Critical aspects of green hydrogen production from renewables

Renewable hydrogen production can have an important role in the context of global decarbonization and to increase the penetration of renewable energies (REs) in the electric system. Renewable hydrogen can contribute to balance REs production fluctuations, reduce curtailment, and ensure grid stability by acting as a long-term storage of large amounts of energy. Green hydrogen can be converted into other fuels or used as feedstock in industry where its production is now mainly based on fossil fuels. Research interest in electrolysis has risen in the last decades since it is the main process to produce hydrogen by water splitting from renewable electricity. The research focus aims to increase conversion efficiency and reduce costs. When coupled with intermittent and variable sources, electrolyzers are subjected to part-load and dynamic operation. This can accelerate the electrolyzer stack degradation and worsen its performance. In the first part of this thesis, an alkaline electrolyzer is modelled in Matlab by considering electrochemical and thermal aspects. In addition, a degradation model is proposed to include the stack degradation, and the performance reduction due to electrolyzer variable operation and number of on/offs. Currently, in the literature, very few electrolyzer models including stack degradation are deployed in simulations of electrolysis systems. The electrolyzer model is then applied to a case study in which an electrolysis system is coupled to a wind farm for green hydrogen production. Because of RE intermittency, the sizing of the electrolysis plant is a trade-off between the maximization of RE utilization, high electrolyzer utilization factors and cost minimization. In the analyzed case study, the sizing of the electrolysis system and the size and number of separated electrolysis groups into which it is divided has been investigated by performing a techno-economic analysis of several configurations. On one hand, the system is more flexible by having a greater number of separated groups. On the other hand, each electrolysis group must have its own balance of plant, which results in higher capital costs. At the same overall electrolysis capacity, the presence of two separated groups resulted to be enough to significantly increase hydrogen production compared to the configurations with only one larger group and minimized the specific cost of hydrogen production in the case of an overall electrolysis nominal power greater than about two-thirds of the nominal wind power. In the second part of the thesis, the potential, sustainability, and feasibility of hydrogen production through electrolysis are investigated in future scenarios of the Italian electric grid with increasing PV and wind capacities. Both electrolytic hydrogen production with solely RE curtailment (green hydrogen) and electrolytic hydrogen production also with additional electricity from natural gas-based plants are analyzed and compared from techno-economic and CO2-emission points of view. Very low utilization factors of the electrolyzers were obtained (up to a maximum of 22.35% in the highest RE penetration scenario), and consequent high specific hydrogen production costs, by assuming to produce only green hydrogen. By considering additional capacities of electric storage (up to 200 GWh) to cover the electric demand, the RE curtailment available for electrolysis is further reduced and the maximum green hydrogen production potential results far lower than the current national consumption. By allowing the use of additional non-RE electricity for electrolysis, the specific cost of hydrogen can be reduced but CO2 emissions increase. Under the theoretical hypothesis of producing all the current national hydrogen demand through electrolysis, specific CO2 emissions resulted higher than SMR ones, with the sole exception of the highest RE penetration scenarios and at the highest installed electrolysis capacity considered and, consequently, at the highest costs. Furthermore, electrolysis from renewables potentially competes with the electric grid decarbonization since the electricity used could be alternatively stored and provided back to the grid when electricity demand exceeds RE production.