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 seven-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.
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