Multi-scale modelling of a solar reactor for the high-temperature step of a sulphur-iodine-based water splitting cycle

Abstract

The 3-step sulphur-iodine-based thermochemical cycle for splitting water is considered. The high temperature step of the closed-material cycle consists of evaporation, decomposition, and reduction of sulphuric acid to SO2 using concentrated solar process heat. This step is followed by the Bunsen reaction and HI decomposition. The solar reactor concepts proposed for the first step are based on a shell-and-tube heat exchanger filled with catalytic packed beds and on porous ceramic structures to directly absorb solar radiation and act as reactor. The design, modelling and optimisation of the solar reactor using complex porous structures relies on the accurate determination of their effective heat and mass transport properties. Accordingly, a multi-scale approach is applied. Ceramic foam samples are imaged using high-resolution X-ray tomography to obtain their exact 3D geometrical configuration, which in turn is used in direct pore-level simulations for the determination of the morphological and effective transport properties. These are incorporated in a volume-averaged (continuum) model of the solar reactor. Model validation is accomplished by comparing numerically simulated and experimentally measured temperatures in a 2 kW reactor prototype tested in a solar furnace. The model is further applied to analyze the influence of foam properties, reactor geometry, and operational conditions on the reactor performance.