In recent years, significant progress has been accomplished in the development of CSP (Concentrating Solar Power) systems making use of heliostat fields, with central receivers on top of a tower, capable of achieving irradiances well above 2000kW/m². Such high solar radiation fluxes allow the efficient conversion of solar energy at temperatures well beyond 1000ºC, which are needed for the application of two-step thermochemical cycles using metal oxide redox reactions, either for use in thermochemical energy storage or for the production of solar fuels (Romero and Steinfeld, 2012).

In that respect, nonstoichiometric oxides become a well proven material for solar fuels production. Ceria (CeO₂) is currently considered the state-of-the-art redox material because of its rapid redox kinetics and long-term stability, being able to maintain its crystal structure. For typical operating conditions of the reduction step at 1500°C and 0.1 mbar, and the oxidation step at 900°C and 1 bar, thermodynamics predict a δ = 0.04, where δ denotes the non-stoichiometry – the measure of the redox extent. Thermodynamic analyses of solar hydrogen processing cycles with ceria provide peak solar-to-hydrogen efficiencies in the range of 12–20 %, though the associated challenge involves optimized gas-to-gas heat recovery systems and strict oxygen partial pressure control during the reduction step. The best on-sun results as of today are reported with the framework of the European Sun-to-Liquid project in a solar tower plant for the thermochemical production of kerosene from H2O and CO2 (Zoller et al., 2022)

The 50 kW solar reactor, mounted on top of the solar tower, consisted of a cavity-receiver containing a reticulated porous structure made of ceria which was directly exposed to a mean solar flux concentration of up to 2,500 suns (see figure below). Approximately 5,000 standard liters of syngas were produced with full selectivity during 55 hours of on-sun operation, yielding a maximum solar-to-syngas energy efficiency of 4.1% without applying heat recovery. It was observed that a limitation on the energy conversion efficiency in the reactor persists through the inhomogeneous temperature distribution, which is created when the material is heated up during the reduction step. A part of the material reached critical temperatures of 2000 K before the rear part of the reticulated porous ceramic (RPC) structure had achieved high enough temperatures for efficient reduction. Follow up research is oriented to produce 3D-printed structured redox materials with porosity gradient leading to conversion efficiencies of 10% and designing integrated solar reactor+heat recovery system with attached rear chamber for heat exchange for increased efficiency up to 15% at TRL5 (Project Sun-to-Liquid II) and more refined definitions of KPI (Project Suner-C).