Re: [EnergyPlus_Support] Re: DayLighting Sky clearness

[Chris <chris@xxxxxxxxxxxxxxxxxxxxxxxxxxx>]

The sky luminance model uses more variables than just the sky clearness 
(i.e., sun zenith angle, sky brightness, diffuse irradiance). To 
complicate the picture, these variables are all partially correlated. 
Also, depending on what part of the sky your window would be facing, an 
increase in luminance might correctly result from a decrease in sky 
clearness. For instance a whitish sky (due to light cloudiness or haze; 
thus average sky clearness) will have a much higher luminance than a 
dark blue patch of sky (resulting from high sky clearness) .

Chris



>> I am trying to do some daylighting analysis, and adjusting the sky clearness parameter in the design day specification.
>>
>> I have noticed that as I decrease sky clearness from 1 down to 0.25 , that occasionally the light level *increases* as the sky clearness *decreases*
>>
>> This is totally unexpected and cannot think of any explanation how this could be. Explanations? Could this be correct or is there some error in the model or just inaccuracies in how energy plus calculates it.
>>
>> For an example, the same point, in the same room, just as sky clearness changes, the lux level is.
>>
>> 1.     Lux 274
>> 0.75   Lux 215
>> 0.5    Lux 241
>> 0.25   Lux 125
>>
>> I have rerun every simulation and am getting the same results.
>>
>> Help? Thanks
>>
>>     


-- 
Chris A. Gueymard, PhD
Solar Consulting Services
P.O. Box 392
Colebrook, NH 03576, USA
Tel. (603) 237-8550
Fax  (603) 237-5314
http://www.SolarConsultingServices.com



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