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Source of illuminance values in EnergyPlus daylighting calculations

asked 2017-11-22 07:20:59 -0500

dhollman's avatar

updated 2017-12-30 14:44:22 -0500

I am looking for information about what source of illuminance values are used by EnergyPlus daylighting calculations. So far I have found two documentation sources but they seem to be inconsistent:

(I) In the Auxiliary Programs document it specifically states that the 3 illuminance values are “not currently used in EnergyPlus calculations”. Field: Global Horizontal Illuminance ... It is not currently used in EnergyPlus calculations. Field: Direct Normal Illuminance ... It is not currently used in EnergyPlus calculations. Field: Diffuse Horizontal Illuminance ... It is not currently used in EnergyPlus calculations.

This would seem conclusive except for…

(II) In the Input / Output Reference there is one section which touches on this but I’m not sure what it means:

Section: Missing Weather File Data

The weather description of data contains “missing” descriptors … EnergyPlus will fill in “missing” value with something “appropriate”. … the following are used:

Data item  Supplied Value  Units 
Illuminance Values  0.0  lux (or CD/m2)

I don’t understand how it could assume zero values for illuminance; using any single fixed value does not seem possible. The idea that this data could be "missing" seems contradictory to the statements about weather data files above.

I can’t find any other documentation of illuminance values from weather data. There is a related question here but it is unanswered.

Trying to read between the lines, I assume that EnergyPlus (A) ignores illuminance data in the EPW weather file but (B) calculates illuminance at each time step based on general solar radiation (from the weather file). Is this correct? What is the calculation method if so?


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answered 2017-11-22 09:44:10 -0500

Yeah, I have no idea what's going on with those "missing data" handlers. You know things have gone off the rails when the units note is patently incorrect (i.e., in your example above, lux /= cd/m2). ¯_(ツ)_/¯

Anyway, you are correct in that with EnergyPlus, the photometric values are derived from the radiometric values in the weather file. Check out Chapter 7 of the Engineering Reference, particularly Sec 7.2.5 and its neighbors, for details.

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Thanks. For others who may view this question, the relevant text is:

7.2.5 Direct Normal Solar Illuminance
For purposes of calculating daylight factors associated with beam solar illuminance, the direct normal solar illuminance is taken to be 1.0 W/m2. The actual direct normal solar illuminance, determined from direct normal solar irradiance from the weather file and empirically-determined luminious efficacy, is used in the time-step calculation.
dhollman's avatar dhollman  ( 2017-11-22 10:33:17 -0500 )edit

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Asked: 2017-11-22 07:20:59 -0500

Seen: 294 times

Last updated: Nov 22 '17