Passive Thermosyphon

SUMMARY: Thermosyphons can make effective passive cooling systems.

Passive

Passive cooling is one tough proposition – the problem is that without some form of forced air flow, the surface area to accomplish effective cooling becomes vary large. For conventional air cooled heatsinks, spreading resistance also comes into play, effectively limiting how large the cooling surface can be. In water cooling, spreading resistance limitations do not come into play, but the size of a passive radiator can be bulky.

One of the neat characteristics of a thermosyphon is that condensation can take place in just about any vessel – a radiator, a statue, a ball, two glass plates, glass on a fish tank, decorative sheet metal stampings, etc. As long as it’s sealed and thermally conductive, it can work as a condenser.

We decided to fashion a totally passive thermosyphon to demonstrate “proof of concept”. We built a simple prototype out of tin flashing material .007″ thick:

Passive

The total footprint of the condenser is 20″ x 34″ x 0.1″ thick, with total weight of about three pounds – sized to cool 75 watts. This assembly was hooked up to the P4 test rig, an Acorp 4S845A motherboard with a modified P4 1500 to read CPU case temps. Air flow in the room was totally still – no fans to move air.

The Test

Device

Case Temp

Ambient Temp

C/W

CPU Diode Temp

Passive Thermosyphon

41.5 ºC

20.5 ºC

0.36

37 ºC

Under stress, the passive thermosyphon turned in performance comparable to some low noise, fan-assisted heatsinks; even though it’s not optimized, we could probably knock off a few more degrees. At idle, case temps dropped to 36.3 ºC and the CPU diode showed 30 ºC. “Normal” use should fall somewhere between these two boundaries.

Conclusions

Effective passive cooling is a tough nut to crack. Thermosypons have some interesting characteristics which make this a viable technology for totally passive cooling – the “no-noise” approach.

On another note, I have received emails asking me when we might come to market with a product. We are continuing to investigate sourcing options and it does take some time to tie the loose ends up. Our pricing objective is for a retail price under $50, and this entails some keen sourcing. We may or may not make a totally passive thermosyphon product. In addition, we are considering DIY kits as another option. Stay tuned!

-JoeC

Disclosure: Joe Citarella has a financial interest in a company developing thermosyphon products for electronic chip cooling.

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