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Re: [EnergyPlus_Support] Summer Design Day -- Direct Solar Radiation Calculation

Linda,

This is not "pure" ASHRAE, and seems highly convoluted (diffuse 
radiation is calculated twice). I'm curious to see how the code selects 
A, B, C and the clearness number (the original ASHRAE coefficients) for 
locations outside of the USA, since they are not defined by ASHRAE in 
such a case. I would not use these calculations for locations in the 
southern hemisphere or in dusty/hazy areas, due to the potentially large 
errors involved.

Anyway, this is in need of complete overhaul. The 2009 ASHRAE Handbook 
of Fundamentals will have a completely new model with worldwide 
application, which I hope will find its way to E+.

Chris

>
>
> It currently uses a really old ASHRAE clearsky model, hopefullly you 
> can read Fortran:
>
>          IF(CosZenith >= SunIsUpValue) THEN
>            ! Sun is up.  Calculate solar values
>            TotHoriz = DesDayInput(EnvrnNum)%SkyClear * A * (C + 
> CosZenith) * EXP( -B / CosZenith)
>            HO=GlobalSolarConstant*AVSC*CosZenith
>            KT=TotHoriz/HO
>            KT=MIN(KT,.75d0)
>            Wthr%DifSolarRad(Hour) = TotHoriz * (1.0045d0 + KT * 
> (.04349d0 + &
>                                                     KT * (-3.5227d0 + 
> 2.6313d0 * KT)))
>            IF (DesDayInput(EnvrnNum)%SkyClear .GT. 0.70d0)    &
>                      Wthr%DifSolarRad(Hour) = TotHoriz*C/(C+CosZenith)
>            Wthr%BeamSolarRad(Hour) = &
>                                 
> (TotHoriz-Wthr%DifSolarRad(Hour))/CosZenith
>            Wthr%DifSolarRad(Hour) = MAX(0.0d0,Wthr%DifSolarRad(Hour))
>            Wthr%BeamSolarRad(Hour) = MAX(0.0d0,Wthr%BeamSolarRad(Hour))
>          ELSE
>
> when the sun is not up -- these are set to 0.0
>


-- 
Chris A. Gueymard, PhD
Solar Consulting Services
P.O. Box 392
Colebrook, NH 03576, USA
Tel. (603) 237-8550
Fax  (603) 237-5314
http://www.SolarConsultingServices.com



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