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re: a 16' cube
9 mar 2001
chuck simmons wrote:
>...you give the transmission of the window material as 90%. however,
>transmisson for transparent materials is always given for perpendicular
>incidence.
the cube walls would be 0.020" polycarbonate with an index of 1.6
and a max incidence angle of about 45 degrees in january. what's
the sun-power-weighted average transmittance?
>a cube shaped building is poor since reflection from the windows could
>be 20% or more from each layer of multiple glazing.
say 10%, with this single layer at normal incidence.
>you have not taken account of the spectral relationship of transmission.
polycarbonate's better than glass in that respect...
>the reflector bothers me another way. fresh aluminum on glass is about
>95% reflective. it degrades to less than 90% in a year or two and is
>often as low as 80% in five years time depending on various factors. 90%
>is not a conservative estimate for sustained reflectivity of aluminum on
>glass or mylar.
i disagree. nielsen sells this stuff to hydroponic growers and claims
the initial reflectance is more than 90% and it "lasts for years..."
duane johnson uses it for outdoor heliostats in minnesota with lifetimes
in years. it would be indoors in this application, inside a building
that is likely to be quite dry. there are better materials for outdoor
use, eg 3m's old sa-85 film, which had a 0.25 mil layer of polyurethane
over the aluminum, but they sold that for more than $1/ft^2, vs $0.09.
>your collector efficiency at the parabola focus needs attention. flat
>black paint, even the special paints used inside optical instruments,
>reflect a bit.
the sun hits the polycarbonate target with air on one side and a few
inches of water on the other and a black surface beneath the water.
the water (what's its refractive index?) makes the water-polycarbonate
surface less reflective (1% vs 4%?) polyethylene is cheaper and more
diffusive (what's its index?), and probably better with underwater pvs.
>since there will be loss from unintercepted solar radiation and heat
>loss into the air and surroundings, a heavy north wall well insulated
>from the outside might be considered. a masonry or rock wall comes
>to mind. this would capture some lost heat.
sounds pretty useless and expensive compared to making the target
as thin as possible, so it loses minimal heat at night.
some people look for problems, and some people look for solutions.
nick
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