Thermodynamic Analysis of Isothermal Redox Cycling of Ceria for Solar Fuel Production

TLDR

The study performs a thermodynamic analysis of continuous fuel production by isothermal redox cycling of ceria in a single solar reactor, assessing how operating temperature, pressure, and heat recovery affect solar‑to‑fuel conversion efficiency. Ceria is partially reduced in purified nitrogen and reoxidized with steam or CO₂ to generate H₂ or CO, with sweep gas and oxidizer flows preheated by product gases and heat recovery and temperature swing influencing the process. Under a 3000× solar concentration and 1773 K, 95.5 % heat recovery is required for 10 % H₂ and 18 % CO efficiencies, while a 150 K temperature swing cuts sweep gas flow and raises H₂ efficiency to 31.6 %.

Abstract

A thermodynamic analysis of continuous fuel production by redox cycling of ceria in a single solar reactor under isothermal conditions is presented. Ceria is partially reduced in a sweep gas flow of purified nitrogen and reoxidized with either steam or carbon dioxide to produce hydrogen or carbon monoxide, respectively. The sweep gas and oxidizer flows are preheated by the product gases. The influence of selected process parameters, including operating temperature, pressure, and the effectiveness of heat recovery, on the solar-to-fuel conversion efficiency is determined. For a solar concentration ratio of 3000, typical of state-of-the-art solar dish concentrators, and operating temperature of 1773 K, 95.5% of the available gas-phase heat must be recovered to reach conversion efficiencies of 10% and 18% for hydrogen and carbon monoxide production, respectively, assuming the flow rate of inert sweep gas is equivalent to that in a counter-current flow arrangement of gas and ceria. The efficiency depends strongly on the gas-phase heat recovery effectiveness and the sweep gas flow rate. Introducing a temperature swing of 150 K between reduction and oxidation steps strongly reduces the sweep gas flow rate and increases the efficiency from 10% to 31.6% for hydrogen production.