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

  1. Top | #21
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    sssivity of common paints runs around 80% , you can fact check me.

    High emisivity substances can get into the mid 90% or so. Carbon soot being high.

    In the 80s I lived west of Portland Or in the valley wth few trees. In July-August I ran a fan at night. In the morning when I left I closed te windows, taped reflective Mylar space blankets on the windows, and closed the blinds.

    When I got home the temperature was comfortable without air conditioning..


    It is an old question. I don't think color has much effect on temperature rise.

    I watced a show about people who built passive homes in the desert in Arizona. Done right he temperture stays fairly constant inside. Large thermal mass structure that cools at night and rises slowly during the day.

    There are many passive techniques.

    You could build a space at a a wall, like a cmney hat would create convective updrafts.

    Cover your house in mirrors.......

  2. Top | #22
    Loony Running The Asylum ZiprHead's Avatar
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    There's a house in Switzerland/Sweden (?) covered in mirrors to no ruin the views.

    When conservatives realize they cannot win democratically, they will not abandon conservatism. They will abandon democracy.

    Poverty exists not because we cannot feed the poor but because we can't satisfy the rich.

  3. Top | #23
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    Quote Originally Posted by skepticalbip View Post
    Quote Originally Posted by barbos View Post
    Quote Originally Posted by steve_bank View Post
    So a house in space will not heat up from solar radiation?

    Depends what's underneath of paint. If there is a layer of good insulation then color of the roof will have no effect.
    Of course having white surface reduces requirements on insulation.
    Temperature control in space is different than on Earth. On Earth most heat transfer is through conduction and convection and not so much through radiation. In space the only method of transferring heat is through radiation.

    The black body radiation curve is also an absorption curve so an object in space will reach thermal equilibrium at the point where its absorption equals its radiation. So (assuming solar radiation is the only heat source) the black body radiation curve tells us that a black object in space will reach thermal equilibrium at a lower temperature than a white or silvered object. Having only lived on Earth in a dense atmosphere, this is rather counter intuitive.
    If this were right wouldn’t space suits (and spacecraft) be black?

  4. Top | #24
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    Quote Originally Posted by ZiprHead View Post
    There's a house in Switzerland/Sweden (?) covered in mirrors to no ruin the views.

    It gives me pause for reelection....People in mirror houses should not piss of neighbors.

  5. Top | #25
    Veteran Member Treedbear's Avatar
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    Quote Originally Posted by ZiprHead View Post
    There's a house in Switzerland/Sweden (?) covered in mirrors to no ruin the views.

    Must take a toll on the local birds.

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    Quote Originally Posted by skepticalbip View Post
    Quote Originally Posted by barbos View Post
    Quote Originally Posted by steve_bank View Post
    So a house in space will not heat up from solar radiation?

    Depends what's underneath of paint. If there is a layer of good insulation then color of the roof will have no effect.
    Of course having white surface reduces requirements on insulation.
    Temperature control in space is different than on Earth. On Earth most heat transfer is through conduction and convection and not so much through radiation. In space the only method of transferring heat is through radiation.

    The black body radiation curve is also an absorption curve so an object in space will reach thermal equilibrium at the point where its absorption equals its radiation. So (assuming solar radiation is the only heat source) the black body radiation curve tells us that a black object in space will reach thermal equilibrium at a lower temperature than a white or silvered object.
    Are you positive this is not a misinterpetation of Kirchhoff's law of thermal radiation? Most materials do not emit and absorb energy equally well at all wavelengths/temperatures. A material that appears black in the visible spectrum will absorb much sunlight (since sunlight has a maximum in the visible spectrum), but since it emits in all directions, not just in the direction of the sun, it's own temperature will stay well below the sun, and thus the maximum of its own emissions will be at much longer wavelengths. An object that is a good absorber and emitter at visible wavelengths may well be a poor emiter in the infrared range, and conversely a material that reflects most light in the visible range may be "black" in the infrared, and thus be a good emitter in the range of its actual equilibrium temperature. Which entails that such a material has a lower, not higher equilibrium temperature.

