ASSESSING THE ENERGY AND WATER IMPACTS OF SEAWATER ELECTROLYSIS FOR GREEN HYDROGEN PRODUCTION

Green hydrogen from renewable energy sources is a promising energy carrier for a low-carbon future, with applications in energy, industry, transportation, and chemical sectors. In a scenario with high electrolyzer installed capacities, it is important to assess the impact of water consumption on freshwater resources. In fact, an annual water usage of about 20.7 Gt is expected for the predicted 2.3 Gt of hydrogen produced globally. In those areas where freshwater is a limited resource, high hydrogen production can deeply impact the local water system. A promising solution for hydrogen production without freshwater consumption is represented by seawater electrolysis. The two approaches to hydrogen generation from seawater are indirect seawater electrolysis and direct seawater electrolysis. While indirect seawater electrolysis is a two-step process, in which seawater is first treated and purified in a desalination unit before electrolysis, direct seawater electrolysis is a novel technology that uses seawater directly as the electrolyzer feed. Within this context, this study investigates the water and energy consumption of green hydrogen in the energy sector, with a focus on seawater electrolysis as a sustainable alternative to freshwater usage. The analysis was carried out considering the entire power-to-power process, involving renewable energy generation, water electrolysis, and electricity generation using solid oxide fuel cells. The water and energy impacts of both indirect and direct seawater electrolysis are assessed and compared across nine countries with varying levels of water stress and different energy mixes. The findings of this study reveal that substituting fossil fuels such as coal, oil, and natural gas, with green hydrogen in the energy sector can significantly reduce water withdrawal. With respect to energy production from conventional fossil-fuelled generators, water withdrawal for the entire power-to-power process is reduced between 30% to 90% when electrolyzers are powered by photovoltaic systems, and between 37% to 91% when electrolyzers are powered by wind systems. The highest reductions are observed when substituting oil, particularly with wind energy coupled to alkaline electrolyzers. Moreover, seawater electrolysis represents a viable solution for regions with limited freshwater resources. Indirect seawater electrolysis, employing reverse osmosis desalination before water splitting, demonstrated a minimal energy consumption increase of 1.8% compared to direct seawater electrolysis. Consequently, direct seawater electrolysis is favourable only in specific applications.