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re: solar panel glazing
11 jul 2000
eugene bleiers wrote:
>hello !
greetings eugene,
>i'm building flat panel solar collector for water heating. will there
>be any significant improvement in efficiency if i put two sheets of
>glass over the panel...
not much. the extra layer of glass reduces the solar energy
transmission as well as the heat loss.
>maybe some new plastic material is better?
i like a single layer of polycarbonate plastic, us r1 with
90% solar transmission. comes in rolls 49 inches wide and
50 feet long with a 10 year guarantee. about $16/m^2.
>i live in latvia, and even in summer air temperature can drop
>to +10 °c or +5°c .
a single layer of polycarbonate might collect 720 w/m^2 in full sun
(am2 at 800 w/m^2.) with 60 c water and 5 c outdoor air and perfect
side and back insulation, you might lose (60-5)/r0.18 = 312 w/m^2 of
heat for a 51% collection efficiency. two layers might collect 648
w/m^2 and lose 156 for 61% efficiency. maybe more single glazing is
better. can you find some average monthly latvian weather data?
>also, i couldn't find any diy info on solar collectors.
>does everybody buy factory made panels?
no, although air heaters are easier to build than water heaters. why
not make one a part of some useful structure, eg a solar attic above
a house or a bus shelter with a parabolic north roof (i've seen these in
southern france) with something like nielsen's mylar mirror ($0.09/ft^2
in 4' rolls from http://www.snomo.com/mylar.html) under sheet metal
attached to parabolically-kerfed rafters? there's less direct beam sun
than overall sun, maybe only an average of an hour a day in the winter,
but concentration can lower cost and improve thermal efficiency, even
at higher water temperatures.
nick
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