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I am looking to separate the diffuse and direct component of global irradiance and found the Erbs model to do this in pvlib (see pvlib.irradiance.erbs) however, I am getting very strange results. I would expect the Direct Normal Irradiance (DNI) to be lower than the Global Horizontal Irradiance (GHI); or am I missing something? Values of GHI are not above 800 W m^2 for these days.

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EDIT: As per Cliff H advice, I have limited the solar zenith to less than 85 arc degrees; the results have improved however, there are large spikes in DNI values that do not appear very reasonable, e.g. start of 07-16.

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Kievit
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DNI > GHI is common at low solar elevation. GHI decreases much faster than DNI as solar elevation decreases. For example, think of a clear day with the sun right near the horizon. DNI will large because it's measured on a plane normal to the sun vector, but GHI will be near zero.

The values of DNI that are much greater than 1000 W/m2 are likely at very high zenith, since the Erbs model basically divides by cos(zenith). In practice, I limit using decomposition models like Erbs to zenith<85 degrees, to avoid the non-physical results.

Kievit
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Cliff H
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  • Thanks for this. I have limited the zenith to < 85 arc degrees. This has definitely improved the results however, I still get funny artefacts. – Kievit Nov 12 '18 at 22:53
  • The results appear credible to me. The Erbs model is estimating the diffuse component of GHI, and returns DNI by a closure calculation, i.e., DNI = (GHI - DHI) / cos(Z). The empirical equation that obtains DHI from GHI is a fit through a fairly broad scatter of data, so it should be regarded as one of many possible values of DHI that correspond to the observed GHI value. – Cliff H Nov 14 '18 at 16:13
  • Thanks. I did not consider that DNI assumes the surface perpendicular to the rays. As this is slightly different, I have asked a new question [here](https://stackoverflow.com/questions/53372519/how-to-estimate-dni-and-dhi-for-a-horizontal-surface-from-ghi) on how to estimate DNI and DHI for a horizontal surface from GHI. – Kievit Nov 19 '18 at 12:31
  • @Kievit could you share how did you limit the solar zenith to < 85 arc degrees? I have an issue here: https://stackoverflow.com/questions/55775990/python-pvlib-error-erbs-got-an-unexpected-keyword-argument-min-cos-zenith – k.ko3n Apr 21 '19 at 19:34