# Thread: This Ultra-White Paint May Someday Replace Air Conditioning

1. Originally Posted by skepticalbip
Originally Posted by Loren Pechtel
Originally Posted by Worldtraveller
There are a few misconceptions about color and the effect it has on temperatures. (Background/disclaimer: I am an aerospace engineer, and thermodynamics was my favorite topic in college and what I studied the most, but I wound up working in a different field.)

The equilibrium temperature is independent of the color or reflectivity, of the material. If I recall, and a quick skim of Kirchoff's law again makes me think I'm remembering correctly, the material itself is barely affected by the color, and the emission characteristics are mostly dependent on the material.

However, the heating rate can be varied in a material or surface by the color and condition of the surface coating. Since a home, or spacecraft isn't left to sit in the sun indefinitely, the color of the roof on a house, or the color of the radiators on a spacecraft have a significant effect on the heat cycles it experiences.

So given enough time with no change in conditions, the color doesn't matter. Given a continuous heating/cooling cycle, the color makes anywhere from a nominal, to a significant difference.
Yup--observe the popularity of white cars in desert climates.

As for space use--there are two more factors at work:

1) In many cases you can control the color. Make a craft that's white on one side, black on the other. Point the white to the sun, it's cooler. Point the black to the sun, it's warmer.

2) Space also has the issue that it's harder to get rid of heat.
Getting rid of heat in space is easy and free with a ship designed with that in mind. Creating heat is expensive as it requires power, a precious resource in space.
Look up the cooling system on the ISS and you get an estimate for kw/m^2 of radiator area.

A ST scale system would likely be impossible. The aerospace guy can weigh in. I read that the SST could not stay up unless the cargo bay doors opened exposing the radiators.

2. Originally Posted by skepticalbip
Originally Posted by Loren Pechtel
Originally Posted by Worldtraveller
There are a few misconceptions about color and the effect it has on temperatures. (Background/disclaimer: I am an aerospace engineer, and thermodynamics was my favorite topic in college and what I studied the most, but I wound up working in a different field.)

The equilibrium temperature is independent of the color or reflectivity, of the material. If I recall, and a quick skim of Kirchoff's law again makes me think I'm remembering correctly, the material itself is barely affected by the color, and the emission characteristics are mostly dependent on the material.

However, the heating rate can be varied in a material or surface by the color and condition of the surface coating. Since a home, or spacecraft isn't left to sit in the sun indefinitely, the color of the roof on a house, or the color of the radiators on a spacecraft have a significant effect on the heat cycles it experiences.

So given enough time with no change in conditions, the color doesn't matter. Given a continuous heating/cooling cycle, the color makes anywhere from a nominal, to a significant difference.
Yup--observe the popularity of white cars in desert climates.

As for space use--there are two more factors at work:

1) In many cases you can control the color. Make a craft that's white on one side, black on the other. Point the white to the sun, it's cooler. Point the black to the sun, it's warmer.

2) Space also has the issue that it's harder to get rid of heat.
Getting rid of heat in space is easy and free with a ship designed with that in mind. Creating heat is expensive as it requires power, a precious resource in space.
Heat is easy to get rid of if you don't care how heavy your spacecraft is.

3. Originally Posted by steve_bank
Originally Posted by skepticalbip
Getting rid of heat in space is easy and free with a ship designed with that in mind. Creating heat is expensive as it requires power, a precious resource in space.
Look up the cooling system on the ISS and you get an estimate for kw/m^2 of radiator area.

A ST scale system would likely be impossible. The aerospace guy can weigh in. I read that the SST could not stay up unless the cargo bay doors opened exposing the radiators.
Yup. If the doors weren't open it had to use water for cooling--and it couldn't do that for too long before it ran out.

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