Form Energy, the Massachusetts-based developer of 100-hour iron-air batteries, has signed its first international deal. The company will deploy a 10 MW / 1 GWh battery system in County Donegal, Ireland, through a partnership with FuturEnergy Ireland. The project, called Ballynahone Energy Storage, is expected to come online in 2029 and will be used to reduce renewable curtailment and stabilize Ireland's national grid. The agreement was announced on March 17 at Form Energy's headquarters in a ceremony timed for St. Patrick's Day. Irish Minister for Foreign Affairs Helen McEntee attended the signing and called the partnership a demonstration that "Ireland and the United States need to be able to work together to take on today's challenges, meeting the needs of affordable energy with new technologies." The Ballynahone project will sit near Buncrana in County Donegal, adjacent to the Trillick substation, in one of Ireland's windiest regions. Why Ireland Needs 100-Hour Storage Ireland has been one of Europe's wind energy success stories. Renewables met roughly half of the country's electricity demand in December 2025, with wind carrying most of that load. But the very success of wind power has created a new problem: curtailment. When the grid can't absorb all the electricity that wind farms produce, operators are forced to shut turbines down. That wasted energy represents both lost revenue for generators and a missed opportunity for decarbonization. Short-duration lithium-ion batteries can help smooth out gaps that last a few hours, but they can't carry the grid through multi-day lulls when Atlantic weather patterns stall. Ireland's 2024 Electricity Storage Policy Framework singled out long-duration energy storage as essential for future-proofing the power system, and the Ballynahone project is a direct response to that policy signal. Form Energy claims its own analysis shows that integrating multi-day storage could reduce Ireland's renewable curtailment and lower energy generation costs by more than 25% annually. FuturEnergy Ireland CEO Peter Lynch put it bluntly: the technology "is set to fill a critical gap in Ireland's power system." Ireland generates roughly half its electricity from renewables, but curtailment remains a persistent challenge during high-wind periods. How Iron-Air Batteries Work Iron-air batteries store energy through a reversible rusting process. During discharge, iron pellets in the anode oxidize by absorbing oxygen from the air, releasing electrons that flow through an external circuit. During charging, an electrical current reverses the reaction, converting iron oxide back to metallic iron and releasing the oxygen. The electrolyte is a water-based solution, and the raw materials (iron, water, air) are among the cheapest and most abundant on the planet. The trade-off is round-trip efficiency. Iron-air batteries lose more energy per charge-discharge cycle than lithium-ion systems, which typically operate at 85-90% efficiency. Form Energy has not published exact figures, but industry estimates place iron-air round-trip efficiency in the range of 40-50%. The company's argument is that low material costs and extreme duration make the economics work for applications where you're storing cheap surplus wind or solar for days at a time, not cycling twice a day like a lithium-ion peaker. Iron-air batteries use a reversible rusting process: iron oxidizes during discharge and is reduced back to metal during charging. The Ballynahone Project in Detail The system will be built southwest of Buncrana, adjacent to the Trillick substation in County Donegal. The project's power rating is 10 MW with 1,000 MWh (1 GWh) of energy capacity, giving it a discharge duration of 100 hours at full power. For context, the typical utility-scale lithium-ion battery in Ireland or the U.S. today stores 4 hours of energy. Ballynahone would store 25 times as much. FuturEnergy Ireland submitted the planning application in September 2024 and received approval from Donegal County Council two months later. The permit is valid for 10 years and covers a facility designed to operate for 30 years. The companies had been collaborating for several years before the formal announcement. FuturEnergy Ireland is jointly owned by Coillte, the state-backed forestry company, and ESB, Ireland's state-owned electric utility. The developer is targeting 1 GW of renewable energy capacity by 2030, and it sees multi-day storage as a way to unlock more value from its existing wind portfolio by preventing curtailment. Form Energy's Growing Order Book Ireland is Form Energy's first project outside the United States, but it's far from the company's only deal. Form now holds more than 65 GWh of commercial projects under agreement globally. The biggest is the 30 GWh deal with Xcel Energy and Google announced earlier this month for a data center in Minnesota, making it the largest battery storage project by energy capacity ever announced. An 8.5 GWh project with the U.S. Department of Energy, Massachusetts, and other New England states is also in the pipeline. The company began commercial production at its Weirton, West Virginia manufacturing plant in 2025 and delivered its first pilot system to Great River Energy in Minnesota last year. That facility, which occupies a former steel mill, is designed to produce iron-air battery modules at scale. Form has not disclosed the plant's annual capacity in GWh terms, but the growing order book suggests the company will need to expand manufacturing significantly to meet demand. Form Energy's Weirton, West Virginia factory began commercial production in 2025 and delivered its first pilot system to Great River Energy. The Broader Long-Duration Race Form Energy is not the only company chasing multi-day energy storage. Dutch startup Ore Energy grid-connected a 100-hour iron-air pilot project at an EDF lab in France in February 2026, offering a direct comparison to Form's technology on European soil. U.S. startup Noon Energy recently unveiled a demonstration of its reversible solid oxide fuel cell for multi-day baseload storage, taking a fundamentally different chemical approach to the same problem. The concept is also gaining traction among grid modelers. A recent analysis by Open Energy Transition found that multi-day storage "can lower system costs, strengthen grid support, and boost resilience during multi-day gaps" when modeled against the German grid. The researchers concluded that the same logic applies across Europe, where wind-heavy grids face similar curtailment challenges. Ireland itself is experimenting with other approaches to grid stability. Earlier in March, the country opened its first hybridized grid-stabilization system at Shannonbridge in County Offaly, combining a synchronous condenser with a 180 MWh lithium-ion battery. That project, developed by Lumcloon Energy and Hanwha Energy, can export 20 MW for nine hours. Ballynahone, by contrast, would discharge at 10 MW for 100 hours, storing more than five times as much energy for applications where sustained output matters more than peak power. What to Watch The Ballynahone project won't come online until 2029, so the near-term question is whether Form Energy can scale its manufacturing fast enough to serve a 65+ GWh pipeline. The Weirton plant is operational but modest. The company will likely need additional factory capacity, and whether that comes through expansion in West Virginia or new facilities overseas remains to be seen. For Ireland, the project is a test case for whether 100-hour storage can deliver on its theoretical promise in a real grid with real weather patterns. If Ballynahone performs as modeled, cutting curtailment and lowering system costs by double-digit percentages, it could open the door to multi-GWh iron-air deployments across the island and beyond. The economics will be the deciding factor. Iron-air's low material costs are compelling on paper, but the technology still n