MODELING AND OPTIMIZATION OF A MULTI-TUBULAR SOLAR RECEIVER FOR SOLAR-DRIVEN HIGH TEMPERATURE ELECTROLYSIS
High-temperature co-electrolysis of water and CO2 into hydrogen and CO provides a pathway for direct solar fuel processing if solar energy is used to simultaneously provide the electricity and the thermal energy. The design of a solar receiver for the concurrent direct steam generation and CO2 heating in a pressurized high temperature environment, coupled to a high temperature co-electroyzer, is novel and challenging as it constitutes a coupled heat and mass transfer problem incorporating phase change. We developed a coupled heat and mass transfer model of a multi-tubular reactor for the concurrent direct steam generation and CO2 heating able to provide target electrolyzer temperatures (800-1100 K) and pressures (10-15bar). The flow boiling inside a single tube is modeled by solving the coupled 1D, steady-state mass, momentum and energy conservation equations incorporating the two-phase phenomena by semi-empirical correlations. Due to the non-uniform distribution of the heat flux at the receiver aperture, the multi-mode heat transfer inside the receiver cavity is a 3D problem in nature. The 1D two-phase flow model can extrapolate the 3D heat flux profile through the use of flow pattern maps allowing to estimate local flow pattern, liquid and vapor fractions, stratified and dry angles, and liquid film thickness. The estimated 3D heat transfer profile in the tube can subsequently be incorporated into a coupled 3D model of the multi-tubular reactor.