Evaluating the Global Horizontal Irradiance Projected by the Global Forecast System (GFS) Model in Diverse Climatic Zones in South Africa

Abstract As the deployment of solar power utilizing devices increases, accurate forecasts of solar energy become increasingly important for managing and stabilizing the electricity grid. This study investigates the validity of 7-day ahead global horizontal irradiance (GHI) forecasts for South Africa provided by the Global Forecast System (GFS). The clear-sky index was used to interpolate GFS mixed 3-hourly and 6-hourly averages to generate hourly forecasts. The model’s performance was validated against radiometric GHI data from 15 locations spread across South Africa’s six climatic zones. The results showed that the GFS model can satisfactorily forecast GHI in South Africa. Overall, the model performed best in interior climatic zones and relatively poorly in coastal climatic zones. The accuracy of the GFS model performance decreases as the forecast lead time increases. The model overestimates GHI in overcast conditions while it underestimates GHI in clear and partly cloudy conditions. On average, the model underestimates GHI in all seasons. Forecasting errors are highest in summer and lowest in winter and moderate in autumn and spring. The inadequacy of the GFS model to precisely determine the cloud quantities is the major source of error; errors increase as the cloud fraction increases. Despite this, the study’s findings suggest that GFS forecasts may be used for forecasting solar photovoltaic output for energy planning in South Africa. Significance Statement This paper provides the first comprehensive assessment of Global Forecast System (GFS) forecasts of global horizontal irradiance (GHI) in six climatic regions of South Africa, exploring how cloud cover, site location, climatic zone, season, forecast horizon, and hour of the day impact the model’s performance. This research was carried out in a country where GFS GHI models have never been validated before and that presents a wide range of climatic conditions. The results allow the accuracy of other global and mesoscale GHI forecasting models to be compared against the current results in the region. The outcomes of this study indicate the possibility of using GFS GHI forecasts for forecasting solar photovoltaic output for energy planning. South Africa has an electricity supply deficit, and being able to accurately forecast the output of the increasing capacity of solar PV is one of the things needed to stabilize the grid.