Running pseudo-subhourly simulations with PVsyst

[Bruno Wittmer]

At the WCPEC8 conference of 2022 in Milan, Italy, PVsyst presented a study on sub-hourly clipping effects.:

"A model correcting the effect of sub-hourly irradiance fluctuations on overload clipping losses in hourly simulations", 2022, André Mermoud, Bruno Wittmer, Michele Oliosi, Agnès Bridel, Adrien Villoz.

In this study, in order to estimate the clipping on a minute-level, a method was used, where minute-level weather data was split into 60 sets, one for each minute in the hour. Each set was then simulated in hourly steps, and the results of the 60 simulations were aggregated to obtain the equivalent of a minute-level simulation.

Concerning this procedure, which is also described in the paper, the following question was asked by a PVsyst user:

Quote

I have a question: when your article indicates that a one-minute weather file can be split into 60 hourly files, do we just keep the minute-level values as if they were the hourly averages or can we specify sub-hourly files with the corresponding minutes in the timestamp labels?

I'm asking that because I tried importing sub-hourly complete weather series (which are aggregated by PVsyst into hourly values) with no problem. However, if I try to split sub-hourly data into custom files based on their timestamp offset (from the beginning of the corresponding hour) and specify that the data is in power (not energy) units, PVsyst gives warnings about missing records (which is reasonable) but doesn't seem to take into account the minute values in the timestamps for time shift purposes, i.e the data still needs shifting.

Also, the GHI seems to be interpreted as an energy value and filled with zeroes for the missing records even when it's specified in power units in the custom file format definition. I tried splitting my data into 15-minute and 10-minute intervals, which gives a quarter (resp. a sixth) of the yearly GHI and energy yield in hourly output files.

 

[Bruno Wittmer]

It is the first case in the question of the user that is correct. You import each of the 60 sets of minute-level data as if it were hourly data. You therefore need to use the correct time shift, and since the data will be interpreted as hourly values, you need to specify a multiplier in the conversion format if the data is in  energy units.
PVsyst will run the simulation as a normal hourly-steps simulation, therefore you need to do a renormalization of the energy variables in the results to get the equivalent of a minute-level simulation.

Please note, that this approach does not take into account the thermal inertia of the PV modules. This leads to overestimated temperature fluctuations in the PV modules. In the PVsyst study this was handled by fixing the module temperature for each full hour to a value obtained with a simulation on aggregated hourly weather data, as explained in the paper.

[Debbie]

This paper references a pending update to PVsyst that will address sub-hourly clipping.  Is there an expected timeline for release of this update?

Thanks very much,
Debbie