DNICast - Validation of DNI nowcasting results

Abstract

To enable efficient plant operation, Direct Normal Irradiance (DNI) irradiance maps in high spatial and temporal resolutions for up to 30 minutes ahead are of interest. Ground-based All Sky Imagers (ASI) can be used to detect, track and predict 3D positions of clouds possibly shading the plant. Within the framework of the DNICast project, an ASI-based nowcasting method was developed and applied to data measured on Plataforma Solar de Almería (PSA). The ASI derived nowcasts are validated in this report with special focus on the effects of temporal and spatial aggregation on the deviations found comparing the nowcasts to ground measurements. In most publications, nowcasted irradiance maps are only validated against singular ground measurements and commonly only for one-minute averages. These validation metrics are of minor relevance for Concentrating Solar Power (CSP) and most Concentrating Photovoltaics (CPV) plants as spatially aggregated irradiances (e.g. over a whole subfield) and temporally aggregated irradiance are more important. This is due to the thermal inertia of the heat transfer fluid and the interconnection between several CPV collectors. Therefore, spatial and temporal aggregation effects are investigated in this publication in detail.