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Energy-saving coating without pigment keeps the surface beneath cool

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Re: Energy-saving coating without pigment keeps the surface beneath cool

#21

Post by londons_explore »

shanebellone wrote: post #18
londons_explore wrote: post #15

Earlier quoted context omitted.

There are benefits to all these approaches, and combining them all gives best results. But specifically on coatings, it's true that conduction and convection also play a role, and radiation is only part of the picture. However, it is theoretically possible to design a coating that can cool a material by 45C (~80F) compared to ambient at night, and practically a cooling of 4C (8F) has been achieved [1]. 4C is pretty s…

Are they attempting to generate electricity from these coatings yet? Sounds like both the "keep warm" and "keep cold" coatings offer that possibility.

Sadly it's very hard to generate much energy from small differences in temperature.

Re: Energy-saving coating without pigment keeps the surface beneath cool

#22

Post by shanebellone »

londons_explore wrote: post #21
shanebellone wrote: post #18

Earlier quoted context omitted.

Are they attempting to generate electricity from these coatings yet? Sounds like both the "keep warm" and "keep cold" coatings offer that possibility.

Sadly it's very hard to generate much energy from small differences in temperature.

"Sadly it's very hard to generate much energy from small differences in temperature."

Nighttime electric power generation at a density of 50 mW/m2 via radiative cooling of a photovoltaic cell

https://aip.scitation.org/doi/full/10.1063/5.0085205

Sure, but it does provide some interesting opportunities for embedded technology (LEDs, security cameras, Bluetooth, etc.).

Re: Energy-saving coating without pigment keeps the surface beneath cool

#23

Post by Maursault »

londons_explore wrote: post #11
hammyhavoc wrote: post #10

Earlier quoted context omitted.

Tell us more.

When you put a hot thing near a cold thing, they exchange heat and the temperatures even out. The heat moves via light - infrared light usually. Cooler things emit less light overall, and the light they do emit tends to have a longer wavelength (thats why the embers of a fire might glow red, but a hotter fire might be orange or yellow). For each color of light that something might emit or absorb, there is a 'shinynes…

> Reflective of short wavelengths, and non-reflective of long wavelengths. That will help it stay cool

Why wouldn't a surface that reflects heat stay cooler than something non-reflective that absorbs heat?

Re: Energy-saving coating without pigment keeps the surface beneath cool

#24

Post by stdbrouw »

londons_explore wrote: post #15
stdbrouw wrote: post #13

Earlier quoted context omitted.

Can a coating on the outside of a material really have an appreciable impact on heat transfer from inside to outside, at night? I get that a coating can block or admit light or radiation coming from somewhere else, but surely if the material itself is hot then thermal conduction within the solid will make the coating itself hot, and shed heat, too? Wouldn't you need some sort of "heat shield" offset from the outer wa…

There are benefits to all these approaches, and combining them all gives best results. But specifically on coatings, it's true that conduction and convection also play a role, and radiation is only part of the picture. However, it is theoretically possible to design a coating that can cool a material by 45C (~80F) compared to ambient at night, and practically a cooling of 4C (8F) has been achieved [1]. 4C is pretty s…

That's 45C "with the absence of nonradiative heat exchange" though, but you're right, that is still impressive. Thanks for explaining.

Re: Energy-saving coating without pigment keeps the surface beneath cool

#25

Post by jabl »

Maursault wrote: post #23
londons_explore wrote: post #11

Earlier quoted context omitted.

When you put a hot thing near a cold thing, they exchange heat and the temperatures even out. The heat moves via light - infrared light usually. Cooler things emit less light overall, and the light they do emit tends to have a longer wavelength (thats why the embers of a fire might glow red, but a hotter fire might be orange or yellow). For each color of light that something might emit or absorb, there is a 'shinynes…

> Reflective of short wavelengths, and non-reflective of long wavelengths. That will help it stay cool Why wouldn't a surface that reflects heat stay cooler than something non-reflective that absorbs heat?

As mentioned, non-reflective surfaces are good emitters as well as adsorbers. So the argument, AFAIUI, is that a surface which is non-reflective in the infrared will radiate away more heat during the night. Crucially including heat that has been adsorbed in other wavelengths and then stored in the material during the day.

Re: Energy-saving coating without pigment keeps the surface beneath cool

#27

Post by smusamashah »

> "A raisin's worth of structural paint is enough to cover the front and back of a door,"

> Chanda uses a Boeing 747 as an example in his team's study. To cover these planes, approximately 500 kilograms of paint is needed. Chanda estimates that using structural paint, manufacturers could cover the plane with only 1.7 kilograms.

Can someone explain how this will look like in real life? I have a hard time imagining painting something that big with such tiny supply of paint. Brushing can not possibly work, but even if its sprayed over, how can so little spread over such large area?

Re: Energy-saving coating without pigment keeps the surface beneath cool

#28

Post by goda90 »

smusamashah wrote: post #27

> "A raisin's worth of structural paint is enough to cover the front and back of a door," > Chanda uses a Boeing 747 as an example in his team's study. To cover these planes, approximately 500 kilograms of paint is needed. Chanda estimates that using structural paint, manufacturers could cover the plane with only 1.7 kilograms. Can someone explain how this will look like in real life? I have a hard time imagining pai…

I don't know, but maybe it has to do with the viscosity of the paint? If it sticks to the surface well, but not itself, then it could be easier to achieve a thinner layer.

Re: Energy-saving coating without pigment keeps the surface beneath cool

#30

Post by Bluestrike2 »

stdbrouw wrote: post #13
londons_explore wrote: post #11

Earlier quoted context omitted.

When you put a hot thing near a cold thing, they exchange heat and the temperatures even out. The heat moves via light - infrared light usually. Cooler things emit less light overall, and the light they do emit tends to have a longer wavelength (thats why the embers of a fire might glow red, but a hotter fire might be orange or yellow). For each color of light that something might emit or absorb, there is a 'shinynes…

Can a coating on the outside of a material really have an appreciable impact on heat transfer from inside to outside, at night? I get that a coating can block or admit light or radiation coming from somewhere else, but surely if the material itself is hot then thermal conduction within the solid will make the coating itself hot, and shed heat, too? Wouldn't you need some sort of "heat shield" offset from the outer wa…

> Wouldn't you need some sort of "heat shield" offset from the outer walls for your approach to work at night, if at all? Or if not, why are we still covering homes with big chunks of styrofoam, polyurethane and mineral wool?

If a home is well-built, the wall assembly already includes a physical gap between the structural sheathing or exterior insulation (if present) and whatever cladding materials you're using.[0] That gap allows for airflow and drainage behind your cladding, so that the water that gets behind it--and it will--can drain out and everything can dry.

That gap would help create something of a thermal break (the cladding isn't attached directly to the house but to furring strips with a very low surface area), but it would still heat up the air in the gap itself. In that case, reflective paints would probably still be beneficial.

That said, I think their greatest benefit would probably be in roofing. Metal roofs, for example, also require an air gap[1] behind they can get extremely hot.

0. https://buildingscience.com/documents/enclosures-that-work/h...

1. https://www.youtube.com/watch?v=yesjnBnPt0A

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