Is it possible to estimate GHI or DHI from only DNI

Hi

I would like to know if it is possible to estimate the global horizontal irradiance or the diffuse horizontal irradiance from only knowing the direct normal irradiance? I have all the other data to create my TMY3 file but I only dont have the GHI or DHI values.

Thanks

You can use the NREL DISC model to estimate DNI and DHI when you only know GHI.

I am not aware of any tools to estimate GHI and DHI from the DNI.

If you are planning to use the TMY3 file with one of SAM's CSP system models that uses only DNI, you may be able to create your TMY3 file using GHI and DHI data from the base file with your DNI data. (Replacing the GHI and DHI values with zeros or a flag like -999 may cause SAM's weather file reader to fail.)

Best regards,
Paul.

Does the NREL DISC model actually permit estimation of DHI? It only seems to estimate DNI.

Thanks

The DISC model estimates the DNI from GHI. Once you have values for both of those radiation components, you can estimate the DHI using a method like the one in this DNI-GHI to DHI calculator ( XLSX 2 MB ). It uses sun angle equations and the GHI to determine the global radiation incident on a surface normal to a line drawn from the sun to the surface, calculates the diffuse radiation on the normal surface by subtracting the DNI value from the GHI value, and then calculates the equivalent diffuse irradiance on a horizontal surface. Best regards, Paul.

I would like to know if it is possible to run the SAM without the diffuse horizontal irradiance??

For the detailed photovoltaic model, you can choose which two components of the solar radiation you want the model to use:

1. On the Location and Resource input page, click the Plus sign next to "PV Albedo and Radiation" at the bottom of the page to expand the panel.

2. Under "Weather File Irradiance Data," choose the radiation component you want SAM to use.

Best regards,
Paul.