Underground energy storage using man-made CO2 hydrate geo-capsules

The increasing reliance on renewable energy sources presents challenges due to their intermittent and variable nature, necessitating efficient energy storage solutions. Underground Mechanical Energy Storage (UMES) has emerged as a promising approach, utilizing subsurface reservoirs to store pressurized fluids. However, the deployment of UMES systems is constrained by the availability of suitable natural subsurface formations. This study explores a novel concept of forming impermeable CO₂ hydrate geo-capsules within porous soil deposits at depths of 50–400 m for fluid-based UMES applications, leveraging gas hydrate-bearing sediments’ unique properties of extremely low permeability and increased stiffness. An analytical mechanical model is developed to describe the overall capsule response to energy storage, incorporating a spherical multi-layered elasto-plastic solution for internal fluid injection. The analytical model is validated through numerical simulations, which are also used to extend the investigation to varying lateral earth pressure conditions (k₀≠1). A solution space mapping is then performed to evaluate the influence of key geometrical and stiffness parameters on the mechanical response of hydrate geo-capsules, followed by an assessment of pressure retention over time. The framework is further extended to estimate the energy storage capacity of a single or multiple capsules, considering a hybrid Pumped-Hydro Compressed Gas Energy Storage (PH-CGES) configuration. This study provides a first-step assessment of hydrate-based UMES feasibility and establishes a baseline for future investigations.