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

[Feng Kelvin <kelvin.tju@xxxxxxxxx>]

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. 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.

          
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

           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))

Here it calculate diffuse radiation and direct radiation from previous horizontal radiation term.

So, E+ direct solar radiation calculation is calculated from horizontal radiation. And diffuse + direct = horizotal in E+ Fortran code.

This procedure is different from ASHARE handbook. Current procedure calcuate Direct Normal radiation (EDN) first, then calculate diffuse radiation from EDN. I copy my C++ code (developed based Handbook Chp31.16) in comparison with existing procedure . I guess the error come from the procedure to calculate Direct Radiation.

if (beta_r <= 0 ) {
    // the sun has not rised up
    theta_r = 0;
    return 0;
}
else
// the sun is up
{
    // edn, direct normal irradiation (or solar irradiance)
    double edn = (m_A / exp(m_B / sin_beta)) * m_clearnessNumber;
    // e_diff, diffuse radiation incident on surface
    
    // ? calculate incidental angle theta
    // ? get cos_theta ... skip here....

    double e_diff;
    // if surface is vertical, then use "Y" ratio
    if (abs(tilt-90) <= 2 ) {
        // y, sky diffuse radiation ratio
        double y;
        if (cos_theta > -0.2)
            y = 0.55 + 0.437 * cos_theta + 0.313 * std::pow(cos_theta, 2);
        else
            y = 0.45;
            e_diff = m_C * y * edn;
    }
    // non-vertical surface
    else
    //For non-vertical surfaces,
    e_diff = calculateVFSS(tilt) * m_C * edn;
}

then we calculate ground reflectance radiation.

So my conclusion is, existing E+ method compared with ASHRAE Handbook method, will have much lower direct radiation value (so does diffuse radiation) in the morning time. And the error at noon time is not that big. That is why we have lower heat load in E+'s results compared with Handbook method in the morning time but still can have similiar peak load. 

Those are all based on personal evaluation. If there is any problem, please kindly let me know.

-- 
Best Regards

Kelvin Feng

On Thu, Jul 23, 2009 at 9:52 PM, Chris <chris@xxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

 

Linda,

I'm not sure what implementation problem you found (maybe we should
discuss this offline), but in any case we have not resolved the
requester's problem. The portion of code you sent does not provide the
direct irradiance nor the values of A, B, C and clearness index, so we
cannot compare to his results. Can you please double check for all this?

Chris

>
>
> It would be nice to include the ASHRAE 2009 calcs BUT there are some
> table lookups that won't be appropriate for all locations.
>
> Will/would be difficult to implement. But gets put into the mix of
> other things on the priority list.
>
>

--
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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