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followup regarding a thermosiphon panel

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Posted by Tater on October 30, 2006, 11:34 pm
 
well, Its been up a month and I thoguht I would share.

I built a 24sq ft thermosiphon panel on the south side if my garage
wall to see if there was a possibliltyof using for home heating.

well, results have not been astounding. two matched thermometers have
shown that there has not been a  tempature change in the garage at all.
the panel appears to work, although i think i need to tweak a few
things.

the exit vents on the panel do siphon in sunligt, but it doesnt seem to
be getting anywhere hot enough, and I belieleve the reason for that is
the glazing i used. one website said to use 6mil film or less(the
thinner the clearer) and i used 4mil film, but it was not what anyone
would call clear. plans to use polycarbonate panels are in use, but I
have not been making it a high priority, and it seems sources in my
area are mythic :)

I promised myself that i would do a folluwup, so here it is. hopefully
i can find a suplier of clear polycarbonate so i can replace the milky
white film and see a tempature boost.


Posted by SJC on October 30, 2006, 11:59 pm
 


   If you can find clearer film, you might try some wood strips and a second
layer.
You would be making a low cost multiwall using poly clear film. It would pass
the IR through, but the air gap would be an insulator to keep heat in.


Posted by Gary on October 31, 2006, 3:12 am
 Tater wrote:

I think you would be better off to measure the output of the collector panel,
and see if its efficiency is in the reasonable range.  You just need to measure
the temperature rise from inlet to outlet, and the outlet air velocity.  The
procedure I use is here:
http://www.builditsolar.com/References/Measurements/CollectorPerformance.htm
To measure air velocity, the Dwyer $5 van meter, or something like a Kestrel
wind meter work fine.
http://www.builditsolar.com/References/Measurements/measurments.htm

Once you know the output of the collector, you can compare that to the
calculated heat loss for the house, and get some idea how much of your heat load
   collectors of a given size would supply.

Thermosyphon collectors should do fine on efficiency, but they are sensitive to
following the design rules, which are roughly:

- Fairly tall -- 7 or 8 ft is good.
- collector depth about 1/15 or more of height (for an 8ft high one, 96/15 =
6inches deep)
- absorber that absorbs well, but has low flow resistance (2 layers of black
window screen, expanded metal lath, ...  (something like felt is to tight for a
thermosyphon collector)
- The air flow path is through the absorber
- The air flows from the south (glazing) side to exits on the north side.
- The entry and exit vents area at least half of the cross sectional area.

And a glazing the transmits light :)

Most of these design rules are aimed at keeping the flow resistance down. The
only thing driving the air through the collector is the buoyancy of the heated
air.  This is not a powerful force, so the flow resistance must be low to get
good air flow.

Its easy to violate the depth, vent area, and absorber requirements -- if you
do, the collector won't work well.  If you want a collector that is more
forgiving, then add a fan.  The beauty of the thermosyphon (to me) is that if
you follow the somewhat picky design rules you end up with an inexpensive,
efficient, no moving parts (except the 2 cent back draft dampers), zero
maintenance collector -- this can be done for just over $ per sqft with all new
materials (compared to $0 per sqft for commercial collectors).

Mine (which I am guessing is typical) with full sun, gives a 50F to 60F
temperature rise, and delivers 120 to 140ft/min air velocity through vents that
are one half the cross section area.  Based on the measured output compared to
the looked up solar input, I believe that my thermosyphon (which is certainly
nothing fancy) does as well as commercial forced air collectors.  I don't see
why yours should not do just as well.

Gary





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









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Posted by Tater on November 1, 2006, 10:53 pm
 
Gary wrote:

hi gary, you are gonna love this. two identical thermometers one at
inlet, one at oulet. zero difference in temp.

I dont think the air velocity is strong enough to spin a vane on a wind
meter :)

putting my hand as close as possible to the output vents does register
heat. it also registers the mildest of air movements. the plastic bag
one way flaps do move open to about 45 degrees.

I really got to get some clear polycarbonate on the panel, but have
been comming up empty on that end.


mine is six feet, the roof slopes so i could not get it any taller,
when i expand i should be able to go 7ft for the next 4 ft wide panel,
then 8 ft for the next 20 ft of panels


a bit less for mine, used 2x4s for depth


just like the one you described in your article. only difference was
that i used plastic film instead of polycarbonate(as described in
another article)


each or total? I approximated the same as what yours were(per your
article)


light does get through, but the film is milky white, and I am guessing
that it is blocking a bit of IR. like I said, i want to get
polycarbonate, but havent found a local source.


It is not. only one thing appears different from yours. and am
currently looking to get that changed.

*IF* it was working correctly, my follouw-up would have been about me
stockpiling materials to expand the one 4ft panel to 4-5 more of them
to heat up a garage half the size of the shop you heated in your
article.

as it is, I am glad I went with this approach rather than shelling out
hundreds of dollars to completely converting my garages south wall to a
solar collector. now is my time to perfect the design, then let 'ol Sol
carnk out the BTUs


Posted by Jeff on November 2, 2006, 2:02 am
 Tater wrote:

I'm doubtfull that this is the only reason, but you can run some tests
to see where you are.

   Try measuring the temperature at the absorber. You need to see if the
collector is heating up.

   If that is fairly hot, try forcing air through.

   SunTuf, or the Lowes equivalent polycarb was the easiest thing I
found in building my collectors. Everything else was trouble!

   Jeff




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