How to fix Rserie and Rshunt parameters ?

[André Mermoud]

Rshunt is the inverse of the slope of the I/V curve around Isc.

According to our measurements at sun, it has an exponential behaviour (increases when the irradiance diminishes).

For the crystalline modules that we have measured, this behaviour may be approached with an exponential paramater RshExp = 5.5, and an intercept Rsh(0 W/m²) = 4 times the Rsh(1000 W/m²).

Rserie is one of the 4 unknown parameters when establishing the one-diode model using a reference I/V curve (i.e. imposing that the curve passes through the 3 reference points (0,Isc), (Vmp, Imp), (Voc, 0) at STC). When we avail of a measured I/V curve, Rserie may be determined by minimizing the errors between the measurement and the modelled I/V curve.

NB: Rsmax is the maximun possible value allowing the curve to pass through the 3 reference points, i.e. to solve the equations.

Now for crystalline modules or CIS, the manufacturer's data don't specify an I/V curve, so that PVsyst has to do some hypothesis for proposing default values:

- 1/Rshunt is taken as a given fraction of the (Isc-Imp)/Vmp value,

- Rserie is chosen for getting a fixed value of the Diode Ideality factor Gamma in the equations.

These values have strong influence on the Low-light efficiency.

Therefore when we avail of reliable low-light efficiency data, the Rserie value may be chosen for approaching them at best.