Measurement Of Circumsolar Radiation

An instrument system has been developed to measure the flux of energy from the sun and the circumsolar region (the small-angle region around the sun) as a function of angle, wavelength, and atmospheric conditions. The measurements are necessary to accurately predict the performance of solar thermal conversion systems using focusing collectors, and to determine whether pyrheliometer data are adequate for estimating this performance. The instrument system consists of a "scanning telescope", several conventional solar instruments, and a digital electronics control and recording system. The telescope makes a scan of ±3°, passing through the center of the sun. The angular resolution is approximately 1.5 minutes of arc from the center of the sun out to two solar radii, and 5 minutes from there out to 3°. The background caused by direct solar light scattering within the telescope itself has been reduced to less than 1% of the intensity of the circumsolar radiation. Spectral dependence of the radiation is determined using filters. A measurement program using this telescope will be carried out at various locations in the West and Southwest. Additional instruments are being constructed for deployment for the accumulation of data at potential sites for future solar power plants.