[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
RE: [EnergyPlus_Support] Re: modelling night sky cooling
Danny,
Downwelling IR radiation from the atmosphere is
radiation from the gaseous molecules in the atmosphere
which absorb and reradiate IR. This IR not homogenous
across the IR spectrum but concentrated in bands at
which the most important molecules (H20, CO2,
O2)absorb and radiate. An IR thermometer is designed
to operate in the "atmospheric window", a band from
about 10 to 14 microns, where the atmospheric
molecules have the least absorption and radiation.
Therefore when you point it at the clear sky, it isn't
"seeing" much of the downwelling IR, therefore giving
an inaccurate measure of the effective sky
temperature. Also keep in mind that Tsky is an
"effective" temperature, not a true temperature at any
particular location in the sky. The instrument by
which to measure downwelling IR is a pyrgeometer. Look
at:
http://www.answers.com/pyrgeometer?cat=technology
Ed Cox
--- Danny Bishop <bish.dj@xxxxxxxxx> wrote:
> Hi Linda
>
> Is it generally accepted that this is an accurate
> way to calculate sky
> temperature? Can you describe what you mean by
> Horizontal IR from sky is? We
> have tried measuring it by pointing an IR
> thermometer at the sky, and the
> night time temperatures are very low - much lower
> than I saw on the output
> variable from a simulation in E+. Is this an
> appropriate way to measure
> night sky temperature?
>
>
>
> Cheers
>
> Danny
>
>
>
> From: EnergyPlus_Support@xxxxxxxxxxxxxxx
> [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx] On
> Behalf Of Linda Lawrie
> Sent: Tuesday, 19 February 2008 1:59 AM
> To: EnergyPlus_Support@xxxxxxxxxxxxxxx
> Subject: Re: [EnergyPlus_Support] Re: modelling
> night sky cooling
>
>
>
> At 11:28 PM 2/17/2008, bish.dj61 wrote:
> >1) Does the sky temp come with a standard weather
> file?
>
> Sky temperature is calculated from conditions
> presented on the
> weather file or design day. Horizontal IR from Sky
> is (or can be
> present) on weather files. If not, it is calculated
> as follows:
>
> ! Missing, use sky cover
> OSky=OpaqueSkyCover
> TDewK=MIN(DryBulb,DewPoint)+TKelvin
> EmissSky= (.787 +.764*LOG((TDewK)/TKelvin))*(1. +
> .0224*OpaqueSkyCover- 0.0035*(OpaqueSkyCover**2) +
> .00028*(OpaqueSkyCover**3))
> SkyTemp=(DryBulb+TKelvin)*(EmissSky**.25)-TKelvin
> where
> TKelvin is the conversion from C to Kelvin, i.e.,
> 273.15
>
> when HorizIR is present on the weather file,
> SkyTemp=(HorizIRfromSky/Sigma)**.25 -TKelvin
>
> !References:
> !
> !George N. Walton, "Thermal Analysis Research
> Program Reference Manual,"
> !NBSIR 83-2655, March 1983, p. 21.
> !
> !G. Clark and C. Allen, "The Estimation of
> Atmospheric Radiation
> for Clear and
> !Cloudy Skies," Proc. 2nd National Passive Solar
> Conference
> (AS/ISES), 1978, pp. 675-678.
>
> !HIR = ESKY * SIGMA * (TOUT**4)
> !
> !where
> !
> !HIR = horizontal IR intensity (W/m2)
> !ESKY = sky emissivity
> !SIGMA = Stefan-Boltzmann constant = 5.6697e-8
> W/m2-K4
> !TOUT = drybulb temperature (K)
> !
> !The sky emissivity is given by
> !
> !ESKY = [0.787 + 0.764*ln(TDEW/273)]*[1 + 0.0224*N -
>
> 0.0035*(N**2) + 0.00028*(N**3)]
> !
> !where
> !
> !TDEW = dewpoint temperature (K)
> !N = opaque sky cover (tenths)
> !
> !Example: Clear sky (N=0), TOUT = 273+20=293K, TDEW
> = 273+10=283K:
> !
> !ESKY = 0.787 + 0.764*0.036 = 0.815
> !HIR = 0.815*5.6697e-8*(293**4) = 340.6 W/m^2
>
> Linda
>
> EnergyPlus WebSite: http://www.energyplus.gov
> Owner: EnergyPlus_Support list
> Member: EnergyPlus Development Team
>
> [Non-text portions of this message have been
> removed]
>
>
>
>
>
> [Non-text portions of this message have been
> removed]
>
>
____________________________________________________________________________________
Looking for last minute shopping deals?
Find them fast with Yahoo! Search. http://tools.search.yahoo.com/newsearch/category.php?category=shopping
The primary EnergyPlus web site is found at:
http://www.energyplus.gov
The group web site is:
http://groups.yahoo.com/group/EnergyPlus_Support/
Attachments are not allowed -- please post any files to the appropriate folder in the Files area of the Support Web Site.
EnergyPlus Documentation is searchable. Open EPlusMainMenu.pdf under the Documentation link and press the "search" button.
Yahoo! Groups Links
<*> To visit your group on the web, go to:
http://groups.yahoo.com/group/EnergyPlus_Support/
<*> Your email settings:
Individual Email | Traditional
<*> To change settings online go to:
http://groups.yahoo.com/group/EnergyPlus_Support/join
(Yahoo! ID required)
<*> To change settings via email:
mailto:EnergyPlus_Support-digest@xxxxxxxxxxxxxxx
mailto:EnergyPlus_Support-fullfeatured@xxxxxxxxxxxxxxx
<*> To unsubscribe from this group, send an email to:
EnergyPlus_Support-unsubscribe@xxxxxxxxxxxxxxx
<*> Your use of Yahoo! Groups is subject to:
http://docs.yahoo.com/info/terms/