Solar thermochemical production of hydrogen: Steady-state and dynamic modeling of a Hybrid- Sulfur Process coupled to a solar tower

Kurzfassung

Hydrogen, being an environmentally friendly energy carrier is considered a future solution of the current energy problems, provided that it is produced from water and by applying renewable energy sources. Its productions from water at moderate temperatures can be realized by means of indirect routes, called “thermochemical cycles”, a series of chemical steps by which the net result is the splitting of water into hydrogen and oxygen. The Hybrid Sulphur Cycle (HyS) is capable of producing hydrogen on large-scale using water as feedstock and holds promise of high overall efficiencies. When coupled with solar power, the hybrid sulphur cycle effectively converts solar radiation into chemical energy in the form of hydrogen. The type of solar configuration chosen is the Central Receiver System (CRS) which belongs to the point focussing group of concentrating solar power systems. The focus of this study has been to design and engineer a process model for the hybrid sulphur cycle within the constraints of a CRS with an incident power rating of 50 MW. An overall process model and the constituent process sections for HyS have been rigorously designed and simulated. All process sections engineered are full-fledged and satisfactory in performance. The net production rate of hydrogen is 0.25 kg/s. The process model at atmospheric pressure in the reactor predicts an energy consumption of 360 kJ/mol H2 for the decomposition of sulphuric acid and 210 kJ/mol H2 for concentration of dilute sulphuric acid. The process model was analysed in the form of different parametric studies identifying a strong influence of the sulphuric acid decomposition temperature and operational pressure. Due to fluctuation of the insolated power core components of the process like the solar reactor for sulphuric acid decomposition can hardly be operated at constant operating conditions. Therefore, accurate modelling of the transient behaviour was essential to predict the performance of the system under real conditions. The model has been validated by experiments and proved to be applicable to simulate the process in the solar reactor. Validation at steady state and for non-stationary operation was performed with the experimental data available from experiments in a solar furnace. The simulation of the transient behaviour of the system provided a satisfactory coincidence with experimental data for the time-dependence of the temperature distribution of the system as well as for the chemical performance.