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Solar heat reflection?

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Posted by MBS on December 12, 2003, 3:44 am
 
I am curious about which surfaces or coatings on a metal would reflect
the most solar heat. I know that a mirrored surface would relect the
most visible light but what about heat? I.e., would the underside of
the metal be the coolest if it had a mirrored surface or a glossy
white surface facing the sunlight?

Thanks!

Posted by Steve O'Hara-Smith on December 12, 2003, 6:08 am
 
On 11 Dec 2003 19:44:24 -0800
mbstruss@bellsouth.net (MBS) wrote:

M> I am curious about which surfaces or coatings on a metal would reflect
M> the most solar heat. I know that a mirrored surface would relect the
M> most visible light but what about heat?

    There's almost no difference between light and radiant heat.
If you're dealing with non radiant heat then metals will just conduct it
no matter what colour you make it. Use mirrors for focussing radiant energy
and white coatings for dispersive reflection such as inside an artificially
lit greenhouse.

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Posted by Duane C. Johnson on December 12, 2003, 1:11 pm
 Hi MBS;


Technically, Infra Red light is just like visible
light when it comes to mirrors. IR light has longer
wavelengths. As a rule of thumb, if it's shiny to
visible light it will be shiny to IR light.


That's actually a separate question. White surfaces
also reflect light, but unlike a mirror, white
reflects light in all directions.

Lets say the material is placed between a wood stove
and a wall. And we are interested in protecting the
wall from getting to hot. Clearly, the white surface
would reflect light out and away to remain cooler
than a mirror surface which would reflect light back
to the stove just to have it return again.
(This is very theoretical and assumes that all
parameters are equal or equivalent, which they are not.)

If you are just interested in keeping the material cool
one has to compare the ratio of reflectivity and
emissivity of the chosen surface. Both mirror and white
surfaces can approach 100% reflectivity. Practical
surfaces fall far from this.

If I had to make a guess I would say it was easier for
a mirrored surface to retain a higher reflectivity and
lower absorptivity than an equivalent white surface over
a longer period of time. Mainly because a dirtier white
surface looses reflectivity more than a mirrored surface.
It's the smoothness that counts.

I have a lot of data pertaining to these subjects at:
http://www.redrok.com/concept.htm#absorptivity
 

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