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temp glazing and cheap solar collector - Page 4

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Posted by Tater on November 22, 2006, 9:26 pm
 

Jeff wrote:

Oh i am sure the absorber is getting hot, I can feel the radiant heat
from it, but the airflow doesnt match suggested, and i dont have one of
the fancy IR guns to check the absorber temps, or even an
indoor/outdoor thermometer to fish a prope into! not that i am worried,
i got plenty of other things to keep me busy, so it is just a matter of
waiting until i got a day when my mood and motivation match, and go
tinker with it till i know whats going on

I'd bet 10% of my paycheck that if I put a fan in there that my garage
wouldshow a significant heat change vs the outside. that poly just
looks too milky for me to believe i am getting the heat i should be
though, but it is getting heat.


Posted by Jeff on November 25, 2006, 2:18 pm
 
Jeff wrote:

 >   After I finish my other projects...
 >
 >   I have a south facing wall that gets some sun (some shade) in the
 > winter, it has two large jalousy(sp?) windows. Normally I cover these
 > windows for the winter.
 >
 >   Here's what I'm thinking:
 >
 > Cover most of this wall with some temporary glazing. Air could be pulled
 > out one window, with the cold air entering from the other. There's about
 > 10' between windows.

Here's the current thinking:

Take unused floor heater and use it as an air intake. Force the air
through a duct to outside and below the two windows,  that would center
feed a manifold that would be on the sun side of the single or double
window screen absorber (about 24' * 8' high). I think a 10" galvanized
round duct with many small "vents".

   The hot air return would be the windows.

   Glazing would be clear poly, I'll build it to upgrade to SunTuf or
sheet polycarb.

   Questions about sizing remain. Any ideas would be helpfull (or
corrections to my thinking), here's about what I think.

1) I'm thinking that duct vents into the absorber should be small
compared to the duct area. I think the vent area should be on the order
of the duct area for each half manifold section. That could be 10  3"
holes for a 10" diameter 12' long manifold half section.

2) Absorber screen distance from glazing is confusing. It seems to me
this distance should taper, with less distance at the top and more at
the bottom. Perhaps the full depth minus an inch at the bottom and the
reverse at the top. Total collector depth would be 12".

3) One layer of screen or two? I'm thinking because of the large
absorber area that 2 screens would be better. More even distribution
through screen because of increased air resistance.

   Got the walls filled with blown in cellulous. Lots of loose bits
everywhere!

   Jeff


 >
 >
 > Rough idea:
 > 1) Add a solar absorber across (either black screen or shade cloth) the
 > entire area. The cold air inlet window would be framed out to the
 > absorber, so that the air inlet would be on the north side of the
 > absorber screen.
 >
 > 2) The return window would just be behind the screen. Probably a window
 > fan pulling air through.
 >
 >  Since initially I'd like to take this down during the off season, some
 > temp glazing might be in order. What would that be? Polyethelene glazing
 > springs to mind, but isn't that IR transparent? Wouldn't IR opaque be
 > better. How about vinyl (like shower curtain liners)?
 >
 >   Suggestions welcome (Nick, Gary, et all). Especially to tell me this
 >  idea is foolish and not to do it!
 >
 >   Jeff

Posted by Gary on November 27, 2006, 8:36 pm
 Jeff wrote:

Hi Jeff,
Sounds like a good plan to me.


For an 8 by 24 ft collector (192 sf), it seems like about 3cfm per sqft or about
600 cfm would be about right.
This would provide a 40F temperature rise with 250 BTU/sf-hr (full) sun if the
collector is operating at 50% efficiency.
You probably don't want any more flow than this, or the temperature rise won't
be enough to make the air feel warm --  a bit less might be OK.  A 2 speed or
variable speed fan, or adjustable damper would allow some adjustment.

You will need something like a squirrel cage blower that produces 600 cfm at
whatever the pressure drop is for you collector (I would guess around 0.2 inches
of water).

