Hydrostor’s 500 MW / 4,000 MWh Willow Rock Energy Storage Center in Kern County, California, has received final permitting approval from the California Energy Commission, clearing the final regulatory hurdle and enabling construction to begin as early as 2026.
The project will deploy Hydrostor’s advanced compressed air energy storage (A-CAES) technology, storing surplus grid electricity by using turbines to compress air and inject it into a large, water-filled cavern roughly 2,000 feet underground. Heat generated during compression is captured for later use, while displaced water is stored in a surface reservoir. When demand peaks or extreme weather stresses the grid, the system reverses: reheated compressed air expands through turbines to deliver up to eight hours of continuous discharge, providing long-duration capacity to support grid reliability and affordability amid rising demand from data centres, artificial intelligence workloads and broader electrification.
At full output, Willow Rock is expected to supply enough electricity to power more than 400,000 homes for over eight hours. Hydrostor estimates the project will bring more than US$500 million in investment to East Kern County and the greater Antelope Valley over its planned 50-year operating life.
Company executives described the permit approval as a pivotal milestone, unlocking the Willow Rock project while advancing Hydrostor’s multi-gigawatt development pipeline across North America. That pipeline includes another late-stage project in New South Wales and forms part of a global 7 GW portfolio spanning the US, Australia, Canada and the UK.
Local backing for the project has also strengthened, with Willow Rock securing a franchise agreement and a conditional letter of support from the Kern County Board of Supervisors, reinforcing its role as a long-term economic and energy-resilience asset for the region.
