Novel Passive Radiative Cooling

It moves to thermal equilibrium with the sun at 5800K subtending an angle of 0.01 radians and the sky at an effective temperature ~200K.

Absolutely. Its performance would be significantly degraded if it was facing a cloudbase at typically 270K instead of clear blue sky.

It is real cooling since the thing has to reach thermal equilibrium with the incident and emitted radiation and has been carefully engineered to be nearly perfect black at NTP thermal wavelengths and very reflective at visible, near IR and absorbed thermal wavelengths.

The bulk of the suns radiation is reflected back and it is tremendously good at radiating thermal energy into space through the relevant transparent wavelength window. It is a clever design but I doubt if it will ever have any commercial impact unless it can be very cheaply fabricated some other way (like rolls of coated plastic film).

It depends a lot on how much cloud is above you and how dry the air is. Clear sky detectors at observatories rely on the difference in temperature between the cloudbase and clear sky to alert astronomers if the conditions improve or deteriorate. These days weather forecasts are so good they are barely needed except to warn observers if unexpectedly forming clouds are about to ruin a long time exposure towards the end of a session.

You really don't have a clue abut physics do you?

It is necessary for them to do that or the plate would be seeing the radiation from nearby CO2 and H2O at ambient air temperature. They also have to cover it to prevent convection (although it seems to me that since it would be cooler than ambient putting it in a shallow box to keep the cold air it produced over the device would be OK too).

Things like that tend to self destruct. The original limelight and later the domestic gas mantle exploited that feature of quicklime.

Selective emissivity coatings to block IR wavelengths are used to enhance the efficiency of sodium street lamps for instance (otherwise they would emit a strong IR line). InO is used as a selective IR mirror.

Almost certainly. It is surprising how effective passive self shading structure can be made by taking a look at nature. Cacti for instance.

Thermal management paint has been around for a while now. Critically for observatories the problem is to make sure that the dome does not super cool and drip colder air into the dome slit causing turbulence on a clear night.

The old domes were painted dazzling white to keep heat of the sun out during the day. Modern ones are now slightly metallic grey white with a paint that manages thermal radiation and is neutral at night. They have to use more internal thermal insulation and active cooling.

I think that is a bit unkind. It is a novelty and technically rather difficult to achieve.

Hi, Martin,

Stefan's law (the T**4 thing) is integrated over all frequencies, whereas in that paragraph I'm talking about sky temperature at a given frequency (assumed to be well below the Planck peak).

Two of the factors of T in Stefan's law come from the phase space volume (at constant frequency) expanding like T**2 due to the peak shift, and the fourth comes from the mean energy per photon being proportional to T.

At a fixed frequency, the radiation goes like T. See e.g.

Cheers

Phil Hobbs

Worth a Read, while not "cooling" using a quirk of thermodynamics, we have enough desert home owners here that this is worth posting:

"Cool Roof" paints, can lower roof temp as much as 80' over a pure asphalt roof.

Steve

Making and announcing these sorts of novelties is cool... if folks would stop there. But they usually (or whoever writes the press releases?) go on to conjecture about applications, and introduction dates (peaking at 10 years!) I think most of these guys are envisioning spinoffs and SBIR+VC money.

Enough to design electronics. I can always hire a physicist when I need to.

Then they should have expressed it that way. Stick to the physics.

Lol! So it is the researcher's fault that you don't understand the physics involved?

Of course, anyone's fault but your own.

Not that you will listen to them if they don't phrase their advice in a way that boosts your self-esteem.

People who understand a bit of physics understood what was being said. You didn't.