CAS-Climate: Bubble generation and ripening in underground hydrogen storage

The international energy agency identifies hydrogen as one of the key pillars for decarbonizing the global energy sector by mid-century. This is because most renewable energy sources such as wind and solar are intermittent, requiring storage during times of high production to ensure availability during times of low production. Hydrogen is an excellent medium for storage, but very large volumes will be required. Surface facilities and underground salt caverns provide some capacity, but not enough for what is needed. Deep geologic formations comprised of porous rocks are a promising solution because they offer large capacity with demonstrated success in storing other fluids (e.g., carbon dioxide). What is not well understood is how much of the hydrogen injected during summertime can be recovered during winter and how hydrogen purity is impacted by the presence of a co-injected fluid, called cushion gas. The seasonal injection-withdrawal cycles will likely generate and trap many hydrogen and cushion-gas bubbles in the rock that subsequently exchange mass with each other through a process known as Ostwald ripening. Both can contribute to the loss and purity degradation of hydrogen. This award aims to understand the basic mechanisms controlling entrapment and ripening of multicomponent bubbles relevant to hydrogen storage. This understanding will help select suitable geologic storage sites and compatible cushion-gasses. Hydrogen is widely regarded as a key pillar for decarbonizing the global energy system. To buffer the intermittency burden of wind and solar at scale, vast quantities of hydrogen must be stored seasonally. Existing solutions (e.g., tanks and salt caverns) do not provide nearly enough capacity for what is needed. Geologic hydrogen storage in deep formations meets the demand, but very little is understood about how hydrogen interacts with pre-existing and co-injected fluids, called cushion gas. The goal of this award is to understand how cyclically injected multicomponent gases become trapped within a porous microstructure and how they evolve through diffusive mass exchange, or Ostwald ripening. An integrated experimental and modeling plan is proposed to gain fundamental insights into: (1) the factors that control the spatial distribution of trapped bubbles during cyclic injections and withdrawals; (2) how bubble sizes, shapes, and compositions evolve due to ripening; and (3) how collective bubble equilibration impacts the macroscopic storage capacity and flow resistance of the porous medium. The educational plan will train one PhD student and engage underserved K-12 students through in-person/virtual museum exhibits. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.