If each side of the manifold takes 300 cfm of flow, then a 10 inch diameter
manifold would be plenty -- flow velocity of about 500 fpm, and a pressure loss
of only 0.005 inches of water.  An 8 inch duct might be OK.
http://www.connel.com/freeware/airduct.shtml

If you want the manifold to act like a plenum, then the holes should be small
enough to have a pressure drop across them that is larger than the pressure drop
along the length of the manifold, but you also don't want them too small, as
that will reduce flow.
If you have 10 holes, the flow per hole would be about 60 cfm.
Experimenting around with this calculator:
http://www.flowmeterdirectory.com/flowmeter_orifice_calc.html
It looks like it might take more than 3 inch diameter holes to keep the pressure
loss across the holes less than 0.05 inches of water.  Anyway, you might 1) find
a better pressure drop across orifice calculator to give a better estimate of
hole size, or 2) start with ten 2 inch holes, and if that's not enough to get
good flow add 10 more 2 inch homes.
I suppose that you might think about building a rectangular manifold into the
bottom of the collector, and provide a continuous slot (or holes) to distribute
the air to the collector space.  It might be fairly easy to make the slot width
variable to get an even flow everywhere.



The taper seems sensible, but not sure how much difference it really makes.
The 12 inch depth seems like more than is really needed, especially with a fan
forced collector?


Yes, at least 2 I think.
I think that the absorber having enough resistance to provide even flow through
the full absorber is important.
It would be nice if you could figure out some way to assess how even the flow
through the absorber is, and pass this info on to the waiting world :)  Maybe
very light tufts? Or, a smoke pencil on a stick that you could position at
various places?  Or, a way to measure the absorber temperature with an IR
thermometer to look for hot (low flow) spots?

Don't forget that you will need a backflow damper of some kind to prevent
reverse flow at night -- very important.

I think this thing could potentially produce a lot of heat for little money.

Gary


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









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Posted by Jeff on November 28, 2006, 4:30 am
 Gary wrote:

   Thanks, that sounds right to me. And thanks for the ball park figures.

   I had found that already and it is where I got my sizing from. Some
of the calcs are inscrutable to me! A little too big should not be a
problem.perhaps a 10" square plenum.

OK, this will take a while to go through. Particularly since I need to
install the java virtual machine!


Not so far!

  or 2) start with ten 2 inch holes, and if that's not enough

   That's pretty  cool. I had thought galvanized duct but now I'm
thinking a rectangular wooden duct. It could be the structural bottom of
the collector and I could line the bottom of the plenum with poliso with
   the reflective side up of course! For that matter the sides and top
could be polyiso, not that the top needs to be insulated, but it would
be easy to make alterations. The polyiso would improve the wall smoothness.


   The windows open out 12". I had thought that during summer I would
take off the glazing and use the windows for ventilation.  Now, I'm
thinking that the remaining several layers of screen would impede that.

Maybe 4" on the sun side of the absorber and 6" in back. I don't think
the back side is critical and the 4" sounds like plenty of air space in
front. Ditch the taper as that was just to force more air through the
top of the collector and that is questionable it would do that, or need
to be done.

  Perhaps I'll make the screen removeable.

especially with

   I think you are right. The denser the more even the airflow through,
  Bill Kreamer does the black felt, but then I would lose all view.
Seems like I need to make a test strip to see how the airflow is.


I've been wanting to get an IR thermometer I think the temperature
reading is probably the way to go. Wonder what effect the glazing will
have on the reading. Perhaps a little flap of paper that speads from the
absorber, sort of like the leaves of an electroscope.

   It seems to me that the temp vertically is where it will be uneven.
Perhaps some riser tubes off the manifold that would go up half way.

Good idea. I had just planned on closing the windows!

  Seems like that may be so. Thanks for taking so much time and giving
me so many ideas.

   Cheers,
Jeff


Posted by Gary on December 4, 2006, 10:04 pm
 
Hi Jeff,

...


A quick experiment with a dark colored piece of alum sheet in the sun.  I
measured temperature of the sheet without glazing, and then with Acrylic,
Polycarbonate, Vinyl, and Polyethylene glazing materials -- idea being that if
the temperature measured with the IR scanner is close to the alum sheet, than
the IR scanner is seeing through the glazing to the alum, and if its closer to
the glazing temperature, then the IR scanner is seeing glazing temperature.

Results:

Bare Alum        110F
Polyethylene        101F
Acrylic            60F
Vinyl            85F
PVC            82F
Polycarbonate        81F

This is not to scientific, but it looks like the polyethylene is the only one
that appears to be reading the radiation from the alum (or pretty close anyway).
This would seem to agree with the transmission spectra I have seen for
polyethylene and polycarbonate.

So, maybe if you covered with one layer of polyethylene, you could use an IR
temperature scanner to get an idea where the hot/cold spots are on the absorber?

Gary




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