Techno-Economic Evaluation of Alkaline Electrolyzers for Green H2 production from Seawater

This thesis aims to investigate the topic of green hydrogen production through the electrolysis reaction from desalinated saltwater to analyze the current technical and economic issues. First, a general overview of the electrolyzers currently available on the market and under development is given, which reveals that the best option for hydrogen production at sea are alkaline electrolyzers, as they represent the cheapest and most efficient technology in this field. Furthermore, it is shown that it is possible to produce the electrolyte on-site through the Chloralkali process, avoiding the risks and costs of its manual replacement. Then, the advantages and disadvantages of hydrogen production at sea are highlighted, pointing out that the use of saltwater for hydrogen production will be crucial in the future to avoid problems of freshwater scarcity and environmental impact. For this reason, all water purification techniques are analyzed, detailing the advantages and disadvantages of each technology. In parallel to the theoretical analysis, an experimental part is carried out in which a synthetic seawater mixture is prepared and distilled to evaluate the salt rejection rate following a serial distillation. Chromatography results show that a single distillation is the right compromise between residual salinity and cost, and that only a more efficient process, such as electrodialysis, can further reduce the salinity level after the first distillation. From these results, a saltwater solution with the salinity values related to the first distillation is created and mixed with different quantity of distilled water to simulate the operation of the electrolyzer at different times. In the meantime, various trials and errors are carried out before the functional alkaline electrolyzer could be printed in the correct way with the 3D printer. The cell is initially printed entirely with PLA, but with poor results as this material is not very resistant to caustic soda and high temperatures. Then the cell is designed and printed with polypropylene, using PLA as the support structure, and the results are much better due to the better physical and chemical properties of this material. Once obtained the functional cell, it is connected to the entire system in the test bench to produce hydrogen and evaluate the voltage increase over time due to the degradation of the electrolyte. The results obtained reveal for the first time in the available literature that the degradation of the electrolyte follows a reasonably accurate logarithmic pattern, which allows to calculate the electrolyte renewable time and its management cost at sea. In contrast to all previous analyses, these results show that the electrolyte must be changed only 5 times during the entire life cycle of the electrolyzer to maintain high efficiency. From this analysis, it can be also noticed that the additional cost of the electrolyte management cost, calculated for the first time in the available literature in line with what it is currently available in the market, is not excessively high with respect to the levelized cost of hydrogen of the case study production plant (6.35 €/kg). The electrolyte management cost results in fact equal to 0.12 €/kg in the worst-case scenario and represents the 8% of the total plant costs, and a percentage lower than 2% with respect to the total levelized cost of hydrogen. Nevertheless, further innovations in the technology can further lower costs by making seawater electrolysis even more competitive.