[Radiance-general] [help] Does the "direct horizontal irradiance" in GenCumulativeSky mean the "direct normal radiation" in .epw weather file?

[Jia Hu]

Hi,

I am not familiar with this program. I just skim its code. This program use
Horizontal Irradiance values, Not normal. In its "cPerezSkyModel.cpp" file,
you can see this:

if (m_SolarAlt > 0)
Ibn=Ibh/sin(m_SolarAlt);

It converts "direct horizontal" (lbh) to "direct normal" (lbn) and then use
lbn in other parts in this class.It is better to re-calculate "Direct
horizontal irra" which would be served as an input

Another option is to revise some parts of the code. Change the above code to
the follows and rebuild to get a new .exe file:

if (m_SolarAlt > 0)
Ibn=Ibh;


In your input weather file (txt), the first column is "global horizontal"
and the second column is "direct normal". Then you set -G option in its
command.

I have not tested the second method.

Best,

Jia



On Thu, Jul 21, 2011 at 2:42 PM, Ji Zhang <hope.zh at gmail.com> wrote:

> Hi, Jia, Thanks for the clarification!
>
>
> Well, if this is the case, does it mean that:
>
> Either
> 1.GenCumulativeSky requires "direct horizontal irradiance" as input for
> calculation, but we can only feed in the "direct normal radiation" as that
> is what we can get from EnergyPlus weather file. Then, GenCumulativeSky will
> "internally" transform the "direct normal radiation" into "direct horizontal
> irradiance" by applying the formula: "Direct Horizontal Irradiance = Direct
> Normal Irradiance *  Sin(Sun_Altitude)".
>
> Or
> 2. What is used in GenCumulativeSky's internal calculation is actually
> "direct normal irradiance". Then what is stated in the "Solar Irradiation
> Modeling" document is not correct.
>
>
> So, Jia, Can you kindly help to check if the above is correct?
>
> And Dear Darren Robinson, if you can receive this email, can you also
> kindly comment?
>
> Thanks!
>
> Ji
>
>
>
>
> On Fri, Jul 22, 2011 at 2:14 AM, Jia Hu <hujia06 at gmail.com> wrote:
>
>> Yes, you are right. Sorry for my typo.
>>
>>
>> On Thu, Jul 21, 2011 at 2:12 PM, Ji Zhang <hope.zh at gmail.com> wrote:
>>
>>> Hi, Jia, I'm confused...
>>>
>>> Should it be "Direct Normal Irradiance = Direct Horizontal Irradiance /
>>> Sin(Sun_Altitude)" ?
>>>
>>> - Ji
>>>
>>>
>>>
>>>
>>> On Fri, Jul 22, 2011 at 12:35 AM, Jia Hu <hujia06 at gmail.com> wrote:
>>>
>>>> Hi:
>>>>
>>>> The relationship is: Direct Normal Irradiance = Diffuse Horizontal
>>>> Irradiance / Sin(Sun_Altitude)
>>>>
>>>> Regards,
>>>>
>>>> Jia
>>>>
>>>> On Thu, Jul 21, 2011 at 6:41 AM, Ji Zhang <hope.zh at gmail.com> wrote:
>>>>
>>>>>  Dear Radiance experts,
>>>>>
>>>>> According to the "Solar Irradiation Modeling" document inside the
>>>>> GenCumulativeSky package (downloaded from
>>>>> https://documents.epfl.ch/groups/u/ur/urbansimulation/www/GenCumSky/GenCumSky.zip),
>>>>> the GenCumulativeSky.exe program needs a climate file containing two columns
>>>>> of data, the first column being either the "global horizontal irradiance" or
>>>>> "direct horizontal irradiance" and the second column being the "diffused
>>>>> horizontal irradiance".
>>>>>
>>>>> In a typical EnergyPlus weather file (.epw file), there's the hourly
>>>>> data on "global horizontal radiation", "direct normal radiation" and
>>>>> "diffuse horizontal radiation".
>>>>>
>>>>> May I ask:* Is the "direct horizontal irradiance" as required by
>>>>> GenCumulativeSky the same as the "direct normal radiation" as recorded in
>>>>> the EnergyPlus weather file?*
>>>>>
>>>>>
>>>>> To my understanding, the "direct normal radiation" means the "amount of
>>>>> solar radiation in Wh/m2 received within a 5.7° field of view centered on
>>>>> the sun" (refer to http://rredc.nrel.gov/solar/pubs/tmy2/tab3-2.html).
>>>>>
>>>>>
>>>>> So, the "effective" direct horizontal irradiance, or the effective
>>>>> solar radiation falling vertically on a horizontal surface, should be a
>>>>> cosine-weighted "direct normal radiation" according to the solar elevation
>>>>> angle at a given time. Therefore, the "direct horizontal irradiance" as
>>>>> required by GenCumulativeSky does NOT mean the same as the "direct normal
>>>>> radiation" as recorded in the EnergyPlus weather file.
>>>>>
>>>>> Clarification is appreciated!
>>>>>
>>>>> - Ji
>>>>>
>>>>> _______________________________________________
>>>>> Radiance-general mailing list
>>>>> Radiance-general at radiance-online.org
>>>>> http://www.radiance-online.org/mailman/listinfo/radiance-general
>>>>>
>>>>>
>>>>
>>>> _______________________________________________
>>>> Radiance-general mailing list
>>>> Radiance-general at radiance-online.org
>>>> http://www.radiance-online.org/mailman/listinfo/radiance-general
>>>>
>>>>
>>>
>>> _______________________________________________
>>> Radiance-general mailing list
>>> Radiance-general at radiance-online.org
>>> http://www.radiance-online.org/mailman/listinfo/radiance-general
>>>
>>>
>>
>> _______________________________________________
>> Radiance-general mailing list
>> Radiance-general at radiance-online.org
>> http://www.radiance-online.org/mailman/listinfo/radiance-general
>>
>>
>
> _______________________________________________
> Radiance-general mailing list
> Radiance-general at radiance-online.org
> http://www.radiance-online.org/mailman/listinfo/radiance-general
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.radiance-online.org/pipermail/radiance-general/attachments/20110721/126ce118/attachment.html>