Three breakthroughs that earned Fourth Power its $19M Series A

Congrats, we did it! Solar and wind are now the cheapest sources of electricity in the U.S. Now we just have to sit back and let the market do the work, right?

Not so fast. It’s true that wind and solar are the cheapest when they’re providing electrons. But the wind doesn’t blow everywhere all the time, and the sun sets every day, so renewable project developers have to find ways to store that power.

The big winner has been lithium-ion batteries, which dominate the market, mostly because they’re manufactured at massive scale, driving the cost down. They’re modular and quick to respond to demand, making them a flexible addition to the grid.

But for all their strengths, lithium-ion batteries aren’t great at storing large amounts of power at low cost. In the last few years, researchers have raced to develop novel ways to beat them at the grid-scale game.

Asegun Henry has been working on one alternative for longer than that, though when he first started, he wasn’t trying to compete with lithium-ion batteries. He was trying to find a way to gather the sun’s energy and transport it in the form of heat at higher temperatures and efficiency than before.

“At that time, we were thinking about solar fuels,” the MIT professor told TechCrunch+, referring to the process of making liquid fuels from carbon dioxide and sunlight. “Then it kind of pivoted to liquid metal-based, concentrated solar power.” Liquid metal would allow a concentrated solar power plant to capture even more of the sun’s energy.

Concentrated solar power received a burst of attention in the early 2010s when a slew of them were built around the world. But after a few years, the cost of photovoltaic (PV) solar panels dropped dramatically. Concentrated solar rapidly fell from favor.

“PV really was winning,” Henry said. So instead of trying to beat them, he figured out a way to join them.

Henry’s designs form the basis of a new company, Fourth Power, which he founded last year. Fourth Power’s thermal battery stores energy in the form of graphite blocks kept at around 2,400°C (4,350°F). The blocks are charged by molten tin, which is heated using electricity from solar panels or wind turbines. When it’s time to discharge the battery, the tin absorbs heat from the graphite blocks and pipes it through carbon stacks until they glow white hot. Special photovoltaic panels tuned to infrared light absorb the thermal energy those stacks emit.

The whole system is kept free of oxygen to prevent corrosion and wrapped in enough insulation that it only loses about 1% of its energy per day.

Fourth Power’s thermal battery draws on three breakthroughs, all of which came out of Henry’s lab at MIT, he said. The first is a pumping and plumbing system made of graphite that could handle the extreme temperatures required. The second is thermal photovoltaic cells that are over 40% efficient. And the third is a method to keep those PV cells clean when they’re in the presence of the superheated materials.

Henry chose thermal PV over a more traditional steam turbine because no turbine has been built to withstand those temperatures. Thermal PV also has a faster response time — it doesn’t have to build up a head of steam before it’s able to start producing electricity — and it’s the cheapest way to prove his system works at scale. Plus, he said that he thinks he can boost the solar panels’ efficiency even further.

“My hope is that five to ten years from now, we will be getting calls from Siemens and GE, and they’ll be asking me, ‘Why didn’t you use a turbine instead?’ And then my answer will be different. I can say, ‘Well, if you’re ready to invest the money to develop it, then let’s do it.’”

The system could be used to provide industrial heat, too, though Henry said that would have to come later. The grid has a “plug-and-play interface,” he said. Industrial facilities, though, vary greatly. “Every time you want to make a new installation, you’ve got to sync a whole bunch of extra engineering costs to reconfigure the system for that bespoke installation.”

Asegun Henry poses in his lab.

Asegun Henry. Image Credits: Ryuji Suzuki/MIT

Henry founded Fourth Power when it became clear that he had done all he could in his lab. “It needs to be a company because I need to pay someone to run 1,000 hour tests over and over so we can have large long datasets of how things behave in a more industrial context,” he said. “It’s not ethical to ask a graduate student to run 1,000 hour tests.”

There’s more than that, of course. Having founded a company, Henry is able to raise the money needed to build a 1-megawatt-hour pilot that he says will prove each piece of the system can run at scale. Along those lines, he and CEO Arvin Ganesan, who was previously head of global energy and environmental policy at Apple, have raised a $19 million Series A round. The round was led by DCVC with participation from Breakthrough Energy Ventures and the Black Venture Capital Consortium.

Fourth Power plans to have its pilot up and running in two years, Henry said. It’ll be built at MIT’s Bates Lab about a half hour north of Boston. It’s a quick timeline, though one Henry is confident in because he’s already explored the riskiest scientific questions in his lab.

The startup says it can get the price down to $25 per kilowatt-hour, which would be significantly cheaper than lithium-ion batteries and on par with other long-duration storage technologies like iron-air batteries being made by Form Energy.

Given Fourth Power’s competitive cost estimates and its quick timeline, it’s little surprise that two prominent investors in climate tech decided to back the startup’s large Series A. There’s still a risk that something will go wrong, of course. Perhaps the insulation won’t be as tight as hoped or maybe sealing the facility against the corrosive effects of oxygen will be a little more challenging than predicted.

But even if the timeline slips a bit, Fourth Power will still be in a strong position to capitalize on the booming energy storage market, which is expected to double by the end of the decade, according to BloombergNEF.

Given that growth, the race for Fourth Power is less about beating its competition and more about building its commercial-scale plant, which will be able to store 1 gigawatt-hour of electricity. “That’s where the economics are great,” Henry said. If he’s right, the economics will look pretty good for Fourth Power’s investors, too.