Solar-driven thermochemical water splitting: 3D energy flow analysis of a volumetric fixed-bed reactor design

Abstract

A promising alternative to established methods for solar hydrogen generation is direct water splitting via thermochemical two-step redox cycling, which has the potential for highly attractive solar-to-fuel efficiencies. This paper presents results of a 250-kW prototype fixed-bed reactor, utilizing a hemispherical-shaped porous metal foam absorber. A complex multi-physical simulation model has been developed with fine local resolution of the size of thin slices of absorber blocks. The focus of this paper is on local energy flow analysis of hot spots, characteristic behavior of absorber temperature distribution in case of nonuniform irradiation and resulting influence on efficiency. Further, balancing effects of surface temperature inhomogeneities are analyzed by varying thermal conductivity of reactor materials, thereby reducing both surface and radial temperature differences: increasing the conductivity of the used tape material by a factor of 5 (or 20) the overall plant efficiency can be substantially improved by roughly 50% or 100%, respectively.