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



Feng,

>
>
>   Thanks Linda for the Fortran code. Thanks Chris for reading and 
> following this question.
>
> I read the code, and not sure about some variable name stand for. But 
> some equations are understandable. I copy the code and put my 
> explanation below, my equations are from ASHRAE Handbook 2005 Chp31.16 
> (Handbook)
>
> IF(CosZenith >= SunIsUpValue) THEN
>            ! Sun is up.  Calculate solar values
>            TotHoriz = DesDayInput(EnvrnNum)%SkyClear * A * (C + 
> CosZenith) * EXP( -B / CosZenith)
>
> Here, calculate horizontal radiation. I assmue CosZenith is similar 
> with Handbook's sin(beta) which is solar altitude angular calculation.

Yes, cos(Z)=sin(beta).

> We can see that it still has A * exp(-B / CosZenith) term but with one 
> more (C + CosZnith) term which I think is diffustion calculation.

Actually, the uncorrected diffuse horizontal irradiance is Diffuse=C*A 
exp(-B/cosZ) and the uncorrected direct horizontal irradiance is 
cosZ*DirectNormal = cosZ*A exp(-B/cosZ). The sum of the two gives you 
the global horizontal irradiance, denoted TotHoriz, after 
multiplication/correction by a clearness number CN. This equation is 
actually incorrect. The correct ASHRAE equation is Global = 
CN*DirectNormal*cosZ + Diffuse/CN^2.
Linda has not provided the values of A, B, C or CN actually used in E+, 
so this is another layer of concern.

>
>            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)))
>
> I am not sure what going on here. It is not ducumented in E+ nor in 
> current version ASHRAE Handbook

It is not from ASHRAE. Kt is the so-called clearness index= Global/ET 
Global. This empirical equation is a way to obtain Diffuse from Global. 
I don't know why Diffuse is recalculated this way.

Chris

-- 
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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