[TRNSYS-users] Surprising phenomenon with Type 67

[fdominguezm at uma.es]

Dear trnsys users,

I hope this helps:

* If you look at Type67 mathematical formulation, you will notice that it
is supposed that the surface (on which you want to calculate the
radiation) extends to infinity, so that the whole sky portion behind it is
completely obstructed (not seen by the surface). This is what modifies the
diffuse-radiation component, regardless of the obstruction mask. If you
want to get an unaltered diffuse component, you have to modify the source
code, or write a new code solving the actual geometry.

* The mask must be defined with many points (more than 16, for example),
independently of its shape, even in the case of a constant height profile.
Type67 divides the azimuth into the same number of segments that you use
in the mask. If the are not enough segments, type67 cannot properly locate
the position of the sun. In future releases type67 should use an internal
sub-division of the azimuth, in order to avoid this problem.

With best regards

Fernando Domínguez
University of Málaga (Spain)



> Hi David,
>
> I would like to add the following comments to Dave's answers.
>> I've realised the following experience: I've used Type67 without
>> shadings (angular height equal to 0°) and Type34 without shadings
>> (overhang projection and wingwalls projection equal to 0).
>> Direct and diffuse radiation should not change. Actually, diffuse
>> radiation vary. It's sometimes even greater after Type34 than before!
> You got me very worried with this possible bug. I am attaching a very
> simple project. I used the types 15 and 67, in your project; I made two
> modifications: the slope in Type15 was set to zero in the file that you
> sent us; I set it to 90, and I got rid of the equation component.  I
> think that Type67 is working fine.
>> Another phenomenon : I've tried the experience with a "detailed"
>> angular obstruction data file (24 surface angles) and a simplified one
>> (4 surface angles): it turns out that with the simplified obstruction
>> data file, diffuse radiation after Type34 varies with a greater
>> amplitude.
> I agree with you on this topic.  Until we release a new version, I would
> recommend that your shading mask is more detailed than just a few angles.
>
>> Finally, questions about basic use type 67 follow :
>> 5 - I cannot use more than 20 orientations with type 67 eventhough the
>> max is 40 in the proforma. Can you help me?
> In the attached example, I used 37 orientations.
>> 6 - Besides, I did not manage to use 2 type 67 at the same time. I
>> made a few tests using 2 type 67 and it always generate an error of
>> reading of the external file, while both types 67 work alone. Is there
>> a bug somewhere?
>>
> Regards,
>
> Diego
>
> --
> Diego A. Arias
> TRNSYS Coordinator
> Solar Energy Laboratory
> University of Wisconsin - Madison
> 1500 Engineering Drive
> Madison, WI 53706
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