pvlib.irradiance.dirindex

pvlib.irradiance.dirindex(ghi, ghi_clearsky, dni_clearsky, zenith, times, pressure=101325.0, use_delta_kt_prime=True, temp_dew=None, min_cos_zenith=0.065, max_zenith=87)

Determine DNI from GHI using the DIRINDEX model, which is a modification of the DIRINT model with information from a clear sky model.

DIRINDEX [1] improves upon the DIRINT model by taking into account turbidity when used with the Ineichen clear sky model results.

The pvlib implementation limits the clearness index to 1.

Parameters:

ghi : array-like

Global horizontal irradiance in W/m^2.

ghi_clearsky : array-like

Global horizontal irradiance from clear sky model, in W/m^2.

dni_clearsky : array-like

Direct normal irradiance from clear sky model, in W/m^2.

zenith : array-like

True (not refraction-corrected) zenith angles in decimal degrees. If Z is a vector it must be of the same size as all other vector inputs. Z must be >=0 and <=180.

times : DatetimeIndex

pressure : float or array-like, default 101325.0

The site pressure in Pascal. Pressure may be measured or an average pressure may be calculated from site altitude.

use_delta_kt_prime : bool, default True

If True, indicates that the stability index delta_kt_prime is included in the model. The stability index adjusts the estimated DNI in response to dynamics in the time series of GHI. It is recommended that delta_kt_prime is not used if the time between GHI points is 1.5 hours or greater. If use_delta_kt_prime=True, input data must be Series.

temp_dew : None, float, or array-like, default None

Surface dew point temperatures, in degrees C. Values of temp_dew may be numeric or NaN. Any single time period point with a temp_dew=NaN does not have dew point improvements applied. If temp_dew is not provided, then dew point improvements are not applied.

min_cos_zenith : numeric, default 0.065

Minimum value of cos(zenith) to allow when calculating global clearness index kt. Equivalent to zenith = 86.273 degrees.

max_zenith : numeric, default 87

Maximum value of zenith to allow in DNI calculation. DNI will be set to 0 for times with zenith values greater than max_zenith.

Returns:

dni : array-like

The modeled direct normal irradiance in W/m^2.

Notes

DIRINDEX model requires time series data (ie. one of the inputs must be a vector of length > 2).

References

[1] Perez, R., Ineichen, P., Moore, K., Kmiecik, M., Chain, C., George, R., & Vignola, F. (2002). A new operational model for satellite-derived irradiances: description and validation. Solar Energy, 73(5), 307-317.