Optimized Ceria Structures for Enhanced Efficiency in Solar Thermochemical Fuel Production

Kurzfassung

The world’s dependence on fossil fuels and its consequent release of CO₂ is the primary driver of climate change, with the transportation sector contributing approximately 15% of net global emissions. While achieving net-zero CO₂ emissions in passenger car transport is viable via electric motors powered by renewable electricity, this is not yet possible for large vehicles and long-haul travel, particularly in aviation. To address this challenge, a two-step thermochemical cycle shows the potential to make aviation and other fuel-intensive sectors more sustainable by producing synthetic fuels from CO₂ and H₂O using solar energy. This thermochemical cycle takes place in a solar reactor filled with a porous structure made of ceria. The ceria porous structure is directly exposed to concentrated solar radiation, which is absorbed through the structure’s volume, facilitating the efficient high-temperature heat transfer directly to the reaction site. Tailoring the topology and shape of the ceria porous structure can enhance the penetration of solar radiation deeper into the structure, resulting in uniform heating and lower losses, without compromising the structure’s total active mass. In this work, the topology of the ceria structure is optimized using a novel voxel-based Monte Carlo ray-tracing algorithm that simulates radiative heat transfer in a voxel (three-dimensional pixels) discretized representation of the structures. The algorithm identifies ray-solid intersections by querying the voxels’ value (solid or void) along the ray’s trajectory. This approach eliminates the need to solve the 3×3 system of equations required for each ray - solid’s surface combination in a standard ray tracer, resulting in a significant increase in speed (approximately 90 times faster). The new algorithm is verified by estimating the optical properties of known porous media and by simulating the complete radiation heat exchange within a cavity. The results are compared to those obtained by a standard MC ray tracer and the analytical radiosity method. Optimized ceria structures feature hierarchically ordered channels and circumvent the exponential radiative attenuation characteristic of isotropic topologies. These structures achieve a higher and more uniform temperature profile when exposed to concentrated solar radiation compared to state-of-the-art reticulated structures. The higher overall temperatures achieved by these structures improve their redox performance, doubling the amount of carbon monoxide (the fuel) for the same solar energy input. A complete solar reactor is designed using the optimized ceria structures. The ceria structures form a cavity that maximizes the available volume for ceria loading within the solar reactor while avoiding excessively high radiative flux zones. The modular cavity design allows it to be self-supported, maintaining its integrity and stability without requiring additional materials or bonding. The modular design also isolates propagating cracks within the structures. To fabricate the optimized ceria structures, a novel manufacturing technique is implemented for the direct 3D printing of the ceria structures. The 3D printed ceria structures are subsequently vacuum coated and infiltrated to enhance their mechanical robustness and long-term stability. The assembled solar reactor is tested at ETH’s High-Flux Solar Simulator, where up to 5.13 kW radiative power enters the solar reactor’s aperture. The operational parameters of the two-step thermochemical cycle are adjusted, focusing on the solar-to-fuel energy efficiency as the key performance indicator. Experimental results reveal that the 3D printed solar reactor doubles the fuel yield of the benchmark solar reactor containing a reticulated porous ceramic ceria structure. Furthermore, a record solar-to-fuel energy efficiency of 6.29% is achieved, surpassing the previous record established in a significantly larger 50 kW solar reactor. These findings contribute to the economic viability of solar fuels, bringing sustainable aviation closer to reality. While this thesis focuses on the use of ceria and its two-step thermochemical cycle, the methodologies presented can be applied to optimize other absorber materials utilized in various directly irradiated solar reactors.