NOCT definition
Some practitioners, and most PV module manufacturers, usually specify the NOCT coefficient ("Nominal Operating Cell Temperature"), which is the temperature attained by PV modules with free air circulation all around under standard conditions defined as:
\(G_{\mathsf{incid}} = 800 ~\frac{\mathrm{W}}{\mathrm{m^2}}, \quad T_{\mathsf{amb}} = 20 ~\mathrm{°C}, \quad W_s = 1 ~\mathrm{m/s}, \quad \text{Open circuit condition}\)
The exact NOCT conditions are defined in the standard IEC/TS 61836.
Equivalence with the U-factor
The NOCT factor is related to our loss factor \(U\) by the thermal balance: $$ U (T_{\mathsf{SS}} - T_{\mathsf{amb}}) = \alpha G_{\mathsf{inc}} (1 - \eta) \mathrm{, where} ~ U = U_c + U_v W_s $$ which gives, under NOCT conditions $$ (U_c + U_v \cdot 1 ~\mathrm{m/s}) (\mathrm{NOCT} - 20 ~\mathrm{°C}) = \alpha \cdot 800 ~\frac{\mathrm{W}}{\mathrm{m^2}} \cdot (1 - 0) $$
where the open circuit condition implied \(\eta = 0\).
However, under operating conditions, electrical energy is drawn from the module, and this thermal balance is affected by the efficiency value, which is of the order of \(15 ~\%\). PVsyst also proposes this operating NOCT, derived from our \(U\)-value definition, for the user's choice.
Validity of the specified NOCT
To our understanding, the NOCT value for a module depends only slightly on the front cover (glass) and the back material of the module (plastic, glass), which are largely the same for almost any module with respect to heat convection. It could vary slightly according to IR losses, but the IR emissivity is about the same for any material used in the PV collector.
As confirmation of this, an NREL team measured side by side, in the sun over several days, three modules with manufacturer NOCT values of \(42.4\), \(49.7\), and \(52.3\), and these three modules showed exactly the same temperature within \(0.2 ~\mathrm{°C}\).
On the other hand, the NOCT does not include any information about the mounting mode (free ventilated, integrated and insulated, etc.). It is always given for a "bare" module.
Therefore, the NOCT is not useful for evaluating module temperature under simulation conditions, which is why we did not include it in the PVsyst parameters.