Technical analysis and economic evaluation of solar reactors for sulfuric acid cracking for the thermochemical production of hydrogen

Kurzfassung

The Hybrid-Sulfur-Cycle (HyS), coupled with concentrated solar power, is a high- potential candidate for energy-efficient, renewable mass production of hydrogen. Sulfurous acid is electrochemically processed into sulfuric acid and hydrogen, which requires virtually one third of the electric energy in comparison to conventional water electrolysis. In a second step, sulfuric acid is decomposed thermally into sulfurous acid in order to be reprocessed. Two fundamentally differing approaches are conceivable to provide solar heat for the evaporation (at temperatures up to 500 °C) and decomposition (at temperatures up to 1000 °C) of sulfuric acid. In the first scenario, the corresponding reactors are directly irradiated and operated intermittently, thus avoiding the need for an additional heat carrier medium. In the second, the interposition of an efficient heat storage enables continuous operation of the entire chemical plant. Suitable reactor concepts for the solar sulfuric acid decomposition are under development by DLR (Germany) and SRNL (USA) and have been demonstrated in laboratory or even in a representative environment. This work undertakes a systematic comparison of both concepts in order to provide a guideline for the definition of further development towards the industrial scale. Thermodynamic models are developed at same level of detail, focused respectively on one-dimensional and temporal resolution, in order to determine the related mechanisms of loss, and to extrapolate achievable full load hours, yield, and hydrogen production costs. As a basis for this comparative analysis, the thesis addresses requirements concerning heat recovery, increasing sulfuric acid concentration level prior to its reprocessing, and the operating pressure, ensuring the efficient integration of both concepts into an overall process. It emerges from the analysis that the indirectly heated system promises significantly higher yields: 1. Assuming air as heat carrier, the fully integrated, indirectly heated reactor concept can be operated efficiently over the required part load range. 2. The thermal inertia of the relatively complex, directly irradiated system results in substantial losses, primarily caused by daily cold start-ups. Moreover, depending on local irradiation conditions and the extent to which the chemical subsystem will allow for operation with strong gradients, relative yields are likely to be yet further diminished. If ambitious development goals for the sulfur depolarized electrolyzer can be met fully, the HyS-process with the indirectly heated configuration reveals the potential to achieve hydrogen costs close to 4 €/kg, as long as thermal receiver efficiencies, utilization levels and economies of scale are fully exploited. In this case, the share on the hydrogen costs associated to the solar recycling of sulfuric acid is 2,2 €/kg. In the long term, if the process is to provide an economically more viable alternative to water electrolysis using renewable electricity, this value must be substantially lower.