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Passive Solar Heater Box Design - Page 4

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Posted by David L. Jones on July 2, 2005, 7:41 am
 
Gary wrote:

Thanks Gary.
The aluminium "flyscreen" (as we call it in Australia) window mesh
sounds like a great idea!
Now would it be better to have multiple angled screens within the box,
or just one big angled screen (eg. air flows in one side, sucked out
the other)
I'm thinking the multiple screen would be better as there would be more
air diffusion and hence greater speading over the surface area?

Dave :)


Posted by Gary on July 2, 2005, 8:14 pm
 
David L. Jones wrote:

Hi Dave,
I'm not sure.
It might improve the performance to have the 3 layers of screen spaced
a little bit apart from each other.
I went out and looked at mine, and the even though the layers are all
in contact with each other at the edges where they are stapled in,
they separate a little bit from each other as soon as you get away
from the edges.  So, I think you automatically get a little separation.



Gary

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









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Posted by David L. Jones on July 2, 2005, 11:44 pm
 Gary wrote:

I am going to use the 3 layers touching each other, but what I meant
was seperate screens of three layers each within the box. e.g. the air
has to pass through each screen in sequence to get from one side of the
box to the other side.
I have to get the air flowing from one side of the box to the other, so
the choice is either multiple screens within the box or one big screen
which is slightly angled within the box, e.g. attached to the top
corner on the top of the box and the bottom corner on the bottom side
of the box.
Anyone got any better ideas?

Thanks
Dave :)


Posted by David L. Jones on July 3, 2005, 7:58 am
 Gary wrote:

I had a look at some flyscreen today, and it's probably got 5-10%
coverage at most. Obvious given that it's designed to be as "see
through" as possible. Even with 3 layers on top of each other, the
total coverage would still be less than 30%, worse if the pattern
happens to line up.

How can such a thing possibly be an efficient collector if it has a
very poor surface area percentage? Most of the light will just go
straight through.
I thought the basic principle of solar collectors is simply the amount
of square surface area you have light collecting on? Flyscreen seems to
be extremely poor in this respect.
Sure you could paint the back of your box black to collect the rest of
the light, but I don't want to have to do that.

Am I looking at the wrong stuff, or am I misunderstanding something?

Thanks
Dave :)


Posted by livefree on July 3, 2005, 1:42 pm
 wrote:

if the pattern

Hi David,

I used furnace filter material, and spray painted the first batch from
both sides, then found dipping in some thinned black point was easier
and much cheaper.  (this filter material came in a roll, and seemed
much cheaper than even a single layer of window screen)  Anyway, I'd
bet the screen blocks a little more than 5-10%, and then what is
reflected off the back wall is partially absorbed again from the back
side of the screen, so that helps.  I used foil faced polyiso' on the
back wall and put the black side facing the sun on the first set of
windows, then opted for the silver side on the second batch to reflect
more through the more efficient mesh collector.

I "stacked" my sloped sections, mostly just because I couldn't find
furnace filter 39" wide, so I had to run the roll "sideways"instead of
one single top to bottom run.  In hindsight this seems somewhat better
because the air should move through at 3 times the speed.  One
downside of this is that it allows the heated air to contact the cold
window after the first section of mesh.  An advantage of the speed is
I think you get more even flow and less chance of stagnation spots,
and less air is able to "cling" to the mesh.  (I like using vague non
scientific terms so I don't use the wrong scientific term  lol)

I sloped my filter sections toward the sun, but may change that next
time so the air against the glass is cooler.  I don't understand air
flow well enough to say if that is really best though.  More
experienced people seem to say it is :)  My case is a little different
since it has the 3 sloped sections, so where the air goes exactly
after the first and second sections becomes important.

Another good thing about sloping down a little (instead of up like
mine) would be getting better use of the mesh, since the sun would
come in at more of an angle (greater angle of incidence?), hence
passing through more mesh.

I move about 500cfm through my collector (about 125 sqaure feet of
glass) and have gotten an increase from 72 to 135.  ( that was
probably January with snow) My disappointment is that there are so few
good sunny days in the winter.

Good luck,
Bill




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