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Issues with finding the clearness index and extraterrestrial irradiance using PVLIB functions

I recently ran into an issue with calculating the clearness index and the extraterrestrial irradiance using the PVLIB functions. Basically, the numbers do not tally up.

This is my raw data that I ran the function for (my Datetime is already timezone aware):

My raw data

I then ran the below code to get the clearness index:

latitude = -22.24
longitude = 114.09
times = pd.DatetimeIndex(test_data['Datetime']) #Converting to Datetime Index
solpos = solarposition.get_solarposition(times, latitude, longitude)
test_data['apparent_elevation'] = solpos.apparent_elevation.to_numpy()
test_data['zenith'] = solpos.zenith.to_numpy()
#    data_file = data_file[data_file.apparent_elevation > 0] #For now, not removing data during night time
test_data.reset_index(drop = True, inplace = True)
extr_irr = pvlib.irradiance.get_extra_radiation(pd.DatetimeIndex(test_data['Datetime']))
clearness_index = pvlib.irradiance.clearness_index(ghi = np.array(test_data['GHI']), solar_zenith = np.array(test_data['zenith']), extra_radiation = np.array(extr_irr), min_cos_zenith=0.065, max_clearness_index=2.0)
test_data = pd.DataFrame(test_data)
test_data['clearness_index'] = clearness_index
test_data['GHI_ET'] = extr_irr.to_numpy()

Now analysing the calculated data:

calculated data

Here we can see that for example in the first row,

GHI/GHI_ET does not equal the clearness_index value.

Could anyone point out what I am missing or doing wrong here?

Thanks

Solution

  • The problem is in this line:

    test_data['GHI_ET'] = extr_irr.to_numpy()

    You need to multiply the extraterrestrial irradiance by the cos(zenith) in order to obtain the value for the horizontal plane. If you look inside the clearness_index function you'll see it's done there as well.