All,
Attached is the illuminance (global horizontal) for Boise, ID, from
TMY2 data. Note the valley in the graph on the left bottom corner
which is caused by the very low illuminance values for January (the
values are in the hundreds, compared to tens of thousand in February).
I know for a fact that in Boise January is not (much) darker than
December or February.
Furthermore, the corresponding radiation data do not have that dip in
January. The illumination values are calculated based on the radiation
data, right? So if there is nothing wrong in the radiation data, is
this related to the model used in converting radiation data into
illumination values?
I also checked another site in Idaho, it has the same dark January.
Any thoughts?
Thanks,
Ery
Hi Ery,
I don't know if this was also a problem with TMY2 or if you meant TMY3,
but TMY3 files have the problem you observed. See the notice listed on
this website:
http://rredc.nrel.gov/solar/old_data/nsrdb/1991-2005/tmy3/
For what it's worth, certain simulation engines (like EnergyPlus) don't
use that data from the weather file.
Hope that helps,
Scott Horowitz
Scott,
Thanks for the info. I know that the solution can be as simple as that.
This data is of course important only for the (annual) daylighting
calculation. Daysim is using this data. I suspect even EPlus or eQuest
will use this data for daylight control.
Thanks,
Ery
Hi Ery,
I think illumination value can't be calculated from radiation data.
Different skymodel and altitude of the sun provide different efficacy.
Thanks,
Eddy Santosa
Anyone seen this error and know how to fix it?
Timothy Howe, MS, LEED(r) AP