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re: some sunspace heatflow basics
21 nov 1996
carl windnagle wrote:
>nick@ufo.ee.vill.edu (nick pine) writes...
>> t = (i+taag/rg+68q)/(ag/rg+q) where
>> i is the total solar input in btu/hr, eg 300 ag, perhaps reduced
>> by a sunspace glazing transmission of less than 100%,
>> ta is the ambient temperature, eg 37.5 f in hartford
>> ag is the sunspace glazing area, eg 100 ft^2
>> rg is the sunspace glazing r-value, eg r1, and
>> q is the fan cfm, eg 1000 cfm.
>what do you assume as the glazing transmission effieciency for the greenhouse
>plastic that you recommend?
the latest nraes-33 greenhouse engineering book lists 92% as the transmittance
of a single layer of poly film. i often assume 100%, to simplify calculations.
>when i drive past a greenhouse it looks bright, which indicates to me that
>it is reflecting a lot of light either before it enters the greenhouse or
>back out of the greenhouse after it entered. does the low-cost greenhouse
>plastic really transmit enough light that a solar collector will work
>efficiently behind it?
i think so. that's a good observation. you might measure the light intensity
inside and outside with a light meter too. most us commercial greenhouses use
2 layers of film in the winter, not 1, inflated with a small blower, and if
the greenhouse is full of plants on a cold day (eg orchids, at 90 f and 90%
rh :-) you may see reflection from condensation on the inside of the glazing.
ir reradiation should not be much of a problem for a greenhouse used as a
sunspace, full of room temperature air, but it may be more of a problem for
a higher temperature thermal store inside, behind its own glazing. so, that
smaller thermal store glazing might better be glass or polycarbonate plastic,
with more "greenhouse effect." or perhaps another layer of poly film, with
dark shadecloth or metal screen in the air gap north of it to reduce ir loss.
nick
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