The Hottest Battery in the World
Fourth Power, a startup spun out of MIT, is preparing to launch a commercial thermal battery that stores electricity as heat in massive carbon blocks raised to approximately 4,350 degrees Fahrenheit — nearly half the temperature of the Sun's surface. The technology, developed by MIT heat transfer professor Asegun Henry, represents a fundamentally different approach to grid-scale energy storage that may offer significant cost and duration advantages over lithium-ion batteries for long-duration applications.
The company's name comes from the Stefan-Boltzmann law: at these extreme temperatures, doubling the heat increases light output by a factor of 16 — to the fourth power — dramatically improving the efficiency with which heat can be converted back to electricity through thermophotovoltaic cells.
How Thermal Energy Storage Works
The system operates on a conceptually simple but technically demanding principle. When excess electricity is available — from solar panels in the middle of the day, or wind turbines during off-peak hours — it heats the carbon blocks using electrical resistance. The carbon is maintained in an insulated enclosure where it retains thermal energy with losses of only about one percent per day.
When electricity is needed, the hot carbon blocks emit intense thermal radiation. This radiation is captured by thermophotovoltaic cells — specialized semiconductors that convert heat radiation into electricity, functioning like solar panels but for thermal energy. The TPV cells convert the radiation to electricity with efficiency above 40 percent, a record that Henry's team demonstrated in laboratory conditions. Heat transfer between the carbon blocks and the TPV cells is managed by a system of molten tin pumps — an innovation that earned Henry a Guinness World Record for the hottest liquid pump in 2017.
Why Carbon Blocks Instead of Metal
The choice of graphite carbon as the storage medium is central to the system's economics. Most thermal storage approaches use metals like iron or aluminum, which become expensive and structurally challenging at the temperatures needed for high-efficiency conversion. Graphite can withstand extreme heat without melting or corroding, doesn't react with the molten tin heat transfer fluid, and is abundant and relatively inexpensive as a raw material.
This materials advantage is what allows Fourth Power to target storage costs significantly below lithium-ion at scale. The company estimates that at commercial deployment scale, its technology can provide long-duration storage at a fraction of lithium-ion costs — critical for the utility and grid-scale market where duration matters as much as round-trip efficiency.
The Long-Duration Storage Gap
Lithium-ion batteries have transformed short-duration grid storage — systems that need to store energy for two to four hours to smooth out renewable variability. But as the grid increasingly relies on solar and wind power, the need for storage covering multi-day periods of low generation is growing. Fourth Power's system is specifically designed for this gap: a base configuration provides 10 hours of storage, and adding more storage modules extends duration linearly. A full-scale installation would provide 25 megawatts of power and 250 megawatt-hours of storage.
The company plans to demonstrate a one-megawatt-hour pilot system later in 2026, with full commercial deployments to follow as the technology is validated at scale. If the demonstration confirms projected costs and performance, thermal energy storage at these temperatures could become a key part of the infrastructure that makes renewable energy reliable across seasons and weather patterns — the long-sought solution to the intermittency problem that has been the central challenge of the clean energy transition.
This article is based on reporting by Interesting Engineering. Read the original article.
