Re: [EnergyPlus_Support] A question about sky temperature

[Linda Lawrie <linda@xxxxxxxxxx>]

As noted earlier in this group (available by searching at the yahoo 
group site):

It uses the Horizontal IR from sky as a basis (we calculate this on
the weather files and design days)

          ! Calculate sky temperature, use IRHoriz if not missing
          IF (IRHoriz == "missing") THEN
!         Missing, use sky cover
            OSky=OpaqueSkyCover
            TDewK=MIN(DryBulb,DewPoint)+TKelvin
            ESky= (.787 +.764*LOG((TDewK)/TKelvin))*
                              (1. + .0224*OSky - 0.0035*(OSky**2) + 
.00028*(OSky**3))
            SkyTemp=(DryBulb+TKelvin)*(ESky**.25)-TKelvin
          ELSE  ! Valid IR from Sky
            SkyTemp=(IRHoriz/Sigma)**.25 -TKelvin
          ENDIF

Sky IR is calculated as:
         ! Determine Sky Temp ==>
         ! Function of DryBulb, DewPoint, OpaqueSkyCover
         ! 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/m2

         !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.



At 01:17 AM 6/25/2007, wanglei wrote:
>   After reading the document of InputOutputReference, we can know 
> the sky temperature is calculated using the flowing formula:
>   sky Temperature=power((Horizontal IR/SigMa),0.25)-273.15
>who can give me the more detail literatures about how to define the 
>value of  Horizontal IR and calculate the sky temperature.

Linda

EnergyPlus WebSite: http://www.energyplus.gov
Owner: EnergyPlus_Support list
Member: EnergyPlus Development Team



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