pvlib: Standardizing Diffuse Irradiance Components and Extending ModelChain's Optical Loss Modeling

This project aims to improve the handling of transposed diffuse irradiance and optical loss modeling in the pvlib-python package. Currently, pvlib’s transposition models provide inconsistent output, with some providing only total sky diffuse radiation (e.g. isotropic), while others allow for outputting the isotropic, circumsolar, and horizon components. Due to this, the ModelChain workflow only allows users to define a single angle of incidence (AOI) model, and then applies the resulting incident angle modifier (IAM) to total irradiance, limiting physical accuracy. The proposed GSoC project will start by standardizing the outputs of diffuse transposition models already implemented in pvlib, introducing a consistent interface to return circumsolar, isotropic, and horizon components. Building on this, the project will extend ModelChain to support component-specific IAM calculations, enabling more accurate modeling of optical losses by distinguishing between direct and diffuse contributions. These enhancements will improve both the flexibility and accuracy of pvlib simulations while maintaining full backward compatibility. The project will include comprehensive testing and documentation to ensure reliability and ease of use.