Dear TRNSYS Users, I often use TRNSYS for buildingsimulations. For the Solar Radiation Processor I always use the Perez skymodel (parameter 3=4). For a special case I had to use the Isotropic skymodel (parameter 3=1). I remarked that there was a great difference in the diffuse radiation on a square that is South East orientated (tilt = 35 °). With the Perez skymodel, the annual total diffuse radiation on this square (tilt 35 °, orientation South East) is 3,0 GJ/m2 a year. With the Isotropic skymodel, this radiation is "only" 2,4 GJ/m2 a year. This is a difference of 0.6 GJ/m2 a year. When I do the same exercise for a sqaure that is South orientated, then the result for the Perez Model is 2.6 GJ/m2 a year and for the Isotropic skymodel 2.4 GJ/m2 a year. The difference is only 0.2 GJ/m2 a year. To explain this, I made a graph of the day-to-day total diffuse radiation for the different skymodels and orientations (see attached file Graph.zip (zipped Excel-file)). You can see that for the South East orientation with the Perez sky model there is a "sky-high" diffuse radiation on day 299. I think this cannot be correct. This could be caused by incorrect input for Total Horizontal Radiation, the Diffuse Horizontal Radiation or the Beam Radiation. To check this, I plotted in the same graph these radiations (also day-to-day totals). In the graph you can see that the input is totally normal on day 299, so the input can not cause the "sky-high" diffuse radiation. Does anybody has equal experiences and does anybody know an explenation for these strange results ? With kind regards, Maurice Jong <<Graph.zip>> =========================================================== E-Mail : jong@ecn.nl - Maurice Jong. ECN - Energy Research Foundation Department: Wind, Group DEGO Telephone: (+31) 224 564938 Fax: (+31) 224 563214 P.O. box 1 1755 ZG Petten Country: The Netherlands ===========================================================
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