Speaker
Description
Introduction
Highly concentrated solar radiation can be used in solar-thermochemical water splitting to produce hydrogen as an energy carrier. This production pathway shows a high efficiency potential as an alternative to PV coupled to electrolysis. In solar tower plants, receiver-reactors (like the R2Mx reactor [1]) convert high temperature heat to chemical energy in a two-step redox cycle. At the heart of the process is the redox material, commonly ceria ($\mathrm{CeO_2}$). Firstly, in the reduction step, the redox material is reduced at >1500°C and at low oxygen partial pressure. Secondly, in the oxidation step, at around 1000°C it splits $\mathrm{H_2O}$ to $\mathrm{H_2}$.
Operating the reactor under the extreme temperature conditions is a great challenge that requires detailed monitoring and accompanying simulations. Furthermore, the technology is still in development with optimizations in design and operation playing an important role. For all this, knowledge of the state variables, temperature and reduction extent, of the active redox material is crucial. IR thermography as known from high temperature industries like steel and petrochemical industry can deliver temperature data under controlled conditions. To control the measurement environment, one needs to restrict the background radiation. Secondly, the unknown emissivity of the redox material poses a problem. So far, only measurements at temperatures below 1244°C [2] without attention to reduction extents and room temperature measurements at different reduction extents [3] have been done. These suggest a strong dependence of the emissivity on both the reduction extent and the temperature.
Methodology
The methodology consists of two parts. (1) The determination of the emissivity and (2) the development of the analysis procedure for the extraction of the state variables, temperature and reduction extent, from the measured irradiance. The latter is done in close cooperation with (3) the application in the reactor environment.
Conclusion
To summarize, the contribution of the work will be high temperature emissivity data at different reduction stages for an often-used redox material, needed for measurement interpretation and optical modelling. Furthermore, a methodology to apply this to the in-situ characterization of the redox material state during reactor operation is developed.
References
[1] S. Brendelberger, “R2Mx plant model for solar thermochemical hydrogen production at MW scale,” Int. J. Hydrogen Energy 91, pp. 1407–1421, 2024.
[2] L. Gaillard, A. Aouali, P.-M. Geffroy, B. Rousseau, “Développement d'un dispositif expérimental pour la mesure de l'émissivité normale spectrale d'une céramique de CeO2 en conditions de thermochimie solaire,” available online at https://www.sft.asso.fr/sites/default/files/congres/2025/66_doi.pdf, 2025.
[3] S. Ackermann, A. Steinfeld, “Spectral hemispherical reflectivity of nonstoichiometric cerium dioxide,” Sol. Energy Mater. Sol. Cells 159, pp. 167-171, 2017.
