Turning extreme heat into large-scale energy storage

Thermal batteries can efficiently store energy as heat. But building them requires a carefully designed system with materials that can withstand cycles of extremely high temperatures, without succumbing to problems like corrosion, thermal expansion, and structural fatigue.

Many thermal battery systems move high-temperature gas or molten salt around through metal pipes. Fourth Power, founded by MIT Professor Asegun Henry SM '06, PhD '09, is turning these materials inside out, using molten metal to transport the heat, which is stored in carbon bricks. Henry's approach earned him a Guinness World Record for the hottest liquid pump back in 2017 - important because when you double the absolute temperature of a material, to the point where it glows white-hot, the amount of light it emits doesn't just double, it increases 16 times (or to the fourth power).

The company is harvesting all that light with thermophotovoltaic cells, which work like solar cells to convert light into electricity. Henry and his collaborators broke another record when they demonstrated a lab version of a thermophotovoltaic cell that could convert light to electricity with an efficiency above 40 percent.

Fourth Power is working to use those record-breaking innovations to provide energy for power grids, power producers, and technology companies building power-hungry infrastructure like data centers. Henry says the batteries can provide anywhere from 10 to over 100 hours of electricity at a storage cost that is significantly cheaper than lithium-ion batteries at grid scale. The company is currently cycling each section of its system through relevant operating temperatures - which are nearly half as hot as the sun - and plans to have a fully integrated demonstration unit operating later this year.

"Explaining why our system is such a huge improvement over everything else centers around power density," explains Henry, who serves as Fourth Power's chief technologist. "We realized if you push the temperature higher, you will transfer heat at a higher rate and shrink the system. Then everything gets cheaper. That's why we pursue such high temperatures at Fourth Power. We operate our thermal battery between 1,900 and 2,400 degrees Celsius, which allows us to save a tremendous amount on the balance of system costs."

A career in heat

Henry earned his master's and PhD degrees from MIT before working in faculty positions at Georgia Tech and MIT. As a professor at both schools, his research has focused on thermal transport, storage, renewable energy, and other technologies that could lead to improvements in sustainability and decarbonization. Today, he is the George N. Hatsopoulos Professor in Thermodynamics in MIT's Department of Mechanical Engineering.

Heat transfer systems are usually made out of metals like iron and nickel. Generally, the higher temperature you want to reach, the more expensive the metal. Henry noticed ceramics can get much hotter than metals, but they're not used nearly as often. He started asking why.

"The answer is often pretty straightforward: You can't weld ceramics," Henry says. "Ceramics aren't ductile. They generally fail in a catastrophically brittle way, and that's not how we like large systems to behave. But I couldn't find many problems beyond that."

After receiving funding from the Department of Energy, Henry spent years developing a pump made from ceramics and graphite (which is similar to a ceramic). In 2017, his pump set the record for the highest recorded operating temperature for a liquid pump, at 1,200 Celsius. The pump used white-hot liquid tin as a fuel. He chose tin because it doesn't react with carbon, eliminating corrosion. It also has a relatively low melting point and high boiling point, which keeps it liquid in a large temperature range.

"The idea was, instead of making the system from metal, let's move liquid metals," Henry says.

The challenge then became designing the system.

"Typically, a mechanical engineer would come up with a design and say, 'Give me the best materials to do this,'" Henry says. "We flipped the problem, so we were saying, 'We know what materials will work, now we need to figure out how to make a system out of it.'"

In 2023, Henry met Arvin Ganesan, who had previously led global energy work at Apple. At first, Ganesan wasn't interested in joining a startup - he had two young kids and wanted to prioritize his family - but he was intrigued by the potential of the technology. At their first meeting, the two connected over shared values and fatherhood, as Henry surprised Ganesan by bringing his own young children.

"I had a sense this technology had the promise to tackle the twin crises of affordability and climate change at the same time," says Ganesan, who is now Fourth Power's CEO. "As energy demand becomes more pronounced, we either need to deploy harder and deeper tech, which is also important, or improve existing tech. Fourth Power is trying to simplify the physics and thermodynamic principles to deliver an approach that has been very well-studied for a very long time."

The system Fourth Power designed takes in excess electricity from sources like the grid and uses it to heat a series of 6-foot-long, 20-inch thick graphite bricks until they reach about 2,400 Celsius. At that point the system is considered fully charged.

When the customer wants the electricity back, the bricks are used to heat up liquid tin, which flows through a series of graphite pipes, pumps, and flow meters to thermophotovoltaic cells, which turn the light from the glowing hot infrastructure back into electricity.

"You can basically dip the cells into the light and get power, or you can pull them back out and shut it off," Henry explains. "The liquid metal starts at 2,400 Celsius and then cools as it's going through the system because it's giving a bunch of its energy to the photovoltaic, and then it circulates back through the graphite blocks, which act as a furnace, to retrieve more heat."

From concept to company

Later this year, Fourth Power plans to turn on a 1-megawatt-hour system in its new headquarters in Bedford, Massachusetts. A full-scale system would offer 25 megawatts of power and 250 megawatt hours of storage and take up about half a football field.

"Most technologies you'll see in storage are around 10 megawatts an acre or less," Henry explains. "Fourth Power is more like 100 megawatts per acre. It's very power-dense."

The power and storage units of Fourth Power's system are modular, which will allow customers to start with a smaller system and add storage units to extend storage length later. The company expects to lose about 1 percent of total heat stored per day.

"Customers can buy one storage and one power module, and that's a 10-hour battery," Henry explains. "But if they want one power module and two storage modules, that's a 20-hour battery. Customers can mix and match, which is really advantageous for utilities as renewables scale and storage needs change."

Down the line, the system could also be run as a power plant, converting fuel into electricity or using fuel to charge its batteries during stretches with little wind or sun. It could also be used to provide industrial heat.

But for now, Fourth Power is focused on the battery application.

"Utilities need something cheap and they need something reliable," Henry says. "The only technology that has managed to reach at least one of those requirements is lithium ion. But the world is waiting for something that's much cheaper than lithium ion and just as reliable, if not better. That's what we're focused on demonstrating to the world."