Temperature consideration in PVsyst

[AdiM]

I have a very basic question regarding the simulation run by PVsyst.

The temperature data of a location that we feed in PVsyst, is only ambient temperature. Now considering module output reduces with temperature, how does PVsyst determine what exactly is the cell temperature at that location throughout the year>?

[André Mermoud]

The cell's temperature at sun is established using a little energy balance model.

The parameters of this model (U-values, possibly accoding to the wind velocity) are of your responsibility, and defined in the "Detailed Losses".

The model is explained in the help "Project design > Array and System losses > Array thermal losses".

[lonav]

Hi André,

as meteo data suppliers provide wind velocity measured at 10m high and wind data inputs for PVsyst standard format meteo file need to be values also at 10m, does the programm recalculate wind velocity for the height of the array (~2m for a free-mounted system, for example) in order to compute array thermal losses? In the help it mentions the Uc value of 29W/m²K has transferred the wind-dependent coefficient assuming average wind velocity of 1.5m/s, but is that value a 10m high measurement? Do I need to recalculate wind velocity if it's not measured at the height of my array?

Thank you in advance for your help.

[André Mermoud]

The effect of the wind on the PV array temperature is expressed as the Uv [W/m2K / m/s] factor, which is a proportionnality factor for the Wind speed.

Now PVsyst cannot provide reliable values for this factor, as we don't have reliable measured references.

Therefore the only way to get this value is to measure it on site (long-term measurement).

Now if you determine this factor with a reference wind velocity measurement at 10 m from the ground, you will get a Uv value.

If you admit a scaling factor for a measurement at another altitude, you will obtain a Uv factor scaled (inversely) by the same amount.

An (old) study of the WMO proposes a scaling facor (Hellmann model) as:

Vh = V10 (0.233 + 0.656 log10 (h+4.75) )

With this expression, the speed at 1.5 m will be 25% less than the speed at 10 m. Therefore the Uv value will be 25% more for a same result.

[SIEyE]

Regarding the effect of the wind on the PV array temperature, the wind loss factor Uv, is there any special consideration for large installations, for example 100 MW? what value es recommended for such installations?