Techno-economic modeling and optimization of solar-driven high-temperature electrolysis systems

We present a techno-economic analysis of solar-driven high-temperature electrolysis systems used for the production of hydrogen and synthesis gas. We consider different strategies for the incorporation of solar energy, distinguished by the use of differing technologies to provide solar power and heat: (i) thermal approaches (system 1) using concentrated solar technologies to provide heat and to generate electricity through thermodynamic cycles, (ii) electrical approaches (system 2) using photovoltaic technologies to provide electricity and to generate heat through electrical heaters, and (iii) hybrid approaches (system 3) utilizing concentrated solar technologies and photovoltaics to provide heat and electricity, respectively. We find that system 3 generates hydrogen at a high efficiency (gSTF = 9.9%, slightly lower than the best performing system 1 with 10.6%) and at a low cost (Cfuel = $4.9/kg, lowest cost of all three systems) at reference conditions, providing evidence for the competitiveness of this hybrid approach for scaled solar hydrogen generation. Sensitivity analysis indicates an optimal working temperature for system 3 of 1350 K, which balances the increased thermal receiver losses with the reduced electrolysis cell potential when increasing the temperature. Lower working pressure always favors high system efficiency and low cost. The working current densities for thermoneutral voltage were determined for various temperature and pressure combinations, and trends for efficient and cost-effective thermoneutral operation were identified. The water conversion extent was optimized to avoid mass transport limitations in the electrodes while ensuring large fuel generation rates. For synthesis gas production, a H2/CO molar ratio of 2 can be achieved by tuning the inlet feeding molar ratio of CO2/ H2O, temperature, and pressure. This study introduces a flexible simulation framework of solar-driven high-temperature electrolysis systems allowing for the assessment of competing solar integration approaches and for the guidance of the operational conditions maximizing efficiency and minimizing cost, providing pathways for scalable solar fuel processing. 2017 Elsevier Ltd. All rights reserved.