[kuzee]

Thanks for explaining. I am also surprised by this possibility given my limited physics knowledge.

Do clothes substantially reduce the effectiveness? Is true line of sight required - visible or near visible spectrum?

Secondly, can you expand a bit about the humidity/condensation aspect? I get the impression that condensation represents inefficiency, and that this somehow avoids having to cool air as a middle layer to cooling a person.

[tripletao]

A cubic meter of air around room temperature and pressure has mass around 1000 g, and isobaric specific heat capacity around 1 J/(g*K). So cooling that air from 30 C to 25 C requires us to suck out 5 kJ. But at 30 C and 100% RH, air holds roughly 30 g/kg of water, while at 25 C it can hold only 20 g/kg. So as we cool the air, 10 g of water will condense, releasing about 20 kJ that we also have to suck out. There's no way to cool the air without condensing the water. In my example, the "latent" heat from the water condensing was much bigger than the "sensible" heat from air cooling. In practice it seems the latent heat is typically around 25% of the total[1], since we're probably at <100% RH, and since heat probably leaks in faster than moisture does (since air leaks let in both heat and moisture, but conduction through the walls/windows lets in only heat). The latent heat would be a bigger contributor in the poorly-sealed or outdoor cooling applications that these researchers seem to be targeting.

The idea of a radiant cooling tunnel appears to be old, but not very popular because it's not very effective. The cold plates leak cold into the air, just becoming a more cumbersome version of conventional air conditioning. The cold plate also can't run colder than the dew point, since the condensed water would drip and make a mess. The innovation here seems to be that they've placed a thermally non-conductive but transparent (to the thermal radiation) membrane between the cold plate and the user. There's a thin layer of cold, dry air between the cold plates and the membrane, but the membrane stops the cold from leaking out into the room air.

It's analogous to the sun shining through a well-insulated window on a winter day--you still feel the radiative heating from the sun, even as the window keeps the warm and cool air separated. Normal window glass wouldn't work for the cooling case, since the heat source isn't the sun (~6000 K) but the human (~300 K), so the wavelengths are much longer, around 10 um, and window glass is opaque there. Plastics with transmission around there are known, though, like the ones used to make the Fresnel lenses for PIR motion detectors. In any case, that membrane lets them keep the cold plate colder than the dew point without condensing water onto it, and also decreases loss of cooling by convection into the air.

It would work best on exposed skin. It should still work with clothes, as long as the clothes aren't too thermally insulating.

1. https://www.energy.gov/sites/prod/files/2013/12/f5/issue7_se...