Long duration storage 2026: the four day battery becomes bankable

The 4 day target and the 48E credit stack #

Long duration storage was a thesis between 2018 and 2023 and became a procurement line in utility integrated resource plans between 2024 and 2026. The LDES Council 2024 Net Zero Power update fixed a working definition of 4 day median discharge, with 10 to 24 hour systems for the diurnal shift and 100 plus hour systems for multi day reliability. The same report sized 2040 LDES need at 1.5 to 2.5 terawatts and 85 to 140 terawatt hours globally, with 400 to 600 billion dollars of cumulative capex.

The Inflation Reduction Act made the unit economics work. Section 48E, the technology neutral investment tax credit that took over from legacy Section 48 in 2025, applies to standalone storage at a 30 percent base rate. The energy community, domestic content, and low income community adders stack to 40 to 50 percent in qualifying locations. The DOE Long Duration Storage Earthshot, launched July 2021 and reaffirmed in the 2023 LDES pathways study, set a 90 percent reduction in levelized cost of storage versus a 2020 lithium ion baseline by 2030 for systems above 10 hours.

FERC Order 841, finalized February 2018 and fully compliant across the seven US ISO and RTO markets by 2022, removed the structural barrier. Storage can now bid into energy, capacity, and ancillary services markets on the same terms as a generator, which is what made the merchant or hybrid PPA business case for LDES legible to project finance.

Form Energy: iron air clears the bankable threshold #

Form Energy is the most important single project finance reference in the LDES category in 2026. The Lincoln Maine first commercial deployment, with Great River Energy and Versant Power, energized in late 2024 at 1.5 megawatts and 1.5 gigawatt hours of energy capacity, the 100 hour duration that defines the iron air product. The electrochemistry is reversible iron oxidation, the chemistry that rusts a nail, in a cell with a potassium hydroxide electrolyte. The bill of materials is iron, water, and air, with no lithium, cobalt, nickel, or vanadium.

The 2025 to 2026 order book moved the technology from demonstration into the asset class. NYSEG (Avangrid) contracted an 8 megawatt and 640 megawatt hour project in upstate New York for 2027 energization. Georgia Power filed for a 15 megawatt and 1,500 megawatt hour installation in its 2025 IRP update for 2028 energization. Xcel Energy, Dominion in Virginia, and Duke in the Carolinas have each disclosed pilot scale procurements of 5 to 20 megawatts. Form's Weirton West Virginia factory, a brownfield steel mill site, is targeting 500 megawatts nameplate per year by end of 2026.

Form's installed cost target at scale is below 20 dollars per kilowatt hour of energy capacity, roughly an order of magnitude below lithium ion on an energy basis. Power capacity costs are higher per kilowatt because the conversion stack is sized small relative to the energy reservoir, the architectural feature that makes 100 hour duration feasible. The first Lincoln Maine block is closer to 60 to 80 dollars per kilowatt hour installed, with the glide path to 20 conditional on the Weirton ramp and the iron pellet supply chain through Cleveland Cliffs.

Energy Vault gravity and EOS zinc bromine #

Energy Vault commissioned the Versant La Sterne gravity storage system in Switzerland in March 2025. The architecture stacks composite blocks made of locally sourced soil and reinforced material in a tower, lifting them with regenerative motors during charge and lowering them during discharge. Versant La Sterne is the first commercial scale plant outside China and the United States and follows the Rudong gravity plant in Jiangsu, grid connected late 2023. Energy Vault's product mix is now hybrid, pairing the gravity tower with a lithium ion block for frequency response while the gravity stack handles the 4 to 12 hour shift.

EOS Energy Enterprises took its Z3 zinc bromine aqueous flow battery to nameplate production at the Turtle Creek Pennsylvania factory in 2024 at a 1.25 gigawatt hour annual run rate, with planned 8 gigawatt hour capacity by 2027 conditional on a DOE Loan Programs Office commitment of 398.6 million dollars announced August 2023 and finalized 2024. The Z3 chemistry is non flammable and rated for 12 hour duration. EOS's order book through Q1 2026 stands above 6 gigawatt hours of letters of intent, with named offtakes including Indian Energy, Pine Gate Renewables, and a portfolio of California community choice aggregators. The challenge is factory ramp execution, not order book or technology.

Invinity Energy Systems, formed from the merger of redT and Avalon, sells the third generation vanadium redox flow battery (VRFB) product, with reference deployments at Energy Superhub Oxford in the UK and a 7 megawatt hour California Energy Commission sponsored deployment at Chappell Dairy. The chemistry has the longest cycle life in the LDES catalog (above 25,000 cycles with no measurable capacity fade), making levelized cost competitive at 8 to 12 hour duration despite higher upfront capital cost driven by vanadium pentoxide pricing.

Hydrostor compressed air and Highview liquid air #

Hydrostor's advanced adiabatic compressed air energy storage (AAA CAES) is the single largest LDES project class by gigawatt hour announced in 2024 to 2026. The Goderich Ontario commercial demonstration, in operation since 2019, is the operating reference. Silver City in Broken Hill New South Wales reached FID in 2024 at 1.6 gigawatt hours over 200 megawatts, with commercial operation targeted 2027 to 2028. Willow Rock in Kern County California, contracted with Central Coast Community Energy and PG&E, sits at 500 megawatts and 4,000 megawatt hours.

