Iron-air batteries, flow batteries, and the technologies reshaping how grids store clean energy
The rapid growth of solar and wind power has created an unprecedented demand for energy storage. Lithium-ion batteries — refined through decades of electric vehicle development — have become the dominant short-duration storage technology, with costs declining more than 85% since 2010 to approximately $115/kWh in 2024. But lithium-ion has limits: it struggles economically at durations beyond 4–8 hours, raising questions about how grids can store clean energy for overnight, multiday, or seasonal applications. A new generation of grid storage technologies is addressing this gap.
Iron-Air Batteries: Rust as an Energy Store
Form Energy’s iron-air battery technology replicates the chemistry of rusting at electrochemical speed. During charging, an electric current reverses the oxidation process, converting iron oxide back to metallic iron. During discharge, the iron reacts with oxygen from the air (essentially rusting), releasing electrons and generating electricity. The electrochemistry is simple, the materials — iron and air — are virtually unlimited in supply and extremely cheap, and the system can theoretically sustain discharge for 100+ hours. Form Energy, backed by Breakthrough Energy Ventures and Mitsubishi, has begun commercial production of iron-air batteries and has signed supply agreements with major utilities including AES Corporation and Georgia Power.
Flow Batteries: Energy Stored in Tanks
Vanadium Redox Flow Batteries (VRFBs) store energy not in solid electrodes but in liquid electrolytes held in external tanks. Because energy storage and power output are independently scalable — larger tanks for more energy, bigger cells for more power — flow batteries are highly flexible for grid applications. VRFBs have demonstrated cycle lives exceeding 20,000 cycles with minimal degradation, making their lifetime economics attractive despite higher upfront costs. The dominant challenge is the high cost of vanadium electrolyte. Iron-chromium and zinc-bromine flow battery variants use cheaper materials and are gaining commercial traction for 4–12-hour storage applications.
The Pumped Hydro Incumbent
Despite all the innovation in battery chemistry, pumped hydroelectric storage remains by far the dominant form of grid-scale energy storage, representing approximately 68% of global storage capacity as of 2023. In pumped hydro, surplus electricity pumps water to an elevated reservoir; when power is needed, the water flows back down through turbines. Round-trip efficiency is 70–85%, and projects last 50–100+ years. The limitations are geographic — suitable reservoir sites are limited — and the permitting and construction timeline can extend to decades. Advanced ‘closed loop’ pumped hydro designs that don’t require a natural water body are expanding potential sites.
Compressed Air Energy Storage
Compressed air energy storage (CAES) pumps air into underground caverns — typically salt formations — under high pressure. When energy is needed, the compressed air is released to drive turbines. APEX CAES is developing a 324 MW / 16,000 MWh facility in Texas capable of 48-hour discharge — one of the longest durations of any grid storage technology. CAES is particularly well-suited for seasonal storage and time-shifting large amounts of renewable generation, though it requires suitable geological formations.
The 2024 battery storage year was extraordinary: the U.S. power grid added battery storage equivalent in capacity to approximately 20 nuclear reactors over four years, according to analysis by The Guardian. The global energy storage market is projected to grow at a compound annual growth rate of 21% through 2030. Green hydrogen and ammonia offer the most promising pathways for truly seasonal storage — shifting summer solar surplus to winter heating demand — but their round-trip efficiency (roughly 30–40%) makes them economically complementary to high-efficiency batteries rather than competitive with them for shorter durations.
The storage landscape is not a winner-take-all market. Different technologies serve different durations and use cases: lithium-ion for sub-4-hour applications, flow batteries and iron-air for 8–100 hours, and hydrogen-based systems for seasonal storage. Successful grids will likely deploy all of them.