Adapting the Spencer model for diffuse solar radiation in Badajoz (Spain)

Reliable estimations of solar global radiation and its partitioning direct-to-diffuse are demanded for the use of solar energy as renewable energy source. While solar global radiation is widely available at radiometric stations, measurements of the diffuse component are rather scarce. Therefore, sound models to obtain reliable estimations of the partitioning direct-to-diffuse are required. In this study, the Spencer's model is applied to original measurements performed at a radiometric station in Badajoz (Spain). This model proposes a linear relationship between the ratio diffuse to global solar radiation and the clearness index, defined as the ratio solar global irradiance at ground level to solar irradiance at the top of the atmosphere. Additionally, the regression coefficients were computed and compared to those proposed by Spencer for our latitude. Results indicate that Spencer's model reflects the general trend of the measurements. The calculated regression coefficients are similar to those proposed by Spencer, showing relative differences between 0.3% and 20%.