Oak Ridge National Laboratory is investigating whether thousands of abandoned coal mines scattered across the United States could serve as underground reservoirs for pumped storage hydropower, effectively turning defunct fossil fuel infrastructure into giant water batteries. The research program pairs computational modeling with site screening and economic analysis to determine which mines could store and release energy on demand. If the concept proves viable at scale, it could address two problems at once: the country’s growing need for grid-scale energy storage and the economic void left behind when coal mines shut down.
How Water Batteries Work Underground
Pumped storage hydropower is a simple idea with enormous scale. During periods of excess electricity, water is pumped uphill to a higher reservoir. When demand rises, that water flows back down through turbines to generate power. The technology already accounts for more than 90% of utility-scale storage in the United States, but nearly all existing systems rely on above-ground reservoirs built between mountains or hills. That limits where new projects can go and drives up construction costs, especially in regions without suitable topography or where new surface reservoirs face land-use conflicts.
The ORNL research program flips the model by looking underground. Abandoned coal mines already contain large voids at different elevations, connected by shafts and tunnels. Water could cycle between upper and lower mine chambers, generating electricity on the way down and storing it on the way back up. Because these sites already exist and many are already connected to power grids and road networks, the startup costs could be significantly lower than building new reservoirs from scratch. As researchers at the International Institute for Applied Systems Analysis noted in a January 2023 analysis, mines already have the basic infrastructure and grid connections that can reduce capital expenses and shorten construction timelines compared with greenfield projects.
Engineering Barriers Below the Surface
The concept sounds elegant, but the engineering reality is harder. A technical assessment of coal-mine storage identifies two primary barriers. First, water quality and mineral chemistry pose serious risks. Coal mines contain sulfides and heavy metals that react with water, potentially degrading turbine equipment and creating environmental contamination. Acid mine drainage is a familiar problem in coal regions; in a pumped storage setting, that chemistry would be interacting with high-value mechanical systems and potentially circulating beyond mine boundaries if not contained. Second, the structural integrity of subsurface tunnels and chambers is uncertain. Decades of neglect, ground shifting, and water infiltration may have weakened the very voids that would need to hold pressurized water reliably for years.
ORNL’s technical approach uses computational hydrodynamics and chemistry models to simulate how water would behave in these environments before anyone commits to construction. The research program also includes site-scale reservoir modeling and techno-economic analysis to determine whether the numbers work at individual locations. Those simulations are paired with geotechnical surveys and, where possible, historical mine maps to reconstruct the three-dimensional layout of shafts and galleries. Rye Development, a private hydropower company, is conducting feasibility work with ORNL technical assistance, signaling that the concept has attracted commercial interest alongside federal research dollars. The gap between a promising simulation and a functioning underground power plant remains wide, but the structured research program is designed to identify deal-breakers early.
From Neutrons to Mine Caverns
Some of the most detailed information about how rock, concrete, and steel behave under long-term stress comes from national laboratory facilities. ORNL operates advanced neutron scattering instruments that can probe the microscopic structure of materials, including those used to reinforce mine shafts or line underground reservoirs. By studying how these materials respond to pressure, temperature changes, and chemical exposure, researchers can refine models of how an underground pumped storage system would age over decades of operation. That kind of data is critical when the cost of failure is not just lost generation but potential flooding or contamination in surrounding communities.
Those material insights feed into broader systems studies. A detailed energy storage analysis associated with ORNL’s mine-focused work looks at how underground pumped storage could complement other grid assets, from transmission lines to utility-scale batteries. The same report, available through an open technical portal, examines scenarios where mine-based reservoirs help smooth the variability of wind and solar power by shifting large blocks of energy from low-demand hours to evening peaks. In those models, underground systems do not replace batteries or demand response; instead, they provide multi-hour to multi-day capacity that is difficult to achieve economically with electrochemical storage alone.
Federal Policy Clears a Path
Federal regulators have already started building the permitting framework these projects would need. The Federal Energy Regulatory Commission issued guidance for closed-loop pumped storage development at abandoned mine sites, acting under authority granted by America’s Water Infrastructure Act of 2018. That guidance, filed under docket AD19-8-000, created a defined permitting pathway for mine-based projects that did not previously exist. Without it, developers would face the same lengthy review process as conventional hydropower dams, a timeline that can stretch beyond a decade and add substantial uncertainty for investors evaluating novel underground designs.
The Department of Energy has also moved forward. The Lewis Ridge initiative in Kentucky received a categorical exclusion under the National Environmental Policy Act, clearing its Phase 1 work through DOE’s Office of Clean Energy Demonstrations. That NEPA determination means the initial feasibility and site characterization work can proceed without a full environmental impact statement, though later phases would likely require deeper review. The project represents one of the first concrete federal actions tied specifically to coal-mine pumped storage and serves as a test case for how environmental regulators will weigh legacy contamination, groundwater interactions, and community concerns against the potential climate and reliability benefits of large-scale storage.
Real Projects Already in the Pipeline
Several mine-based pumped storage proposals have advanced beyond the concept stage. In New York, the Mineville Energy Storage Project would use a decommissioned underground complex for a closed-loop system, and FERC staff have already issued a draft environmental impact statement for the project under docket P-12635-002. In California, the Eagle Mountain Pumped Storage Project plans to use two existing mining pits as reservoirs at an inactive mine site, with its own FERC docket under review. These are not theoretical exercises. They involve real permitting timelines, environmental reviews, and engineering plans that must confront issues like seismic risk, water sourcing, and potential impacts on nearby protected lands.
FERC maintains a map of issued preliminary permits for pumped storage projects, updated as recently as February 2026, that shows the breadth of active proposals across the country. The pipeline suggests that mine-based storage is part of a broader resurgence in pumped hydro interest driven by the expansion of wind and solar generation, which produce power intermittently and need storage to balance supply with demand. ORNL’s own program notes that repurposed coal mines could expand reliable capacity in regions that lack suitable sites for conventional reservoirs, effectively turning legacy fossil infrastructure into a backbone for a more flexible, low-carbon grid.
Why Existing Coverage Misses the Hard Part
Most reporting on this topic emphasizes the appealing symmetry of turning coal infrastructure into clean energy assets. That framing is accurate but incomplete. The real question is not whether the physics works; pumped storage hydropower is a proven technology with decades of operational history. The hard part is whether specific abandoned mines, each with unique geology, water chemistry, and community context, can host projects that are both safe and economically competitive with other storage options. That requires detailed engineering studies, robust public engagement, and regulatory decisions that grapple with legacy pollution as well as future climate benefits.
Even if only a fraction of candidate mines prove suitable, the stakes are significant. Underground pumped storage could offer multi-gigawatt-hours of dispatchable capacity while creating skilled jobs in regions hit hard by coal’s decline. But the same characteristics that make these sites attractive—existing voids, grid connections, and industrial history—also embed risks that cannot be papered over by optimistic headlines. The ORNL-led research effort, backed by federal policy shifts and early-stage private projects, is essentially a nationwide experiment in whether those trade-offs can be managed in a way that delivers real climate and economic gains without repeating the environmental mistakes of the coal era.
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*This article was researched with the help of AI, with human editors creating the final content.
