Negative irradiance values when PV modules go underground

[kandersolar]

Originally posted in SunPower/pvfactors#141:

Currently there is nothing stopping a user from specifying an array much wider than it is tall and proceeding to model it at steep tilts that cause the lower module edge to go to negative heights. When that happens, negative irradiance values are returned. For example, for static solar position and irradiance, here's how the returned rearside irradiance varies with surface tilt for an array with pvrow_width/pvrow_height=3 showing how irradiance goes negative when pvrow_width/2 * sin(surface_tilt) > pvrow_height:

Code to reproduce above plot (v1.5.2):

Click to expand!

from pvfactors.geometry import OrderedPVArray
from pvfactors.engine import PVEngine
import pandas as pd
import numpy as np
import matplotlib.pyplot as plt

ts_inputs = pd.DataFrame({
    "timestamps": 0,
    "DNI": 1000,
    "DHI": 200,
    "solar_zenith": 45,
    "solar_azimuth": 90,
    "surface_tilt": np.linspace(0, 90, num=1000),
    "surface_azimuth": 90,
    "albedo": 0.2,
})

pvarray_parameters = {
    "n_pvrows": 3,
    "pvrow_height": 1,
    "pvrow_width": 3,
    "axis_azimuth": 180,
    "gcr": 0.5,
}

# tilt when part of the module goes underground
boundary_tilt = np.degrees(np.arcsin(
    pvarray_parameters["pvrow_height"] / (pvarray_parameters["pvrow_width"] / 2)
))

pvarray = OrderedPVArray.init_from_dict(pvarray_parameters)
engine = PVEngine(pvarray)
engine.fit(**ts_inputs)

engine.run_full_mode()
grear = pvarray.ts_pvrows[0].back.get_param_weighted("qinc")
plt.plot(ts_inputs["surface_tilt"], grear)
plt.axvline(boundary_tilt, c='k', ls='--')
plt.xlabel("Surface Tilt [degrees]")
plt.ylabel("Rearside Irradiance [W/m2]")

This behavior has tripped people up at least twice:

• https://groups.google.com/g/pvlib-python/c/go5Qlft3-i4/m/1buEfYbxAAAJ
• https://stackoverflow.com/questions/71696509/typeerror-when-using-pvfactors-timeseries-in-pvlib-v0-7-1-with-pvfactors-v-1-5-0?noredirect=1#comment126887184_71696509

I think it would be good to raise an error, or at least emit a warning, in this situation. Perhaps a check like this in OrderedPVArray.fit?

        # Check whether the lower module edge ever goes underground
        lower_edge_height = self.height - 0.5 * self.width * np.sin(np.radians(surface_tilt))
        if np.any(lower_edge_height < 0):
            msg = ("Input pvrow_height is not large enough to prevent modules "
                   "from colliding with ground. Check pvrow_width, "
                   "pvrow_height, and surface_tilt.")
            raise ValueError(msg)