Illuminance source data for Daylight calc in DesignBuilder

asked 2018-01-26 11:27:20 -0600

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updated 2018-01-26 13:36:26 -0600

__AmirRoth__

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Hi all, Referring to DesignBuilder v4.7, from which source are taken illuminance data when you are performing a daylight calculation,? I mean, the calculation gives as output lux values on the working plain height selected, but which are the starting values of illuminance? (This excludes option "6- CIE overcast sky (specify illuminance)" in wich you can put illuminance value at zenith point). I've a feeling that the initial illuminance data are built only from latitude and longitude of the location, that is, they do not take into account any irradiance or illuminance data contained in the EPW file. Can someone confirm what i'm guessing? (in the help i've not found reference about this issue) Thank you Alessandro

answered 2018-01-27 02:33:20 -0600

Andy Tindale
121 ●3 ●2

DesignBuilder v4.7 daylighting calculations don't take into account the illuminance/irradiance data in the epw file. By default for the overcast sky options the sky illuminance is taken as 10,000 lux. I'm not sure about the illuminance used in the sunny sky options, but they are standard CIE sky distributions.

In v5.2 there are options to define the sky illuminance distribution using either the CIE skies (the only options in v4.7) or a set of "Perez all weather" options:

Brightness and clearness (enter delta and epsilon)
Enter beam and diffuse horizontal irradiance in W/m2
Enter beam and diffuse horizontal illuminance in lux

With options 2 and 3 it is straightforward to copy values from the weather file to define the sky illuminance.

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I'd like to add a warning that not all EPW weather files have illuminance data. It all depends on whether illuminances were calculated in the source weather file from which the EPW was created. EPW is simply a derivative data format, which is one of the reasons I refrain from using that as a way to describe weather files.