Re: [EnergyPlus_Support] Daylighting

[fred buhl <wfbuhl@xxxxxxx>]

Thanks Peter (Ellis). The criteria are described quite well in the 
Engineering Document under the heading "Time-Step Luminance".
The reference is Perez et alia, "Modeling Daylight Availability and 
Irradiance Components from Direct and Global Irradiance",
Solar Energy 44, pp 271-289, 1990.

Fred Buhl

Ellis, Peter wrote:

> 
>Hello Peter,
> 
>The sky type is selected using a sky weighting factor determined by SkyClearness and SkyBrightness:
> 
>3.0 < SkyClearness < 4.0  -->  interpolate between "clear sky" and "clear turbid sky"
>    SkyWeight = MIN(1.0,(SkyClearness-3)/3)
>    ...if SkyWeight = 1, then it's a clear sky
> 
>1.2 < SkyClearness < 3.0  -->  interpolate between "clear turbid sky" and "intermediate sky"
>    SkyWeight = (SkyClearness - 1.2)/1.8
>    ...if SkyWeight = 1, then it's a clear turbid sky
> 
>1.0 < SkyClearness < 1.2  -->  interpolate between "intermediate sky" and "overcast sky"
>    SkyWeight = MIN(1.0, MAX(0.0, (SkyClearness-1)/0.2, (SkyBrightness-0.05)/0.4))
>    ...if SkyWeight = 1, then it's an intermediate sky 
>    ...if SkyWeight = 0, then it's an overcast sky
> 
> 
>"Sky Clearness for Daylighting Calculation" and "Sky Clearness for Daylighting Calculation" are output variables that you can report out, if you wish.  
> 
>The final daylighting factors are interpolations/weighted averages (using SkyWeight) between the daylight factors calculated for each discrete sky type.  
> 
>SkyClearness is calculated using BeamSolarRad and DifSolarRad come from the weather file:
> 
>      Zeta = 1.041*SunZenith**3
>      SkyClearness = ( (DifSolarRad + BeamSolarRad)/(DifSolarRad + 0.0001) + Zeta )/(1.+Zeta)
> 
> 
>SkyBrightness is calculated as follows:
> 
>SkyBrightness = (DifSolarRad * 93.73)* AirMass / ExtraDirNormIll(Month) 
> 
>REAL, PARAMETER, DIMENSION(12) :: ExtraDirNormIll=  &     ! Monthly exterrestrial direct normal illuminance (lum/m^2)
>      (/131153.,130613.,128992.,126816.,124731.,123240., &
>        122652.,123120.,124576.,126658.,128814.,130471./)
> 
> 
>Let me know if you have further questions.
> 
>Peter Ellis
>NREL
> 
> 
>-----Original Message-----
>From: EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx]On Behalf Of Peter G. Loutzenhiser
>Sent: Wednesday, May 11, 2005 1:54 AM
>To: EnergyPlus_Support@xxxxxxxxxxxxxxx
>Subject: [EnergyPlus_Support] Daylighting
>
>
>Dear All,
>
>I am interested in the daylighting algorithm described in the Engineering 
>Auxillary manual and by Winkelmann and Sellkowitz (1985).  What is the 
>criterion (or what are the criteria) for selecting the sky conditions (there 
>are four in EnergyPlus and two in DOE-2.1e), and what specific parameters are 
>these conditions a function of?
>
>Many thanks,
>
>Peter
>
>
>
>
>
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