LDES Technology Candidates

Long-duration energy storage needs to hold power for 10 to 100+ hours. No single technology dominates because the physics splits into four fundamentally different approaches, each with distinct cost profiles and siting constraints.

The four families: mechanical (pumped hydro, compressed air, gravity systems), electrochemical (iron-air, zinc-bromine, flow batteries), thermal (molten salt, heated rock), and chemical (hydrogen in salt caverns). Pumped hydro remains the largest source of grid storage today, roughly 60% of combined pumped-hydro-and-battery capacity as of 2024, but requires specific geography: two reservoirs at different elevations. Compressed air needs underground caverns. Iron-air batteries promise $20/kWh storage costs but are pre-commercial. Hydrogen stored in salt caverns offers the lowest per-kWh capital cost for truly seasonal durations (weeks to months) but suffers 65-75% round-trip energy losses.

Matching technology to duration. A grid planner needs storage for two scenarios: 12-hour overnight backup and 100-hour seasonal reserve.

Can the same technology serve both?

Unlikely. Iron-air or flow batteries are candidates for 12-hour backup: moderate round-trip efficiency (45-70%), declining cost per added hour, and no geographic constraint. For 100-hour seasonal storage, hydrogen in salt caverns is the leading candidate: the marginal cost of an additional hour of duration is nearly zero (just more cavern volume), and the input electricity is often curtailed surplus valued near zero. The 65-75% energy loss matters less when the input is essentially free.

The LDES market will likely be a portfolio, not a winner-take-all race, because each duration bracket favors different physics.

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