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Good selective coatings for solar absorber -- black chrome plating

Q. Black Chrome Plating is supposed to be useful for Solar Applications but could anybody explain how a black coating can have good absorption and poor emissivity, kind of goes against what we all learned about blackbody radiation. Can Black Chrome Plating be done on all substrates or does it have to be done on a particular base metal?

Thanks in advance.

Firdosh S [last name deleted for privacy by Editor]
- Bangalore, India


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.

tom & pooky   toms signature
Tom Pullizzi
Falls Township, Pennsylvania


MARTIN M [last name deleted for privacy by Editor]
- NORTHVILLE, Michigan

"Solar power Your Home"

on AbeBooks

or Amazon

(affil links)

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
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.

Ted Mooney,
Ted Mooney, P.E.
Striving to live Aloha - Pine Beach, New Jersey

Multiple threads were merged: please forgive repetition, chronology errors, or disrespect towards other postings [they weren't on the same page] :-)

Q. Sir,

I am doing in Energy System Engg. I found your information regarding selective & non-selective surfaces for solar collectors very useful in presenting a seminar. So please send more information regarding selective & non selective surfaces of solar collectors, its properties, coatings, absorbers, methods of coatings, procedures and its life.

Thanking you sir,

Koti Prakash j.l
- Hospet, Karnataka, India

thumbs up signApologies 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 :-(


Ted Mooney,
Ted Mooney, P.E.
Striving to live Aloha - Pine Beach, New Jersey

RFQ: I need to have metal sheet plated (or perhaps treated) to yield a reasonable performance solar collecting surface that would remain stable for a few years if the surface is periodically heated to 300 °C in air. Probably the black chrome platings described in the literature are more than adequate. I don't believe any of the spray-on paint type coatings are stable enough. My effort is investigative (applications: heat sterilization, solar cooking, others), and an initial order would be small.

What is specifically needed: Quantity: 4 Single sided coating with above described surface on: Sheets of metal 24 in. X 24 in. X .030 in. (approx. dimensions) Material: steel, copper, or aluminum (choice of metal is not very important, although if my effort results in production, the cost of copper would probably be to great. One additional specification: The sheet metal is to be rolled into a cylindrical/conical shape, thus putting the coating under tension. The coating has to be able to survive such a forming process. Possibly this means that the metal has to be first formed to its (nearly) final shape, and then coated.

Martin G [last name deleted for privacy by Editor]
inventing solar collector - Dearborn, Michigan, USA

Ed. note: Sorry, this RFQ is old & outdated, so contact info is no longer available. However, if you feel that something technical should be said in reply, please post it; no public commercial suggestions please ( huh? why?)

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.


Ted Mooney,
Ted Mooney, P.E.
Striving to live Aloha - Pine Beach, New Jersey

Q. I am fabricating flat plate solar water heaters. I need a selective coating to apply to the plate, to absorb as much energy and emit as little energy as possible, and to withstand temps up to 220° F. Need a black matte selective coating. I know Thurmalox was a brand to use but I don't find any more info specific to this application. Please help with your info in this.

Thank you,

Jesus Bermudez
Varnier - San Diego, California, USA

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.


Ted Mooney,
Ted Mooney, P.E.
Striving to live Aloha - Pine Beach, New Jersey


RFQ: I need 15 square meters of solar selective (high visible absorbance, low infrared emissivity) black chrome coated copper sheet of the type used in solar water heaters- thickness between .004" to .030". I would prefer a minimum sheet dimension of 20", but narrower strips could be acceptable. This material is needed for development of a solar heater. Please quote, stating specifications of the material you can supply.

Martin G [last name deleted for privacy by Editor]
inventor - Dearborn, Michigan, USA

Ed. note: Sorry, this RFQ is old & outdated, so contact info is no longer available. However, if you feel that something technical should be said in reply, please post it; no public commercial suggestions please ( huh? why?)

Q. I am a student in the National University of sciences and technology Pakistan. I am doing a project on solar water heating. We have looked for solar panels but could not find one. I needed help in making one of our own. We have a well equipped science lab at the university. Can you guide me on how to coat a metal with black Chromium or any other suitable selective coating for my purposes .