    If what you said were true, I'd furthermore expect a negative feedback from glaciation on cooling. I believe it is a firmly established principle of climatology that glaciation/increased snowfall during a cooling period creates a positive feedback loop.

  7. Top | #27
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    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.

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    The Wikipedia page on 'Planetary Equilibrium Temperature' sums it up well (https://en.wikipedia.org/wiki/Planet...um_temperature).

    In short, an object in equilibrium with energy input form the Sun will reach a temperature that scales with the fourth root of (1 - albedo). That is, an object that reflects more light reaches a lower temperature than an object that absorbs more light.

    It is true that a black object is a more efficient radiator than a white object, a fact that I think is the source of the confusion, but the black object will radiate at a higher equilibrium temperature.

    This is why space craft and space suits are white and/or covered in reflective mylar. Otherwise they can heat up a lot in the sun, without convection or conduction to remove heat.

  9. Top | #29
    Contributor skepticalbip's Avatar
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    Quote Originally Posted by Shadowy Man View Post
    The Wikipedia page on 'Planetary Equilibrium Temperature' sums it up well (https://en.wikipedia.org/wiki/Planet...um_temperature).

    In short, an object in equilibrium with energy input form the Sun will reach a temperature that scales with the fourth root of (1 - albedo). That is, an object that reflects more light reaches a lower temperature than an object that absorbs more light.
    Absorption is only half the concern when determining what the temperature will be. Thermal equilibrium is reached when energy absorbed equals energy radiated. Venus has the highest albedo of any planet in the solar system at 0.75. Venus being closer to the Sun so receiving more solar flux is part of the reason for its high temperature but mostly it is the extremely high albedo and atmosphere that accounts for the high temperature. OTOH, our moon has an albedo of 0.12 so has an equilibrium temperature of 271K (~28F) given its distance from the Sun but, with no atmosphere, short term temperatures swing from ~ +/- 200F.
    It is true that a black object is a more efficient radiator than a white object, a fact that I think is the source of the confusion, but the black object will radiate at a higher equilibrium temperature.

    This is why space craft and space suits are white and/or covered in reflective mylar. Otherwise they can heat up a lot in the sun, without convection or conduction to remove heat.
    Control of temperature on space crafts deals with more than a concern for overheating. It also is concerned with staying warm enough. Apparently, overheating is an easier concern to deal with than being too cold. Skylab used a tarp to shade the lab when it got too hot. The space shuttle opened its cargo bay doors to radiate away more heat. If the problem had been that they were getting too cold then they would have needed to use power to generate heat internally.

  10. Top | #30
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    Quote Originally Posted by skepticalbip View Post
    Quote Originally Posted by Shadowy Man View Post
    The Wikipedia page on 'Planetary Equilibrium Temperature' sums it up well (https://en.wikipedia.org/wiki/Planet...um_temperature).

    In short, an object in equilibrium with energy input form the Sun will reach a temperature that scales with the fourth root of (1 - albedo). That is, an object that reflects more light reaches a lower temperature than an object that absorbs more light.
    Absorption is only half the concern when determining what the temperature will be. Thermal equilibrium is reached when energy absorbed equals energy radiated. Venus has the highest albedo of any planet in the solar system at 0.75. Venus being closer to the Sun so receiving more solar flux is part of the reason for its high temperature but mostly it is the extremely high albedo and atmosphere that accounts for the high temperature.
    I'm pretty sure it's just the atmosphere. The high albedo actually works in the opposite direction, but it's effect is dwarfed by an atmosphere of mostly CO2 that's many times more massive than Earth's.
    OTOH, our moon has an albedo of 0.12 so has an equilibrium temperature of 271K (~28F) given its distance from the Sun but, with no atmosphere, short term temperatures swing from ~ +/- 200F.
    Earth has a higher albedo than the moon and the exact same average distance from the Sun.

    Earth, without a greenhouse effect, has an equilibrium temperature of 255 K, significantly colder than the moon.

    It seems to me that either Kirchhoff is wrong, or you're misunderstanding him.

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