Thermal design and performance optimization of the four-step Cu-Cl cycle coupled with clean energy for hydrogen production

The four-step Cu-Cl cycle, one of the most promising thermo-electrochemical water-splitting cycles for producing hydrogen, is improved with its energy utilization intensified based on the integration with the Organic Rankine Cycle. Equations are deduced to analyze the influence of reaction temperatures on the system performance. With the reaction thermodynamic equilibrium and the two phase-transitions of molten CuCl considered, the mathematical model is developed and can be applied to conduct a comprehensive analysis of utilities, reaction temperatures, current density, energy/exergy efficiency, and economic performance and facilitate the evaluation and optimization of the improved system. The improved system's heating utility consumption, cooling utility consumption, and energy efficiency are 43.68 MW, 28.76 MW, and 13.57%, respectively. After the optimization, the heating and cooling utility consumptions are reduced by 12.41% and 18.08%, respectively, and the energy efficiency is increased by 14%. Six different clean-energy-based power generation systems are integrated with the improved process. Their optimal LCOHs (Levelized cost of hydrogen) are reduced by 13.29%, 15.35%, 16.05%, 16.53%, 22.93%, and 20.44%, respectively. The integrated system with power generated based on hybrid bifacial photovoltaic (BiPV) and geothermal has the highest exergy efficiency (63.24%) and minimum LCOH (3.30 $/kg); the corresponding hydrolysis, decomposition, electrolysis, and drying temperatures, and current density are 400 degrees C, 520 degrees C, 76 degrees C, 100 degrees C and 2,000 A/m2. The results can guide the further improvement of the Cu-Cl cycle.