Steve O'Hara-Smith wrote:
I think your gut feel is accurate. With a straight (untapered) plenum,
we end up with the cool air moving more slowly than the warm air which,
when you think about it, is pretty much what we want.
Although the temperatures aren't constant, we know that we want the
discharge temperature to be as close to the intake temperature as we can
manage - which means that the ratio of the absolute temperatures will be
(we hope) fairly close to unity...
DeSoto, Iowa USA
I agree that it would be nice to be able to make the vents the same
size and area as the collector cross section. And, further to have
nice rounded aerodynamic corners to help the air make the corner.
It would be nice to find out how much this actually gains from the
vents that are half the cross section area.
Just not sure I want to cut holes that big in my wall :)
The wider vents are worthwhile - the rounded corners (which would
subtract from the height of the glazed area) do not seem worthwhile.
I understand the feeling. It's not just a matter of cutting holes
because a wide opening calls for a pair of jack studs to support a
header over the opening to support the structure above the panel. I have
a construction drawing somewhere here, and I'll put it up on a web page
so people can see what's needed. It's not rocket science, but it does
need to be done right.
I guess it's less a matter of feeling than doing what's needed in order
to have the full benefit to be derived from paying the costs.
You can get an idea what the wide openings look like from the photos on
the web page at the link below.
If you did cut holes that big in your wall, you'd be glad. :-)
DeSoto, Iowa USA
Morris Dovey wrote:
Found it! There's a quick and dirty web page at the link below.
DeSoto, Iowa USA
Bill K here. Sorry to jump in so late in the discussion. You have a
nice project going. I would make several observations.
1. The whole idea of an absorber is to let as little light through as
possible. A metal screen absorber, if you are bent on using that
material, should have at least 5 layers. Rather than 80% open, you
want the composite to be 80-90% closed.
Better yet, use a thick black material with fine fibers. You can try
weed control landscaping cloth. Caveat: shade control material is
generally not very good at stopping light. Try black polyester felt is
available at fabric stores. Get a thickness that stops a good amount
of light. Test by looking carefully through the candidate material at
the sun, with that "80-90% closed" in mind. I have used black
polyester felt as a solar collector absorber material for 20 years
without observing significant material degradation. The material is
best mounted at an angle, as you show above, silicone-glued onto 1"x1"
el "rails" of aluminum flashing.
A felt absorber's finely divided and dispersed mass gives many
benefits: reliance on internal heat-transfer is reduced; the heat-
transfer path (moving along the fiber from a sun-lit fiber site to an
adjacent shaded site) is very short; and the surface area is very
great, as much as fifty times the surface area as 5 layers of screen.
Surface area is the name of the game in air-cooled solar absorbers.
And the micro-turbulence of the air flow is good.
There are advantages of felt that accrue because of the small diameter
of the fibers. A sun-lit fiber transfers its heat in several ways.
First, through outer-surface heat exchange. Unlike liquid-cooled
absorbers, a goodly portion of the total heat transfer occurs directly
at the site at which the heat is generated, i.e. without moving
anywhere by means of internal conduction. Second, heat is conducted
internally along the fibers, but only a very short distance. Because
the bulk of sun-lit interception sites are very short due to the
random crossing and shading of the fibers in the material matrix, a
virtual tripling of the initial sunlit surface area is available (for
increased heat exchange) within one fiber diameter of distance in
either direction along the fiber. The increased surface area being "a
fiber diameter away" means that smaller fibers are better.
Another advantage of the short conduction path is that using a
material that is less conductive than metal does not hurt
2. The planar back wall of the collector has a poor heat exchange
capability compared with the absorber. Ideally, it should reflective,
and serve as a mirror to turn un-intercepted light back to the
absorber, which has the qualities to do the job.
3. It's not the vent cross section area per se that matters, but the
flow rate per glazing area. Efficiency is better at higher flow rates.
The flow rate should range from 70 to 120 cfm for a 4'x8' collector.
You will probably need a fan to do this.
Best of luck on your very interesting project - Bill Kreamer