3 ms·

It's the cooling equivalent of a radiant heater, and you need line-of-sight to the walls to feel cooled. The thermal interface is deliberately inefficient for c

It's the cooling equivalent of a radiant heater, and you need line-of-sight to the walls to feel cooled. The thermal interface is deliberately inefficient for convection (to avoid wasting power chilling the air), but efficient for radiation. Radiation is proportional to absolute temperature T^4. So compared to a glowing radiant patio heater, I'd expect to need maybe (1000^4 - 300^4)/(300^4 - 270^4) ~ 350x the area for the same heat transfer in the opposite direction, thus their big tunnel instead of a little filament. Even with their cold source at absolute zero, they'd still need >100x.

sparker72678 6y ago
And you still have to pump the heat out to somewhere else. The whole premise with this system is that you can keep the temperature higher so you have to pump less heat, which I'm extremely skeptical of, at least for indoor environments.
- URSpider94 6y ago
  The biggest gain is that you don’t have to pump out all of the heat of condensation of all of the moisture in the air. Condensing out a pound of water is 200x more energy-consuming than lowering the temperature of a kg of air by 10 degrees C.
kuzee 6y ago
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 6y ago
  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_sensible_loadcontrol.pdf https://www.energy.gov/sites/prod/files/2013/12/f5/issue7_se...

sparker72678 6y ago

And you still have to pump the heat out to somewhere else. The whole premise with this system is that you can keep the temperature higher so you have to pump less heat, which I'm extremely skeptical of, at least for indoor environments.

URSpider94 6y ago
The biggest gain is that you don’t have to pump out all of the heat of condensation of all of the moisture in the air. Condensing out a pound of water is 200x more energy-consuming than lowering the temperature of a kg of air by 10 degrees C.

kuzee 6y ago

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 6y ago
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_sensible_loadcontrol.pdf https://www.energy.gov/sites/prod/files/2013/12/f5/issue7_se...