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

[Ji Zhang]

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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