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re: gee, somebody actually read our paper...
26 oct 1996
will stewart wrote:
>nick pine wrote:
>> >> consider a 1 m cubical solar closet (fig. 1a) sitting outdoors on an
>> >> average 0 c december day near philadelphia, pennsylvania, usa. the
>> >> south wall of the closet, i.e., the solar air heater, receives about 3
>> >> kwh/m^2/day of solar heat, i.e. ein = 3 kwh/day, more if there is snow
>> >> on the ground or a white surface or a shallow frozen reflecting pool
>> >> in front...
>> >this value is closer to 2.7 kwh/day and that is with a snow covered ground.
>> let's see. the nrel _solar radiation data manual for flat-plate and
>> concentrating collectors_ (printed with a renewable source ink on paper
>> containing at least 50% wastepaper :-) says the 24 hour average december air
>> temperature in philadelphia over the last 30 years has been 2.1 c, not 0 c,
>> with a daytime high of 6.3 c. the nice folks at nrel say the average amount
>> of sun that has fallen on south walls for the last 30 years in december in
>> phila has been 2.9 kwh/m^2/day, 1.8 min, 3.6 max, with an uncertainty of
>> +/-11%, and a standard deviation of 7.3%, given a 20% ground reflectivity,
>> using the perez model (perez, r.; ineichen, p.; seals, r.; michalsky, j.;
>> stewart, will. (1990), "modeling daylight availability and irradiance
>> components from direct and global irradiance." solar energy, 44(5),
>> pp. 271-289.)
oops, i seem to have made a mistake in this paragraph. the last author was
stewart, r., not stewart, will.
>original data from the
>"solar radiation data manual for flat-plate and concentration
>collectors"
>nrel/tp-463-5607 de93018229
>
>december:
> [+/-]
>solar radiation, kwh/m^2/day percentage uncertainty:^11
>avg. 3.1 min. 2.8 max 3.4
yes will, for a south-facing collector at a tilt of latitude + 15 degrees, in
philadelphia, unlike the vertical window above, which gets 2.9 kwh/m^2/day as
i wrote, with a ground reflectance of close to 20%, vs higher snow or ice or
concrete or white pebble or white plastic deckboard or paint reflectances.
>> our single layer of 0.020" polycarbonate glazing with a nominal 92% solar
>> transmittance might have passed about 85% of this.
>
>what do you base this estimate on?
it's a guess. you might call it engineering judgement. i wouldn't.
perhaps you'd like to work out a more exact answer, and post it.
>>our instruments will be living outdoors again this winter, in a larger box,
>>a 10'x 12' cabin at a youth hostel. the sunspace frame is now up and covered.
>>it's a quarter cylinder, about 8' tall and 8' deep and 12' wide, made with 4
>>curved spaced beams, each beam made with 2 12' 1x3s (costing $1.44 each :-)
>>with a 1x3 spacer block every 2' and some drywall screws.
>what is your data logging strategy, assuming you are going to collect
>empirical data to support your assertions?
same as last winter. we will use our $2500 lambert data logger/controller with
easy help menus and data collection screens and non-volatile data and program
memories, and a 19.2 modem, to collect data continuously and record averages
every 5 minutes from sensors for the amount of sun, wind, and 6 temperatures.
one more this winter, for solar dhw. the controller section will again drive
solid state relays to open and close motorized dampers, etc, and it will also
measure the total electrical power used in the structure, including its own,
an average total power consumption of about 11 watts. we will download the
data every few days and play with it, using spreadsheets and other computer
programs, with occasional changes to control algorithms and configurations,
and we will probably write another paper. we might let other people call up
the data logger too, if we like them.
cheers,
nick
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