Hi!! Just wanted to report that we ended up finding a solution good enough for us. The solution is as follows:
- We start by building a sky, using gendaylit, and a simplified scene like the one I described above.
- We measure the GHI in 20 locations in the scene, leaving a margin of separation with the edges of the ground.
- We calculate a GHI normalization factor, defined as measured_ghi / target_ghi, where target_ghi is the GHI value defined in the meteorological data. The measured_ghi value is the average of all previous measruements.
- We then calculate a corrected diffuse horizontal irradiance, by doing corrected_dhi = dhi + ghi * (ghi_normalization_factor - 1). This way, we increase the DHI by something like 5 W/m2 for example. The normalization factor is usually very close to one.
- Finally, we rebuild the sky using gendaylit and the corrected_dhi value. If we measure GHI again by repeating the procedure above, we get the correct value (on average).
It was not easy to come up with this procedure, it took a lot of trial and error. The process of measurement in 20 locations was necessary so that the “randomness” of rtrace averages out. When calculating the results of the individual hourly intervals for an entire year, the errors average out and we end up with the correct value.
Thanks Greg for your help!!