Subject: SOLAR COLLECTING TUBES - BLACK OR SHINY?

From: [email-masked] (Henry Polvi)
Date: Sat, 11 Jan 97 09:44:04 GMT

Recently i observed a solar collecting device consisting of concentric tubes,
the inner contains water to be heated, this has a relfective mirror like coating.
The annular space is vacuum, the outer tube is clear pyrex glass.

My question, wouldn't it work better if the inner tube was coated flat black instead of being reflective?
The black is more absorptive and thus more heat should transfer to the water inside, no?

Opinions anyone please?

Henry Polvi
Toronto, ON

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From: [email-masked]
Date: Sun, 12 Jan 1997 09:24:40 -0500

In article <[email-masked]>, [email-masked] (Henry

> The black is more absorptive and thus more heat should transfer to the
water inside, no?

My experience has been that with black coating things get extremely hot. 
Perhaps in this situation only a certain amount of heat could be
controllable within economic reasons or ???
-Harry Sadler

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From: [email-masked] (Andrew McKegney)
Date: Mon, 13 Jan 1997 05:21:23 GMT

>Toronto, ON

If the solar tubes you are refering to are the units on the post
office just North of the Gardiner (west of the old race track), those
panels (tubes) are actually black inside. They operate by filling with
water, and when this happens, it changes the refractive index of the
two tubes (water between them) and they look black. When empty they
look silvery.

Andrew McKegney
Oakville Ontario.

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From: [email-masked] (John Q. Glasgow Jr.)
Date: Mon, 13 Jan 1997 09:30:08 GMT

>Toronto, ON

I agree. I don't have a degree in the field, but I did have a solar
heat system installed in a house. It used a flat black color for the
backing material. There were copper tubes that were installed on top
of the backing material. It had a clear tempered glass top panel. The
theory for the system was that the sun could shine through the clear
tempered glass top, strike the flat black backing material, and that
in turn heated the copper tubes and the 50/50 antifreeze/water
solution flowing through the tubes. Flat black has the greatest
absorption of all colors.

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From: [email-masked] (MingusP)
Date: 14 Jan 1997 18:15:32 GMT

The reason that the inside is shiny and the outside is black is so that
energy from the sun will be absorbed (black reflecting less then white or
any other for that matter). One the energy (heat ) has been absorbed color
has little to do with the rate at which heat is passed through the wall.
On the inside of he cylinder you want to retain heat as much as possible
so the it is shiny. The mirror like coating bounces the heat energy back
and forth. Remember you are trying to heat the water inside not keep the
walls of the container warm.