Advances in the Modeling of All-Sky Radiative Transfer for Solar Energy Applications

Multiple radiative transfer models simulating atmospheric radiation under all-sky conditions have been developed and used in broad range of applications such as satellite remote sensing and climate studies. Compared to other applications, solar energy has unique requirements from radiative transfer models and thus has particular prerequisites in the model design. For instance, conventional radiative transfer models only provide surface irradiances in direct and downwelling directions. However, solar energy forecast requires rapid computation of solar irradiances over inclined surfaces as solar tracking systems frequently orient photovoltaic (PV) panels. This study briefly introduces the recent advances in the development of all-sky radiative transfer models for solar energy applications. A Fast All-sky Radiation Model for Solar applications (FARMS) was developed to compute diffuse horizontal irradiance (DHI) and direct normal irradiance (DNI) by parameterizing output from Rapid Radiation Transfer Model (RRTM). The accuracy of FARMS is comparable to or better than two-stream approach but is about 1000 times more efficient. To extend the capability of FARMS, cloud bidirectional reflectance distribution functions (BRDFs) and bidirectional transmittance distribution functions (BTDFs) were simulated and combined with a clear-sky model to compute all-sky radiances. Integration of the radiances over inclined surfaces provides accurate solar irradiance in the Plane-of-array (POA). POA irradiances are also simulated in spectral bands to meet the needs of solar energy studies.