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Design temperatures

The number of modules in series is based on the compatibility of the array voltage in any operating situations, with the input voltages range of the inverter. The PV array voltages are depending on pre-defined design temperatures which are part of the project's settings, page "Design conditions". Except for the "lower temperature for absolute voltage limit", these temperatures are not crucial and you can let them at their default values in most cases.

The default values (initiaization for each new project) may be redefined in the Hidden Parameters, topic "System design parameters".

NB: these parameters are used at design time only. They are not involved in the simulation in any way.

TOperMin and TOperMax during operation

  • TOperMin represents the cell temperature during usual operating conditions in winter. This is used for the maximum number of modules in series, in order to stay below the VmppMax input value of the inverter (by default 20°C).
  • TOperMax represents the cell temperature during usual operating conditions in summer, under full sun. This is used for the minimum number of modules in series, in order to stay above the VmppMin input value of the inverter (by default 60°C).
  • TOper is an intermediate value, used sometimes in the program or report for characterising usual operating conditions (default 50°C).

TMin: lower temperature for absolute voltage limit

This is indeed a very important parameter, that you have to adjust for your own site conditions. The Voc(TMin) limit is indeed a requirement corresponding to a safety condition.

The array voltage Voc should never overcome, in any case:

  • neither the inverter's VabsMax (absolute input voltage, corresponding to the input electronics circuit voltage limit),
  • nor the absolute array maximum voltage admitted for your PV modules (usually 1000V or 1500V).

The reference temperature for this evaluation is the lowest temperature even observed on this site.

  • In middle-Europe climate (and not in altitude), a usual best practice is to choose -10°C. But this should be decreased for high altitude or northern regions.
  • In other situations, you should take the lower temperature ever observed during the day (possibly with a condition of irradiance superior to 10 W/m2, i.e. first hours in the morning) for this site.

Here we consider the ambient temperature, not the cell temperature at sun. Because the sun may suddenly appear on the PV array (which is still cold) when the inverter is not yet working (i.e. submitted to Voc voltage). Remember that the array Voc voltage is only slightly dependent on the irradiance.

These conditions are a common practice, adopted by everybody. They are specified in the norm IEC TS 62738 (2018), paragraph 7.2.1. You are not advised to cheat with this evaluation, because it may affect the safety of your installation and the warranty conditions.