A US-based startup, Fourth Power, is developing thermal batteries that use extremely high temperatures to store energy more efficiently and at lower costs than traditional lithium-ion battery.
The company was founded by MIT professor Asegun Henry, whose research focuses on heat transfer and energy systems.
As renewable energy sources like solar and wind continue to grow, the need for reliable energy storage is becoming more urgent. These sources do not produce electricity continuously, creating supply gaps. Thermal batteries could help bridge this gap by storing excess electricity and releasing it when needed.
Fourth Power’s approach is different from conventional systems. Instead of relying on chemical reactions, the company stores energy in the form of heat. This heat is stored in specially designed carbon or graphite bricks at extremely high temperatures. The stored heat can later be converted back into electricity when required.
How Thermal Battery Works at Extreme Temperatures
The system developed by Fourth Power operates at temperatures ranging between 1,900 and 2,400 degrees Celsius. These temperatures are nearly half as hot as the surface of the Sun.
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At such extreme heat levels, materials behave differently, and only carefully selected components can survive repeated heating and cooling cycles.
Traditional thermal systems often use molten salt or hot gases that flow through metal pipes. However, metals can corrode, expand, and weaken under extreme heat.
To overcome these challenges, Fourth Power has adopted a unique design. The system uses molten metal, specifically liquid tin, to transport heat instead of gases or salts.
The liquid tin flows through pipes made of graphite, a material that withstands very high temperatures without degrading rapidly. The heat is stored in large graphite blocks, which act as thermal reservoirs. When electricity is needed, the stored heat is transferred back into the liquid metal.
Asegun Henry explains, “Instead of building everything with metal, we decided to move liquid metals through materials that can handle extreme heat.” This design shift allows the system to operate at much higher temperatures than traditional solutions.
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One key innovation in this system is the use of thermophotovoltaic cells. These cells work similarly to solar panels, but instead of sunlight, they convert intense heat into electricity.
When the graphite blocks become white-hot, they emit a large amount of light energy. The thermophotovoltaic cells capture this light and convert it into usable electrical power.
Henry highlights the importance of high temperatures in improving efficiency. He says, “When you double the absolute temperature, the light emission increases sixteen times. That gives us a huge advantage in power density.”
His team has already demonstrated thermophotovoltaic cells with efficiencies above 40 percent, a significant milestone in this field.
Cost Advantage Over Lithium-Ion Battery
One of the biggest challenges in energy storage today is cost. Lithium-ion batteries are widely used, but they remain expensive at large scales. They also have limitations related to lifespan, safety, and resource availability.
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Fourth Power aims to address these issues by offering a cheaper and more scalable alternative. The company claims its thermal batteries can provide electricity for durations ranging from 10 hours to more than 100 hours. This makes them suitable for grid-level storage, where long-duration energy supply is critical.
The system is also highly modular. Customers can choose the number of storage and power units based on their needs. For example, a single power unit combined with a single storage unit can deliver around 10 hours of electricity. Adding more storage units increases the duration without requiring major redesigns.
Henry explains, “Customers can mix and match modules based on how much storage they need. This flexibility is very useful as energy demand keeps changing.” The modular design also allows for gradual expansion, which reduces upfront investment costs.
Another advantage is energy efficiency over time. The system is expected to lose only about 1 percent of its stored heat per day. This makes it competitive with other storage technologies, especially for long-duration applications.
The system’s physical footprint is also relatively small. A full-scale installation could deliver 25 megawatts of power and 250 megawatt-hours of storage while occupying about half the area of a football field. This high power density makes it attractive for urban and industrial use.
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The journey of Fourth Power began with years of academic research. As Henry studied thermal systems at MIT, he later continued his work as a professor. His research explored how heat can be used more effectively for energy storage and sustainability.
One of his early breakthroughs came in 2017, when he developed a high-temperature pump capable of operating at 1,200 degrees Celsius. This achievement earned a Guinness World Record for the hottest liquid pump. The pump used liquid tin, which remains stable across a wide temperature range and does not react with carbon materials.
Henry says, “We started by choosing the right materials first and then designed the system around them.” This approach helped overcome traditional engineering challenges, especially those related to material limitations.
The company gained further momentum in 2023 when Arvin Ganesan joined as CEO. Ganesan previously led global energy initiatives at Apple and was drawn to the technology’s potential. He saw it as a solution to both rising energy costs and climate change.
Ganesan explains, “This technology can address affordability and sustainability at the same time. It simplifies well-known principles to create a practical solution.” His leadership has helped move the company closer to commercial deployment.
Fourth Power is now preparing to launch a demonstration system at its facility in Bedford, Massachusetts. The initial system will have a capacity of 1 megawatt-hour. This pilot project will test the full integration of all components under real-world conditions.
The company plans to scale up rapidly. Future systems could be deployed for utility grids, industrial operations, and large data centers that require a continuous power supply.
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Beyond electricity storage, the technology has the potential to serve multiple applications. It could be used as a power plant that converts fuel into electricity or even as a source of industrial heat for manufacturing processes.
Industries such as steel, cement, and chemicals require high-temperature heat, which is currently generated using fossil fuels. Thermal batteries could provide a cleaner alternative by storing renewable energy and delivering heat when needed.
For now, Fourth Power is focusing on proving the reliability and cost-effectiveness of its battery system. Utilities are looking for solutions that are both affordable and dependable. According to Henry, lithium-ion batteries have only partially met these requirements.
He says, “The world needs something cheaper and just as reliable as lithium-ion batteries. That is what we are trying to deliver.”
As global energy demand continues to rise, innovations such as thermal batteries could play a key role in shaping a sustainable future. With Fourth Power succeeding, extreme heat may soon become one of the most valuable tools in the transition to clean energy.
