Solar radiation resource assessment by means of silicon cells

Abstract

To provide an accurate solar resource assessment, radiometric stations measuring global, diffuse, and direct irradiance must be widespread. Nevertheless, the high capital costs of thermopile instruments, usually used in pyranometry, represent an important obstacle. Therefore, silicon photovoltaic sensors have emerged as a more accessible alternative than standard thermopile sensors. However, their temperature and spectral responses must be taken into account in order to match silicon cells and thermopile responses. Global and diffuse irradiance have been measured from early 1990 to the end of 1992 in Almería, southeastern Spain, by means of thermopile and photovoltaic sensor. Polar axis shadowbands were employed to measure the diffuse irradiance. These 10-minute coincident data sets, covering a complete range of atmospheric conditions, have been used to develop a correction procedure for the silicon detector measurements. Before the correction procedure was applied severe discrepancies were found, especially for the diffuse irradiance measurements performed under clear skies. Results of the correction method applied to an independent data set show a remarkable improvement. After correction, the comparison of silicon cell measurements with those obtained by means of thermopile pyranometers leads to a root mean square deviation of about 4% and 5% over the mean value of global and diffuse horizontal irradiance, respectively.