Assessment of diffuse radiation models for cloudy atmospheric conditions in the Azores region

• Diffuse radiation models applied to the Azores region showed to be inadequate.
• Clearness index only correlations underestimated diffuse irradiance above 300 W m−2.
• Boland–Ridley–Lauret model showed to be the most satisfactory.
• A more suitable diffuse radiation model for the Azores region is required.
• A simple correction to Boland–Ridley–Lauret model is applied to improve the results.

Measurements of diffuse, global and direct solar irradiance are key to a proper assessment of the potential of solar energy technologies. Unfortunately, worldwide measured irradiance databases usually consist of only global solar radiation data, often with limited spatial coverage. The estimation of the diffuse fraction from global radiation is therefore essential. Solar radiation models for prediction hourly diffuse fraction from global radiation are reviewed in this paper in order to explore their applicability to the Azorean region. Since models of diffuse fraction exhibit some degree of location dependence, a correction to the best performance model to improve the results for the studied region by considering the zenith angle is also presented.