Technical analysis and economic assessment of solar reactors for the decomposition of sulfuric acid for the thermochemical generation of hydrogen

The hybrid sulfur (HyS) cycle, coupled with concentrated solar power, is a high-potential candidate for energy-efficient mass production of hydrogen. Sulfurous acid is electrochemically processed into sulfuric acid and hydrogen in a first step, which effectually requires one third of the electric energy needed for conventional water electrolysis. In a second step, sulfuric acid is decomposed thermally into sulfurous acid in order to be reprocessed. Two approaches are conceivable to provide solar heat for the evaporation (up to 500 °C) and decomposition (up to 1000 °C) of sulfuric acid: (1) The reactors are directly irradiated and operated intermittently, avoiding the need for an additional heat carrier. (2) An interposed thermal cycle with heat storage enables continuous operation of the entire chemical plant. Suitable reactor concepts are under development by German Aerospace Center and Savannah River National Laboratory (USA) – demonstrated in laboratory and, partly, 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. 1D transient thermodynamic models are developed 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, 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 – exemplarily investigated with air as heat carrier – promises significantly higher yields. Its fully integrated reactor concept can be operated efficiently over the required part load range, whereas daily cold start-ups of the bulky, directly irradiated system, imply substantial losses. Relative yields are likely to be yet further diminished, depending on local irradiation conditions, transient thermomechanical loads and the reactors' durability. If ambitious development goals for the sulfur dioxide-depolarized electrolyzer (SDE) can be met, and provided that thermal receiver efficiencies, utilization levels and economies of scale of the solar plant are largely exploited, hydrogen costs are estimated to be close to 4 €/kg; with a share of 2.2 €/kg for the solar recycling of sulfuric acid. In the long term, if the process is to provide an economic alternative to water electrolysis, this value must be substantially lower.