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Re: Shading sunspace ceiling/roof - inside vs outside?

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Posted by nicksanspam on June 3, 2005, 7:42 pm
 


What are the philosophical reasons? IIRC, greenhouse shadecloth is made
from old soda bottles... 80% shade costs about 14 cents/ft^2 and lasts
10 years or so and might save the heat equivalent of 2 gallons of oil per
square foot per year. A linear foot of 8' tall x 8' deep sunspace glazing
covered all over with 80% shadecloth would gain about 0.9x0.2x8(790+1940)
= 3931 Btu on an average 71.8 F June day in Phila. With some thermal mass
and an 8' x 1 ft^2 chimney, we might have 24hx16.6x1xsqrt(8)dT^1.5 = 3931,
so dT = 2.3 F, and the sunspace would only be 2.3 F warmer than outdoors.

Then again, Big Fins under glass might make hot water for showers...

I improved the aesthetics of black shadecloth over sunspace glazing by
painting vertical white stripes on it with a narrow roller. You can also
find shadecloth in light colors with flowery designs.


Sure, altho warm air rises.


Glass does not transmit IR well. It would still reflect under shadecloth,
especially with a few spacers between the cloth and the glass. You might
pull it tight over the spacers with a horizontal roller or bungee cords
through grommets.


You might look for a brise-soleil (sunbreaker), often found over the
near-horizontal back windows of sports cars. You might build one from
4" thinwall PVC pipe sliced in half lengthwise with latex paint on top.

Nick


Posted by Loren Amelang on June 4, 2005, 8:50 pm
 
On 3 Jun 2005 15:42:34 -0400, nicksanspam@ece.villanova.edu wrote:


I doubt it. Soda bottles are PET. Shadecloth is either HDPE or
polypropylene. I haven't seen any vendor claim any recycled content,
not even of milk jugs or grocery bags.

Granted, my objections are mainly aesthetic...  


Save heat? As in reduced air conditioning costs, I guess? This is an
offgrid home, no AC.


So I'm guessing 790 btu/sqft comes in the vertical part and 1940 in
the horizontal part? And those values come from a table for a specific
latitude? The 0.2 is what gets through the shadecloth, but what is the
0.9? And how does the ambient temperature affect this? On a 72 degree
day I have no problem - it is those 95 F days that prompt the shade
project.

That doesn't quantify re-radiation downward from the shadecloth, or
whether it is black or white or aluminized. Has someone demonstrated
those are insignificant, or is there just not a formula sophisticated
enough to include them?

Actually, I'd be tempted to sacrifice aesthetics for aluminized
shadecloth if I thought it would maintain its reflectivity, but the
reflectivity of my aluminized plastic bird-scare tape barely lasts the
month required to mature and harvest fruit. And if it wasn't waving in
the wind, the collected road dust and grime would defeat it even
sooner. (No summer rain in California.)

I guess my real issue here is how much of the extra heat that would
enter the sunspace if the shading was just under the glass rather than
over it would end up stored inside the structure. The sunspace ceiling
has insignificant thermal mass. If the same amount of solar radiation
is kept off of the thermal mass of the floor and of the concrete wall
between sunspace and living space, does it matter if the ceiling is
warmer during the day?

The hot air in the top of the sunspace shouldn't convect downward to
heat the floor and walls. It seems the only way for that heat to get
to the massive floor and walls would be by re-radiation from the shade
material. Having the shade material above the glass would reduce that
a bit, but the shade would be proportionately hotter - especially if
it was black synthetic.


Is that still for each lineal foot of your example sunspace? So that
the chimney would need to extend the full length of the space? I guess
it is an enclosed chimney? My flue is 4' wide by 8' tall by 8" deep
with a glazed front at 26 degrees off of vertical, and a suspended
absorber plate in the center, plus another 3' vertical section above
that. Not a match for ambient temps above about 85.

...

I thought clear glass did, around 85%, and that's why low-E coatings
help to trap heat.

Loren

Posted by nicksanspam on June 4, 2005, 9:44 pm
 

And longitude (Phila.) The numbers are long-term averages from NREL's
Solar Radiation Data Manual for Buildings ("Blue Book"), which you can
find on line.


