Geologic Energy Storage: Banking Energy Underground

As the United States moves away from fossil fuels, it will rely more on renewable energy — and because renewables can produce power unevenly, energy must be stored for times when supply falls short of demand. Batteries are one answer. Geologic energy storage — storing energy underground — may hold vastly more energy for much longer, and it's flexible: many materials can store chemical, thermal or mechanical energy in many underground settings.

Above-ground equipment 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.

What it is

Geologic energy storage means putting surplus energy into underground settings such as rock formations, to meet seasonal demand, keep strategic stockpiles, or provide baseload power when renewables vary. Much of the technology is still in research and development, though several industrial-scale underground storage projects already operate in the United States and abroad.

Three kinds

Examples of chemical, mechanical and thermal storage in possible underground settings in a sedimentary basin. USGS.

Where

Some methods use natural, permeable rock; others reuse sites from mining or gas production. Settings include:

• depleted or abandoned gas reservoirs;
• abandoned mine tunnels and shafts, lined or unlined;
• purpose-drilled boreholes or shafts;
• mined caverns in salt;
• freshwater or saline aquifers.

Most can host more than one kind of storage. But state and federal rules — especially on who owns underground pore space and on protecting drinking-water aquifers — may apply, and together with grid access they shape site choice and economic risk.

Why it matters for the energy transition

• Batteries start delivering power quickly but hold relatively little energy for short periods. Geologic storage starts more slowly but holds far more, far longer.
• A grid with long-duration storage of more than 100 hours could cut electricity costs the most, and geologic storage can provide 100 hours or more.
• Batteries mainly store electricity; geologic methods can store chemical and thermal energy directly — heat for buildings, for example — without converting it to electricity.
• Chemical and thermal storage don't depend on the critical minerals batteries need, which may be in short supply.

Typical storage capacity versus discharge time for geologic and non-geologic methods: non-geologic methods store up to megawatt-hours, some geologic methods giga- to terawatt-hours. Modified from Crotogino and others (2017) and Matos and others (2019). USGS.

Hydrogen underground

Hydrogen — as a gas, a liquid or bound in molecules like ammonia — can store a lot of chemical energy, and using it in fuel cells or by burning it is relatively clean compared with fossil fuels, so it's the focus of much research.

• It can be stored mainly in solution-mined salt caverns and depleted gas reservoirs, among other settings.
• Because the tiny hydrogen molecule moves easily through permeable rock and can react chemically with its surroundings, sites need extra screening: how well the cap rock seals, possible reactions with the host rock or groundwater, and microbes that could turn some hydrogen into methane and reduce efficiency.
• How the hydrogen is made — from renewable or non-renewable sources — isn't expected to affect how much a site can hold.

The USGS role

The USGS gathers the science needed to assess geologic energy resources and has long studied undiscovered hydrocarbons, geothermal energy and carbon sequestration. A 2018 National Academies report suggested that assessing storage potential across U.S. basins could become a new, strategically important priority for the USGS Energy Resources Program.

An initial USGS assessment could focus on natural gas and hydrogen (chemical), compressed air and solid-mass gravity (mechanical) and geothermal (thermal) storage:

For each setting the USGS would apply screening criteria — such as depth, cavern spacing, host-rock properties and groundwater — within a peer-reviewed assessment method. Economic or engineering studies could follow. A consistent national picture of storage capacity could help policymakers, tribal leaders and public and private decision-makers in areas that could support geologic energy storage.

Sources

Based on U.S. Geological Survey Fact Sheet 2022–3082, "Geologic energy storage"; a work of the United States government in the public domain.