Estimation of the circumsolar ratio in a turbid atmosphere

International audienceRoutine measurements of the direct normal irradiance (DNI) are not sufficient for optimal design of concentrating solar technologies. Due to a generally larger aperture angle of pyrheliometers or equivalent pyranometric systems when compared to that of concentrating collectors, the measured irradiance is overestimated as it includes the irradiance from the solar disc and a larger circumsolar region. The angular distribution of the direct and circumsolar radiances, i.e. the sunshape, can have a significant effect on the performance of concentrating collectors. Therefore, optimal design of concentrating solar technologies requires accurate measurements or estimations of the DNI and the sunshape. Published models are available for reproducing the representative sunshape for a given circumsolar ratio (CSR), i.e. the ratio between the circumsolar irradiance and the sum of the circumsolar and solar disc irradiances. The objective of this study is to estimate the CSR over a cloudless turbid atmosphere using a published sky radiance model and a Radiative Transfer Model (RTM). Using 10 months of solar irradiance and aerosol optical depth measurements, results show that there is an underestimation in the CSR computed by means of the sky radiance model when compared to that computed by the RTM. Also, a high correlation coefficient of 0.87 was found between the CSR estimated from both models, implying that modifications to the sky radiance model are possible to accurately estimate the CSR