Circumsolar Radiation and Beam Irradiance Measurements for Focusing Collectors

Abstract

Depending on the present atmospheric conditions a considerable part of the direct solar radiation can be scattered into the circumsolar region closely surrounding the solar disk. This forward scattered radiation, the circumsolar radiation, is widely detected by pyrheliometers, while concentrating solar collectors can only use a concentrator specific fraction of it. This lower response to circumsoalr radiation can lead to systematic overestimations of the collector performance. when thin clouds mask the sun, this overestimation can be greater than 10% although the direct normal irradiance (DNI) is often above 200 W/M2 in such case. The fraction of the measured DNI that is caused by circumsolar radiation varies depending on the day conditions. The average of this fraction depends also on the location. Thus, representativ site specific information on the circumsolar radiation is necessary for the accurate calcuation of the expected yield of a concentrating solar plant. Beam irradiance data for concentrating solar collectors should be accompanied with information on the circumsolar radiation. However, such information is not available for locations that are now of particular interest for concentrating solar power (CSP) or concentrating photovoltaics (CPV). As a first step to overcome this complication, a reference system consisting of the Sun and Aureole Measurement System (SAM), an Aeronet sun photometer and post processing software has been developed and is used at a small number of sites. In order to provide a sufficient database, an alternative method for the determination of the sunshape is presented using less expensive and maintenance intense equipment. This method makes use of a pair of pyrheliomters with different angular acceptance functions. By comparison of the measurements of the two pyrheliomters, the amount of circumsolar irradiance can be determined. The system was tested and verified using the reference system. As a result, the necessary information for CSP and CPV systems, and a basis for the development of of modeling methods for circumsolar radiation, can now be achieved.