The subsurface air battery: rethinking long-duration storage with compressed air in subsurface fractures

Scalable long-duration energy storage requires subsurface storage concepts that are not constrained by natural caverns or surface topography. Here, we propose a compressed-air-based subsurface pumped energy storage system that uses subsurface fractures as deformable geological storage spaces. A coupled fracture–flow–leak-off model is developed to evaluate storage capacity and cycle efficiency by incorporating fracture deformation, real-gas compressibility, nonuniform fracture pressure, wellbore and fracture-flow friction, and pressure-dependent gas leak-off. Results show that gas compressional potential energy dominates fracture-based compressed-air storage, contributing approximately 83 % of the total stored energy, about five times the fracture-related mechanical energy. The storage capacity of a single subsurface fracture can be scaled from several to tens of megawatt-hours by adjusting fracture geometry and reservoir depth. Compared with water-based SPES, compressed air greatly improves energy density potential but imposes stricter sealing requirements: achieving 80 % cycle efficiency requires reservoir permeability of approximately 100 nd or lower in shallow penny-shaped fractures and below 10 nd in deep elliptical fractures. Compared with cavern-based CAES, an illustrative equivalent-capacity case indicates a substantially higher effective subsurface energy density for compressed-air SPES, approximately 34 kWh/m 3 versus 4 kWh/m 3 . These findings identify compressed-air SPES as a high-energy-density extension of fracture-based energy storage for low-permeability, well-confined formations.