Evaluating direct and diffuse solar radiation components through global radiation measurements from three fixed directions

Solar systems are deemed to play a pivotal role within the process which should lead to the reduction in the carbon intensity of the worldwide energy use. However, for the analysis of solar system performance, the availability of data regarding the various components (direct and diffuse) of solar radiation is crucial. Nevertheless, the measure of either direct or diffuse solar irradiance requires sun tracker apparatuses (pyrheliometers) or shadow devices (e.g. shadow-band pyranometers) which involve mechanical appliances needing constant maintenance and articulated measurement protocols; therefore, these data are usually derived from global solar radiation measured records, by using empirical functions. With a view to trying to address these issues, the article proposes a methodology for the assessment of the direct and diffuse components of solar radiation from both global solar irradiance and infrared radiation data, all measured on three planes differently tilted and oriented. Therefore, no tracking or moving devices are involved, whereas the real configuration and weather conditions of the site (Sky View Factor, clouds, climate conditions, presence of reflective surfaces, etc.) are taken into account. Therefore, the results should possibly characterize the site, so that its actual potential for solar system installation can be properly assessed. Comparisons with different correlation models demonstrate good agreement, showing correlation coefficients ranging from 0.75 to 0.89 for diffuse radiation and from 0.97 to 0.99 for direct radiation. Notably, the proposed method tends to produce lower values for diffuse radiation. These discrepancies may be due to the peculiar features of the site.