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

[Jia Hu]

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
>>>
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>>>
>>>
>>
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