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Passive Solar Heater Box Design - Page 5

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Posted by Gary on July 3, 2005, 2:48 pm
 
David L. Jones wrote:

Hi Dave,

The screen I use has a spacing of 0.075 inch between wires, and the
wires are about 0.01 inches thick.
If you look through three stacked layers onto a brightly lit scene,
you can still see through it, but indistinctly.

I did a rough test with my light meter and got these results:

Full sun    1      (ie 1 is full sunlight with no screen)
1 layer        0.67   (fraction of light getting through 1 screen)
2 layers    0.46
3 layers     0.32
4 layers    0.21

For the light that does get through, some is absorbed by the back wall
of the collector, and heats the air that flows by it. and some gets
reflected back toward the screen, and would have to go back through
all three layers of screen to get out.  I think that in the end, very
little gets out?  If 0.32 gets through the 3 layers, and 0.8 of that
gets absorbed by the back wall, the (0.2)(0.32) = 0.06 gets reflected
back towards screen, and (0.06)(0.32) = 0.02 might get through the
screens and escape the collector -- pretty small.  I suppose you might
argue that not having so much absorbed by the first layer of screen
lets it run a little cooler, and reduces losses out the glass (sounds
like another test :-))

The way I arrived at 3 layers was to do side by side tests using 2
bays of my collector.  I compared 1 layer to 2 layers, and 2 layers to
3 layers, and 3 layers to 4 layers.  The optimum combination of
temperature and airflow occurred between 3 and 4 layers, but closer to
3 layers.  This is for a collector without forced air flow.
I also did as good a test as the instrumentation I have allows of the
efficiency of the collector.  I did this by picking a clear day at
solar noon, and looking up the solar input in a reference, and then
measured the temperature rise through the collector and the airflow
velocity to get the thermal output.  The result was a good efficiency.

It does seem odd that after 30 years of making air collectors that
there is still a lot of discussion on whats the best absorber.  I've
looked for a good comparison test in books and on the Internet, and
found very little.  Maybe somebody knows of some published comparisons
that would help resolve this?
I also read Encerwal's comments on the solid absorber, and while the
arguments make sense, I have never seen any test data to support them.
My own prejudice is that you can probably make a solid sheet
absorber collector that works well if you get the airflow path and
velocity right, and it seems to me I have seen efficiency numbers on
commercial, solid absorber air collectors that were good?
In the end, for me, the deciding factors were more that the window
screen was easy to get, easy to install, and has no life or smell
issues.  There may well be something out there that works 5% better
thermally, but I'm not even sure which of the other candidates it
might be.

Another feature that I like about the screen is that it does not have
the solid black look from the outside (my wife would say the ugly
black look :-)).  If the back wall of the collector is a medium color,
you can still see just a bit of it through the screening, and (to me)
it looks better.  This probably applies more to collectors mounted on
a vertical wall.

As Nick says, "Perfection is the enemy of good" (or something like
that) -- its better to build something that performs in the "good"
range than to think forever about getting it perfect :-)

Go Build It!

And, please send some pictures and a description to post at BuildItSolar.

--


Gary

www.BuildItSolar.com
gary@BuildItSolar.com
"Build It Yourself" Solar Projects











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Posted by David L. Jones on July 4, 2005, 2:20 am
 
Gary wrote:

Your dimentions above indicate only a 13% surface area for the wire. I
can't see how you could possibly measure 0.67 with a (measured?)
surface area of 13%!

There must be something wrong with your light meter measurement or data
somewhere...


It's going to be very small if your walls are black, but my walls won't
be black!
In that case I think the losses might be very high.

Flyscreen surprisingly isn't that cheap either, only about half the
cost of a solid aluminium plate. But I really do like the flyscreen
idea, I'm just worried about those losses. I'll have to keep thinking
about it!

Dave :)


Posted by David L. Jones on July 4, 2005, 4:42 am
 

David L. Jones wrote:

Oops, minor error, I think it's more like 23%, not 13%. Still very low!

Dave :)


Posted by Gary on July 4, 2005, 5:17 am
 David L. Jones wrote:

Right -- I get 25%, which is
(0.075-0.005-0.005)(0.075-0.005-0.005)/(0.075)(0.075) = 75%
transmission, or 25% blocked.

The light meter shows 68% transmission on the first layer, 70% on the
2nd layer ...
I think this is pretty reasonable agreement with the light meter
readings.  It could be that you get a little less than the 75%
transmission because of the winding of the wires over each other.  Or,
my diameter measurement of the wire could be a thousandth off.  I
would trust the light meter more than my measurements of the screen
geometry.

The thing is by the time you take two trips through three layers at
70% transmission you are down to 0.7^6 = 10%  not counting whats
absorbed at the back wall.  The back wall of my collector is actually
the barn siding, which is light grey, and is listed as 50% absorbent
-- you can see how light the color is in the pictures.

Have you thought about how big you are going to make the collector?
Seems like people often lose track of the fact area is probably the
largest single factor in how much heat you collect.

I think that as a service to the solar thermal community, you should
really build two of these collectors, and do side by side comparisons
of different absorbers :-)


--


Gary

www.BuildItSolar.com
gary@BuildItSolar.com
"Build It Yourself" Solar Projects









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Posted by David L. Jones on July 5, 2005, 2:28 am
 Gary wrote:

The polycarb sheeting material I can get comes 1m wide, so I was
thinking of making my collector based on that width, perhaps 2m in
length. So 2 sqm total to begin with as a trial.
I figured I can always connect another one in parallel.

Although 2m x 2m is sounding good too, but I figure that for this size
it would be best to "snake" the heat around the box using guides rather
than just one big open box. That way the air would be forced through
the entire collection area of the box.
Probably not a bad idea for the smaller box too.

Dave :)


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