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re: a 16' cube
9 mar 2001
chuck simmons wrote:
>> 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?
>essentially, i was asking you...
i invite you to answer that question.
>> >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 don't get normal incidence at any time during the day. that is the
>point.
who cares? what's the sun-power-weighted average transmittance?
>> >...90% is not a conservative estimate for sustained reflectivity
>> >of aluminum on glass or mylar.
>> i disagree...
>without seeing measurements on these materials, i have only experience
>with deposited aluminum films on surfaces.
you might get a sample and measure how the reflectance changes over
10 years inside a dry building.
>the index of refraction of water is 1.33 near enouge. you have an
>air/polycabonate interface and then a polycarbonate/water interface.
so the polycarb-water reflectance is 100((1.6-1.33)/(1.6+1.33)^2 = 0.85%.
>...i would think that eliminating both reflective interfaces would
>improve efficiency.
there are many things that could be done to improve efficiency, but
i'm more interested in cost-effectiveness, with oil at $1.50/gallon.
>i don't think polyethelene is a good idea. being more diffussive means
>it is a scatterer making it probably more lossy yet.
the nraes-33 greenhouse engineering book lists glass with an 88% solar
transmission, polycarbonate with 91-94% and polyethylene at 85%, but
polyethylene film is sooo cheap, $10 for a 4'x16' collector duct, and
scattering makes more uniform illumination for 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...
>> sounds pretty useless and expensive compared to making the target
>> as thin as possible, so it loses minimal heat at night.
>it is useless if the windows are not covered by a reflective material at
>night.
but they wouldn't be. this structure would get cold inside at night,
and the heat in the target would be lost, while the tank stays warm.
nick
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