Next-Day Solar Irradiance Forecasting: A Preliminary Study in Limassol

This study presents a preliminary evaluation of a next-day solar irradiance forecast system in Limassol, Cyprus, over six months. For the evaluation, forecasted Global Horizontal Irradiance (GHI) was compared with high-quality ground measurements. The system showed strong correlation with measured values at both hourly and daily timescales. Hourly forecasts exhibited a correlation coefficient (r<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup>) of 0.84 and a Mean Bias Error (MBE) of 23.4 W/m<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> (4.8%), while daily energy forecasts showed improved performance with an r<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> of 0.86 and an MBE of 0.3 kWh/m<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> (4.7%). The Root Mean Squared Error (RMSE) was 123.7 W/m<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> (25.4%) for hourly forecasts and 0.8 kWh/m<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> (13%) for daily forecasts, indicating that daily aggregation reduces error magnitudes by averaging out short-term fluctuations and discrepancies. Some discrepancies were due to the model’s limited ability to capture cloud and aerosol variations. Future work will focus on investigating these factors to enhance the forecast system’s accuracy and reliability.