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re: need higher output fan for my solar collector
23 oct 2001
bill kreamer  wrote:

>...to prevent thermosyphoning, and (if you were to simply stop up the
>openings) collector stagnation and high-temperature damage, you will
>need to use another control method.  in the summer the most effective/safest
>way to stop the heat is to remove or cover the collectors.  all it takes is
>a white canvas cover, or a light-colored lightweight shallow-plywood-box
>cover with latch hooks.

opening vents to the outdoors might be less trouble, one at the bottom
and one at the top.

just noticed an interesting paper, "development and testing of low-cost
solar energy collectors for heating air," by n.k. bansal and r. uhlemann
of the program group for systems research and technological development,
kernforschungsanlage juelich, postfach 1913, 5170 juelich 1, f.r.g in
solar energy vol. 33, no. 2, pp 197-208, 1984. they talk about optimizing
pore size and matrix thickness for porous polyester absorbers, with
electron microscope photos. 

their inflatable(?) collector covers were "transparent uv stabilized pvc
foils, 0.6 mm [0.024"] thick, with 83% solar transmittance." the sheets have
an average 7% transmittance to ir over 7-20 micrometers. they say "the lower
transparent sheet... helps to double the lifetime of the collectors; when 
the absorptance of the top cover becomes too high, the collector is simply
inverted so that the bottom pvc sheet acts as a cover."

their collectors were long, 9 to 18 meters, by about 1 meter wide. the
absorber was 2 mm (0.079") thick. in one run, they measured 49% efficiency
with 774 kg/h of airflow and an 18.2 c temperature rise (with 20.3 c air
in) with 876 w/m^2 of sun and 3 cm of polyethylene back insulation.
increasing the back insulation to 6 cm raised the efficiency to 61%.
the pressure loss over the 9.1 m^2 collector was 1 mm h20 at 360 kg/h
of airflow and 4.5 mm at 960 kg/h.

they say their collector costs 32 dm/m^2, or 80 dm/m^2 with the frame and
back insulation (in 1983) vs 400 dm/m^2 for "a conventional collector
fabricated from blackened metallic scraps as the matrix" with a glass cover
and metal frame and back insulation. they compare the cost of collected
energy and say "it is clearly seen from the graph that even if one assigns
a shorter lifetime to the present collector, its economics is seen to be
much better than that of the conventional one," eg 0.04 dm/kwh vs 0.12,
with a 4 year lifetime. 

nick




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