Detection of clear sky instants from high frequencies pyranometric measurements of global horizontal irradiance

Omaima El Alani,Abdellatif Ghennioui,Hicham Ghennioui,Fatima-Ezzahra Dahr,S Krit

doi:10.1051/e3sconf/202129701028

Abstract

Solar resource assessment by clear sky models is of great importance in the solar energy field: verifying the performance of photovoltaic systems during stable conditions, clouds effects evaluation, the determination of geographical areas where irradiation is more uncertain and the preparation of forecasts with sky cameras. But before using these models they must be validated against high performances soil measurements. Since there is no radiometric sensor that measures clear-sky radiation, then historical clear-sky time periods must be identified only from long-term all-sky irradiation records. The contribution of this study is to exploit the ground measurements, analyze them and retrieve the information they contain concerning the clear sky instants. The study will be performed by comparing the clear sky instants identified by an algorithm proposed by Reno and Hansen with a physical clear sky model. This comparison is made using high frequency global horizontal irradiation (GHI) data from high performances meteorological station installed at Benguerir in Morocco.

Highlights

The knowledge of the solar irradiation available on the ground is of great importance for different applications of solar energy
It is necessary to identify clear sky times from all-sky pyranometric measurements to obtain data corresponding to the clear sky model data, because when validating these models an accidental presence of clouds during an assumed cloudless period could have adverse consequences leading to erroneous results
We chose the McClear model [14], that is a clear-sky model based on the libRadtran radiative transfer model and that exploits aerosol properties, total water vapor and ozone content supplied by the Copernicus Air Monitoring Service (CAMS) and provides time series of irradiation for any location in the world at any time from 2004 to the current day 2

Summary

Introduction

The knowledge of the solar irradiation available on the ground is of great importance for different applications of solar energy. These clear sky models need to be validated against similar time series, obtained from ground-based radiometric observations at sites. We will use the algorithm proposed by Reno and Hansen [5] to identify clear sky moments from the pyranometric measurements of GHI only because, it’s the most important for photovoltaic applications This algorithm requires the use of a clear sky model, and the model chosen is that of Ineichen [17]. The strength of this model is that in addition to the use of normal extraterrestrial irradiance, solar zenith, air mass as input, it takes Linke Turbidity factor, which is a factor representing the effects of absorption and diffusion by aerosols, and absorption by water vapor. The second validation step will consist of feeding the clear-sky detection model with output of a clear-sky model to test the behavior of the different algorithm in this situation

Ground-measured GHI data

Clear sky model

Clear sky detection

Clear sky model validation

Conclusion