Comment by blincoln | Hacker News Reader

Comment by blincoln | Hacker News Reader

I wonder how Teflon (PTFE) compares to this stuff. In the DIY multispectral photography community, a lot of people use it as a reference material because not only is it pure white to our eyes, it also reflects (diffusely) UV-A, UV-B, and SWIR very well. I found a chart and paper[1] showing that a proprietary variation of it called Tetratex continued that theme well into the SWIR range, but unfortunately the authors didn't measure it into the MWIR or LWIR ranges.

It's easiest to find at the hardware store as very thin tape for wrapping pipe threads, but I bought a few bars of it (about 5mm x 30mm x 300mm) on eBay years ago, so I know it comes in a form that would be more convenient for making into roofing or siding material. I don't know how it holds up to weather, though. According to that paper, it needs to be at least 0.5mm thick to reflect longer wavelengths, so anyone who's curious about testing it should find the thicker stuff like I did.

[1] https://www.researchgate.net/figure/Reflection-coefficient-f...

> I don't know how it holds up to weather, though.

It does very well. The main terminal of Denver International Airport has an enormous PTFE-coated fiberglass fabric roof. It's decades old now and has weathered hail, high winds, and Colorado sun without any significant wear that I know of.

EDIT:

The same product has since been used in a bunch of other stadiums and other large structures, including the Georgia Dome where it survived a near-miss by a tornado with only minor tears:

https://spinoff.nasa.gov/Spinoff2009/ip_2.html

> "The material is pound-for-pound stronger than steel while weighing less than 5 ounces per square foot. It offers up to 24 percent solar translucency while providing as much as 75 percent solar reflectance"

TLDR

But for teflon to passively cool, it would have to be strongly absorbent in the IR.

The way it works is this: the sun’s rays are at 5500 K. So you pick the set of materials that reflect light in the visible gap (the atmosphere takes care of the rest).

This keeps the material from getting heat.

Then, from this set, you pick a material that is strongly absorbent in IR.

If it absorbs in IR, itll emit there too, there y cooling itself

Specifically, it needs to absorb/emit in a frequency range called the “sky window”, where the atmosphere is transparent. In this range, the object is basically seeing the cold of space at a few Kelvin, so that almost no energy is absorbed and a lot is emitted.

The trick is to design a system where the total power absorbed across the solar irradiance spectrum is less than the power that is emitted in the sky window, so that the system is a net power sink (in this case to the tune of 100W per sq. meter or so).

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