[TRNSYS-users] Diffuse radiation

[Michaël Kummert]

Esther,

>   When working with radiation processor type 16a in "tilted surface mode 
> 4", which corresponds to Perez model for the calculation of diffuse 
> radiation, I get  very high values for diffuse radiation.They are as 
> high as 2400 W/m2 ,which makes my simulation of a PV array doesn´t work .
> I have tried the same simulation with mode 1" isotropic model" and 
> everything is ok whereas with model 2 and 3 the same problem appears 
> with diffuse radiation values.
> Does it mean that "tilted surface model 2,3 and 4" can´t be used for all 
> radiation horizontal values? Or perhaps am I doing something wrong?
> Thanks a lot.

Tilted radiation modes 2 to 4 are different from mode 1 in that they 
assume a non-isotropic distribution of the diffuse radiation. Hence they 
use the measured values of direct / global / diffuse radiation 
(depending on the horizontal radiation mode) in some way to calculate 
the diffuse radiation on a tilted surface.
This makes those modes more sensitive to inconsistencies in the weather 
data (beam higher than global, global higher than extraterrestrial, 
etc.) and to other errors.
I would recommend that you check the connections of your Type 16, as 
well as its "synchronization" with the data file (do you get radiation 
when the sun is supposed to be down?). The Perez model is generally 
accepted as the most accurate model and it should definitely not give 
you diffuse radiation values of 2400 W/m2 when properly configured to 
read a good quality data file.

Kind regards,

Michaël Kummert



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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 at engr.wisc.edu

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