Geologic Energy Storage
Schematic cross section showing examples of chemical, mechanical, and thermal geologic energy storage methods in potential underground settings in a sedimentary basin. This illustration is a higher resolution version of figure 2 of USGS Fact Sheet 2022-3084.
Geologic energy storage methods may be divided into three broad categories:
• Chemical methods, where energy is stored as potential energy in chemical bonds. These methods include storage of methane or natural gas, natural gas liquids, and hydrogen.
• Mechanical methods, where energy is stored as potential energy using materials or fluids. These methods include compressed air energy storage, with constant or variable temperatures; gravity energy storage using suspended loads; and pumped hydroelectric energy storage.
• Thermal methods, where energy is stored as a temperature difference in materials or fluids to be used later for heating, cooling, or industrial processes such as drying.
Some applications may use natural, permeable rock formations, but others rely on new or existing resource-extraction activities, such as mining or gas production. Different geologic settings for energy storage include the following:
• Depleted or abandoned gas reservoirs;
• Abandoned mine tunnels and shafts, both lined and unlined;
• Purpose-drilled boreholes or shafts;
• Mined caverns in salt formations; and
• Freshwater or saline aquifers.