spelland74 Posted November 17, 2016 Share Posted November 17, 2016 Hello,I have just sent an email to email@example.com with multiple files. This concerns the separation model used by PVsyst to calculate diffuse (and direct) irradiance when GHI only is imported using the ascii meteorological data import tool.Here is the situation:-We are importing GHI only (because this was adapted to ground measurements) and importing using ascii import. PVsyst calculates diffuse and direct irradiance.-I am checking whether the diffuse and direct calculated by PVsyst make sense. My understanding based on the PVsyst help was that the Liu and Jordan algorithm is being used. -However, I calculated diffuse irradiance using that algorithm, and the values are very different from what is in PVsyst (I used the clearness index from PVsyst). In terms of annual total, the PVsyst DIF value is 27% higher than what I find using Liu and Jordan. -The equations I used are fits to Figure 7 in the original (1960) Liu and Jordan paper taken from a 2015 article by Christian Gueymard (see reference below). I checked that the equations indeed match Figure 7 extremely well. Here are the equations (Kt is clearness index):If Kt<=0.75(DIF/GHI)=1+0.006381*Kt-3.2315*Kt^2+2.2448*Kt^3+0.081882*Kt^4else(DIF/GHI)=0.16I was wondering if you could please help us in understanding what’s going on. Ideally, if you can give more details about the equations you were using, that would be much appreciated.Best regards,SophieGueymard 2015: https://www.researchgate.net/publication/283570186_Extensive_worldwide_validation_and_climate_sensitivity_analysis_of_direct_irradiance_predictions_from_1-min_global_irradiance Link to comment Share on other sites More sharing options...
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