Solar Redox Cycles Research Articles

A techno-economic and environmental evaluation of the integration of direct air capture with hydrogen derivatives production

Carbon-neutral fuels are key to decarbonizing hard-to-abate sectors. Solar redox cycles can produce them by creating oxygen vacancies in a metal oxide capable of splitting water and CO2. The resulting synthesis gas can be processed into a liquid fuel like methanol. To close the carbon cycle, feedstock CO2 can be captured from the atmosphere with direct air capture (DAC), but the synergies between synthetic fuel production and DAC are largely unexplored. In this work, four integration strategies between DAC and solar redox cycles are proposed. Each of them is modeled with Aspen Plus and HFLCAL and compared with a techno-economic and a cradle-to-gate life cycle assessment. The optimal configuration, with a levelized cost of 7.9 ± 0.4 USD2022/kgMethanol and a climate change impact of −450 ± 30 g CO2e/kgMethanol, uses solid DAC powered by waste heat. Therefore, the study recommends the integration of DAC in the production of synthetic fuels.

Thermochemical oxygen pumping for improved hydrogen production in solar redox cycles

Solar thermochemical cycles are promising processes for the efficient production of renewable hydrogen at large scale. One area for process optimization is the high temperature reduction step. The oxygen released during this step has to be removed from the reactor in order to increase the reduction extent of the redox material. If low partial pressures of oxygen are required, the removal of oxygen can result in a significant energy penalty for the process. Two options for oxygen removal are mainly considered so far: the use of sweep gas and vacuum pumping. Here, a third promising option is discussed - thermochemical oxygen pumping. This approach shows large energy saving potentials especially at low partial pressures of oxygen. In this study, the interaction between splitting material and pumping material is theoretically analyzed for the conditions of a demonstration campaign previously published. The presented model approach is able to capture the main mechanisms of the interaction between the two materials and the gas phase and provides predictions of the thermochemical oxygen pumping effect on the reduction extent of the splitting material. A parametric study shows the importance of the optimization of the relative material amounts. Furthermore, the influence of using different perovskite materials on the energy consumption of such a process is addressed in a more generic thermodynamic analysis. The results indicate, that by using perovskite-based redox materials, the lower limit of oxygen partial pressures for solar thermochemical cycles from an energy demand perspective might be pushed well below 10−10 bar. At low oxygen partial pressures, thermochemical pumps seem to be far more efficient than mechanical pumps, and their efficiency can be further improved by recovering the heat released during the oxidation of the pumping material.