Nowcasting of Irradiance Using a Network of All-Sky-Imagers

Abstract

Electric power grids are increasingly penetrated by generation from photovoltaic (PV) systems. The volatility of solar irradiance can therefore cause problematic fluctuations of grid voltage and frequency. For local low voltage power grid sections highly accurate nowcasts of the short-term local PV-power generation may enable higher PV penetration by employing short-term flexibility options while saving investments in grid-infrastructure. Systems of up to four All-Sky-Imagers (ASI) have been used successfully to nowcast direct and global irradiance as well as the production of solar power plants. In Oldenburg and the surrounding area these systems are enhanced by interconnecting 34 All-Sky-Imagers in a measurement network called Eye2Sky. Intermediate results can be combined and exchanged between systems with overlapping observation areas but also between systems up to 100 km apart. To demonstrate the potential of the approach three state-of-the-art 2-ASI-systems from the network are selected. Nowcasts of the individual systems are averaged weighting each prediction by its expected accuracy in the single points of the forecast domain. In a validation period of 86 days the network outperforms a single 2-ASI-system. For all lead times greater than zero, significantly lower root mean square errors are found compared to the 2-ASI system. For highly variable time intervals the root mean square error is reduced by up to 50 %. The network´s combined nowcast is found to reduce forecast errors related to the often uncertain cloud edge positions and further systematic errors found in single 2-ASI-systems.