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Plastic glazing

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Posted by Charles Beener on October 7, 2004, 3:53 pm

   I have a 100 Sq. Foot solar hot water heat thats is covered with old
sliding glass doors. Some of the glass is fogy and 50 % has a IR block
coating on it. My question to the group is how much better would two
layers of  plastic glazing be ? What kind of plastic is good type to
use? Where is a good place to buy 8' by 40' of it ?

   Write now I'm getting around 30K BTU s per 5 hour of good sun . I
have pictures and info on my system at.



Posted by Gary on October 8, 2004, 6:44 pm
Charles Beener wrote:

Hi Charles,
Interesting system.

If you use plastic, Polycarbonate with a UV resistant treatment on the
outside is probably a good choice.  It has a high temperature
capability, and long life in the sun.

As to the comparative efficiency of glass and plastic glazing:
Regular double pane glass transmits about 76% of sunlight, and has an
R value of about 1.9.  It sounds like your glass might be a fair bit
worse than this.

Twinwall Polycarbonate glazing transmits about 83% of sunlight, and
has an R value of about 1.6 -- I guess the lower R value might be due
to the ribs that connect the inner and our sheets.
Here is a link with some info on twinwall PolyCarbonate:

A single layer of Polycarbonate will transmit about 90% of sunlight,
and has an R value of about 1.

Here is a very rough efficiency comparison for these glazing materials:

Isun = 250 BTU/hr (Sunlight -- Dec, lat 40, midday, sunny)
Tamb = 35  F      (average ambient daytime outdoor temperature)
Tcol = 120 F      (average collector temperature)

efic = (sun transmitted to absorber - convective loss)/total sunlight

Dbl Glass efic = (250*0.76 - (120-35)/1.9  )/ 250
                =  (190 - 45) / 250 = 58%

Dbl PC efic   =  (250*0.83 - (120-35)/1.6) / 250
               =  (208 - 53) / 250 = 62%

Single PC efic = (250*0.9 - (120-35)/1)/ 250
                = (225 - 85) / 250 = 56%

Not a whole lot of difference?

The single glaze does pretty well -- its convection losses are higher,
but its high light transmission helps to make up for this.

These calcs ignore radiation loses from the absorber plate out the
glazing, but these should not be large with your selective coated
absorber.  The calcs also assume well insulated back of collector, and
does not include other losses in the system (e.g. the pipes etc).

Maybe others have different thoughts on these relative efficiencies??

Single glaze corrugated Polycarbonate is available at home depot type
places for about a $ per sqft.  Single glaze flat sheet is available
in rolls -- not sure where you get this.  The twinwall is available
from glass suppliers and greenhouse supply places -- its fairly pricey.

If you switch to plastic glazing, you want to leave more room for
thermal expansion than with glass.  If you use single glazed
corrugated PC panels, you will probably need some light horizontal
support strips to control thermal deflection of the panels -- take a
look at the picture near the end of the first pdf link at this site to
see an example of corrugated PC support:
Also, if you google for something like "greenhouse construction" you
will find info on using and installing plastic glazing.


Posted by Gary on October 8, 2004, 9:35 pm
 Gary wrote:

Hi again,

I was wondering how you liked the absorber plates from
Any problems with them?
Would you recommend them?

Thanks -- Gary

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