Analysis and Optimization of Innovative Solar-driven Electrochemical and Thermochemical Processes for Green Hydrogen Production

In the context of the global transition towards carbon neutrality, hydrogen has emerged as one of the most promising energy carriers due to its high energy density, versatility, and potential for decarbonizing sectors that are difficult to electrify. Among the different production pathways, renewable hydrogen generated using solar energy represents one of the most sustainable long-term solutions. This doctoral dissertation addresses this challenge by investigating, modelling, optimizing, and critically comparing different solar-driven hydrogen production technologies. The main objective is to provide a comprehensive assessment of the technical feasibility, energy efficiency, operational flexibility, sustainability, and economic viability of innovative electrochemical and thermochemical processes integrated with renewable solar energy. The research follows a progressive evolution in the utilization of solar energy, beginning with systems powered exclusively by photovoltaic (PV) electricity, progressing to hybrid electro-thermal configurations combining PV and concentrated solar thermal (CST) energy, and ultimately investigating purely thermochemical processes driven entirely by CST. This approach establishes a coherent framework for comparing hydrogen production technologies at different stages of technological maturity while highlighting the progressive replacement of electrical energy with renewable thermal energy. The investigation begins with PV-powered alkaline electrolysis (AE), considered the benchmark technology for renewable hydrogen production. Particular attention is devoted to the dynamic modelling and analysis of a 25 kW industrial alkaline electrolyzer integrated with a real 25 kWp PV plant. The developed model evaluates the response of the electrolyzer under variable solar irradiation, demonstrating that although AE is currently the most mature and economically competitive technology, its integration with intermittent renewable electricity remains limited by relatively low current density, moderate energy efficiency, and reduced operational flexibility under dynamic operating conditions. The research subsequently investigates high-temperature electrolysis through Solid Oxide Electrolysis (SOE), representing the transition from purely electrical to hybrid electro-thermal hydrogen production. Detailed mathematical models were developed to simulate and optimize an integrated SOE system supplied simultaneously by PV electricity and CST energy. Different grid-connected and stand-alone operating strategies were analysed together with full-load, partial-load, and night-time operating modes through comprehensive techno-economic assessment. The results demonstrate that, despite higher capital costs and durability challenges, SOE significantly outperforms conventional AE in terms of energy efficiency, hydrogen purity, and overall system performance, highlighting the advantages of combining renewable electrical and thermal energy sources. The final part of the dissertation explores innovative thermochemical pathways that completely eliminate the electrical energy requirement by exploiting CST energy as the sole external energy source. First, the novel Nickel-Iodine-Sulfur (NIS) thermochemical water-splitting cycle is rigorously modelled and optimized, demonstrating important advantages over conventional thermochemical cycles through lower operating temperatures and improved process integration. To support the practical implementation of this technology, an innovative solar-driven fluidized-bed reactor is proposed for carrying out the most energy-intensive reactions of the cycle. Inspired by advanced solar fluidized-bed systems employed for metal oxide reduction and biomass gasification, the proposed reactor offers promising improvements in solar heat transfer, temperature control, and reaction kinetics. The research further extends the thermochemical approach by investigating an innovative process integrating waste biomass hydrogasification with solar-driven steam reforming. The proposed concept exploits CST energy to convert waste biomass into renewable hydrogen and methane while providing flexibility in the final product according to market demands and process operating conditions. The experimental activity focuses on the hydrogasification stage, demonstrating the feasibility of producing methane suitable as feedstock for the subsequent solar reforming process and highlighting the potential of combining renewable carbon resources with CST energy for sustainable fuel production. Overall, this dissertation provides a comprehensive and systematic comparison of four representative solar-driven hydrogen production pathways spanning different levels of technological maturity and solar energy integration. The comparative analysis demonstrates that, while AE remains the most mature and economically competitive technology for PV integration, the incorporation of concentrated CST energy substantially improves the overall performance of solar-driven hydrogen production. When the complete solar-to-hydrogen conversion chain is considered, the overall efficiency increases from 7.5% for AE to 11.4% for the NIS thermochemical cycle, reaching 14.2% for hybrid SOE. These findings highlight the significant advantages of hybrid electro-thermal systems, which combine renewable electrical and thermal energy to achieve the highest overall efficiency while bridging the gap between mature electrochemical technologies and emerging thermochemical pathways.