Alil Sheikh
Student(doing a project on solar energy) - Islamabad, Punjab, Pakistan

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

Engr. M N Tahir
- Islamabad, Pakistan

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 [on eBay or Amazon] 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.


Ted Mooney,
Ted Mooney, P.E.
Striving to live Aloha - Pine Beach, New Jersey

Q. I am a PhD student working on solar selective surfaces. Just want to get in contact with people, institutions and organizations working in this field of study. Want to know latest development in solar selective surfaces. Also interested to know the history of solar selective surfaces and leading manufacturers of these surfaces. Interested to know the related magazines and books on the subject. Anyone else involve in the study of solar selective surfaces.I am also interested to know about its application on solar thermal systems. Manufacturing techniques of solar selective surfaces.

Sana Ullah
Student- PhD - Islamabad, Pakistan

Q. I am keenly interested in the these selective solar absorbers and would like to know more about them.

Ngengere Ngari
- Nairobi, Kenya

RFQ: In approximately 1980, a friend and I built a flat roof mounted water solar system on a roof 30'(N/S) x 60'(E/W) consisting of 14 - 4'x 8' copper "selective surface" collectors purchased from Phelps Dodge Copper Co. This building was a laundromat, and had a 3,000 gal insulated storage tank in the building. It was a total drain back system, thus no anti-freeze was ever needed even in cold temperature extremes (-30 F). An efficient heat exchanger allowed us to heat all incoming city water to above 120° for up to 4 weeks at a time in the summer without using any additional heat source, and pre-heated water in winter to between 70° and 90°, even on cold sunny days in January. The whole array (14) collector insulated boxes were built as one unit and faced south, in summer at a 35 degree angle with the roof, and in winter were raised to a 65 degree angle. We were located 17 miles north of the 45th parallel in Northeastern Wisconsin.

I now live in New Ulm MN, and have a laundromat which I would like to do the same thing with, but cannot seem to find the same type of collector panels to build a similar system. My thanks in advance for any help in locating a solar supply company that I can buy individual components and build my own system once again.

Norman E [last name deleted for privacy by Editor]
Laundromat Owner - New Ulm, Minnesota, USA

Ed. note: Sorry, this RFQ is old & outdated, so contact info is no longer available. However, if you feel that something technical should be said in reply, please post it; no public commercial suggestions please ( huh? why?)

Q. There are many possible selective coatings.
For solar thermal collectors, are there any processes or products that let the home owner / experimenter do the coating? Please describe the process and any precautions to take. Are there generally available services for such coatings?

Peter P Nuspl
solar design, installation - Bumpass, Virginia


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 [on eBay or Amazon] ; 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.

Ted Mooney,
Ted Mooney, P.E.
Striving to live Aloha - Pine Beach, New Jersey

Q. Hi all,

I am looking for selective coating for the Solar Panel Absorber (Aluminum or copper sheets). What kinds and where can I get them?

It is a solar panel with copper tubing inside for solar water heating.


Hobbyist - Hong Kong

A. Best coating for solar heating is black chrome.

Tom Gallant
- Long Beach, California, USA

Q. For solar collectors is a black aluminum oxide finish superior to matte paint for the absorber? Does black aluminum oxide have act as a better selective coating then paint? Thank you.

Adam Prince
- Brick, New Jersey

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.

Ted Mooney,
Ted Mooney, P.E.
Striving to live Aloha - Pine Beach, New Jersey

Can we do black chrome in a lab?

Q. Hi,
What are the conditions required to add black chrome to the absorber sheet? Is it something like a spray? Or does it need a lot of processes to be finally a coated absorber?

The thing is, I need to add black chrome to the sheet metal at my labs by myself ... Can I?

Mahmoud Khawaga
- Cairo, Egypt
July 31, 2013

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.


Ted Mooney,
Ted Mooney, P.E.
Striving to live Aloha - Pine Beach, New Jersey
July 2013

May 7, 2014

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.

David Long
- Nampa, Idaho USA

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