Solar Fuels Production with Redox Thermochemical Processes

M. Syrigou, D.A. Dimitrakis, S. Lorentzou, A.G. Konstandopoulos
Centre for Research & Technology Hellas (CERTH),

Keywords: solar fuels, solar hydrogen, thermochemical redox processes, water splitting


Concentrated Solar Power technology is a promising component in the vision of a sustainable energy future. A multi-scale solar facility has been constructed in order to gain valuable insight in solar fuels production research and identify the key factors governing the operation and the efficiency of such technologies. The solar facility consists of a High Flux Solar Simulator (HFSS) and a recently constructed solar furnace for lab-scale and actual solar field experiments respectively. The 18kWth solar simulator consists of 11 Xenon Short Arc Lamps that deliver high-flux thermal radiation approximating natural sunlight, either on one or two focal targets. The constructed 20kWth concentrating solar furnace allows the achievement of very high solar concentrations, similar to those achieved by parabolic dishes while at the same time tackles the challenging technical aspects (moving parts, sealing etc.) encountered when operating at the temperature ranges of 1000-1400oC. The solar aided hydrogen production, takes advantage of the fact that redox materials exist in multiple valence states, establishing a two-step cycle. Redox materials at high temperature (~ 1400oC) release oxygen atoms from their lattice structure and subsequently the inlet stream flows through the reactor at lower temperature (~1000oC). Thermochemical Water Splitting (WS) and/or Carbon Dioxide Splitting (CDS) is attained as oxygen fills the available empty sites, thus solar hydrogen, carbon dioxide and syngas is produced. Experiments under solar simulated conditions have been performed in a simple cavity-tubular reactor. Nickel ferrite in various formulations (bulk, extruded honeycomb monolith or foam structured) is placed inside an inert ceramic tube. The reactor unit is settled in the middle of a cavity and the incoming irradiation, provided by the solar simulator, enters the cavity through a windowless aperture. The incident radiative flux is gradually increased to avoid local cracks and failure of the material. Several experiment sets have revealed the reaction mechanism and the impact of the operational conditions, such as the temperature, the pressure and the inlet composition. All three reactor types have been examined to identify the optimum structure that maximizes the products yield, the process efficiency and the lifetime of the catalysts. On the same time, in silico models have been developed simulating the reactor unit performance. Computational modeling of the tubular reactor enables the investigation of different operation scenarios or/and alternative configurations. Reliable kinetic expressions that sufficiently describe the products evolution profiles, under various operational protocols, have been obtained through kinetic analysis taking into account the reactor structure. Scaling-up, an integrated energy reactor model simulating WS/CDS has been developed and validated. The novelty lies on the capability of the developed tool to take into account the energy interaction between the components (solar reactor and cavity), thus the entire reactor unit has been successfully simulated. The products yield augmentation and the optimization of the process are the primary aims of computational modeling. In this framework, various test cases are simulated and the optimum values of the key parameters of the process are obtained via parametric analyses.