Abandoned coal mines in the US will be transformed into giant water piles

A new method can efficiently and safely assess whether abandoned coal mines can be transformed into pumped hydroelectric energy storage systems.

Scientists at Oak Ridge National Laboratory (ORNL) have developed advanced modeling tools that could pave the way for transformation of thousands of abandoned coal mines in the United States at large underground energy storage facilities.

Innovation focuses on hydrodynamic and geochemical models high-fidelity systems designed to assess whether former mining sites can be safely and efficiently converted into pumped hydroelectric energy storage (PSH) systems.

Often described as “piles of water”, PSH facilities store energy by pumping water to a higher altitude when electricity is abundant and releasing it through turbines to generate power during peak demand periods.

The PSH already represents more than 90% energy storage capacity on a utility scale in the US, but its expansion has been limited by geography. Conventional systems require significant differences in altitude to create the gravitational “height” needed to drive the turbines. This restriction has made many regions unsuitable for development.

ORNL’s approach reinvents the model by moving the lower reservoir underground. Instead of building large new surface infrastructure, engineers would use deep wells and tunnels of decommissioned coal mines to store water. This strategy could unlock energy storage potential in flatter regions and significantly reduce construction costs by leveraging existing infrastructure.

However, converting coal mines into hydroelectric power plants presents serious technical challenges. Underground environments are chemically reactive and structurally complex. Trace minerals can interact with water, corroding turbines and other equipment. Additionally, high-pressure, fast-moving water can destabilize aging tunnel walls, raising concerns about fractures or collapses, says .

To address these risks, ORNL researchers have developed detailed simulation tools capable of modeling water flow patterns within mine shafts and predict chemical reactions between water and native minerals. The models allow engineers to assess corrosion risks, structural integrity and long-term operational stability before construction begins.

“Underground hydro storage is an exciting opportunity, but we need overcome challenges such as chemical erosion and structural stability,” said Thien Nguyen, a senior researcher at ORNL.

With the technical modeling framework established, the team now focuses on techno-economic and comprehensive systems efficiency. These studies will evaluate financial viability, best operational practices and overall performance potential in specific locations.