DNI calculation

[Suraj Bhanu Jat]

Hello all,

I have data data available for GHI, DHI and zenith angle but not for DNI. Can anyone please suggest the calculation of DNI using other 3 features.

Thanks and regards,

Suraj Bhanu Jat

[Jurand Bien]

Hi Suraj,

All information you need you are able to find here: 

https://pvlib-python.readthedocs.io/en/stable/reference/generated/pvlib.irradiance.disc.html

The application how to use it I've presented in post:

https://ekordo.pl/prognozowanie-uzysku-energii-z-instalacji-fotowoltaicznej-na-podstawie-prognozy-pogody-z-modelu-gfs-czesc-i/

I know it is in Polish but now having chatgpt or google translator I think it is not a problem;)

Regards

Jurand 

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[Suraj Bhanu Jat]

Thanks, I'll check it out. 

[Saurabh Aneja]

Hi Suraj - if you have the features as described, see here:

https://pvlib-python.readthedocs.io/en/stable/reference/generated/pvlib.irradiance.dni.html

[Adam R. Jensen]

The relationship between the three irradiance components can be expressed by the closure equation:

GHI = DHI + DNI * cos(zenith)

Given that you have two of the three irradiance components, the third can be calculated. Thus it is not advisable to use a model such as the DISC!

The function that Saurabh linked to implements the closure equation but implements some clever limits for high solar zenith angles to reduce unfeasible high values near sunrise/sunset. The reason unfeasible values tend to occur is that cos(zenith) approaches zero at sunrise/sunset and small measurement uncertainties in GHI/DHI are magnified as DNI = (GHI-DHI)/cos(zenith).

[Mark Mikofski]

One of the goals of pvlib (and it's affiliation with the PV Performance Modeling Collaborative https://pvpmc.sandia.gov/) is to be a source of truth. Python is ideal for this because the code is plain text, less cluttered, uses whitespace, and is generally perceived as more humanly readable. From the help page for pvlib.irradiance.dni linked above, you may find 2 links:
* [source] - https://pvlib-python.readthedocs.io/en/stable/_modules/pvlib/irradiance.html#dni

* view on GitHub - https://github.com/pvlib/pvlib-python/blob/main/pvlib/irradiance.py#L2987-L3055

both lead to the source code where you can see the formula that Adam referenced:
    # calculate DNI
    dni = (ghi - dhi) / tools.cosd(zenith)

You can also find this same formula in the modeling steps pages on the PVPMC website:

https://pvpmc.sandia.gov/modeling-steps/1-weather-design-inputs/irradiance-and-insolation-2/global-horizontal-irradiance/

I hope you find these tools and this Google Group helpful!