Sound technical content, curated with aloha by
Ted Mooney, P.E. RET
Pine Beach, NJ
The authoritative public forum
for Metal Finishing since 1989
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Good selective coatings for solar absorber -- black chrome plating
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RFQ: Hello,
I'm working on a project of solar hot air to power Wood-drying kiln.
I want to know what are those blue selective coatings you speak about. I found TINOX:
www.almecosolar.com/en/products/solar_absorber/tinox_energy_cu
-- but it does not seem to be available in North America. Do you know of other U.S. and Canadian suppliers.
Thank you
- Sainte-Brigitte-de-Laval, Quebec, Canada
May 20, 2014
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 note: Sorry, this RFQ is old & outdated, so contact info is no longer available.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?)
RFQ: I previously bought SOLKOTE HI/SORB to paint 1,200 sq meters of corrugated aluminum as part of a thermal solar cell used to dry meat products. I am building another thermal solar cell (similar size) but can't find the original manufacturer/vendor of SOLKOTE.
Is this product still available and, if not, what other bulk solar absorbing paint would replace it?
- Albury, NSW, Australia
May 21, 2014
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 note: Sorry, this RFQ is old & outdated, so contact info is no longer available.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 Simon. We don't compare brands here (why?), but if you can't find your Solkote, then Thurmalox is another brand of solar selective paint intended to work in a similar way.
Regards,
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
May 2014
A. Hi Simon,
You can find Solkote product here: www.solec.org
They ship all around the world. Good luck in your project
Cyril
- Sainte-Brigitte-de-Laval, Quebec, Canada
May 22, 2014
December 23, 2014
A. Hi, Trying to build a better collector, possibly using selective coatings as they appear to be better. I have built many and read a great deal about others that have built both hot air and water panels.
You can find quite a bit of info at these two sites:
www.builditsolar.com/
simplysolar.supporttopics.com/
Gary from builditsolar did some comparisons between different flat and glossy paints compared to Solcote (I think), and found very little difference between the "selective" paint and normal flat black paint. A lot of the guys like the rustoleum "ultra flat black" camo paint.
While some people complain about "off gassing" smells, I've never heard of the complaint of fogging glass from off-gassing or seen it. Our usual fix to off-gassing is to paint the surface and let it bake/stagnate for several days in the sun before we use it to warm the air in the house.
My current thought is to purchase something like BlackFlex, a selective coating applied to a thick foil with very high absorption and very low emissivity
www.cleardomesolar.com/solarflex.html
Another product that works well but has not been tested for selectivity is Cinefoil, as used for photography. It has high absorption but unknown emissivity (but works well in the applications I have seen.
www.bhphotovideo.com/c/product/44690-REG/Rosco_RS100113_Matte_Black_Cinefoil.html
Not trying to dispute, but trying to add real word experience by people trying to build better collectors with available materials.
Dan
- Duluth, Minnesota, USA
December 2014
Hi Daniel. Thanks for the interesting posting. I appreciate your finding that "fogging of the glass" is not really much of a consideration. We took several formerly-separate short threads and combined them onto this one page, and I had responded to each before they were combined -- which left it looking like I kept emphasizing and re-emphasizing this possible "fogging" issue like a mantra, whereas it was actually just something that I read once; I'm glad to hear that it's overstated. Thanks.
Regards,
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
A. Ted,
Thanks for the response. One of the reasons we do not see "fogging" may be our panel temperatures. We make a lot of hot air panels, (we just circulate air though them and heat it up), and the goal is to not have the panel get too hot. Higher flow keeps the heat down in the panel and this lower temperature may keep the off-gassing down. Ideally the panel will be operating at less than 150 °F to run more efficiently so you have less loss out the glazing. Liquid panel temperatures run higher but as long as everything works and the water keeps flowing the temperatures generally stay under 200 °F and we have not seen fogging that we associated with the paint.
Since my goal is to build a better panel and applying Black Chrome at home is generally not an option, are you aware of anything that is available and could be applied with a home or auto spray kit. (such as SolKote in a gallon can) Trying to keep the cost down is a big concern usually, this and availability is why we generally use the cheaper flat black spray paints. Many people have used the high temperature "grill" paint but it does need very high temps to cure properly and may not get to that temperature in the panel. This causes the "smell" to linger a lot longer.
Thanks again, Dan
- Duluth, Minnesota, USA
December 26, 2014
How to Coat galvanized screen black for solar heat applications
Q. Retired US engineer living in Costa Rica. I develop designs for low cost solar heat applications for the third world, using local materials, then make the designs available on the web.
I want to coat galvanized steel screen black to absorb heat in a solar air collector, for food dehydration and cooking. I have been spraying the screen black with hi temp spray paint. Pricey, by local standards... It does the job, but I would rather anodize or electroplate the screen to get it black and hi temp resistant. Easy, cheap, locally available materials... I notice that iron left in hydrogen peroxide leaves the surface a nice black. QUESTION: What might be some simple way to oxidize or otherwise leave my galvanized steel screen a nice high temp resistant (500 °F) black? Need not be particularly abrasive resistant. Battery charger, the right salt or acid added? Gracias...
