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So... black latex paint? Asphalt? Water? http://en.wikipedia.org/wiki/File:Albedo-e_hg.svg http://en.wikipedia.org/wiki/File:Albedo-e_hg.svg The advance is

by mapt 12y ago

So... black latex paint? Asphalt? Water? http://en.wikipedia.org/wiki/File:Albedo-e_hg.svg http://en.wikipedia.org/wiki/File:Albedo-e_hg.svg The advance is not that they "reached 90%", that's absolutely trivial, and nothing to do with a "Solar Power Material". They seem to be claiming that their advance is a black material they can paint onto thermal pipes that is durable at 1000K for an extended period of time in Earth's atmosphere, made out of blends of nanoparticles. "High temperature black paint created that lasts 5-10 years instead of 1 year, reducing maintenance needs for concentrating solar thermal" would be a more honest title.

swamp40 12y ago
It's probably the same material they use to make park slides. If the sun's out, they are usually too hot for the kids to slide on...
nv1729 12y ago
Are you sure? From http://en.wikipedia.org/wiki/Solar_cell_efficiency#mediaviewer/File:PVeff(rev141208).jpg http://en.wikipedia.org/wiki/Solar_cell_efficiency#mediaview... it seems theres a cap at 45%
- edmccard 12y ago
  >it seems theres a cap at 45% That number is for photovoltaic cells converting light to electricity; the 90% in the article is for converting light to heat.
kragen 12y ago
it's actually rather surprising, and important, that they were able to get 90 percent solar absorptivity combined with 30 percent infrared emissivity. this is difficult because absorptivity is the same thing as emissivity, but of course you can have different emissivities at different wavelengths; that's why things are different colors. but the vast majority of opaque solid materials have rather pastel colors, which is to say that their absorptivity (and thus emissivity) varies slowly with wavelength. this is undesirable for concentrating solar power, because it makes it hard to transfer the heat energy from the sunlight to the coolant rather than reradiating it as infrared. all the materials you named with albedos of under .1 are also great absorbers of infrared, which means they're also great emitters of infrared, so they wouldn't work very well for this application. (you also happened to pick three that won't withstand kilokelvin temperatures.) there are a number of 'spectrally selective coatings' already in existence with this unusual combination of emissivities, but they are all very expensive to produce. the longer lifetime is useful too, but not so useful they bothered to mention it in the abstract of their paper. the small particles mentioned are made of a silicon-germanium blend, although they claim you can use different semiconductors for this. they picked that blend because its bandgap is about a volt, so it absorbs photons of more than about an electron volt.
- rab_oof 12y ago
  The latter: would that be "structural absorbtion" related to structural color?
  - kragen 12y ago
    i don't know! i didn't know about structural coloration, https://en.wikipedia.org/wiki/Structural_coloration https://en.wikipedia.org/wiki/Structural_coloration. the crude absorption is not structural (it corresponds to a bandgap in the semiconductor), but clearly they think the structure is important. i didn't read the paper closely enough to know why.
Animats 12y ago
A direct comparison with off-the-shelf high-temp black coatings would have been useful. Here's a list of some known values for black coatings for solar collectors. (http://www.solarmirror.com/fom/fom-serve/cache/43.html http://www.solarmirror.com/fom/fom-serve/cache/43.html) Black chromium plating (absorption 0.87, emission 0.09) is often used for this purpose. It can handle the temperature, tolerates thermal expansion, and, like most hard chrome plating, is a hard plated surface that can be cleaned aggressively, as with pressure washing. There are other black coatings with up to 0.99 absorption, but they aren't as tough. So what this new work has done is improve performance from 0.87 to 0.90. (Why are so many "nanotechnology" articles like this? The title and lede sound like it's some earth-shaking development, and then it turns out it's at best a minor improvement.)

by mapt 12y ago

swamp40 12y ago

It's probably the same material they use to make park slides. If the sun's out, they are usually too hot for the kids to slide on...

nv1729 12y ago

Are you sure? From http://en.wikipedia.org/wiki/Solar_cell_efficiency#mediaviewer/File:PVeff(rev141208).jpg http://en.wikipedia.org/wiki/Solar_cell_efficiency#mediaview... it seems theres a cap at 45%

edmccard 12y ago
>it seems theres a cap at 45% That number is for photovoltaic cells converting light to electricity; the 90% in the article is for converting light to heat.

kragen 12y ago

it's actually rather surprising, and important, that they were able to get 90 percent solar absorptivity combined with 30 percent infrared emissivity. this is difficult because absorptivity is the same thing as emissivity, but of course you can have different emissivities at different wavelengths; that's why things are different colors. but the vast majority of opaque solid materials have rather pastel colors, which is to say that their absorptivity (and thus emissivity) varies slowly with wavelength. this is undesirable for concentrating solar power, because it makes it hard to transfer the heat energy from the sunlight to the coolant rather than reradiating it as infrared. all the materials you named with albedos of under .1 are also great absorbers of infrared, which means they're also great emitters of infrared, so they wouldn't work very well for this application. (you also happened to pick three that won't withstand kilokelvin temperatures.) there are a number of 'spectrally selective coatings' already in existence with this unusual combination of emissivities, but they are all very expensive to produce. the longer lifetime is useful too, but not so useful they bothered to mention it in the abstract of their paper. the small particles mentioned are made of a silicon-germanium blend, although they claim you can use different semiconductors for this. they picked that blend because its bandgap is about a volt, so it absorbs photons of more than about an electron volt.

rab_oof 12y ago
The latter: would that be "structural absorbtion" related to structural color?
- kragen 12y ago
  i don't know! i didn't know about structural coloration, https://en.wikipedia.org/wiki/Structural_coloration https://en.wikipedia.org/wiki/Structural_coloration. the crude absorption is not structural (it corresponds to a bandgap in the semiconductor), but clearly they think the structure is important. i didn't read the paper closely enough to know why.

Animats 12y ago

A direct comparison with off-the-shelf high-temp black coatings would have been useful. Here's a list of some known values for black coatings for solar collectors. (http://www.solarmirror.com/fom/fom-serve/cache/43.html http://www.solarmirror.com/fom/fom-serve/cache/43.html) Black chromium plating (absorption 0.87, emission 0.09) is often used for this purpose. It can handle the temperature, tolerates thermal expansion, and, like most hard chrome plating, is a hard plated surface that can be cleaned aggressively, as with pressure washing. There are other black coatings with up to 0.99 absorption, but they aren't as tough. So what this new work has done is improve performance from 0.87 to 0.90. (Why are so many "nanotechnology" articles like this? The title and lede sound like it's some earth-shaking development, and then it turns out it's at best a minor improvement.)