Sam Altman-backed Exowatt wants to power AI datacenters with hot rocks (techcrunch.com)

Exowatt — a Sam Altman–backed startup — raised a $50M extension to its $70M Series A to scale a modular “hot rocks” concentrated solar system aimed at delivering 24/7 renewable power to AI datacenters for as little as $0.01/kWh. The company is selling container-sized P3 units (a metal box topped with lenses) that concentrate sunlight to heat a special brick thermal battery, driving a Stirling engine and generator. Exowatt says each thermal battery can hold heat for up to five days, units are strung together to match demand, and the company has a backlog of about 10 million P3s (≈90 GWh capacity). The one-cent target, Happi says, becomes achievable at ~1 million units produced per year. Technically, the system uses simple, repeatable components — lenses, ceramic-like bricks, air transfer and Stirling conversion — designed for high-volume manufacturing to exploit learning-curve effects absent in traditional concentrated solar projects. Exowatt claims efficiency on par with PV and slightly better than PV paired with lithium-ion storage, positioning it as a potential low-cost, round-the-clock alternative for energy-hungry AI datacenters. Constraints remain: land intensity, geographic dependence on high insolation, and historical cost competition from PV + batteries. Still, if Exowatt can scale manufacturing as planned, it could materially reduce long‑term power costs and reshape where and how AI infrastructure is sited.