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

[Jia Hu]

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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