anodized aluminum heat absorption properties.

[Dylan]

would a black anodized water tube absorb more heat energy then a raw aluminum water tube in the sun?

What if it was in a glass vacuum?

I've heard a lot of ideas:

the thick oxide layer lowers conduction???

the black body radiation cancels out advantage???

the increased surface area increases convection???

[Jasper Nance]

Absolutely. Paint would also work. Aluminum is quite reflective to IR, so you're not really getting much absorption. 

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Jasper Nance - KE7PHI
Creative and Scientific Imagery
http://www.nebarnix.com/

[Dylan]

do you mean untreated al is reflective to IR or its reflective to ir no matter what.

looks like 1/2 of the sun light energy is in the visible range / half in ir.

[Jasper Nance]

Its shiny in the visible range too ;)

On Sun, Jun 10, 2012 at 5:09 PM, Dylan <dylan....@gmail.com> wrote:

do you mean untreated al is reflective to IR or its reflective to ir no matter what.

looks like 1/2 of the sun light energy is in the visible range / half in ir.

[Aaron Hicks]

Another option would be a composite sheath. Quick-and-dirty: mix 1-hour epoxy, add in a generous quantity of powdered carbon black, roll tube in epoxy and coat as thinly as possible.

-AJ

[Cameron K.]

For this you just need to look at the band structure and reflectivity spectra.

http://en.m.wikipedia.org/wiki/File:Image-Metal-reflectance.png

That's for polished metallic aluminum. An oxide layer will have a different band structure than metal aluminum, as will anodized layers, so that layer will absorb some energy, but then it will reflect back from the metal layer underneath. You will want to compare the absorbance distance (I/I0=0.99) of the oxide and the thickness of the anodization layer. 

Maybe what I said doesn't make any sense for your application...

Cameron Kopas

[Jasper Nance]

Anodized layers (esp with dye) absorb IR VERY well -- just look at the laser etching performance. I don't think much would get through to the metal underneath,and it would be absorbed on its way back up anyway. 

Still, anodizing long sections yourself sounds like a nightmare

[Dylan]

yeah I agree, the lengths are about 4 foot.

Another option I could try id a black matte powder coat.

[Will Bradley]

Can you use another material like steel or iron? Forged plumbing pipes are naturally black.

For surface area, wouldn't you want somewhat of a flat or heatsink design? Assuming you're gathering IR energy from a single direction like the sun, a tube shape is ~50% inefficient.

There's a local powder coating company with pretty good prices near the airport.

[Dylan]

Yeah I'm thinking of a solar heat capture system. Closed loop so I could use oil if needed.

I've never seen black cast iron pipes for sale. I've seen them in old buildings...

as for the round issue. Because the sun tracks across the sky a round collectors always have one face to the sun.

But yes a flat heloistat would be better.