Say What? wrote:
| Morris Dovey wrote:
|| Say What? wrote:
||
||| Great idea but why not dispense with the fancy colors and go with
||| "flat black?" That way you could park the barrel outside the shop
||| on the wall with the southern exposure and plumb it through the
||| wall, using the barrel as a solar collector? With your DC and
||| filter back inside the shop you could be re-heating all that air
||| instead of just moving it around. g
||
|| Thanks, but (seriously) flat black paint isn't adequate for
|| efficient absorption of solar radiation.
|
| Interesting. What is the most efficient finish for absorption? I'm
| seriously confused at this point (not an uncommon problem, I assure
| you, but it boggles the mind that flat black on a surface would not
| suck in the most heat.

I did a bit of experimenting with a single absorber design. Originally
I painted the absorber elements flat black. Then, just because, I left
the elements in their high-gloss (very thin) powdercoat and discovered
that absorber efficiency went way up. It was totally unexpected. It
turns out that the flat black paint acted as a thermal insulator,
inhibited air flow over the surface of the element, and generally got
in the way of everything I wanted to happen.

Also important - it isn't sufficient to merely /look/ black, because
looking black is only an indication that the visible portion of the
spectrum is not being significantly reflected. It's necessary to
ensure that the infrared (which isn't visible) is also being absorbed.
I have a web page at the link below with a graph showing the level of
energy in solar radiation plotted against wavelength (color). It's
worth a look and a quick reading because the energy in the visible
region is only a fraction of the total available.

Let's assume that we've built a collector with a (magic) absorber that
absorbs 100% of the incoming energy. Good stuff, except that as the
absorber absorbs all that energy, it warms up and wants to re-radiate
all that energy as infrared. Now we have the problem of transfering
that energy into the air in the collector box and extracting it
without re-radiating it back through the glazing into the outdoors.
This is where flat black paint /really/ gets in the way because it
simultaneously insulates the heat exchanger _and_ inhibits the free
flow of air.

What I've done with my panels is to integrate absorber and heat
exchanger in such a way as to /use/ the reflective surface of the
absorber to reflect solar energy inward while making it
(geometrically) difficult to re-radiate back through the glazing. That
shiny reflective surface minimizes friction between air and heat
exchanger, and the result is a fast convection current.

| Now, I realize (at least I THINK I do) that the design of
| collectors is certainly more complex and efficient than a black
| object absorbing heat and moving air inside the collector moving it
| around but color-wise, what works better in any given setup?

According to the physics guys, I could produce a moderate improvement
if I could replace the glossy black surface on the absorber elements
with a highly-polished gold plating. The zinger is that the
gold-plated absorber would more than _look_ black - it would _be_
black!

I don't even dream about trying it, but it's fun to think about. :-)

--
Morris Dovey
DeSoto Solar
DeSoto, Iowa USA
http://www.iedu.com/DeSoto/Absorber.html