How is the module temperature calculated?

[Peter Neretnieks]

The module temperature is critical for determining the losses due to the temperature coefficent. If I remember correct the module temperature is calculated by a pre set factor (typically 0,035) times the irradiation which is added to the ambient temperature. I ask since after some evaluation of data from a few sites it seems this factor varies (significantly?) depending on system type and weather. For example a free field system I follow closely in Sweden; the very few all sunny days the factor is approximately 0,025 but the most common value for all these days with a mix of clouds and sunshine is between 0,015 and 0,020.

For a completely integrated system the values are 0,45-0,47 for the few all sunny days, but typically around 0,3 the average Swedish summer day.

An average error in operating temperature of 10 degrees would result in a four percent performance difference.

Any thoughts on this?

[André Mermoud]

In PVsyst the cell's temperature is evaluated from the irradiance (and eventually the wind velocity) using a thermal balance, which is completely explained in the help "Project design > Array and system losses > Array Thermal losses".

It involves namely a thermal loss factor U-value (expressed in W/m2K) which may be splitted into an constant Uc and a wind component Uv * wind velocity.

The U-value should be adapted according to the installation conditions (free air circulation all around, or back insulated modules).