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Constructing collectors for a solar hot water system - Page 5

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Posted by daestrom on September 29, 2007, 12:00 am
 


In short, the answer to your first question is "Yes, foregoing the flat
plate will seriously impede performance."

A mirrored backing would only work for you if it was shaped like a bunch of
parabolic troughs such that it focused all the reflected light on the
back-side of the tubes.  It could be done, but it would be more cost than a
simple flat plate.  A flat mirror would simply reflect most of the sunlight
right back out the glazing into space.

Without a plate or such 'focused mirrors', you're basically reducing the
collection area from the size of the collector frame down to just the area
of the tubing (actually, only half that area is facing the sun).  So you
see, the plate is a very significant part of the whole setup.

In fact, getting really good thermal connections between the plate and the
tubing is something to work on as well.  Just a couple of clamps or such at
a few points (something some DIY's have tried) isn't going to cut it.  Good
systems have continuous contact all along the tubing.  Solder or brazing is
one method, or special bends in the plate so it grips the tubing is another
(some even with thermal conductive 'grease' or paste between tubing and
plate).

If the plate is very thin, then the heat even has a hard time traveling from
the space between tubing to the tubing connection (think of a
cross-sectional view with a lot of the heat flowing along that thin
cross-section).  So there's always a compromise between how thick a plate to
have or how close together to space the tubing.

daestrom


Posted by JohnC on September 29, 2007, 2:00 pm
 
My opinion is, Spend the money and buy professionally made panels.
Save the money on installation . In this manor you will wind up
getting about 10 percent return on your money and you will have
a system that actually produces heat in usable quantities.
I studied and planned for 2 and a half years and 4months part
time labor. We now have a closed system that so far is producing
almost all our hot water needs. We are in San Antonio,Texas
Your location indicates that your heat gathering will have to be
much more prodigious to be of any use.
Can what you discribe work if you learn the methods you seek?
I am thinking,yes. You will need to have surface areas WAY larger
than 33 x 75. If you are only going to use twenty gallons or so
once a week you may have some success. Hook them up in series
and insulate bunches.
http://www.redrok.com/main.htm  has got some great stuff to read...
Cheers!

On Thu, 27 Sep 2007 18:26:36 -0000, mark.fox@gmail.com wrote:



Posted by mark.fox on September 30, 2007, 1:50 pm
 John, I certainly value your opinion. However, this is a project for
getting my feet (really my head) wet. I'd much rather minimize my
outlay at this point, learn some lessons, and then invest real money
and time for the system for our house. Unless it didn't do much at
all, my initial system would eventually end up as part of a livestock
watering system.

Depending on the time of year, I think I agree with you. Actually, I'm
not so sure what I think. During the summer, our days are really long.
In the middle of winter, they are painfully short. However, I'm
thinking that geometry plays into this a bit. When the sun rises/sets
in mid-summer, it's pretty far north, and wouldn't contribute much to
the collector until mid-morning without some sort of tracking system.
At least, that's what my intuition tells me. I really have to play
with some of these websites and programs that people keep pointing out
to me.

Mark


read...


Posted by nicksanspam on January 1, 2008, 3:17 pm
 With 2'x50'x0.018" rolls of Amerimax 69124 brown/white coil for $8 and 17
10'x1/2" copper pipes for $0.35 and 1" pipes for $5.20, the materials
for a 10'x10' bare collector with 17 10'x7.02" fins would cost about
$8+2x$5.20+17x$0.35 = $04, with the 1/2" pipes silver-soldered into
5/8" step-drilled holes in the 1" pipe headers. It might hang on a south
wall, behind some glazing. Screwing it into 3 4'x8' sheets of 1/2" OSB
into 3 4'x8'x1" foil-polyiso boards onto the south wall might add $0.

A draindown version might look like this, viewed in a fixed font:

                              10'
in   -->-------------------------------------------
         |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  
         |  |  |  |  |  |  |  |  |  |  |  |  |  |  | <-- 17 vertical
         |  |  |  |  |  |  |  |  |  |  |  |  |  |  |        10' fins
         |  |  |  |  |  |  |  |4'x8'|  |  |  |  |  | 4'
    57.6"|  | a|  |  |  |  |  |  |  |  |  |  |  |  |  
         |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  
         |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  
         |  |  |  |..|..|..|..|..|..|..|..|..|..|..|  
         |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  
         |..|..|..|  |  |  |  |  |  |  |  |  |  |  |     10'
         |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  
         |  |  |  |  |  |  |  |4'x8'|  |  |  |  |  | 4'
         |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  
         |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  
    57.6"|  | a|  |  |  |  |  |  |  |  |  |  |  |  |  
         |  |  |  |..|..|..|..|..|..|..|..|..|..|..|  
         |  |  |  .  |  |  |  |  .  |  |  |  |  |  |  
         |  |  |  .  |  | b|  |  .  |  |  |b |  |  | 19.2"
         |  |  |  .  |  |  |  |  .  |  |  |  |  |  |  
         |  |  |  .  |  |  |  |  .  |  |  |  |  |  |  
          ------------------------------------------->-- out

         |   2'   |       4'     |         4'      |

Cutting diagram:
                    19.2" 19.2"        57.6"
                   --------------------------------  
                  |     .     .                    |  
                  |     .     .         a          | 2'
                  |     .     .                    |  
               4' |  b  .  b  .....................|  
                  |     .     .                    |  
                  |     .     .         a          | 2'
                  |     .     .                    |  
                  |     .     .                    |  
                   --------------------------------  
                                  8'

This might be inside a 12'x12' frame on a south wall covered by 3 $4 4'x12'
sheets of U0.58 Thermaglas Plus twinwall polycarbonate with 80% transmission.

In 250 Btu/h-ft^2 sun and 34 F outdoor air on an average December day in Phila,
140 F fins with lots of water flow would gain 0.8x250x100ft^2 = 20K Btu/h with
0.8x250(144ft^2-100ft^2) = 8.8K Btu/h of parasite air heating, like this,
viewed in a fixed font:

     ---      1/(1.5x100)
|---|-->|---------www---------- Ta
     ---     |             |
  20K Btu/h  | 140 F       |
            ---            |
             -             |
             |             |
             -             |
     ---                   |
|---|-->|------------------|
     ---                   |      
  8.8K Btu/h               |        
                           |
34 F --------www-----------
      1/(144x0.58) = 1/83.5
                                   which is equivalent to:

                           Ta
     ---        1/1500     |       1/83.5
|---|-->|---------www------*--------www------
     ---     |                               |
  20K Btu/h  | 140 F                         | 34+8.8K/83.5 = 139.4 F
            ---                             ---
             -                               -
             |                               |
             -                               -

with almost no heat loss to 139 F frame air, so back insulation for the fins
would be useless. If we simultaneously collect frame air to warm the house
and lower the frame air temp and raise the overall collection efficiency,
back fin insulation can raise the water collection efficiency.

Nick


Posted by Morris Dovey on January 1, 2008, 7:47 pm
 Nick...

I think you might find OSB particularly unsatisfying for this design
because of difficulty in joining sheets at edges - and because the
stuff reverts to "tree puke" when exposed to moisture.

I'll be happy to send you a small piece of 1/2" OSB so that you can
verify this for yourself...

--
Morris Dovey
DeSoto Solar
DeSoto, Iowa USA
http://www.iedu.com/DeSoto/



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