That's an approximate solar transmission for single pane glass.


It doesn't.


White and aluminized are better, but IIRC, glass is a poor transmitter
for IR longer than 3 microns (T < 1300 F.) You can prove this by holding
a glass pie plate between you and a hot electric stove burner. Then try
some plastic food wrap...


I don't understand that question, but as an aside, putting concrete in
a sunspace used for heating seems inefficient to me. For summer shading,
you might be OK with aluminized (low-e) shadecloth under the glazing.
Or maybe white, to at least reflect most of the solar spectrum back up
through the glass before it turns into longwave heat. I think you want
to avoid anything dark or emissive that radiates heat to the lower part
of the sunspace.
 
The linearized radiation conductance between two objects at average temp
T is 4x0.1714x10^-8(T+460)^3, eg 1.4 Btu/h-F-ft^2 (US R0.7) at T = 130 F.


With an R0.7 thermal resistance. Not much.


Then again, shadecloth above could be freely air-cooled.


Or be larger, if it handles more than a linear foot of sunspace.


That's better, with more height and a glazed front.
 

No amount of this kind of venting and shading can make a sunspace cooler
than the outdoors. Venting at night can, given thermal mass.


I think they help the glass reflect more and emit less heat. Without them,
glass (e = 0.88) absorbs radiated heat better, warms to a higher temperature,
and emits better from the opposite surface to objects on the other side of
the glass, but those coatings don't reduce the already-low IR transmittance
(the original "greenhouse effect") much for glass, IMO.

Nick


Posted by Loren Amelang on June 6, 2005, 1:47 am
 On 4 Jun 2005 17:44:17 -0400, nicksanspam@ece.villanova.edu wrote:


It took a lot of Googling, but I found a great reference that confirms
this  - "GLAZING PERFORMANCE,
A Vital Signs resource package providing background information and
experimental protocols for the study of architectural glazing"
(a 3.2 MB PDF):
<http://arch.ced.berkeley.edu/vitalsigns/res/downloads/rp/glazing/glaz1-bg.pdf>

It goes far beyond what the window manufacturers share, and I've only
begun to explore all the good background and detail information.


The sunspace is not used as living space during really hot weather,
the doors to the rest of the house are closed. The only way for heat
to get from the sunspace to living space is by conductance through the
8" concrete wall (and the closed doors) between them. When heat gain
is desired, the sun shines on the wall. If the shading, whether above
or below the glass, keeps direct sunlight off the wall, most of it
stays relatively cool. It seems the overheated air trapped between
interior shades and ceiling glass would not be likely to transfer much
heat to the concrete far below. Unless there is significant
re-radiation from the shading material...  


If I was starting over now, I'd probably use an active hydronic
system. But for a totally passive heat gain system, the concrete wall
has been quite satisfactory. I consider glazing the exterior surface
to reduce convective and radiative heat loss at night, but pretty much
there is already plenty of heat on any sunny winter day, and it lasts
well through the night.


Absolutely.


Again this is confirmed in the "Vital Signs" PDF. Except they, and all
of the window manufacturer data I've found, map the effect of low-E
coatings between 0.7 and 3 um, above the range of sensible "fire heat"
radiation. Of course the graphs they show are transmittance, not
reflectance/emissivity, so maybe there is a reflectance effect at fire
temperatures, it just doesn't show up against the already low
transmittance.

This is certainly more complex than it was twenty years ago...  

Loren

Posted by nicksanspam on June 6, 2005, 1:24 pm
 

It might be nice to have that ability. Given the concrete, you might
vent it at night and keep it shaded with no venting during the day.


That's another way of saying it loses lots of heat at night through
the glazing and tends to be lukewarm all winter, vs a low-thermal mass
sunspace that makes hot air that flows through the house during the day
and gets cold at night when the airflow stops and it loses little heat
back out through the glazing.


Another solution would be to paint the top of the glass. Greenhouse
suppliers sell permanent whitewash as well as spray-on solutions that
can be washed or weathered-off over time.

Nick


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