What we've got is a demon...

Per Wikipedia: "Maxwell's demon was an 1867 ^[1] thought experiment by the Scottish physicist James Clerk Maxwell, meant to raise questions about the possibility of violating the second law of thermodynamics." http://en.wikipedia.org/wiki/Maxwell%27s_demon

I remember studying the demon in high school and thinking how "cool" it would be... and impossible, to have a little "demon" that could sort air into hot and cold molecules! Wow!

Years later, before cyclonic cleaning was common, I learned how a vortex could be used to clean machine cutting oil. I first figured out how it worked at a manufacturing convention when I was a quality engineer at Gerry Baby Products (now defunct... remember the umbrella baby strollers and baby-backpacks they invented?) They had a display with a clear cylinder and dirty cutting oil going in the top. the oil spun and formed a vortex that went back up a center tube in the opposite direction. The oil stopped momentarily then violently spun in a vortex up the center tube and out. With the slowing at the turn and the spinning vortex the fine metal particles were thrown outward and could not go up the inner tube. Voila, clean oil! We see this same idea all the time now in cyclonic, bag-less vacuum cleaners.

Wait, you say! You were talking about Maxwell's demon... how did you get off onto vortex cleaning??? Well, because the vortex is the demon! Just think, it can separate the heavy metal particles from the lighter oil molecules. Interesting. Later, still at Gerry, I discovered that that cleaver demon can indeed be used to sort air molecules into hot and cold sides, just like Maxwell proposed! Incredible!

Here's how it works... you hook up "shop air" at around 80 psi to a vortex tube. Generally the air enters at one end off center so that it spins. Near the far end some of the air turns a corner and goes up a center tube and out while some continues to the end of the tube and out an adjustable hole. Sound familiar? What is amazing is that the "cold" molecules, that is the slower ones, are more likely to be able to turn the corner and spin up the center, while the fast ones can't quite make the turn, so they exit. See the demon at work? You don't? He just sorted the air molecules into two streams with the cold ones coming out the near center and the hot ones going out the far edge. Very smart that demon.

Vortex cooling is used a lot for various things like spot cooling machining operations and cabinet cooling. Check out one vendor http://www.vortec.com/vortex_coolers.php They even have vortex cooled vests that let people work in 200 degree environments! (with protective clothing.)

Maxwell did not say that the demon did not use energy to open and close the tiny door. It was just that the demon was not putting energy into the air to heat it. One side got cold and one got hot but neither heat nor cold was created by the demon, both were already present in the incoming air. What sort of efficiency do you suppose this process gets?

Heat pumps are similar in that they use some energy to move heat from one place to another rather than creating the heat. A refrigerator is a compression-cycle heat pump that takes heat energy from inside the cabinet and pumps it into your kitchen air. Heat pumps for home heating use an almost identical process and apparatus to move heat from the outside air into your house. You could just as well use a thermoelectric cooler or a vortex cooler. However, the good old compression method is around 65% efficient while the thermocouple is lucky to make 8%! Vortex coolers can drop the air temperature by 80 degrees with an efficiency of up to 16%... not bad I guess... (http://digital.library.adelaide.edu.au/dspace/bitstream/2440/43646/1/hdl_43646.pdf).

You could try to use the heat differential to do work. However, if the hot and cold side are in a fixed volume, then the pressure on the hot side would increase. The demon would need to operate the door more and more, and thus use more and more energy to run the door to keep those fast molecules from passing through to the cold side. A vortex demon needs a free flow of air, not a fixed volume. You could use the hot and cold air streams in the open on a thermocouple to make electricity, but then you would suffer from the poor efficiency of the thermocouple. Hummm... maybe you could use a Sterling cycle engine! Theoretically they can be 100% efficient, but for now, 30% is good... not so good...

Maxwell talked about sorting the molecules, not creating the energy... so if the energy that the cleaver little demon needed to open and close the door was small enough, maybe he could harvest a little heat differential from the hot to cold side to run the door mechanism, and then I could use the rest of the differential to do other things. I could heat my house in the winter and cool it in the summer by just changing some dampers! That would not be violating the Second Law of Thermodynamics any more than using some sunlight to power a photocell (about 10% efficient), after all, sunlight heated the air - right? OK, so it looks like the little vortex is a demon, but he needs too much energy to operate his door... do you know any other demons looking for work???