Welcome to my homepage and first of all sorry for the frumpy “design”. For now, I only put some scientific stuff concerning circumsolar radiation here in the hope someone will find it useful. The content was produced during my times as PhD student at the German Aerospace Center (DLR) and the Meteorological Institute of Ludwig-Maximilians-University of Munich.
Circumsolar Radiation is caused by scattering of sunlight by aerosol or cloud particles. The phenomenon is also known as aureole. In short: For particles that are much larger than the wavelength of the light diffraction plays an important role; it alters the propagation direction of the light only by small angles (scattering it more or less in the forward direction). This causes a bright ring around the sun (compare Fig. 1).
The brightness and width of the aureole is strongly dependent on the properties and the amount of the scattering particles. The larger the particles the more focused is the scattering.
Concentrating Solar Power Plants or Concentrating Photovoltaics use mirrors or lenses to focus the direct sunlight onto an absorber. The sun disc itself as seen from earth has an angular radius of about 0.266∘. The apertures of typical CSTs exhibit “radii” of 0.7∘ to 2.3∘. The devices typically used to measure the direct solar irradiance, pyrheliometers, however, usually have apertures of 2.5∘ angular radius. This can lead to a systematic overestimation of the solar resource which is dependent on the local aerosol and cloud climatology. Figure 2 shows exemplary the radiance profiles along a cut from the center of the sun outwards for three different meteorological situations. For clear skies the direct radiation from the sun disk reaches the ground with little attenuation (998 W/mˆ2); the irradiance in the aureole is negligible (0.1 W/mˆ2 and 0.6 W/mˆ2). In the case with aerosol, the sun disk is clearly darker (621 W/mˆ2), but the circumsolar region lights up. Considering a power plant with a 1∘ aperture, the resource overestimation would be 11 W/mˆ2. In the case with the cirrus cloud the aureole is much more focused. This reflects in higher irradiance values in the sun disk (684 W/mˆ2) and near the sun (90 W/mˆ2), although the optical depth is the same as in the aerosol case. The resource overestimation in this case is 20 W/mˆ2.
In my dissertation I used data from MeteoSat Second Generation to retrieve cirrus cloud properties and compute the circumsolar radiation from them.
If you need more information or you plan to use my look-up tables to compute circumsolar radiation, I encourage you to have a look at following publications:
My research on circumsolar radiation was conducted in collaboration with many colleagues. Specifically I’d like to mention Stefan Wilbert, Robert Buras, Bernhard Mayer and Luca Bugliaro. Special thanks also to my colleagues at the DLR institutes Physics of the Atmosphere in Oberpfaffenhofen, Solar Research in Almería and the Meteorological Institute Munich.