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'Whitest ever' paint reflects 98% of sunlight

bbc.com

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Re: 'Whitest ever' paint reflects 98% of sunlight

#101

Post by quercusa »

beauzero wrote: post #38
ebiester wrote: post #22

Earlier quoted context omitted.

What we really need is white paint in the summer, black paint in the winter - reversible roofs, if you will. That said, in the desert southwest and many places in the world, white roofs are common. I wish it were more common in the southeast...

If you look at old architecture in rural Georgia and Alabama. Houses are on rocks raised above the ground (air circulates underneath), 10 ft ceilings, a huge attic fan to pull cool air in in the evening, tall trees planted next to the house for shade (we have 4 pecan trees), and reflective tin roofs (if still shiny). This helped significantly to cooling in the summer. Winters aren't cold enough to warrant more than a…

Wide eaves help as well to keep summer sun off windows and to provide a place for a porch. A gentle rain on a tin roof is a sure ticket to sleep; a strong one not so much.

I'd never thought about ceiling height - that makes a lot of sense.

Re: 'Whitest ever' paint reflects 98% of sunlight

#102

Post by Arnt »

buran77 wrote: post #44
Arnt wrote: post #29

Earlier quoted context omitted.

Adding: Mirrors don't have to reflect very much, they have to reflect as a picture. That's why one-way mirrors work — people generally don't notice that the light mirrored is much less than 100%. White paint is free to reflect the photons in any direction, so that >98% is a harsher requirement on the share of incoming photons, but a laxer requirement on the photons' direction.

Some mirrors can reflect a lot more light even if in narrower wavelength ranges. But that only makes the matters worse from a practical human perspective, when trying to use them on large surfaces outside. It would feel like staring into the Sun far too often.

Which mirrors are that, and why are they made, what are they used for? Just giant telescopes on remote mountaintops, or is there more?

Re: 'Whitest ever' paint reflects 98% of sunlight

#103

Post by JackFr »

nerdponx wrote: post #43
purple_ferret wrote: post #31

Earlier quoted context omitted.

NYC roofs are definitely painted with reflective paint. They're usually silver-ish though.

Disappointingly few of them, still. It's always depressing to fly over an area and see nothing but black rooftops, devoid of anything but HVAC equipment. I know green roofs have engineering issues but I hope one day I will instead look out my airplane window and see a sea of solar panels, small wind turbines, and rooftop gardens.

The thing is, tar is an inexpensive and excellent sealant.

Re: 'Whitest ever' paint reflects 98% of sunlight

#104

Post by divbzero »

cyberlab wrote: post #4

Someone needs to compile a list of all this fringe tech that I see popping up in my feeds, almost daily now. I know there are techpress blogs that pump out articles like this, but there doesn't seem to be an authoritative compendium of such tech anywhere on the net. I could curate this by hand with 3 hours of free time and some Red Bull I suppose. The reason I want this is because we seem to be swimming in innovation…

Wikipedia’s “List of emerging technologies” [1] provides a glimpse of the breadth of our ongoing innovations.

[1]: https://en.m.wikipedia.org/wiki/List_of_emerging_technologie...

Re: 'Whitest ever' paint reflects 98% of sunlight

#105

Post by analog31 »

appleflaxen wrote: post #76
mrob wrote: post #19

Earlier quoted context omitted.

A mirror reflects with specular reflection. When parallel rays of light hit a flat mirror they remain parallel after being reflected: https://en.wikipedia.org/wiki/Specular_reflection Paint reflects primarily with diffuse reflection. The rays of light are scattered in all directions: https://en.wikipedia.org/wiki/Diffuse_reflection

That's an awesome learning point; thank you. What percent of the light does a mirror reflect in a specular way?

Like the design of paint, the design of a mirror can affect its reflectivity. For an extreme example:

https://www.semrock.com/maxmirrors.aspx

Mirrors that are a cheaper coating on glass or metal can be in the 95% range without too much effort. If you don't care about actually forming an image with a mirror, it can be quite cheap, e.g., plain aluminum (with whatever oxide forms on its surface) or aluminized mylar film.

Re: 'Whitest ever' paint reflects 98% of sunlight

#106

Post by buran77 »

Arnt wrote: post #102
buran77 wrote: post #44

Earlier quoted context omitted.

Some mirrors can reflect a lot more light even if in narrower wavelength ranges. But that only makes the matters worse from a practical human perspective, when trying to use them on large surfaces outside. It would feel like staring into the Sun far too often.

Which mirrors are that, and why are they made, what are they used for? Just giant telescopes on remote mountaintops, or is there more?

Lasers! :)

I'm talking about dielectric mirrors. Wikipedia suggests 99.999% or better reflectivity over a narrow range of wavelengths.

Re: 'Whitest ever' paint reflects 98% of sunlight

#108

Post by contemporary343 »

To claim this is the whitest ever is dubious. First, things like spectralon have existed for a while. Second, a team at UCLA already demonstrated a barium sulfate paint last year: https://www.intelligentliving.co/amp/ultra-white-paint-refle...

I’m sure there’s some good incremental improvements here but this is over the top marketing.

Re: 'Whitest ever' paint reflects 98% of sunlight

#110

Post by adrian_b »

adrian_b wrote: post #48
nvahalik wrote: post #27

Yeah it’s white but does dust and debris still stick to it? If it does, it’ll only be really effective until the next cleaning... and then probably just as good as plain white paint.

They said that they have used an acrylic binder, so this white paint should behave like any other acrylic paint, so it should need periodic cleaning. What is novel is that they have discovered some new method for preparing the barium sulfate pigment and mixing it with the binder, which allows very small pigment particles, possibly having a certain form, and also a very large ratio between the quantities of pigment an…

Now I have read an earlier version of this paper, which is freely available:

https://arxiv.org/abs/2011.01161

The main points are:

1. The more common white pigment titanium white is not good for cooling something exposed to sun, because it strongly absorbs ultraviolet light. (Besides generating heat, this ultraviolet absorption also causes photo-chemical reactions that degrade in time the organic paint binders.)

2. So they have chosen barium sulfate, which reflects not only visible light, but also ultraviolet and near infrared.

3. Barium sulfate not only reflects solar light, but it also emits efficiently in the far infrared, which cools the painted surface.

4. Barium sulfate was traditionally considered a worse white pigment, because it has a lower refractive index than other pigments, like titanium white.

5. The Purdue University team has overcome this disadvantage of barium sulfate by designing a manufacturing process for the pigment that not only ensures a very small average pigment particle size but also a broad distribution of the particle sizes, which results in good reflection properties over the entire solar spectrum.

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