Supercooling of Peltier cooler using a current pulse

Physics.http://link.aip.org/link/?JAPIAU/92/1564/1

Looks odd. Under normal conditions, heat transfer through a Peltier cooler is linearly proportional to current, but the current produces resistive heating in the cooler proportional to the square of the current.

So there is an upper limit to the amount of current that you can usefully drive through a Peltier cooler - a sufficiently high current will produce no cooling effect at all while dumping lots of heat into the exhaust heat sink, and half that current (the figure normally listed on the data sheet) will produce the maximum net cooling heat transfer - any more current will produce more heat in the junction than it transfers through the junction.

Generally, pulsing the current through a Peltier junction is a bad idea - obviously, if you only want cooling for a brief period anyway, you aren't going to run the cooler continuously. One would need to see the original paper to see exactly what the authors think they have discovered, but I'm rather sceptical.

-- Bill Sloman, Nijmegen

Physics.http://link.aip.org/link/?JAPIAU/92/1564/1

I imagine they're relying on the thermal mass.

It would be easy enough to test -- and I will certainly give it a go the next time I am working with Peltiers if someone here doesn't post something about it first. One would think that this is the sort of thing that Peltier cooler manufacturers would put in their datasheets.

The bit about "The improvement due to pulse cooling is about the equivalent of two additional stages in a multistage thermoelectric cooler." is interesting. are they implying that I can replace a six-stage stack with a two-stage stack? Normally I have to make the plates bigger and bigger on the hot side[1] so as to pump out the heat from resistive losses. If I really can replace six stages with two, would the coldest plate be the size of the hottest plate in the stack of six, or the size of the plate right bellow the coldest plate? (Not that I have ever mad a stack six high... Last time I worked on such a system we stacked three on top of a standard freon chiller)

Note [1:] Unless I made another typo, a typical 10W Peltier pumps

10W of heat into the cool side, adds another 20W from I2R losses, and thus needs to shed 30W from the hot side. The next-warmer stage needs to pump that 30W, but adds another 60W of heat doing so. The next-warmer stage needs to pump 90W, but adds another 180W of heat doing so. Then it's 270W in 810W out, 810W in 2430W out, 2430W in 7290W out, and so forth. That's why staged Peltier coolers look like pyramids.

--
Guy Macon

Picture of typical Peltier pyramid:

Peltier FAQ:

Peltier Guide:

Thermoelectric Handbook:

--
Guy Macon

ing=20

vice,=20

0 to 55=20

For the uninitiated ... The heat pumping capacity is maximum at deltaT equal Zero and the heat pumping capacity approaches zero at peak difference temperature which is in the order of 67 degrees per stage. These guys operate close to the peak difference temperature where the pumped heat is almost zero. Meaning the application is rather academic and hardly useable in practise.

Rene

--=20 Ing.Buero R.Tschaggelar -

& commercial newsgroups -

Depends what your idea of practice is. For IR spectroscopy, you're almost always stuck between wishing you could use a Joule-Kelvin (open cycle) refrigerator to get lower temperature, and not wanting to deal with the gas handling, clogged tubes, and vacuum problems. Multistage TECs are one answer, but it's really, really hard to get below, say, -50 C. The reason is (as Guy pointed out) that only the top stage is really working near its maximum delta-T--all the other stages are having to handle the waste heat from all previous stages. Even with spreader plates and spreading out the TECs in the upper stages, it's hard. Since the temperature tuning range is what it is, each degree gets you a wider range, which is often very helpful.

IOW anyone who thinks a multistage TEC is practical is stupid enough to think this idea is practical too. ;)

Cheers,

Phil Hobbs