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Re: negative solar radiation


I found something that works to avoid the radiation peaks at the beginning and/or end of the day. I put a maximum on the radiation when the sun is below 5°, so a kind of solar obstruction.

T1 is the total radiation on the surface, B1 the beam radiation.
Whit this my simulation runs the whole 8760 hours.

There seems to be a time shift in your data, even though the shift angle you use (-10.65) is correct for the location (assuming the file is in local time). If you change the shift to -13.5 progressively, you will notice that the peaks decrease and start happening on the east side instead of the west side.

You are using a short time step (0.1h) with a hourly data file, and unfortunately those problem are likely to happen. They come partly from small "timing" inaccuracies (either in the data or in Type16) and also from the fact that sometimes pyranometers actually record some positive radiation before the official sunrise (or after sunset). In that case, that radiation should be treated as diffuse, obviously. If I am right Type16 does not do that in all cases, which might explain the peaks. When sunrise/set occurs during a hourly timestep, it is not easy to know which part of the diffuse actually occurred when the sun was down).

Your workaround using upper limits on radiation seems OK to me, but you could also zero the beam radiation only and make the total equal to the diffuse for such low sun altitude angles.
I will keep your example for our TRNSYS 16 tests and we will try to get rid of those very high values.

On a side note, or(gt(zenith,85),eql(zenith,85)) can also be written (1-gt(zenith,85)), which is slightly shorter, and in TRNSYS 16 you will have a "LE" function that does that.

Kind regards,

Michaël Kummert


Michaël Kummert

Solar Energy Laboratory - University of Wisconsin-Madison
1303 Engr Res Bldg, 1500 Engineering Drive
Madison, WI 53706

Tel: +1 (608) 263-1589
Fax: +1 (608) 262-8464
E-mail: kummert@xxxxxxxxxxxxx

SEL Web Site: http://sel.me.wisc.edu
TRNSYS Web Site: http://sel.me.wisc.edu/trnsys