McClear_Radiance physical model for estimating clear sky downwelling and upwelling solar irradiance on tilted surfaces

• Physical modelling of downwelling and upwelling diffuse sky radiance. • Model is based on look-up tables established with radiative transfer code libRadtran. • Surface reflectance modelled as non-Lambertian using the Ross-Li model. • Provides global and diffuse irradiances incident on both sides of plane of any tilt. • Validations exhibit particularly accurate estimates. Accurate prediction of downwelling and upwelling surface solar irradiance for various plane orientations is crucial for several solar energy applications, particularly bifacial photovoltaic systems and the architectural design of energy-efficient buildings. In this study, a fast and physical model providing downwelling and upwelling surface solar irradiance under clear skies on any tilted plane is developed and validated, entitled McClear_Radiance. This new model is inspired by the McClear clear sky model, which utilizes reference look-up tables established using the radiative transfer code libRadtran and atmospheric variables from the Copernicus Atmosphere Monitoring Service to estimate global, beam, and diffuse irradiance on a horizontal plane. McClear_Radiance uses look-up tables of sky radiance matched to atmospheric conditions extracted from output variables of McClear. The resulting sky radiances are complemented by a detailed angular representation of surface optical properties based on the Ross-Li Bidirectional Reflectance Distribution Function for non-Lambertian surfaces. McClear_Radiance is approximately 200 times faster than radiative transfer computations. Validations against high-quality pyranometric measurements from the Plataforma Solar de Almería in Spain demonstrate strong agreement. The downwelling global tilted irradiance, for planes with tilt angles from 20° to 45° oriented south or southwest, showed bias and root mean square error values from +1 to +2 % and 2 to 3 %, respectively, relative to the mean reference. A correlation coefficient of 0.998 was obtained. The diffuse tilted irradiance exhibited respective values of +13 to +18 %, 26 to 30 %, and 0.867 to 0.881. These results are comparable to the widely used All-Weather Model of Perez et al.