 35C. At 5V you will move to CoP of 1.9, and you will move 20W per TEC or better, with no increase in power, still at 5V. If you can get the Vantec Stealth fans, give them a try - even at 12V they are only 28db, and at 9V I really can't hear any sound at all.
One other point - I had a hard time getting good contact with the Heatlane Zen. Are you sure that the heatsink makes contact all across the TEC? Also, correct and even pressure is important - there are several references to the right way to do it on this forum, including mine at http://www.xtremesystems.org/forums/...=184686&page=2