Editor’s note: This question was asked in response to Electronics Cooling’s recent webinar by Roger Stout. To view the webinar, click here.
Question: Did you say thermionic coolers are perpetual motion machines?
Answer: No. I tried not to say that. In thermodynamics, we learn about perpetual motion machines of “type 1” that violate the 1st Law of Thermodynamics. That would be like the “zero-cogging” generator I talked about, that creates energy (or outputs more energy than it takes in). A “type 2” perpetual motion machine is one that doesn’t violate the 1st Law, but does violate the 2nd Law – that is, it decreases entropy. In a practical sense, anything that moves heat from a cold place to a hot place is asking for careful scrutiny of the 2nd Law; but there are plenty of actual machines capable of doing this (traditional heat pumps, for instance, such as my hybrid water heater in my garage, or thermionic coolers). Technically speaking, a heat pump or a thermionic cooler doesn’t violate either the 1st or 2nd Laws (and in fact, they couldn’t, or they wouldn’t work at all). The problem is, people seem to think of them as if they violated the second law – that somehow, you can lower the temperature of something without making the temperature of something else go up by an even greater amount. That’s where we often run into trouble in the semiconductor world. You can’t make an overall device more efficient (i.e., run at a lower temperature), simply by inserting a thermionic cooler. You have to give up something. As an electronics cooling guy, what I give up is that there’s more total energy to remove. Unless it was just a very small portion of the silicon that I was trying to cool, at the expense of total waste heat, I’m worse off by adding a thermionic cooler.
-Roger Stout
