Sam Altman-Backed Exowatt Bets Big on ‘Billions of Hot Rocks’ to Supercharge AI Data Centers
In a world where AI’s power demands are skyrocketing faster than a viral meme, one startup is turning to an ancient heat source—rocks—to keep the servers humming. Exowatt, with backing from OpenAI’s Sam Altman, just raised $50 million more to scale its solar-thermal wizardry, aiming to deliver round-the-clock electricity to data centers for a mere penny per kilowatt-hour. If it works, it could slash the fossil fuel footprint of the AI boom and make hyperscalers like Microsoft and Google rethink their energy bills.
Founded in 2023 by Hannan Happi—a former SpaceX engineer turned clean energy trailblazer—Exowatt is laser-focused on solving the AI energy crunch. Data centers already guzzle more electricity than entire countries like Sweden, and with AI training runs projected to consume as much power as the Netherlands by 2027, the strain is real. Happi’s pitch? Ditch pricey lithium batteries for something simpler: superheated “rocks” that store solar energy like a cosmic thermos.
The star of the show is Exowatt’s P3 unit—a compact, modular metal box about the size of a shipping container. Atop it sits an array of lenses that concentrate sunlight, blasting heat onto a proprietary ceramic composite brick (think high-tech lava rock) that soaks it up like a sponge. This thermal battery holds the heat for up to five days, releasing it on demand via a Stirling engine and generator to crank out electricity. Stack thousands of these bad boys together, and you’ve got gigawatt-scale power plants that run 24/7, rain or shine—perfect for AI’s non-stop number-crunching.
“It’s everything designed to be extremely simple,” Happi told TechCrunch, emphasizing how the system sidesteps the complexity that doomed earlier concentrated solar projects. Unlike sprawling desert mirrors or finicky molten salt setups, P3 units are factory-built for mass production, slashing costs through economies of scale. At full tilt—around one million units per year—Exowatt claims it’ll hit that elusive 1¢/kWh mark, undercutting even the cheapest grid power in sunny spots like Texas or Arizona.
Funding-wise, it’s a who’s-who of tech titans. The fresh $50 million extension bumps Exowatt’s Series A to $120 million total, led by MVP Ventures and 8090 Industries, with chops from Atomic, BAM Ventures, and DeepWork Capital. Early angels like Altman (who’s poured billions into energy bets via his personal fund) and Leonardo DiCaprio signal serious conviction. Previous rounds netted $20 million in seed cash from Andreessen Horowitz, and the company’s already locked in a backlog of 10 million P3 units—equivalent to 90 gigawatt-hours of storage—from U.S. data center operators eyeing sustainable upgrades.
Why rocks? Thermal storage isn’t new—humans have used heated stones for cooking since the Stone Age—but Exowatt’s twist is engineering them for industrial muscle. Traditional solar panels + batteries work for daytime peaks but falter at night, driving up costs with rare-earth metals. Exowatt’s approach boasts efficiency on par with photovoltaics, but with longer-duration storage and zero degradation over decades. It’s also dirt-cheap on materials: No exotic chemicals, just abundant ceramics and steel. “We went through all sorts of configurations… How do I reduce structural costs? How do I optimize for this?” Happi explained, crediting iterative prototyping for the breakthrough.
For AI giants, the appeal is massive. Sam Altman has warned that energy shortages could cap AI’s growth, while Microsoft’s Satya Nadella has inked nuclear deals to keep ChatGPT fed. Exowatt positions P3 as the scalable fix: Deployable on data center rooftops or adjacent lots, it overlaps perfectly with sunny build sites in the U.S. Southwest. Early pilots are underway in Florida, where Altman first invested back in 2024, and the company snagged Fast Company’s “Next Big Thing in Tech” award for its potential to “power the AI future.” Challenges remain—land needs, supply chain ramps, and proving durability at scale—but Happi shrugs them off: “We’re not running short of any projects to do.”
| Feature | Traditional Solar + Batteries | Exowatt P3 “Hot Rocks” |
|---|---|---|
| Storage Duration | 4-8 hours | Up to 5 days |
| Cost Target (/kWh) | 5-10¢ | 1¢ at scale |
| Efficiency | 15-20% (round-trip) | Matches PV (~20%) |
| Lifespan | 10-15 years (batteries) | 30+ years (no degradation) |
| Scalability | Gigafactories needed | Modular, factory-built units |
Early buzz on X (formerly Twitter) is heating up, with tech watchers hailing it as a “thermal revolution” for AI. One post quipped: “From cave fires to data fires—rocks are back!” while analysts eye it as a hedge against rising energy regs. As Exowatt ramps to “millions and ultimately billions” of units, it could redefine baseload renewables, turning AI’s Achilles’ heel into a green goldmine. If Happi’s vision pans out, those hot rocks might just be the coolest thing in tech.