Evaluation of a new model to calculate direct normal irradiance based. on satellite images of Meteosat Second Generation

A. Kemper, E. Lorenz, A. Hammer and D. Heinemann

Department of Energy and Semiconductor Research, Faculty V — Institute of Physics Oldenburg University, D-26111 Oldenburg, Germany Corresponding Author, axel. kemper@uni-oldenburg. de

Abstract

We present a method to derive the direct normal irradiance DNI from MSG data. For this we apply the Heliosat method to extract cloudiness from the satellite images. Clouds are causing high fluctuations in the DNI. A new model for the direct fraction of the irradiance is introduced to calculate DNI. The clear sky irradiance is mainly determined by the aerosol optical depth (AOD) and water vapour content, which are taken from suitable climatologies. The accuracy of satellite derived DNI data is analyzed here for Spanish sites.

1. Introduction

Measurements of the direct normal irradiance DNI are needed for the planning of a solar thermal power plant at a given site. Direct solar irradiance is highly variable in space and time. As ground measurements are expensive, such data are rare.

Meteorological satellites operationally scan the Earth’s surface and clouds. So we can derive the direct normal irradiance from their data with a good spatial and temporal coverage. Since 2004, the satellites of the new generation MSG provide images of Africa and Europe every 15 minutes with a spatial resolution of approximately 1 km x 1 km at sub-satellite point.

In this document, we first present a method to derive the direct normal irradiance DNI from MSG data. A new model for the direct fraction of irradiance is used in combination with the Heliosat method applied to MSG data. In a second step we analyse the accuracy of the satellite derived DNI with ground measurements of six Spanish sites.

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