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Re: [EnergyPlus_Support] Weather Data
Here is the formula:
! Calculate Sky IR
!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
!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.
I suppose the manual should be changed to just say "opaque sky cover".
Linda
EnergyPlus WebSite: http://www.energyplus.gov
Owner: EnergyPlus_Support list
Member: EnergyPlus Development Team
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