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Solar Barn Update - Page 4

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Posted by nicksanspam on October 3, 2004, 11:16 am

Got numbers? Even a single layer of screen absorbs in both directions
and reduces reradiation and helps keep warmer air away from the glazing.
If 2 screens absorb 50% of the solar spectrum and a dark back wall absorbs
95% of that, only 2.5% of the sun goes back out the glazing. Reradiation
sounds more serious than reflection, but the screens also intercept that.

Glass won't pass much longwave heat energy (> 3 microns), although
it can intercept and re-radiate. What's the glass temperature?

A dark back wall can make more surface. Why is the wire temp important?

How would adding more screens or blanket material decrease the temperature
of the wire or fabric facing the glass, if the screens or blankets don't
_conduct_ much heat from south to north? The sun hits the south layer first,
and that's where most of the reradiation comes from. An air-cooled wire with
nothing behind it would be cooler than a wire with more hot mesh behind it.
BTW, plastic shadecloth shrinks about 30% when boiled. I wouldn't use it in
a collector if it could reach 212 F.

Have you tried this without a fan?

Sounds good for a freestanding box with cold outdoor air behind the back
wall, but not good for a collector with a warm house as the back wall.


Does that require 10 screens or a thick blanket?

What's the optimal number of screens, with no fan?


Posted by Bill Kreamer on October 4, 2004, 5:37 pm

I have to defer to you on the numbers Nick. But the name of the game in
solar air heaters is high heat transfer surface area. Why let the back wall
do a large percentage of absorbing? It has such a low heat transfer surface
area compared to a fiber blanket. One should make the back wall reflective.
Of course, you've need a substantially opaque absorber to use this

A fiber blanket can eclipse more than 99% of light that strikes the
absorber. Placing the absorber in a reflective cavity helps ensure that
light passing through the absorber is reflected back to it for a second
capture opportunity on high surface area blanket. "Several" layers of screen
just can't cut the mustard, nor would ten layers. For enough bulk opacity
and surface area, you just have to go to a black microfiber blanket. When
you do, performance "jumps."

What's so good about very small diameter fibers? Take a look at some unique
aspects of radiant energy interception, and then, the subsequent
absorber-to-air heat exchange, in a fiber blanket absorber:

The 3D nature of the blanket insures that there is mixture of lighted and
shaded fibers, as in deep jungle, with a preponderance of shaded ones the
deeper you go; deep within the blanket matrix a high portion of sunlit fiber
sites are quite short. Nick, maybe you could do an analysis of how matrix
parameters (fiber diameter, volumetric density, matrix thickness) affect
length of sunlit fiber sites, and their distribution within the matrix.

In any event, the heat exchange will take place either at the initial
interception site, or in either direction axially along a fiber, in either
direction, toward a nearby shaded fiber segment. In a fiber blanket
absorber, the length of a "typical" internal heat conducting path required
to access, let's say, a tripled heat transfer area (that is, the distance
heat must travel from the initial sunlit area to a heat transfer area three
times that size) can be measured in fiber diameters, rather than inches.
That's why it helps to use tiny fibers, e.g. black polyester microfiber felt

With the path length to a tripled heat transfer area (compare it with that
of a fin and tube type solar absorber) that is extremely short, there is no
need for a high conductivity metal absorber material. Placed the blanket
absorber on the diagonal, and give it just enough flow resistance to
equalize cooling across the collector face, and you have a winner. These
features taken together dramatically increase the rate of heat transfer from
absorber to air, and the energy delivered by the system.

What's "dramatically?" My best SWAG? You might see a noonday efficiency, of
a fan driven two-screen plus black-backwall collector, of maybe 40-50%; of a
ten-screen plus black-backwall collector, maybe 45-60%. A fan driven
collector with a fiber blanket absorber in a reflective cavity achieved
above 72% efficiency on an ASHRAE 93-77 test stand.


Collector efficiency is an inverse function of absorber temperature. The
(relatively low surface area) backwall is not a suitable heat transfer
element; it runs too hot. In any case it should be reflective, reflecting
back toward a high surface area absorber. Which, again, screen just isn't.


OK, forewarned is forearmed.


Sure. With or without a fan, you'll get a better collector. The high surface
area keeps it cool enough without a fan. The heat is carried away by the air
from the felt better than from the screen. However, don't let this collector
stagnate. It runs cooler (more heat removed to the room), but stagnates
hotter. Forewarned is forearmed again.


It works just fine, actually. The collector runs relatively cool.





Screen won't give you enough back pressure to distribute the flow across the
collector face. Felt in thicknesses from two layers of dresmaker's felt
(~.060"), up to 1/4" thick (as in some paper process felts) does.


I don't want to hazard a guess. But give black polyester felt a try. Anyone
can email me for collector plans using felt.

Best wishes Nick, and thank you for all your effort for a long time now.
- Bill
Bill Kreamer

Posted by Michael D on September 6, 2004, 8:18 pm

It seems awsome.  Talk about cost effective!  The one suggestion that
I have is that you replace your black mesh absorbers with selective
painted steel foil.

You might still want to have some of the mesh behind it to absorb the
radiant energy from the back of the absorber panels.

Michael D

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