Good selective coatings for solar absorber -- black chrome plating

A. Don't know about the theory, but it works. Black chromium, as applied from proprietary solutions is a very useful finish for solar collectors.

It should be applied over nickel (on steel or copper) to improve the corrosion protection.

A. THE BLACK CHROME IS JUST ABOUT THE ONLY FINISH THAT CAN WITHSTAND THE CONSTANT EXPANSION AND CONTRACTION OF A COPPER PLATE OR TUBING.

A. Thanks for participating, Martin. I don't think that's quite accurate though. A number of platings have outstanding adhesion; black chromium offers not just good adhesion, but high absorbtivity and low emissivity for efficient solar collection, plus (unlike most paints) no volatiles to out-gas and cloud the glass covers over the solar collectors.

Firdosh: Some references like
www.solarmirror.com/fom/fom-serve/cache/43.html
show the absorbtivity of black chrome as about 0.87 and the emissivity as about 0.09, for a very favorable 9.7 ratio.

As a slight simplification, absorbtivity means the propensity of a material to accept and absorb the radiant energy it is exposed to, as opposed to reflecting it. And emissivity is the propensity to emit its contained heat as radiant energy as opposed to retaining it. So an ideal solar collector has high absorbtivity and low emissivity. I don't pretend that I fully understand absorbtivity and emissivity, but it seems that absorbtivity is very closely tied to the color of the material, with black being best, so black chrome has good absorbtivity. Emissivity on the other hand is more closely related to the smoothness and polish of the surface. A highly polished surface has fewer atoms exposed to vibrate and release radiant energy than a sponge would have. So black chrome has the color and polish to exhibit high absorbtivity and low emissivity.

Apologies cousin Koti but we don't offer that service and I doubt that any reader will. If you ask specific questions our readers will usually try to answer them; but asking someone to assemble, package, and e-mail a research package for you has proven to be completely unrealistic. Sorry :-(

Regards,

A. Hi, Martin. I agree that black chrome plating would be the first choice. But chrome plating is brittle, so the parts ought best be plated after forming. Good luck.

Regards,

A. Hi Jesus.

I browsed a library on this topic some years ago and recall that at that time there were three good coatings: best was black chrome plating, then black oxided copper, then regular nickel chrome plating. At that time the author found no black paint that was worthwhile, not just because of optical properties for the coating itself, but because outgassing from the paint clouded the glass cover of the enclosing box. The situation probably has changed since then, with Thurmalox 250 now widely used for solar collectors; but if the collector is enclosed, don't forget to investigate the possibility of any gassing from the coating obscuring the cover glass.

The absorbtivity of Thurmalox 250 is claimed by Dampney to be 0.96, and the emissivity (depending on thickness) about 0.65.
To my limited understanding, this still doesn't compare favorably to black chrome's reputed .87 absorbtivity and .09 emissivity.

Regards,

A. You can use Chromate Conversion coatings followed by dying in black color. You can also go for Black anodizing--a very commercial process.

A. Hi. I appreciate Mr. Tahir's effort to help -- and I could be mistaken and he may know more than I do on the subject -- but I don't think dyed chromate conversion coatings or anodized coatings are good solar selective surfaces.

Alil: Proprietary black chromium plating baths like Atotech's ChromOnyx use additives like (I believe) acetic acid ⇦ this on eBay or Amazon [affil links] and fluorides, free of sulphate, to produce the black color. Still, black paint like Thurmalox 250 may be more appropriate than black chrome plating if you want to apply it yourself.

Regards,

A. Hi Peter. Hazard instruction is a subject for hands-on training. An internet forum is a one-room schoolhouse attended by kindergarten students and post-doctorate students, amateurs working on their dining room table and researchers working under lab hoods in state of the art cleanrooms, so the idea of describing "any precautions" is unfortunately unworkable.

To my understanding, black chromium plating is still the ultimate coating for solar collectors; but it cannot be done by a home owner / experimenter. At the other end of the spectrum, black paint is not useless, but a "low gassing" type like Thurmalox 250 must be selected to prevent fogging of the cover glass in order to get the most out of it. I understand, based on a book I read years ago, that blackened copper is better than paint. Copper can be blackened with liver of sulfur ⇦ this on eBay or Amazon [affil links] ; nothing is without hazard, but it is sold by arts and crafts stores, so it may be "safe enough" if used with caution and good ventilation.

A. Best coating for solar heating is black chrome.

A. I don't know quite what black aluminum oxide would actually be, Adam. Black is not the natural color of aluminum oxide to my knowledge. My understanding is that black chrome is the best collector surface, but blackened copper is very good, and most paints are not very good. Besides not having low emissivity, I understand that most paints outgas and tend to "fog" the glass cover of the enclosing box hampering efficient collection of solar radiation. But there are good books on the subject and I haven't looked at them in years, I'm afraid.

A. Hi Mahmoud. It may not be completely impossible to do black chromium electroplating in your lab, but it will probably be impractical because, yes, it does involve a lot of processes including electroplating from a toxic and carcinogenic chromic acid (hexavalent chromium) bath. But it is generally accepted these days that paints designed for the purpose are often good enough. Thumalox 250 has very high absorbtivity (comparable to black chrome or a bit better), reasonably low emissivity (although certainly not equal to black chrome). They claim it does no outgassing, and it is relatively easy and inexpensive to dispense from a spray can. Good luck.

Regards,

A. In the interest of precision on this topic, compare these absorbance values:
    flat black paint: .96 - .98
    thermalox 250: .96
    black chrome: .87

And these emmisivities:
    flat black paint: .96 - .98
    thermalox 250: .65
    black chrome: .09

Flat black paint, of a high temp variety like BBQ paint, is an excellent absorber. It's high emissivity, however, means that it re-radiates a lot of energy in the infra-red. If used at low temps, it works quite well. At high temps (large values of temp differential between absorber temp and ambient), it will radiate everything it absorbs, leaving nothing to collect.
Thermalox should mitigate this effect to a small degree.
Black chrome, while having a lower absorbance, has an emissivity 1/10 of it's absorbance. So, it's absorbing 10 times more energy than it's emitting. It's high temp performance is very good.
Newer selective coatings are electro-sputter deposited and are blue in color. Their absorbances exceed .90 and emissivities are in range of .05. They exhibit outstanding performance at high temps.