Concept analysis of an indirect particle-based redox process for solar-driven H2O/CO2 splitting

• A new process concept for thermochemical redox cycles is presented and modeled.
• Use of heat transfer particles decouples solar absorption from reduction reaction.
• Decoupling increases flexibility for design and operation.
• Process performance sensitivity to several parameters is analysed.
• Multiple reactors operated at optimized pressures reduce parasitic losses.

The production of solar fuels by thermochemical redox cycles has gathered a lot of attention in the research community over the last years. Still, several challenges are to be overcome to reach high efficiencies with technically feasible process concepts. Critical barriers have been identified for the development of receiver–reactors because of conflicting design and operation requirements for the processes of solar absorption, heat and mass transfer, and the chemical reaction. In addition, thermodynamic studies have indicated the need of solid phase heat recuperation in order to reach high process efficiencies, which adds further complexity to the design. Balancing out the multitude of constrains while respecting technical limitations is a very difficult but necessary task. This study addresses this challenge with the development of a new process concept which includes a solid phase heat recovery approach. The concept is based on decoupling the different process steps by using a particulate redox material in combination with a particulate heat transfer material. A model is introduced to analyse the process performance of the proposed concept. The performance of the system is calculated and assessed for a range of cases, with optimistic and more conservative assumptions for the boundary conditions. While the system reaches peak efficiencies in the range of 30% for optimistic boundary conditions, the peak efficiency drops to just above 15% for the conservative case. Additionally, the implementation of a multi-reactor approach to lower parasitic losses is presented and analysed. By extending the system to multiple reactors working at optimized oxygen partial pressures significant reductions of the vacuum pumping power demand are obtained, resulting in a 20% system efficiency increase. Besides the performance analysis of the concept, its specific challenges and advantages, like the increased flexibility for design and operation, are discussed.