Retired Solar Engineer, rural Costa Rica - Costa Rica
December 7, 2016
A. Galvanized screening can probably not be practically anodized, despite anodized zinc being an interesting surface.
Although black chromium plating, sputtered metal coatings, and some other dark metallic surfaces can be efficient for solar collectors, they are quite unsimple. I'm not aware of any metallic coatings that could be easily applied to your galvanized surface and offer good solar energy properties, but maybe another reader will offer a more optimistic appraisal. There are, however, good thermal paints these days. Good luck.
Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
December 2016
Q. Thanks for responding. Allow me to clarify my design spec.
The implementation is 3rd world. No plating supply houses (Chromium salts are not available unless we can make it here... old bumper??), no home depots, a 'neighborhood' hardware store or pharmacy at best. The focus is to make it doable within these constraints. High efficiency is not the priority. I have worked in high tech environments in the past decades. Now retired doing service work for rural locals here in Costa Rica, with my own challenge to think out of my previous box, to improvise implementations with low tech means. I have done this in other applications with surprising results. It can be quite a challenge and exciting to invent a solution that ends up making a difference in people's lives. I trust there are workable solutions, ...just not there yet. Reaching out to resources of experience on the web to support these folks to dry some cassava so they can make flour and feed their family, and maybe sell some flower to buy a pair of shoes for one of their kids. The black galvanized screen sits in an inclined glazed box, receives heat from the sun and rises up to a cabinet with drying racks in it. An old design. Just trying to improve the solar heat to air transfer characteristics for the dryer with the blackish screen.
Engineers without borders - Costa Rica
December 7, 2016
A. You could "paint" on a solution that discolours the zinc surface, something containing copper would perhaps work, like copper sulphate.
Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK
December 9, 2016
Q. Hi fellows, being a beginner in this field, I want to know why a solar selective absorber (SSA) emits radiation in IR region while IR has much less energy as compared to the Visible region. Absorption is a good thing if your SSA converts that light to heat and transfers it to other side of coating/substrate for other purposes.
adil raza- nanjing, China
February 28, 2018
August 24, 2019
RFQ: I am building a passive solar house with a large Trombe Wall. I am trying to locate a supplier for a selective surface (absorption in visible high, emissivity in infrared low). My calculations show a big advantage in having such. For example, for New Mexico insolation in January, 20 °F outside, R=2.5 f^2hrF/BTU glazing on wall with 0.74 solar transmission, if the emissivity of the wall surface is 0.1, the insulated inside of the wall gets to 61 °FF, but if the emissivity is 0.9, the inside only gets to 40 °F. These ideas have been around since the 1970's. How can I achieve this? Any help appreciated.
Thanks,
- Santa Fe, New Mexico
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. As far as I know black chromium plating is still ideal, but solar adsorbing black paints are probably fine and orders of magnitude simpler and cheaper.
I'm no solar expert but I don't think I agree with Wikipedia that matte surfaces are better; I think the opposite is true because the emissivity of polished surfaces is low. Touch a chrome truck bumper at noon on a summer day and tell us how successful the shiny surface was in keeping it cool by reflecting the heat :-)
Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
August 2019
Q. A rep at Solkote recommended that I use several coats and basically just paint the wall black. Another rep earlier had said to put a very thin (brown not black, 1 mil) coat on aluminum foil to get the emissivity closer to the underlying aluminum (.05). Has anyone measured absorption and emissivity of something like this?
In terms of additional heating to hold the inside temperature at 70F, I calculate that a black wall requires 3.3 kW, while a selective surface wall delivers 0.7 kW (10in concrete, 90 square meters, 20F outside, average insolation 120 W/m^2).
- Santa Fe, New Mexico
August 26, 2019
Q. Hi I am Bhim Kafle, from Kathmandu University, Nepal. I am trying to develop low-cost solar fins with aluminum sheets (similar to what Kirk is trying in Costa Rica).
I tried coating of black chrome in my lab and it works well but the process is lengthy and not cost-effective (Sequentially, each aluminum sheet was cleaned in nitric acid solution, followed by zincating. Then the zincated surface was coated with nickel with electroless deposition followed by electro-deposition of bright nickel, then finally electro-deposition of black chrome).
I am actually looking for a lost technique, that allows me to maintain high absorption in the visible region and low emissivity in the IR region- at least let's say 2 times better than Thurmalox 250).
Kathmandu University - Dhulikhel, Nepal
October 28, 2019
February 28, 2020
Q. Dear friends ,
Many thanks for the enlightening discussions on selective coatings.I am a Metallurgical engineer from Bangalore India and we run a business for various paints and coatings for Metallurgical ions.
We are exploring the coatings for solar applications. From what I read certain questions come to my mind:
Is there any data on the comparative performance of black chrome and selective coatings -- any cost benefit data available?
Any specific reason why selective coatings are spirit based and not water based?
Kindly guide. Many thanks
ALLIED METALLURGICAL PRODUCTS PVT LTD - Bangalore India
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