h a l f b a k e r yWhy not imagine it in a way that works?
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Black paint emits heat much more than white paint, and white paint reflects heat much more than black paint. Thermochromic paint changes colour depending on heat... So why not paint houses in a specialized mixture, so when it gets too hot, the south-facing sides of a house turn white, and the north-facing
sides of a house turn black. And vice versa (and vice versa again, if you're in the southern hemisphere).
May not save *that* much on heating/cooling bills, but would be funky to look at.
Example of thermochromic paint
http://gathering.tw...ew_message/31830667
In this project, they've put thermochromic paint onto a PCB - when current travels through the wires, it heats up slightly and changes the colour of the paint. Goes from black to white in about 3 seconds. [mitxela, Sep 15 2009]
Chameleons do this!
http://www.bbc.co.u...s/p0057tcr#p004rqs8
Impressive ability to use their camouflage for thermoregulation. (possibly only viewable in the uk?) [mitxela, Dec 03 2011]
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Could be cool... or hot I reckon... depending on the need. You might want to extend this thought to roofs as well. Just sayin'... |
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Let me know when you invent this top-secret paint and I'll be sure to pick up 5 or 10 gallons. (+) |
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If you come up with this 'magic' formula, you could even develop other formulas for it to blend in with its environment. Perfect to camoflauge the house, or for hunters and the like... |
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I wouldn't think paint but rather shape changing
surfaces. |
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You've got my bun, but ougn't this idea be in product:paint ? |
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the "chromic" part doesn't have to be in the visible light spectrum... it's IR that causes most heating. |
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to get a noticeable effect, you would have to have a thermal link between the interior space and the exterior paint - this is what most insulation is trying to avoid... |
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Thermochromic paint is already quite diverse in what it can do, I see no reason why this couldn't be done with today's technology, other than perhaps cost. |
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Thinking about it, it may be better to coat the whole house in e-paper and have a thermostat inside which can switch the outside colour from white to black almost instantly. That would easily be doable. You'd also probably be able to display e-books on the walls too, as an added bonus. |
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+- If the color change is able to homeostatically modify the rate of temperature change of the substrate, (Though I am not sure how much energy can be regulated this way) it would also decrease the continual expansion and contraction of the wood. In the US south, this would lead to longer paint jobs, decreased exterior moisture damage, decreased board splitting, caulking etc. |
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//Black paint emits heat much more than white paint, and white paint reflects heat much more than black paint.// Not so. At the temperatures where the emissivities of black paint and white paint are very different, both paints burst into flames. At ordinary building surface temperatures they have fairly similar emissivities (a bit less than 1), as do most non-metallic surfaces. |
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White paint reflects visible (obviously) and near infrared radiation much better than black paint, but those wavelengths correspond (obviously) to the temperature at the surface of the sun, not the surface of the house. |
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I think the mixup is that black paint may absorb more from the underlying strata (ie: the surface which is painted), therefore having more heat to radiate. |
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It must be worthwhile if chameleons do it! (linked video shows one warming up by turning the side of its body facing the sun black and the side facing away white) |
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^ Of what use is the white side ? |
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Typical emissivities for white paint at 300K are around 0.9 to 0.96. Since emissivity cannot be greater than 1, it's impossible for anything to emit heat much more than white paint. |
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Except at night, white exterior surfaces will almost always be cooler than black. Even walls not in direct sunlight are heated by scattered light. |
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Perhaps in highly directional morning/evening light in deserts a dark shade on the non-sun side increases cooling, and a light shade reduces cooling; the difference would be slight, but a chameleon takes what it can. |
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emissivity via radiation is negligible compared to convection and conduction. |
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So instead of thinking whether black or white paint will radiate more heat, think of which one would ABSORB more heat. Obviously black will. |
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So in the summer you would have your house totally white to reflect the sun's rays. In the winter you would have your house black to absorb solar heat. |
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emissivity will be outstripped by absorption during the day. |
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As mentioned you might be able to "tune" the color of sun facing and non sun facing walls such that the surface temperature was uniform decreasing thermal seperation and cracking. |
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//emissivity via radiation is negligible compared to convection and conduction.// |
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Firstly, emissivity is a property of a material, and has a value between 0 and 1 (by definition). I assume you mean emission via radiation. |
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Secondly, a black body emits 459 W/m² at a mild 300K, and 1098 W/m² at 370K (about as hot as a roof in could be expected to get). Even factoring in radiative exchange with the surroundings (small for a roof, but significant for walls) and emissivities, a modest house could easily radiate hundreds of kilowatts. That's hardly insignificant! |
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