How to adjust the NOCT value ?

[André Mermoud]

Some practicians - and most of PV module's catalogues - usually specify the NOCT coefficient (Normalized Operating Cell Temperature), which is defined in IEC 61836: it is the temperature attained by a PV module in specific conditions (800 W/m², T=20°C and wind speed=1 m/s), and this for a non-operating situation (open circuit), for a "nude" rack-mounted module with free air ventilation.

Please don't use the NOCT approach, which is very confusing, but define a Heat Loss Factore U [W/m²K] for your PVsyst simulations.

For me the NOCT value for a module is only dependent (slightly) on the cover (glass) and back kind of the module (glass, plastic), and is useless for the module temperature evaluation. This is the reason why I did not include it in the PVsyst parameters.

As a confirmation of this, a team of the NREL in the USA measured side-by-side, at sun during several days, 3 modules with resp. 42.4°C, 49.7°C and 52.3°C NOCT manufacturer's values, and these 3 modules showed exactly the same temperature within 0.3°C !

On the other hand, the NOCT doesn't include any information about the mounting mode (free ventilated, integrated and insulated, etc). It is always given for a "nude" module. Moreover is is defined for open-circuit conditions, i.e. not-operating conditions; this doesn't make sense as the produced energy affects the energy balance.

If you apply the U-value thermal balance equation to the NOCT conditions, you will obtain an equivalence between the U-factor and the NOCT for specific conditions. But this doesn't make much sense, and is very difficult to interpret. Please completely forget the NOCT approach !!!