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Efficient air solar heating collector absorbers (was Will this increase the amount of solar energy transferred in a solar air heater?)

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Posted by Bill Kreamer on July 16, 2009, 6:20 pm
 
On Apr 16, 2:07 pm, Ecnerwal

I tried to reply, but couldn't see how to do that in Google Groups, so
this is a new post -

The ideas here are good, and I'm sure there are many more out there.
For what it's worth, the following is my experience.

Two important characteristics for an air cooled solar absorber are
very high surface area, and mounting it for front-to-rear air flow.  I
did use furnace filter material for a while, but realized that I could
eliminate the smelly black paint and get much greater surface area by
using a black polyester blanket.  Performance improved dramatically -
the collector ran cooler and the room got hotter - and I've never gone
back.

Please email me.  I will send you a free .pdf file of illustrated
collector plans that use these principles.

Good luck and happy heating.

- Bill Kreamer (Bill.Kreamer@myfairpoint.net)
=====
Here are some design principles that will have a positive effect on
your solar heater project.

1. Flat plate materials (dimpled, corrugated or wavy plate, cans,
etc.) or multiple layers of screen do not provide enough surface area
for an air-cooled solar absorber.  I am now using a black fiber
blanket material (black felt), made of micro-fibers.  This has at
least 50 times the bulk surface area of the materials mentioned
above.  You can use any blanket-like black material as long as you can
see only glints of sunlight through it.  You could try black polyester
dressmaker's felt or black landscaping cloth.  Try the black polyester
felt that is available at fabric stores.  You can try weed control
landscaping cloth. Caveat: shade control material is generally not
very good at stopping light.  Get a thickness that stops a good amount
of light.  Test by looking carefully through the candidate material,
with "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.

A felt absorber's fine fibers and its evenly divided and dispersed
mass give many benefits:

    Internal heat-transfer is de-emphasized.  A goodly portion of the
total heat transfer can occur directly at the site at which the heat
is generated, i.e. without having to move anywhere by means of
internal conduction.

     The heat that is conducted internally along the fibers has to move
only a very short distance.  For that reason that the majority of
directly sun-lit interception sites are very short (due to the random
crossing and shading of the fibers in the material matrix), a nearly
tripled heat exchange surface area is available within one fiber
diameter of distance from many sun-lit interception sites.  The
increased surface area being "one fiber diameter away" means that
smaller fiber diameters are better.

     Because the conduction path is short, using a material less
conductive than metal does not hurt performance.

    The bulk surface area is very great, as much as fifty times the
surface area of 5 layers of screen.

    And, the micro-turbulence in the air flow is good due to vortex
shedding from the cylindrical fibers.

    In use, the path of air through the absorber is very short,
perpendicular to the face of the blanket.  Air first flows parallel to
the glazing, moving easily across the sunward face of the absorber.
The air then turns as it makes a very short pass through the thickness
of the blanket.  This pass through the absorber ia accompanied by a
very slight restriction which helps to distribute the cool inlet air
across the sunward face of the absorber before it dives through.  On
the back side of the absorber, heated air resumes its flow parallel to
the back wall of the collector toward the outlet.

    Each of the fine fibers or "bits" of absorber material gets cooled by
a nearly-equal flow of near-inlet-temperature air.  This absorber and
air flow geometry can be said to be a "massively parallel" flow
pattern.  A scan with an infrared temperature sensor reveals a
startling uniformity, or absence of thermal features, across the face
of the collector.  The absence of significant "hotspots" on the face
of the collector reduces re-radiation losses.

2. You can employ some minimum amount of baffles to guide the air to
all areas of the collector.  But, whether you use baffles or not, it's
important to mount the absorber so it makes a single long diagonal
starting 1/2" from the back wall near the inlet and ending 1/2" from
the glazing near the outlet.  In short, room temperature air enters at
the front of the absorber, floods the entire face of the collector and
then makes a short pass from front to rear through all face areas of
the absorber simultaneously; heated air is segregated at the rear of
the absorber, away from the glazing.  The outlet is behind the
absorber at the end of flow path.

Don't make the mistake of mounting multiple absorbers in "stages,"
which would cause the air to make sequential passes through (or near,
or next to) absorber material.  In this "series" type of flow, the air
gets hotter and hotter as it progresses through the collector; the hot
downstream areas of the collector then lose energy disproportionately
by re-radiating it back through the glazing.

Instead, the cool inlet air should be introduced to the front or
sunward side of the absorber, next to the glazing, and should be
encouraged to flood the entire front face of the absorber.  Equally
important is to seal all around the edges of the absorber so all the
air must pass through it, just once, to reach the outlet.  The best
mounting method is to use 1x1 "L" rails made of aluminum flashing, to
which you glue the absorber blanket using silicone adhesive.

3. Line the inside of the collector box with foil-faced foam
insulation and keep the inner faces reflective; don't paint them
black.  Don't encourage any planar surface inside the collector to
absorb energy except the black absorber.  Let the much-more-efficient
absorber do all the solar energy absorbing and heat transfer
functions.  When the collector's back wall is reflective, this
increases the light-trapping capability of the absorber: light that is
not intercepted on the first pass through the absorber is reflected
back to it by the back wall for a second interception opportunity.

4. Get a fan from Grainger.com.  Use 70-115 cfm for each 24 to 40 sq.
ft. of collector area.

5. Use Grainger's "2E245" snap disc thermostat.

7. If you baffle the collector so the outlet is located approximately
6" above the inlet, this encourages a very slight thermosyphoning
action which prevents stagnation and possible high-temperature damage
in an electric power outage.  This way, the descending and rising legs
of the flow are inside the collector.  Both legs to fall to the same
temperature at night, which minimizes nighttime backflow; it
eliminates the need for an anti-backflow valve.

General tips:

It may not be intuitively obvious, but putting a collector behind a
window does not collect more energy.  It just blocks the view.  Unless
you want to eliminate the window, you should mount it next to, beneath
or above the window.

A window and a hot-air heating solar collector are approximately equal
in collecting ability.  The advantage of the solar collector is that
during non-collecting hours (early morning, late afternoon, evening,
nighttime), it does not leak energy from the house, while a window
does.  A solar collector acts more like a one way valve.

The best combination (far better than big-window passive solar
designs), is to have just enough windows for an aesthetic view, plus
sufficient vertically mounted fan-driven solar heating collectors to
heat in the cold season.  The solar collectors can be (and they may
need to be) covered in the warm season.

For a solar home improvement project, I would recommend substituting
solar collectors for some windows, or simply removing some windows, if
two factors are present: a cold climate, and too many windows.  This
causes excess heat loss at night.  Turn this situation into the one
described in the paragraph above.

Because the diagonal angle gets reduced with very long air flow paths,
using an internal collector depth of five inches does mean that the
practical limit of path length is 16 feet or so (measured along the
flow path, regardless of direction changes).

Both the broad back wall and the narrower side walls of the collector
must be insulated using foil-faced foam.  The back wall can be 1"
thick, or up to 3" thick in very cold climates.  The sides can be 1"
thick.

In general, it does not pay (labor-wise or money-wise) to use heroic
efforts to store heat.  The house contents will store heat for a few
hours with no trouble.

- Bill Kreamer (Bill.Kreamer@myfairpoint.net)

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