PROCESS DESIGN FOR SOLAR THERMO-CHEMICAL HYDROGEN PRODUCTION AND ITS ECONOMIC EVALUATION

Abstract

The search for a sustainable long term massive hydrogen production route is a strong need, considering increasing energy demand, the diminishing fossil fuel reserves and global warming. One important objective of the EU project HYTHEC (HYdrogen THErmo-chemical Cycles) was to evaluate and to improve the potential for hydrogen production using the Hybrid-Sulfur cycle (HyS) driven by solar energy. This cycle comprises two main steps: at 850°C to 1200°C sulfuric acid is decomposed to H2O, O2 and SO2. The oxygen is separated from the gas stream and the SO2 (solved in water) is electrolyzed to produce hydrogen and sulfuric acid, thus closing the cycle. The technical feasibility and the economic potential of solar operation of the HyS process are being analyzed. Plant concepts have been created including the solar supply of heat for the thermo-chemical step and for electric power for the electrolysis step. The cycle is modeled using flow sheet techniques and will allow further optimization of the cycle efficiency. The high temperature heat is assumed to be provided by a heliostat field arranged around a central tower bearing the receiver reactor. Two systems sizes were analyzed: one central receiver system (CRS) with an annual average power of 50 MWth (equal to 140 MWth in peak), the second one six times larger. Both systems are considered to be feasible. The latter case leads to higher hydrogen production costs (HPC), mainly because of exponentially increasing costs of the high tower. Depending on the plant layout HPC of 4.9 €/kg seem achievable at locations with high solar irradiation