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RE: [EnergyPlus_Support] Re: modelling night sky cooling



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

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