Re: negative solar radiation

[Michaël Kummert <kummert@xxxxxxxxxxxxx>]

Jeroen,

> Hello,
> 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.
> lt(zenith,85)*T1+or(gt(zenith,85),eql(zenith,85))*min(T1,extraterrestrial)
> lt(zenith,85)*B1+or(gt(zenith,85),eql(zenith,85))*min(B1,extraterrestrial/2) 
>
> 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