Number of modules in series

[Moataz Yassin]

We are facing a problem regarding the number of modules in series in a project in the gulf area.

all the manual calculations indicates that we can use from 29 to 30 modules in series of power of 570Wp and Voc 51.07

after considering the lowest temperature on site and the irradiance at this temperature the voltage became suitable to use 29 or 30 modules but the PVSYST indicates maximum of 28 

so what considerations does the software uses as we extracted the Tarray and the Irradiance and the string operating voltage and all are within the limits

so do anyone has a reason why PVSYST use less modules ?

[Lazare Fesnien]

Dear Moataz,

In the system tab, when choosing the PV module, PVsyst displays the voltage Voc at -10°C. It is this value which is taken into account for the calculation of the maximum number of modules in series.

Absolute Cell lower temperature for determining the Maximum possible voltage of the array. The default is set to -10°C for most European countries (best practice rule).

For this limit, the cell temperature is considered as the ambient temperature (worst case when the sun suddenly appears on the field).

You should define the lowest temperature ever observed during the day for this site.

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

Please read : https://www.pvsyst.com/help/inverter_array_sizing.htm?zoom_highlightsub=Array+voltage+sizing

 Regards

[Moataz Yassin]

So does this voltage calculated based on the Temperature only or does it consider the irradiance at this temperature also ?

because at the low temperature the irradiance is very low so although the voltage increase due to low temp there is an opposite factor that limit this increase which is the irradiance and this data is from the TMY file 

and regarding the lowest temperature do i have to insert it in the project setting or the data can be used automatically from the TMY file i inserted ? 

[Lazare Fesnien]

The absolute low temperature is not taken by the meteo file.

You can modify it if you have data on it.

Please read our FAQ : https://www.pvsyst.com/help/pvmodule_lowlighttool.htm?zoom_highlightsub=low+light

[dtarin]

On 3/5/2023 at 7:53 AM, Moataz Yassin said:

So does this voltage calculated based on the Temperature only or does it consider the irradiance at this temperature also ?

because at the low temperature the irradiance is very low so although the voltage increase due to low temp there is an opposite factor that limit this increase which is the irradiance and this data is from the TMY file 

and regarding the lowest temperature do i have to insert it in the project setting or the data can be used automatically from the TMY file i inserted ? 

Irradiance is not considered when calculating string lengths - Module Voc_max = Voc x [1 + (Tmin - T_STC) x (Tk_Voc/100)]. Temperature has a small impact on current, but it is mostly voltage that is affected. 

One note to make here, a TMY file is an average of expected conditions and may not be suitable to define a "worst condition" scenario. Time series data over a historical period would be more informative.