pvlib : time convention issue with cumulated GHI

Question

I have a question regarding time conventions in pvlib.

As far as I understand, all the computations are made using instantaneous timestep convention. Instant weather will produce instant electric power.

However, in the weather models like GFS, GHI parameter is cumulated over the last hour. This makes inconsitant the solar radiation and astronomical parameters (zenith, azimuth...).

For example, if I take a look at the ERBS function, used to compute DHI and DHI from GHI :

df_dni_dhi_kt = pvlib.irradiance.erbs(ghi, zenith, ghi.index)

Here, all parameters are hourly timeseries, but the because of the convention, the output may be inaccurate.

• ghi : cumulated radiation over last hour
• zenith : zenith angle at exact hour (instantaneous)
• ghi.index : hourly DateTimeIndex

At the end of the power conversion process, a shift is observed between observations and model (please don't care about the amplitude difference, only time shift matters).

Any idea about using cumulated GHI as input of the library ?

Answer 1

When using hourly data there definitely is a dilemma in how to calculate the solar position. The most common method is to calculate the solar position for the middle of the time step. This is definitely an improvement to using either the start or end of the hour (as shown in your example). However, around sunset and sunrise this poses an issue, as the sun may be below the horizon at the middle of the hour. Thus some calculate the sun position for the middle where the period is defined as the part of the hour where the sun is above the horizon - but that adds complexity.

There's a good discussion on the topic here: https://pvlib-python.readthedocs.io/en/stable/gallery/irradiance-transposition/plot_interval_transposition_error.html