A sky radiance-based approach to diffuse irradiance transposition

The estimation of plane-of-array irradiance on a tilted surface from global horizontal, direct normal, and/or diffuse horizontal irradiance data is the largest single source of uncertainty in PV performance modeling. The most widely used transposition models do not specify the directional origin of the sky diffuse irradiance. However, models for estimating the sky radiance distribution have previously been developed and validated. With an expression for the diffuse radiance distribution, transposition to an arbitrary surface becomes a straightforward numerical integration. While this approach increases model complexity, it allows diffuse reflective losses and diffuse shading effects to be seamlessly included in the transposition step. Initial validation results show that this approach offers comparable accuracy to commonly used transposition models, and additional model development could improve performance.