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As the United States transitions away from fossil fuels, its economy will rely on more renewable energy. Because current renewable energy sources sometimes produce variable power supplies, it is important to store energy for use when power supply drops below power demand. Battery storage is one method to store power. However, geologic (underground) energy storage may be able to retain vastly greater quantities of energy over much longer durations compared to typical battery storage. Geologic energy storage also has high flexibility; many different types of materials can be used to store chemical, thermal, or mechanical energy in a variety of underground settings. The U.S. Geological Survey (USGS) has the capability to research and assess possible domestic geologic energy storage resources to help prepare the United States for the future of renewable energy.
The term ‘geologic energy storage’ describes storing excess energy in underground settings such as rock formations. Storage of energy for later use is needed to supply seasonal demand, ensure strategic stockpiles, or provide baseload power when renewable energy sources are variable. Much of the technology for geologic energy storage is still undergoing research and development ( Example of above-ground infrastructure in Reno County, Kansas, for a natural gas storage cavern hundreds of feet deep in a salt formation. Photograph by Marc L. Buursink, U.S. Geological Survey. Figure 1. Photograph of above-ground infrastructure in Reno County, Kansas, for a natural gas storage cavern hundreds of feet deep in a salt formation
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.
The methods, along with the different underground storage settings, are shown in
Schematic cross section showing examples of chemical, mechanical, and thermal geologic energy storage methods in potential underground settings in a sedimentary basin.
Figure 2. Cross section showing examples of chemical, mechanical, and thermal geologic energy storage methods in potential underground settings in a sedimentary basin
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.
Most of these geologic settings could be used for more than one form of energy storage (
Energy production and consumption in the United States is undergoing a transition from primarily fossil fuels to a mixture that includes greater shares of renewable sources and nuclear energy. Battery storage installations have a short start-up time to deliver power along with relatively short duration and small capacity. In comparison, geologic energy storage methods can retain vastly greater quantities of energy over much longer time periods (
Graph of typical energy storage capacity compared to typical discharge duration for various geologic and nongeologic energy storage methods. Oval sizes are estimated based on current technology. Modified from
Figure 3. Graph of typical energy storage capacity compared to typical discharge duration for various geologic and nongeologic energy storage methods
Hydrogen (either as a gas, liquid, or within another molecule like ammonia) may store a substantial amount of chemical energy. The subsequent use of that energy through electrical fuel cells or combustion is relatively clean compared to fossil fuel usage (
Hydrogen may be produced in different ways, including from renewable and nonrenewable energy sources. However, the resulting hydrogen produced from these methods is not expected to affect the amount of hydrogen that can be stored in a geologic setting.
The USGS acquires and communicates scientific information needed to assess geologic energy resources. In 2018, a National Academies of Sciences, Engineering, and Medicine report (
Initial work on a USGS assessment of geologic energy storage could focus on natural gas and hydrogen (chemical), compressed air and solid-mass gravity (mechanical), and geothermal energy (thermal) storage methods (
Table 1. Summary of currently deployed or likely combinations of geologic energy storage methods and settings in the United States
[X, storage possible; —, not applicable]
Storage setting | Geologic energy storage method | ||
Chemical | Mechanical | Thermal | |
Depleted gas reservoirs | X | — | X |
Solution-mined salt caverns | X | X | — |
Non-potable aquifers | X | — | X |
Abandoned mines | — | X | X |
Development of this assessment methodology could help with consistent delineation of storage capacities across the United States. Ultimately, results from a resource assessment by the USGS may inform public and private stakeholders and aid decisions by policymakers and tribal leaders in geographic areas of the United States that could support geologic energy storage.
For more information, contact the Energy Resources Program at AskEnergyProgram@usgs.gov or visit