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Getting the best reflection for solar-cooking
An ongoing discussion beginning back in 2006 ...2006
Q. I'm working as a volunteer in western Kenya, where I'm trying to start building solar cookers. To get costs down I've been thinking about using galvanized metal sheets instead of the more reflective aluminium sheets. To the question, is galvanized sheet a good reflector, as in how many percent does it reflect? And, very important, can I polish the galvanized sheet to keep it shiny? The mud houses in the rural area have so nice and shiny galvanized metal roofs when they are new, but the old ones look really dull and grey - that's not what I want!
Thank you very much for your answers!
Volunteer in Kenya - Mumias, Western Province, Kenya
A. Galvanized steel it is a coat of zinc and it will get dull, polishing may work for a while. Why not try using aluminum foil it would be a lot cheaper. Good luck and God bless you.
aircraft maintenance - Al Ain, UAE
A. Both aluminum and galvanizing (zinc) are not resistant to oxidation and corrosion. Both will turn dull with time, humidity and rain. Chrome over bright nickel over steel sounds as a suitable and not very expensive solution.Guillermo Marrufo
Monterrey, NL, Mexico
A. While doing research into passive solar heating for homes, I read adding reflectors can substantially increase the gain through windows. The included chart listed percentages of reflectivity for many common materials and finishes. As I recall, the highest reflectivity rating was 97% reflective for white enamel paint. Surprisingly (to me) this beat everything else including mylar, mirrors, polished aluminum, polished stainless steel....
Sheet metal substrate(s) with an 'automotive smooth' coating of white enamel might not only be the cheapest but the best as well.
- Liberty Lake, Washington, USA
Thanks Ned. It wouldn't surprise me if you were right that white paint is more reflective than most polished metals. But the thing is, a white painted surface is white rather than mirror-like because it diffuses the reflection, i.e., the angle of reflection is NOT equal to the angle of incidence.
So, can one focus the reflection sufficiently to make a cooker? I don't know :-(
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
December 12, 2018
Comment for all future readers: Heating a galvanized metal surface releases zinc fumes. These fumes accumulate in the food but are also toxic to breathe.matthew mattis
Grifter - Ardmore,Oklahoma
What is true is that welders heat zinc to white hot, at which temperature the zinc vaporizes into white clouds, and it is easy for a welder to inhale an overdose. "Warm" galvanized zinc surfaces are widely used as ducting for heating & cooling systems, exhaust ducts for furnaces & water heaters, as ash trays in wood stoves, for brake linings, in B-B-Q grills, etc.
It's also true that zinc is not a "safe" surface for food contact because it can dissolve in acidic foods.