Re: [EnergyPlus_Support] Re: modelling night sky cooling

[Linda Lawrie <linda@xxxxxxxxxx>]

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



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