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

[Chris <chris@xxxxxxxxxxxxxxxxxxxxxxxxxxx>]

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



------------------------------------

The primary EnergyPlus web site is found at:
http://www.energyplus.gov

The group web site is:
http://groups.yahoo.com/group/EnergyPlus_Support/

Attachments are not allowed -- please post any files to the appropriate folder in the Files area of the Support Web Site.

EnergyPlus Documentation is searchable.  Open EPlusMainMenu.pdf under the Documentation link and press the "search" button.
Yahoo! Groups Links

<*> To visit your group on the web, go to:
    http://groups.yahoo.com/group/EnergyPlus_Support/

<*> Your email settings:
    Individual Email | Traditional

<*> To change settings online go to:
    http://groups.yahoo.com/group/EnergyPlus_Support/join
    (Yahoo! ID required)

<*> To change settings via email:
    mailto:EnergyPlus_Support-digest@xxxxxxxxxxxxxxx 
    mailto:EnergyPlus_Support-fullfeatured@xxxxxxxxxxxxxxx

<*> To unsubscribe from this group, send an email to:
    EnergyPlus_Support-unsubscribe@xxxxxxxxxxxxxxx

<*> Your use of Yahoo! Groups is subject to:
    http://docs.yahoo.com/info/terms/