The most expensive part of a battery is the part that stores the energy. Fourth Power’s bet is that the cheapest way to store a lot of it is to get something very, very hot and keep it that way for a long time. The Cambridge-based startup, founded by MIT professor Asegun Henry, is building utility-scale thermal batteries that use electricity to heat massive graphite blocks to 2,400 degrees Celsius, a temperature that glows white-hot. The heat is later converted back to electricity using specialized photovoltaic cells. It is a simple idea with a punishingly complex engineering challenge, but the payoff, if the math holds, is a storage system that could cost a tenth of today’s lithium-ion batteries [TechCrunch, Dec 2023].
The physics of a hot brick
Fourth Power’s system is a three-step dance of energy conversion. First, excess renewable power,say, from a midday solar surge,runs through heating elements to warm stacks of six-foot-long, 20-inch-thick graphite bricks to temperatures rivaling the surface of a blast furnace. The graphite acts as the primary storage medium, holding that thermal energy with minimal loss. When power is needed, liquid tin is pumped through channels in the hot graphite, absorbing the heat. This superheated tin then flows past arrays of thermophotovoltaic (TPV) cells, which convert the intense infrared radiation directly back into electricity [Perplexity Sonar Pro Brief].
The company’s wedge rests on the materials bill. Graphite and tin are abundant, domestically available, and cheap compared to the lithium, cobalt, and nickel central to electrochemical batteries. The TPV cells are solid-state with no moving parts, promising longevity. The entire apparatus is designed to be modular and scalable, with units targeting 100 megawatts of power output and 1 gigawatt-hour of storage capacity,enough to power tens of thousands of homes for ten hours [Fourth Power].
Why the grid needs a cheap, long-lasting battery
The economics of renewable energy are now a story of intermittency. Solar and wind are often cheapest when they are most abundant, which is not necessarily when demand peaks. Lithium-ion batteries have become the default solution for shifting power by a few hours, but their cost per kilowatt-hour of storage capacity remains high for the multi-day storage utilities will need as renewables penetration grows. Form Energy, a prominent competitor, is betting on iron-air chemistry for 100-hour storage. Fourth Power is aiming for the flexible middle ground, a system configurable for durations from four to 100-plus hours, arguing its technology can add duration at just 20% of the original install cost [LinkedIn].
The tailwind is regulatory and urgent. Grid operators are scrambling for firm, dispatchable clean power to replace retiring fossil fuel plants and meet decarbonization mandates. The U.S. Department of Energy has set a goal to reduce the cost of grid-scale, long-duration energy storage by 90% within a decade. Fourth Power’s claimed trajectory toward $25 per kilowatt-hour of storage capacity would land squarely in that target zone, undercutting today’s lithium-ion costs by an order of magnitude [Perplexity Sonar Pro Brief].
The team translating lab work to the grid
Founder Asegun Henry is the intellectual engine. A mechanical engineering professor at MIT, his research for over a decade has focused on extreme-temperature heat transfer and thermal energy storage, work partly funded by Department of Energy grants. The company is a direct commercialization of that academic R&D [MIT Energy Initiative]. To steer the company from prototype to power plant, Fourth Power brought on Arvin Ganesan as CEO in 2023. Ganesan’s background is a mix of policy and corporate climate strategy, with previous roles at Apple, the EPA, and in the U.S. Senate [The Org].
The investor roster reads like a who’s who of climate tech conviction capital, suggesting the team has the credibility to attempt the leap from lab to grid.
| Investor | Notable For |
|---|---|
| Breakthrough Energy | Bill Gates-founded climate investment fund |
| DCVC | Deep tech and science-focused venture firm |
| Munich Re Ventures | Corporate venture arm of the reinsurance giant |
| Khosla Ventures | Early-stage hard tech investor |
| Gigascale Capital | Climate tech fund founded by former Tesla CTO |
This group provided a $19 million Series A in late 2023 [TechCrunch, Dec 2023]. In September 2025, Munich Re Ventures led a $20 million Series A Plus round, a signal that an institution which literally prices risk for a living sees a insurable path forward [BusinessWire, Sep 2025].
Where the wheels could come off
Building a first-of-its-kind industrial system that operates reliably at 2,400°C is not a software deployment. The risks are physical, financial, and temporal.
- Materials at the limit. Graphite is stable, but cycling it between ambient and extreme temperatures thousands of times invites fatigue and cracking. The seals, pumps, and piping for molten tin must withstand corrosive, high-temperature flows for decades. Any failure could be catastrophic.
- The valley of death. The capital required to build a full-scale, multi-megawatt demonstration unit is substantial. The recent $20 million extension is likely earmarked for this. The company must then convince a utility to be the first customer for a novel technology, a sales cycle measured in years, not quarters.
- The incumbent’s improvement curve. Lithium-ion costs have fallen dramatically for years. While they may plateau, any further gains erode Fourth Power’s cost advantage. Competing long-duration technologies, like Form Energy’s iron-air battery, are also racing to market with their own compelling economics.
The company’s most plausible answer is that its fundamental materials advantage is too large to ignore. Graphite and tin are orders of magnitude cheaper than battery-grade lithium. If the engineering holds, the cost floor is simply lower.
The next twelve months
The immediate milestone is moving from benchtop prototypes to an integrated, sub-scale demonstration system. The fresh capital from Munich Re Ventures will fund that work. Success means proving the thermal and electrical conversion efficiencies at a meaningful scale and beginning conversations with potential utility partners about a pilot installation. The goal is not to sell thousands of units next year, but to de-risk the technology enough for a first utility to take the bet.
For a sense of the stakes, run the numbers on their target. At $25 per kilowatt-hour, a 1-gigawatt-hour system,enough to power a small city for an evening,would carry a storage capacity cost of $25 million. A comparable lithium-ion system today could cost $250 million or more. The difference is the margin that makes seasonal storage, and a fully renewable grid, economically plausible. Fourth Power is not just building a battery. It is building a financial model for the post-lithium grid. To succeed, it must prove its hot bricks can beat the cold, hard economics of the incumbent battery supply chain.
Sources
- [TechCrunch, Dec 2023] Three breakthroughs that earned Fourth Power its $19M Series A | https://techcrunch.com/2023/12/12/fourth-power-series-a/
- [Perplexity Sonar Pro Brief] Summary of Fourth Power's technology and market claims
- [Fourth Power] Company website and product description | https://gofourth.com/
- [LinkedIn] Fourth Power company page and post on cost structure | https://www.linkedin.com/company/fourth-power
- [MIT Energy Initiative] Podcast interview with founder Asegun Henry | https://energy.mit.edu/podcast/energy-storage-is-heating-up/
- [The Org] Profile of CEO Arvin Ganesan
- [BusinessWire, Sep 2025] Fourth Power Raises $20 Million to Commercialize Low-Cost Utility-Scale Energy Storage Solution | https://www.businesswire.com/news/home/20250916363021/en/Fourth-Power-Raises-$20-Million-to-Commercialize-Low-Cost-Utility-Scale-Energy-Storage-Solution