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re: refers/panel radiation to night sky?
12 oct 1998
warren lauzon wrote:
>...at this point, i still remain unconvinced either way...
this effect seems real to me. i've read that middle-easterners have
long made ice in the desert when the air temp is above freezing by
running water into a shallow pond on the shady side of a wall. in
the morning, they slide the ice layer off and into a nearby pit.
baruch givoni's 1994(?) book "passive and low-energy cooling of
buildings" talks about radiative rooftop surfaces being 10 c cooler
than the air on a still clear night. if the wind blows, it warms up
the surface. he's tried covering the surfaces with (ir-transparent)
poly film to avoid this. i guess that would also help avoid warming
the surface by condensation if it were cooler than the dew point.
satellites use "deep space coolers" to regulate their temperatures.
these are emissive surfaces covered with reflective louvers that open
to expose the surfaces more to 4 k space for cooling, and close to
lower ir loss and keep the satellite warmer.
there are approximate formulas for "effective sky temperature."
here's what duffie and beckman say on pp 157-8 of the 1991 edition
of "solar engineering of thermal processes":
to predict the performance of solar collectors, it will be necessary
to evaluate the radiation exchange between a surface and the sky.
the sky can be considered as a black body at some equivalent sky
temperature ts... the net radiation from a surface with emittance e
and temperature t to the sky at ts is q = eas(t^4-ts^4).
the equivalent blackbody sky temperature... accounts for the facts
that the atmosphere is not at a uniform temperature and that the
atmosphere radiates only in certain wavelength bands. the atmosphere
is essentially transparent in the wavelength region from 8 to 14 um,
but outside of this "window" the atmosphere has absorbing bands
covering much of the ir spectrum. several relations have been proposed
to relate ts for clear skies to measured meterological variables.
swinbank (1963) relates sky temperature to the local air temperature,
brunt (1932) relates sky temperature to the water vapor pressure,
and bliss (1961) relates sky temperature to the dew point temperature.
berdahl and martin (1984) used extensive data from the united states
to relate the effective sky temperature to the dew point temperature,
dry bulb temperture, and hour from midnight t...
ts = ta[0.711+0.0056tdp+0.000073tdp^2+0.013cos(15t)]^0.25,
where ts and ta are in degrees kelvin and tdp is the dew point
temperature in degrees celsius. the experimental data covered a
dew point range from -20 c to 30 c. the range of the difference
between sky and air temperatures is from 5 c in a hot, moist
climate to 30 c in a cold dry climate.
clouds will tend to increase the sky temperature over that for a
clear sky... the sky temperature is critical in evaluating
radiative cooling as a passive cooling method.
let's see. philadelphia has an average daily minimum ta = 67.2 f
(= (67.2-32)/1.8 + 273 = 293 k) in july, with an average humidity ratio
w = 0.0133 pounds of water per pound of dry air, which corresponds to
a dew point of about 65 f (18 c), so at 6 am, when cos(15x6) = 0,
ts = 293[0.711+0.0056(18)+0.000073(18^2)]^0.25 = 280 k or 45 f.
albuquerque, nm has a min ta of 64.4 f (291 k) in july with w = 0.0092
corresponding to tdp = 55 f or 12.8 c, so at 6 am on an average day,
ts = 291[0.711+0.0056(12.8)+0.000073(12.8^2)]^0.25 = 275 k or 35 f.
nick
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