Flexible green hydrogen production through the HyS cycle: process analysis of different concepts for the HySelect demo plant

This work presents the process analysis conducted within the EU funded HySelect project, evaluating different concepts for flexible hydrogen production using the Hybrid Sulfur (HyS) cycle powered by concentrated solar energy. The analysis builds upon the findings of relevant past research projects, incorporating recent technological advancements and the specific innovations proposed within HySelect. For each process section, alternative operation technologies were explored, assessing their advantages and limitations not only in terms of energy efficiency but with a distinct focus on technical feasibility, material compatibility and technological maturity, taking advantage from the sulfur industry consolidated know-how. This allowed to select the most promising options for the construction and testing of the HySelect demonstration plant in a real industrial environment. Notably, while the project developed key innovative technologies for the sulfuric acid decomposition reactor and the sulfur-depolarized electrolysis, for the remaining process sections more conventional technical solutions were selected, such as sulfur dioxide separation from oxygen via liquefaction and sulfuric acid concentration through multi-effect evaporation. Furthermore, drawing on the process analysis, this study presents a preliminary process scheme for the entire demonstrative plant, with the calculation and optimization of the overall cycle thermal efficiency. In the scenarios considered, the resulting cycle thermal efficiency is comprised between 18.5 and 26 %, with the highest efficiency associated with a sulfuric acid concentration in the outlet anolyte around 45 %. While these figures in some cases underperform compared to previous HyS cycle process simulation literature, they offer an unprecedented realistic approach for a real demonstration operation of the cycle, since commercial availability and easy operability of equipment was prioritized over theoretical performance.