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Benoît and
Hermann, Hermann's answer is quite correct on all counts. The reason why it is not possible to choose a different sky model for Type34 is that Type34 does an analytical solution of view factor. It then simply multiplies the unshaded diffuse radiation by the sky view factor that it has calculated and outputs the calculated value as the shaded diffuse radiation. When you have an anisotropic sky, on a clear day, the sky is somewhat brighter near the horizon and near the location of the sun but Type34 does not weight the sky diffuse radiation differently depending on where it is coming from. Kind regards, David Hermann Schranzhofer wrote: Hi Benoît Cogné, as far as I know the problem is that type34 uses the isotropic sky model for calculating the diffuse radiation and there is no other possibility in type 34 (do not ask me why; maybe David Bradley can tell us more about that). Type109 uses the Perez model to calculate the diffuse radiation. And this may cause the differences you got. You can check this by setting Parameter 3 of type109 to 1 (using the isotropic sky model). Than the results should be the same. For detail information of sky models there are a lot of papers available and you can also find some in the documentation of TRNSYS. The Mathematical Reference gives also some comparison and advises concerning the sky models: (page 5-294): "In general, the anisotropic sky models (Hay and Davies, Reindl, and Perez, et al) provide comparable estimates of the total radiation on a tilted surface and are recommended for general use. The Hay and Davies and the Reindl models are computationally simple when compared to the Perez model. The isotropic sky model (mode 1) under predicts the total radiation on a tilted surface and is not recommended for general use; it is included to permit consistency with simulations performed with earlier versions of TRNSYS." We also had some troubles with that and we wonder why type34 does not give the possibility to choose the sky model. Please, if I am wrong correct me (Bradley?)! For your second problem, we use the connection type34<->type56 by creating additional orientations in TRNBUILD. There is an example shipped with TRNSYS called "SunSpace-Shading" showing you the correct connections. I don't know if there are some differences using "fraction of solar shading" (type34) connected with "external shading factor" (type56). I hope it helps a little bit and maybe someone else has more information. Best regards Hermann Schranzhofer. _______________________________________________________________ DI Dr. mont. Hermann Schranzhofer Institut fuer Waermetechnik / Institute of Thermal Engineering TU Graz / Graz University of Technology Inffeldgasse 25 / B, A-8010 Graz Tel.: +43 316 873 7314 Fax : +43 316 873 7305 http://www.iwt.tugraz.at/ _______________________________________________________________ ________________________________________ Von: B.Cogné - dH Technologies [mailto:benoit.cogne@dh-technologies.fr] Gesendet: Montag, 23. Juni 2008 19:32 An: trnsys-users@engr.wisc.edu Betreff: [TRNSYS-users] Overhang Type 34 Dear Trnsys users, I am not fully experienced in Trnsys so my problem may be very simple to solve. Trying to understand how to use the type 34 "Overhang" I made some tests and found strange results. When Overhang and wingwall projections are set to zero, beam radiation and ground radiation to receiver are equal to same data from type 109. This is OK. Sky diffuse is reduced a lot which does not look normal to me. Is there an explanation ? Here attached is this simple test I also need some advice on the way to use the type 34 with the type 56. Should I create in the building an orientation for each overhang and connect the type 34 outputs to the corresponding inputs or should I only use the fraction of solar shading from type 34 as the external shading factor of the window ? I would appreciate an example on the way to connect type 34 properly to type 56 Looking forward for your help Benoît Cogné DH TECHNOLOGIES France --
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