Benchmarking clear sky and transposition models for solar irradiance estimation on vertical planes to facilitate glazed facade design

The present study aims to benchmark the models utilized for estimating the solar irradiance on vertical planes that can facilitate the determination of critical conditions, which are utilized in the glazed facade design. Accordingly, in the first part of the study, the performance of PVGIS databases for Europe for estimating the solar irradiance components on the horizontal surfaces is compared with the on-site measurements at Bolzano airport (Italy). It is demonstrated that PVGIS-SARAH estimates both the global and diffuse solar irradiance components, compared to on-site measurements, with an acceptable error range (+/- 2-5%). The accuracy of isotropic and anisotropic transposition models is then compared for the same location, which demonstrates that the isotropic model tends to underestimate the global vertical incident irradiance by 6-12% of the daily peak value, while an agreement between the estimations of the anisotropic Perez model and the measurements is observed. Next, the performance of Ineichen, Solis and SPECMAGIC clear sky models, transposed with anisotropic Perez model, have been assessed utilizing field measurements on vertical facades at four different locations in Europe. The broadband version of Solis clear sky model, transposed with anisotropic Perez model, demonstrates R-2 scores between 0.85 to 0.95 for the considered locations, indicating that this approach can be utilized as an alternative to open-source/commercially available data sources to estimate the solar irradiance on vertical facades. Finally, for the case study of a triple glazing system, comparing the surface temperatures obtained employing the latter approach with those determined using other commonly utilized irradiance data sources shows a discrepancy of up to 7 degrees C.