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Trombe Wall Variation - Page 4

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Posted by harry on April 23, 2009, 9:08 pm
 

My home-made quad glazed frames are wood (4" X 2") which I think is a
better insulator than plastic..  Re the quad glazing, an interesting
feature is that in the morning there is often condensation on the
outside of the glass (ie dew). The outer glass gets cold enough to
permit this. So they must be keeping the heat in.The insulated
shutters are inside the windows anyway.
  One problem is though you rarely get a failure of double glazing
unit these days (ie condensation between the glass) I reckon it would
be pretty easy for my home-made ones to fail.   I coated the timber
reveals in bitumous paint before assembly to try to avoid this. So
far, it's worked. I nailed the beads in with silicon sealant.
The E-glass is not that much more expensive than ordinary glass.
I think my main heat loss is now from infiltration (ie micro
draughts).   It is incredibly difficult to eliminate these.
I'll see if I can get some pix up on the site.


Posted by daestrom on April 25, 2009, 2:44 pm
 


Window glass is only transparent to the very near infrared.  Wavelengths
above about 5 um it becomes opaque and has a farily high emissivity (about
0.9).  At 'room temperatures' the radiation given off by black/grey bodies
is almost all in the far infrared wavelengths longer than 5 um (very little
is in the near infrared where glass is 'transparent').

Using 0.9 emissivity, the heat transfer between two plates of 1 m^2 where
one is warmed by indoors to about 20 C (293K) and the other cooled by the
outdoors to about 0 C (273K), the radiant heat transfer would be

Q=0.9*sigma*1m^2(293^4-273^4)
 = 92 watts
or about 316 btu/hr.

An approximate conductance is simply

92 Watts = u*1m^2*(293-273)
u = 4.6 W/m^2-K

Of course the gas in between the panes and the film layers on the inside and
outside reduce the actual heat transfer from this.  The conductance of a
vertical air film in free convection is on the order of 5-20 W/m^2-K.

So it would seem the radiant transfer between panes is the same order of
magnitude as the interior film layer of air against the glass.  The
'R-Value' of double-paned 3/4 glass in one reference is 2.38 while for low-e
coated is 3.13.  So clearly the lowering of emissivity has a positive effect
on the insulating affect.

daestrom
http://www.raytek.com/Raytek/en-r0/IREducation/EmissivityNonMetals.htm
http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html
http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html


Posted by Tim Jackson on April 25, 2009, 9:42 pm
 daestrom wrote:

Yes, that's the sort of calculation that led to my retracting my earlier
comment.  But it's actually about half what you say, because the glass
sheets are not at the indoor/outdoor temperatures but are midway through
air layer sandwiches. Discounting layer interference and external
radiation to clear sky, we have layer 1, glass, layer 2, layer 3, glass,
layer 4, so if the layers are all equal the temperature difference
between the glass sheets is about half that between the environments.


Tim

Posted by harry on April 19, 2009, 8:11 pm
 On Apr 19, 5:57 am, "Malcom \"Mal\" Reynolds" <atlas-
bug...@invalid.invalid> wrote:

I think it would get far too hot for most plants. Temperatures can hit
70 or 80degC inside a Trombe wall (ie behind the glass).   Even if you
could find ones that survive I think there would be condensation
problems. Why bother anyway?

Posted by Malcom \"Mal\" Reynolds on April 19, 2009, 11:56 pm
 In article
<32e8392c-359f-435f-81a7-193b39339f5d@k2
g2000yql.googlegroups.com>,
 wrote:


Okay, what if isn't a typical Trombe
wall but a thermal mass that makes the
inside wall of a solar facing hallway
while the outside wall is of course glass


The thinking is incremental gains and
air quality improvements. Adding a
little humidity to the environment isn't
always the worst thing.

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