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PostPosted: Tue Aug 05, 2014 1:10 pm 
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Joined: Mon Apr 16, 2012 7:29 pm
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According to the measurements reported by the specialized laboratories, the maximum power point Pmpp behaves very linearly with the temperature.
The muPmpp parameter specified by the manufacturers is the slope of this linear dependence.

Now the Pmpp temperature dependency as calculated by the one-diode model is not exactly linear.
Until Version 6.25, PVsyst adjusted the slope for getting the correct slope at 25°C (i.e. a tangent adjustment). In these conditions the derive is around -0.03% at 35°C, -0.12% at 45°C and -0.28% at 55°C by respect to the linear hypothesis. The graphics shows that this "error" is almost imperceptible:
File comment: Pmpp differences (linear - model) acc. to temperature
Pmpp_Temperature.png [ 7.88 KiB | Viewed 6722 times ]
From version 6.26, the slope is adjusted in order to get the measured (linear) value at 25°C and 45°C (i.e. a secant adjustment).
This improves the accuracy of the model at the operating temperatures, and results in a diminution of the temperature loss of about -0.1 to -0.2% (of annual yield) with usual PV systems (depending on the climate and the thermal U-factor)
The graph shows the zoomed differences:
File comment: Differences between Pmpp temperature models
muPmpp_Lin_Tangent_Secant.png [ 8.38 KiB | Viewed 6722 times ]
NB1: The temperature behavior is normally a result of the one-diode model, close to the specifications.
However for an exact adjustment of this value PVsyst introduces a slight linear correction of the Gamma value (diode ideality factor) as function of the temperature.

NB2: If you are designing plants in very hot climates, you can choose another secant point than 45°C in the Hidden parameters, topic "PV modules", item "Upper temperature reference for muPmpp default". This point should be in the middle range of your operating temperatures along the year, not the highest !

NB2: In some places of the program - e.g. in the specification dialog ("Model parameters" / "Temper. coeff.") - the muPmpp value corresponds to the Manufacturer's requirement (i.e. secant).
And in other places (page "Basic data", application of the model), it is the real derivative for a given temperature. Therefore there may be some differences when showing this value.

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