Fourth Power Nears Completion of Thermal Battery Prototype That Stores Grid Energy as White-Hot Heat in Liquid Tin
The MIT spinout plans to demonstrate a 1 MWh thermal battery in Bedford, Massachusetts this year, using graphite bricks heated to 2,400 degrees Celsius and liquid tin to deliver grid-scale storage at a claimed $25 per kilowatt-hour, a fraction of lithium-ion costs.
Overview
Fourth Power, a startup spun out of MIT, is preparing to demonstrate a thermal energy storage system that heats graphite bricks to 2,400 degrees Celsius and pumps liquid tin through thermophotovoltaic cells to convert the stored heat back into electricity. The company plans to have a fully integrated 1-megawatt-hour demonstration unit operating at its Bedford, Massachusetts headquarters later this year, according to MIT News. If the prototype performs as designed, the technology could deliver grid-scale storage lasting 10 to over 100 hours at a projected cost of roughly $25 per kilowatt-hour, compared to approximately $330 per kilowatt-hour for lithium-ion systems at equivalent durations.
The announcement arrives as grid operators across the United States confront a widening gap between the hours of storage that lithium-ion batteries can economically provide, typically four to six, and the multi-day reserves needed to keep power flowing during extended periods of low wind and cloud cover. The Machine Herald previously reported on Google and Xcel Energy’s 30-gigawatt-hour iron-air battery project in Minnesota, which addresses the same long-duration storage challenge using a different chemistry. Fourth Power represents yet another approach in what is becoming a crowded race to solve the problem.
How the System Works
The technology relies on three stages. First, excess electricity from the grid or renewable sources heats a series of six-foot-long, 20-inch-thick graphite bricks inside an argon-filled enclosure that prevents oxidation. The bricks reach approximately 2,400 degrees Celsius, nearly half the temperature of the sun’s surface, and glow white-hot.
Second, when electricity is needed, liquid tin, which melts at a relatively low 232 degrees Celsius, is pumped through graphite pipes and across the heated bricks. The tin absorbs the thermal energy and carries it to the system’s output stage. Moving energy through molten metal rather than steam or air is Fourth Power’s core innovation. The company’s founder, MIT Professor Asegun Henry, designed a ceramic centrifugal pump capable of handling liquid tin at extreme temperatures, a device that earned a Guinness World Record, as reported by MIT News.
Third, the glowing liquid tin passes through an array of thermophotovoltaic cells, semiconductor devices that convert infrared radiation into electricity. Henry and his collaborators demonstrated a TPV cell with conversion efficiency above 40 percent, a record for the technology, according to MIT News. The overall round-trip efficiency of the system, from electricity in to electricity out, is targeted at approximately 50 percent.
Cost and Scale Advantages
Fourth Power’s cost projections rest on the cheapness of its raw materials. Graphite and tin are abundant and inexpensive compared to the lithium, cobalt, and nickel used in conventional battery chemistries. The company claims its system can store energy at roughly $25 per kilowatt-hour, a figure that would represent a dramatic reduction from lithium-ion’s cost at grid scale, as detailed by Interesting Engineering.
The system’s power density is another claimed advantage. Fourth Power says its thermal batteries can deliver approximately 100 megawatts per acre, compared to 10 megawatts per acre or less for typical lithium-ion installations, according to MIT News. A full-scale commercial unit would provide 25 megawatts of power with 250 megawatt-hours of storage, occupying roughly half a football field. The modular design allows customers to configure duration by adding storage blocks: one power module paired with one storage module creates a 10-hour battery, while additional storage modules extend the duration further.
Heat loss during storage is estimated at roughly one percent per day, making the technology better suited for durations of days to weeks rather than seasonal storage spanning months.
Funding and Timeline
Fourth Power has raised a total of approximately $39 million across two rounds. The company secured $19 million in Series A funding from Breakthrough Energy Ventures, the climate investment fund founded by Bill Gates, and the Black Venture Capital Consortium, TechCrunch reported. A subsequent $20 million round in September 2025 is funding the construction of the Bedford demonstration unit.
The company has been validating component durability at MIT’s Bates Research and Engineering Center since 2023, according to MIT News. Following the prototype’s completion, Fourth Power plans to partner with utilities for pilot projects designed for commercialization throughout 2026 and 2027.
The Long-Duration Storage Landscape
Fourth Power enters a field that is attracting significant investment and diverse technological approaches. Form Energy’s iron-air batteries, which store energy through a process of controlled rusting and de-rusting of iron pellets, have secured the largest deployment commitment to date with the 30-gigawatt-hour Google-Xcel project in Minnesota. Noon Energy demonstrated a reversible solid oxide fuel cell system capable of over 100 hours of continuous discharge in January 2026. And in the Netherlands, a consortium is piloting three distinct long-duration technologies, including saltwater flow batteries and hybrid thermal-electric systems, to address grid congestion.
Each approach involves tradeoffs. Iron-air batteries offer very low material costs but occupy large footprints and have not yet operated at scale. Noon Energy’s fuel cell approach is compact but remains at the demonstration stage. Fourth Power’s thermal system claims superior power density and material cost advantages but has a lower round-trip efficiency than lithium-ion, meaning more energy is lost in each charge-discharge cycle.
The market for long-duration energy storage is projected to grow from $3.9 billion in 2026 to $9.5 billion by 2035, according to industry forecasts. The U.S. Department of Energy committed $100 million in 2024 to pilot diverse long-duration storage technologies, reflecting a federal consensus that lithium-ion alone cannot support a grid powered predominantly by intermittent renewables.
Whether Fourth Power’s thermal approach can compete with iron-air and other chemistries will depend on the Bedford prototype’s performance. A 50-percent round-trip efficiency means half the input energy is lost as waste heat, a significant disadvantage in applications where electricity is expensive. But if the $25-per-kilowatt-hour cost target proves achievable at scale, the economics could favor thermal storage for the multi-day backup role that lithium-ion batteries are too costly to fill.