Numerical Optimization of a Volumetric Solar Receiver-Reactor for Thermochemical Hydrogen Generation via Decomposition of Sulfur Trioxide

A basic concept for a receiver-reactor for solar sulfuric acid decomposition as the key step of thermochemical cycles for hydrogen production has been developed and realized. A prototype reactor has been built and is specialized for the second part of the reaction, the decomposition of sulfur trioxide. For a detailed understanding of the operational behaviour of the developed reactor type a mathematical model was developed. The reactor model was validated using experimental data from the prototype reactor test operation. The present work deals with the optimization of process and design parameters and the evaluation of the achievable performance of the reactor type. Furthermore the reactor model is used for numerical simulations to predict operational points, which are not easy to realize in experiments due to hardware limitations, to save the experimental effort, and to predict the performance of a large-scale reactor on a solar tower. The results of the simulation confirm a central finding of the experiments: Depending on the operation conditions an optimum of reactor efficiency emerges if one parameter is varied. This is in particular true for the absorber temperature. Two oppositional effects compensate each other in a way that the reactor efficiency exhibits a maximum at a certain temperature: by increasing process temperature the reradiation losses increase disproportionately high whereas the chemical conversion decreases when lowering the temperature. Beyond that influences of other operational parameters like feed mass flow, residence time, and initial concentration of the acid were also analyzed. In a scale-up study the reactor was simulated as being part of the aperture area of a large scale tower receiver. The main differences to the prototype system are the diminished gradients of solar flux on the receiver front face and the reduced thermal conduction losses due to the presence of several neighbor modules at comparable temperature level. This leads to higher chemical conversions and better efficiencies. Reactor efficiencies up to 75% are predicted. Even higher efficiencies are possible if re-radiation losses can be decreased, e.g. by considering a cavity design.