MikeAnderson

Thanks, two follow on questions then: 1) After the steps you just specified, I assume this final Tcell is used in another (final) module performance calculation, correct? 2) Is there an output for "real efficiency"? I believe there is not. If not, then I assume I could determine it at each timestep by using the Tcell and U equations, with all of the other config and output values (mentioned previously in this post), couldn't I. -Mike

Thanks André, I think I may not have been clear enough in my question. I understand that I will need to use site information to determine system specific Uc and Uv values. Before I go to that step, I want to make sure I have the calculation done by PVsyst completely understood, so that I can use it correctly with my site data. To make sure I understand it correctly, and that I am using the correct parameters, I tried performing a simple calculation, but it is not matching the PVsyst output correctly. Can you please consider the following: I am using model input parameters and PVsyst calculated output parameters, as specified below: Uc=20 (model parameter used in simulation) Uv=0 (model parameter used in simulation) Alpha=0.9 (model parameter used in simulation) Tamb=T Amb (from the simulation output) v=WindVel (from the simulation output) Ginc=GlobEff (from the simulation output) Effic=EffArrC (or EffArrR) (from the simulation output - I tried both of these) I then put these values into the equations below, and calculate Tcell: Tcell = Tamb + 1 / U · ( Alpha · Ginc · (1 - Effic) ) U = Uc + Uv · v I would expect the Tcell calculated from above to equal the value outputted by PVsyst: Tcell=TArray (from the simulation output) but it does not. I suspect that this is because the Effic used in the internal PVsyst calculations is not equal to either EffArrC or EffArrR. If this is true, then I'm wondering if there is a way for me to know, or calculate the same Effic used by PVsyst in the Tcell calculation, so that I can duplicate it. Once I am able to duplicate the Tcell calculation done by PVsyst, then I will move forward with the next phase of my work, and use site information, along with measured Tcell, to determine site specific Uc and Uv values. Thanks for working through this with me. -Mike

André, The information above was useful. I am also trying to determine system specific Uc and Uv coefficients from field data. My first step is to make sure that I can duplicate values calculated from a general weather file in PVsyst. I am trying to use the output parameters from PVsyst to calculate Tcell, and I am comparing this to the value calculated by PVsyst. I would like them to be identical, so that I'm sure I am using the correct parameters, but I cannot seem to get them to be identical. I have verified that for my simulation: Uc=20 Uv=0 Alpha=0.9 I am using the following PVsyst output parameters: Tamb=T Amb v=WindVel Ginc=GlobEff (as opposed to GlobInc: per you comment in http://forum.pvsyst.com/viewtopic.php?f=25&t=2739&p=7224&hilit=irradiance+input+to+thermal#p7224) For the efficiency, I have tried both EffArrC and EffArrR Then I plug the above values into the following equations to calculate Tcell Tcell = Tamb + 1 / U · ( Alpha · Ginc · (1 - Effic) ) U = Uc + Uv · v Finally I compare this calculated value (for all hours in the weather file), and compare it to the following. Tcell=TArray (per your comment in http://forum.pvsyst.com/viewtopic.php?f=25&t=3148&p=8046&hilit=cell+temp+in+output#p8046) The differences between my calculated value, and the TArray value are about -0.3 to 0.1 if I use EffArrC, and about 0 to 0.35 if I use EffArrR. Based on your previous comment in this thread, I assume that the efficiency being used internally for this Tcell calculation is different from both of these outputted efficiencies. Is that likely why my values are not matching? is there any way I can use the same efficiency value that PVsyst is using internally, so that I can calculate the same Tcell value? If I did, then I would have a robust method for using site data to come up with accurate Uc and Uv values for similar system types. Another comparison I did, was to use all of the outputted parameters to calculate Effic from the thermal equation, and then I compared this to the two outputted values EffArrC and EffArrR - I did not get a 1:1 correlation for either. Thanks for your help, Mike Anderson