Thermodynamic Analyses of Fuel Production via Solar-Driven Non-stoichiometric Metal Oxide Redox Cycling. Part 2. Impact of Solid–Gas Flow Configurations and Active Material Composition on System-Level Efficiency

Abstract

We present an advanced thermodynamic model for a water-splitting solar reactor system employing Zr-doped ceria as the redox material and inert sweep gas to obtain the desired oxygen partial pressures in the reduction chamber. Conservation of mass and species, conservation of energy, and the Gibbs’s criteria are employed to predict solar-to-fuel efficiencies. Efficiencies vary widely with operating conditions--reactor temperatures and pressures--in addition to material thermodynamic properties, making it difficult to compare the performance of proposed redox materials. We determine the maximum efficiencies theoretically achievable with selected redox materials by simultaneous multivariable optimization of all operational parameters within their meaningful ranges. For the baseline case of zero solid heat recovery and 75% gas heat recovery, the results demonstrate that a modest efficiency improvement can be achieved by doping ceria with 10% and 15% Zr as compared to pure ceria within a narrow reduction temperature range of 1700−1850 K. However, this efficiency benefit is achieved at the cost of low oxidation temperature operation, which may lower the realistic maximum efficiencies if the oxidation step is kinetically limited. Four different reactor flow configurations are considered, including a newly developed model for counter-current flow. It is found that a maximum solar-to-fuel efficiency of 7.8% for water splitting can be attained with state-ofthe-art reactors operated at 1773 K, 95% gas heat recovery, and no solid heat recovery, based on our model assumptions. In terms of potential efficiency enhancement, peak efficiencies of 26.4% and 25.2% can be achieved for inert gas sweeping and vacuum pumping, respectively, at reduction temperature of 1900 K, 95% gas heat recovery, and 90% solid heat recovery. The model results provide insights that help guide reactor design and operation as well as potential redox material selection.