The Erbs function in pvlib (see pvlib.irradiance.erbs) can be used to estimate the direct normal irradiance (DNI) and diffuse horizontal irradiance (DHI) from global horizontal irradiance (GHI) however, where DHI is for a horizontal surface, DNI is for a surface perpendicular to the rays, not a horizontal one. How can I estimate the direct horizontal irradiance given GHI? For example, this may be the case where one wants to separate the diffuse and direct component from a measurement of GHI for the same horizontal surface. Is it reasonable to use the Erbs model, compute DHI, and subtract DHI from GHI? I.e. Direct horizontal irradiance = GHI - DHI?
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Kievit
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I don't fully understand what quantities you're starting with. The question title is "How to estimate DNI and DHI for a horizontal surface from GHI" but your equation for DNI projected into the horizontal plane presumes knowledge of DHI. – Will Holmgren Nov 19 '18 at 22:39
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I think I clarified the question. In other words, I have a fixed radiometer installed on a mast that receives GHI and would like to estimate the direct and diffuse components for the same horizontal surface. – Kievit Nov 20 '18 at 09:03
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Is it reasonable to use the Erbs model, compute DHI, and subtract DHI from GHI? I.e. Direct horizontal irradiance = GHI - DHI?
Yes.
Cliff H
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