AAA CAES uses a purpose mined cavern with water compensated hydrostatic compensation, which keeps the cavern at constant pressure and avoids the salt dome geological constraint of legacy CAES at McIntosh Alabama and Huntorf Germany. Compression heat is captured in a thermal store and reused on discharge, lifting round trip efficiency to roughly 60 percent versus 40 to 45 percent for diabatic CAES. Project capex sits at 1,500 to 2,200 dollars per kilowatt at gigawatt hour scale, competitive with new build natural gas peaking once the 30 percent ITC and a capacity payment are stacked.

Highview Power's liquid air energy storage (LAES) follows a different physics path. Air is compressed, cooled to roughly minus 196 Celsius, stored as a cryogenic liquid, and re expanded through a turbine on discharge. The Carrington 50 megawatt and 300 megawatt hour project at Manchester reached construction milestone in 2024 with first energization targeted late 2026. The advantage is geographic flexibility (no salt dome or hard rock cavern requirement). Highview reports round trip efficiency at 60 to 70 percent once waste heat from a co located industrial host is captured. ESS Inc., the iron flow battery developer in Wilsonville Oregon, occupies the 6 to 12 hour band with installations at SMUD and SoftBank Energy Japan.

Antora hot rocks and Nostromo aquifer thermal #

The thermal LDES segment occupies a different competitive position from electric to electric storage. Antora Energy's hot rocks system charges electrical resistance heaters to bring carbon blocks to roughly 1,800 Celsius, then discharges thermal energy to a co located industrial host (chemicals, cement, food and beverage, paper) and optionally to electricity through a thermo photovoltaic conversion stack. The value is in displacing natural gas at the host, not selling electricity to the grid. The 2025 Fortescue iron ore mine pilot in Western Australia provides the first commercial reference. Antora raised a 150 million dollar Series B in 2024 led by Decarbonization Partners with Breakthrough Energy Ventures and Lowercarbon Capital.

Rondo Energy, a peer, deployed its first commercial heat battery at the Calgren ethanol plant in Pixley California in 2023 and expanded to a 100 megawatt hour scale unit at an Aramco Saudi Arabia host in 2024. Rondo uses brick, Antora uses carbon blocks, but the architectures converge on industrial heat decarbonization with storage as a coupled byproduct.

Nostromo Energy occupies a narrower commercial position. The IceBrick aquifer thermal storage system deployed at the Beverly Hilton and Hilton Beverly Hills in 2024 is the commercial reference, with storage paired to the building's chiller plant to shift cooling load off peak. Nostromo received a DOE Loan Programs Office conditional commitment of 189 million dollars in 2023 for a portfolio of 275 commercial building deployments. The thermal LDES segment is a real revenue category but should not be confused with grid scale electric LDES. The buyer is the building owner or industrial host, not the utility.

2026 to 2030 commercial trajectory #

The order book through Q1 2026 supports a directional view of where the market lands by 2030. BloombergNEF's Energy Storage Outlook 2024 projects global LDES deployments at 50 to 80 gigawatt hours per year by 2030, up from roughly 5 gigawatt hours in 2024. The IEA Renewables 2024 outlook places LDES at 12 to 18 percent of grid storage capacity by 2030 versus less than 4 percent in 2024.

The four utilities to watch are Constellation Energy (a portfolio MOU with Form Energy for up to 1 gigawatt of iron air across its nuclear and renewable hybrid sites), Duke Energy (200 megawatts of LDES in its 2024 Carolinas Resource Plan), Dominion Energy (iron air pilots in Virginia and an IRP amendment for 4 to 8 hour duration), and Southern Company through Georgia Power (15 megawatts and 1,500 megawatt hours in the 2025 IRP update). The four together represent roughly 35 percent of the announced US utility LDES order book through 2028.

The constraint that binds the 2026 to 2028 deployment window is not technology and not capital. It is factory ramp. Form's Weirton factory, EOS's Turtle Creek factory, ESS Inc.'s Wilsonville factory, and Energy Vault's modular manufacturing are each in parallel ramp. Sponsors without a hard slot will slip to 2028 or 2029.

The strategic question in 2026 is no longer whether LDES is bankable. Lincoln Maine, Versant La Sterne, Silver City, and Carrington close that question. The question is which duration band, which chemistry, and which factory slot. The 4 day median target the LDES Council set in 2024 is now the procurement specification across at least seven utility integrated resource plans filed in 2025 and 2026. The Argus and Promethean platforms price these procurements at a regional and corridor specific level. Buyers who move first against confirmed factory slots own firm capacity at the lowest delivered cost. Buyers who wait pay lithium ion plus natural gas peaking against a tightening capacity market through 2030.

Sources #

• LDES Council, Net Zero Power: Long Duration Energy Storage for a Renewable Grid, 2024 update
• US DOE Office of Electricity, Long Duration Storage Shot Technology Strategy Assessment, 2023
• Form Energy, Lincoln Maine first commercial deployment with Great River Energy, press release Q4 2024
• BloombergNEF, Energy Storage Outlook 2024
• IEA, Renewables 2024 Analysis and Forecast to 2030
• Hydrostor, Silver City Energy Storage Centre New South Wales, FID announcement 2024
• Energy Vault, Versant La Sterne Switzerland commissioning, March 2025
• EOS Energy Enterprises, DOE Loan Programs Office conditional commitment 398.6 million USD, August 2023
• Georgia Power 2025 Integrated Resource Plan update, Georgia Public Service Commission docket
• Inflation Reduction Act Section 48E, technology neutral clean electricity investment credit, IRS Notice 2024-41
• FERC Order 841, Electric Storage Participation in Markets Operated by Regional Transmission Organizations and Independent System Operators, February 2018
• Lawrence Berkeley National Lab, Queued Up 2024 interconnection report (storage queue context)