Thermoelectric cooling is pretty inefficient, because the materials need to balance competing requirements:

- Good thermal insulator - Good electrical conductor - Good semiconductor

This is because the hot & cold sides are sandwiched closely together as a PN junction, so once you move heat from one side to the other, it just leaks right back. Mechanical cooling doesn't have this problem, because the hot & cold sides are separated by thin bits of tubing. This makes the thermal leakage a "minor annoyance" in a mechanical system as opposed to "literally the whole problem we're trying to solve" as it is with thermoelectrics.

One work-around is to stack lots & lots of thermoelectric coolers on top of each other. That reduces the temperature difference at each individual PN junction, which in turn lowers the leakage. That's what this team is doing, but using layers that are only a few nanometers thick, so they can fit dozens or hundreds of junctions in a single package.

Twenty years ago there was a company trying to commercialise thermoelectric cooling based on a vacuum gap: https://web.archive.org/web/20031213235132/http://www.coolch...

They claimed 55% Carnot efficiency based on a 30-100 angstrom gap maintained by piezoelectric controllers, and a method to construct large electrodes with matched surfaces so that the gap could be maintained over a large area. It all sounded plausible but never went anywhere as far as I know.

Incidentally that means all their patents will have expired...

What if you use the technology in places where you actually want to maximize heat conductivity?

I'm thinking of the separation walls in counterflow heat exchangers (only useful at the end where the incoming stream is closer to its end temperature than the delta offered by thermoelectrics I guess). Can it do whatever it does across a temperature gradient?

that's an idea but would you not have to power each one which then destroys efficiency anyway