Breaking new ground: emerging underground energy storage technologies

Frequently asked questions

• What is driving interest in underground long-duration energy storage (LDES) beyond lithium-ion batteries and pumped hydro?

  Renewables now supply a significant share of electricity, but their variability creates frequent mismatches between generation and demand. Conventional grid-scale lithium-ion battery energy storage systems typically deliver under four hours of duration and can face degradation, thermal runaway risk and supply chain constraints. Pumped hydro offers longer duration but is limited by suitable terrain and permitting. Underground LDES approaches use existing subsurface assets such as salt caverns, depleted reservoirs and disused mine shafts to store energy at much larger scales and for much longer periods. For B2B buyers, the appeal is the prospect of lower cost per unit of stored energy, reduced reliance on critical minerals and the ability to place storage nearer industrial clusters and grid constraints where it provides the most commercial value.

• Why are salt caverns considered the most promising underground option for hydrogen storage?

  Salt caverns stand out because they can preserve hydrogen purity and minimise leakage. In depleted oil and gas reservoirs or aquifers, hydrogen can react with residual hydrocarbons and certain minerals, potentially consuming hydrogen, creating impurities or changing the reservoir over time, which complicates operations and offtake specifications. By contrast, rock salt is chemically inert with hydrogen, so operators can expect to withdraw gas at a similar purity to what they injected. Salt also behaves in a way that supports containment: under depth-related pressure it undergoes viscoplastic creep, gradually closing micro-fractures and pores, effectively creating a self-sealing store. That matters because hydrogen molecules are small and prone to leakage. The result is a technically attractive pathway for large-scale hydrogen buffering as the hydrogen economy develops.

• How does compressed air energy storage (CAES) work underground, and what makes it cost-competitive?

  CAES absorbs excess electricity by running compressors that pressurise air for storage underground, then later releases the pressurised air to drive a turbine when electricity is needed. The underground element enables very large storage volumes using geological formations, with salt caverns often preferred due to their naturally airtight behaviour and large, solution-mined cavities. Cost competitiveness comes from scaling energy capacity without scaling expensive electrochemical cells, and from leveraging suitable geology rather than building extensive above-ground infrastructure. In the right locations, CAES can potentially be far cheaper per kWh of stored energy than batteries. Commercial viability depends heavily on site specifics, including cavern depth, operating pressure and deliverability, which influence both stored energy and the rate at which the plant can charge and discharge to meet grid requirements.

• What is the difference between diabatic and adiabatic compressed air energy storage (CAES), and why does it matter for decarbonisation?

  Diabatic CAES, used by the two long-running large plants, vents the heat created during compression and then burns fossil fuel to reheat the air during discharge. This wastes energy and ties output to fuel consumption, limiting efficiency and undermining decarbonisation goals. Adiabatic CAES aims to capture, store and reuse that compression heat so the system can discharge without burning fuel. This improves efficiency and makes the technology compatible with renewable-heavy grids and net-zero strategies. It also enhances commercial flexibility: operators can compress air when electricity is cheap or curtailed, then generate when prices are higher, while avoiding the penalty of back-work and heat losses inherent in older configurations. For industrial buyers and utilities, the distinction directly affects emissions, operating costs, and eligibility for green finance and policy incentives.

• How does underground gravity energy storage (UGES) use disused mines, and what are its key benefits and risks?

  UGES converts surplus electricity into gravitational potential energy by using motors to lift heavy weights in a mine shaft, then recovers electricity by lowering the mass to drive generators. Disused mines are attractive because they can avoid major excavation spend and may already have grid connections, hoists and shafts, which can shorten timelines and improve social acceptance by repurposing brownfield assets. UGES can be modular, scaling by shaft and aggregating to larger site capacities, while offering strong round-trip efficiency because it avoids thermodynamic losses from gas compression. However, the main risks are asset condition and retrofit complexity. Many closed mines have uncertain shaft integrity, and equipment such as hoists and cables may need to deliver far more cycles than they ever did in mining, affecting maintenance, reliability and total cost of ownership.