What it is
Long-duration energy storage (LDES) means hours to days, not the four-hour lithium packs that already make money. Form Energy’s iron-air battery is designed for about 100 hours and is in utility pilots (including a Great River Energy project aimed at mid-decade operation). Vanadium and other flow batteries are already in commercial niches; the Pullman, Washington, Turner project is one early flow-battery plant.
Problem it targets
When the wind drops for three days, a four-hour battery is a cushion, not a backbone. Gas peakers fill that gap today. Cheap multi-day storage would let grids raise the renewable share without pretending weather is optional.
How it works
Iron-air cells “rust” iron to store energy and un-rust it to discharge — abundant materials, low energy density, large yards. Flow batteries keep liquid electrolytes in tanks you can enlarge. Both trade bulk and acres for duration. Neither is a physics miracle.
Status and players
Form Energy; flow-battery vendors (UniEnergy / successor chemistries, Invinity, and others); compressed-air and thermal storage as non-battery cousins. IEA and DOE LDES programs track the field.
Risks and limits
Round-trip losses, siting, and whether hydrogen or more transmission is cheaper. Do not count announced gigafactories as delivered MWh. Lithium remains the workhorse for short duration.
Sources
Form Energy technology and utility-pilot pages; UniEnergy Turner project (Commons photo of a real 1 MW / 4 MWh flow system); IEA Grid-Scale Storage and LDES Council overviews.