Fourth Power’s sci-fi thermal batteries could be cheaper than pricey natural gas power plants

Fourth Power’s take on long-duration energy storage sounds like something out of a James Bond film. The thermal batteries come complete with superheated liquid tin and argon-filled hermetically sealed chambers. The company’s goal, though, is far more prosaic — to store electricity cheap enough to make solar and wind an obvious choice for 24/7 power.

Here’s how the technology works: To store energy, electricity from the grid heats blocks of carbon inside insulated chambers filled with argon gas. When power is needed, the system pumps molten tin heated to a scorching 2,400°C (4,352°F) through graphite pipes — the only cost-effective material that can withstand those temperatures. Special solar panel-like devices called thermophotovoltaic cells then convert the heat back into electricity by capturing the white-hot tin’s infrared light.

The four-year-old, Cambridge, Massachusetts, startup has been refining the technology for the last two years, and it’s now preparing to build its first full-scale battery, molten tin and all. If all goes to plan, Fourth Power plans to deliver commercial-scale batteries to customers in 2028 at a cost that could undercut both lithium-ion batteries and peaking natural-gas plants.

“Our projections are for the first-of-a-kind — the first ones to market — that they’ll be cost competitive,” Arvin Ganesan, Fourth Power’s co-founder and CEO, told TechCrunch. “Our comps for these markets are very high.”

In normal operation, Fourth Power envisions charging and discharging daily, providing continuous electricity for around eight hours or more. That’s twice as long as most grid-scale lithium-ion batteries are designed for. The insulation system, made from petroleum coke (a waste product from oil refining), keeps temperatures remarkably stable, losing only 1% of stored energy per day.

The company is currently running extensive testing on smaller versions of the system. “We’re now running cycles to make sure that we’re getting the expected amount of power out and to ensure the system is durable,” Ganesan said. 

Alongside those tests, the company is designing a 1-megawatt-hour demonstration battery. To build that project, Fourth Power has raised $20 million in a round it’s calling a Series A Plus, the company exclusively told TechCrunch. The round was led by Munich Re Ventures with participation from Breakthrough Energy Ventures and DCVC. The startup previously raised a $19 million Series A in 2023.

Ultimately, Fourth Power hopes that once it hits large-scale production, the cost of storing electricity using its thermal batteries will fall to $25 per kilowatt-hour, one-tenth the cost of lithium-ion batteries. 

“There’s not a lot of moving pieces. There’s not a lot of players in the supply chain, which means that you can get to cost targets in a fairly straightforward way,” Ganesan said. “We feel the $25 target is fairly straightforward to achieve.”