KING@KESTREL (12/25/82)
From: Richard M. King <KING at KESTREL> I claim that an extended superconductor (say, a cross-country power cable) need only be refrigerated at one end. A superconductor conducts heat, as well as electricity, with no resistance. (There might be a time delay or a maximum flux due to the speed of doublet motion or the number of doublets - I don't know. It doesn't matter to this argument.) If you cool one end, at any time there will be a superconducting portion and a normal portion. The superconducting portion will grow into the normal region if the refrigerator is more than powerful enough to make up for heat infiltration over the superconducting length. I therefore claim that there need be no provision for liquid flow over the length of the cable. Does anyone know if there is anything wrong with this argument? Dick -------
leichter (12/26/82)
The only problem with the argument is that I can make it the other way just as well: I only need to HEAT one end to get the whole think up to the maximum temperature! So heat flux through the thing is very much an issue: All heat taken up anywhere along the conductor must flow down it and be "taken out" at the cold end. The amount of heat energy involved is going to be huge; it's not easy to insulate against the difference in temperatures involved. (A good Dewar will hold liquid helium for quite a while, but consider its surface to area ratio, and the total surface presented, compared to that of a long wire.) Pumping liquid coolant along the thing essentially distributes the heat removal problem to the point where you can handle it locally. -- Jerry decvax!yale-comix!leichter leichter@yale
norskog (12/28/82)
#R:sri-unix:-492300:fortune:8600001:000:196 fortune!norskog Dec 27 15:20:00 1982 Know what happens if you supercool one end of a wire and not the other? The wire dances around like a snake and breaks. Unless, of course, the material has an expansion coefficient of exactly 0.
KFL@MIT-MC (12/28/82)
From: Keith F. Lynch <KFL @ MIT-MC> What makes you think a superconductor of electricity is also a superconductor of heat? I have never heard of any superconductors of heat. If there was such a thing, its uses would be manifold. For instance you could never burn a hole in it unless you managed to heat the whole piece up to above its transition temperature. Given a big enough piece, that could be made arbritrarily difficult. The closest thing I have heard of to that is superfluid helium, whose viscosity is supposedly zero, and in which heat travels at the speed of sound (which is a lot faster than the speed of sound in air). Still not really a superconductor of heat (Or of electricity; superfluid helium is an excellent insulator!). To the best of my knowledge it is still more of a laboratory curiosity than anything else. If only there were a room temperature substance that conducted heat that well it might be practical to run heat pipes from the equator to the poles, and save on heating and air-conditioning bills world-wide. ...Keith
lemmon (01/06/83)
I think that the heat flux is very relevant. If the cable can only carry a certain heat flux (even with zero temperature drop), then it can only cool a length of itself limited by the heat leakage per unit length. Beyond that length, the flux at the refrigerator would exceed the carrying capacity of the cable, which would then (probably) cease being superconducting. Rather odd cyclic behavior might occur. In any case, there would be a need for refrigerators every so often. Alan Lemmon
bek (01/09/83)
about that long superconductor... The original article claimed that such a wire would need to be cooled only at one end by a refrigerator with sufficient power to remove all the heat absorbed along the length of the wire. I think the concept is basically viable. my comments on opposing articles (so far) Someone thought that a heater at one end (ie. the environment) could heat up the whole wire (via the same logic used to validate the single refrigerator theory.) But note that the cold end has a superconductor working for it while the hot end does not. Thus the argument which says you can heat the wire from one end just like you cool if from the other is flawed. Someone said that if we could effect free heat transfer we would already being so to solve our energy problems. Don't I remember a similar statement made years ago with respect to man, wings, and flying? Someone noted that real wires get epilepsy when subjected to extreme temperature gradients. (probably the most sensible comment made so far, including mine.) OK, so there are start-up problems with the wire. Barrett Koster duke!bek