Project Details
Developer
Location
Eastern Kern County, California, USA
Capacity
500 MW-electric, 4000 MWh-electric
COD
Expected COD: 2030
About This Project
Executive Overview
Willow Rock Energy Storage Center is a 500 MW net / 4,000 MWh net (520 MW gross / 4,160 MWh gross) advanced compressed air energy storage (A-CAES) project under development on 88.6 acres in Kern County, California, developed by Hydrostor. The California Energy Commission certified the project on December 19, 2025 — the final major regulatory approval — with the franchise agreement from Kern County also secured in December 2025. The project comprises four 130 MW generating trains storing compressed air in purpose-built underground caverns at up to 2,500 feet depth, connected to Southern California Edison's Whirlwind substation via a new 19-mile 230 kV generation-tie line. Commercial operation was originally targeted for 2028 and has been pushed to 2030–2031 following three amendments to the primary offtake agreement, driven by permitting delays and inflation.
How It Works & Differentiation
Hydrostor's A-CAES system uses surplus grid electricity to drive motor-compressor trains that compress air and store it in flooded underground rock caverns, where water pressure maintains constant air pressure as the cavern empties and fills — eliminating the efficiency losses associated with pressure variation in conventional CAES. During discharge, the compressed air passes through thermal storage (heated during compression) before expanding through air turbine generators. The system requires no combustion or external heat source, produces zero emissions during operation, and is designed for more than 50 years of service with virtually no performance degradation. The 600 acre-foot water reservoir needed for operation is filled once and self-replenishes through condensation. The technology provides synchronous inertia and black start capability, grid services that battery storage cannot replicate.
Commercialization & Traction
Two offtake agreements are in place: a 25-year PPA with Central Coast Community Energy (3CE) for 200 MW / 1,600 MWh, amended three times with a 15.5% price increase applied following inflation and financing pressures; and a 50 MW agreement with California Community Power, signed February 12, 2026, covering six of its community choice aggregator members. Approximately 250 MW of the 500 MW total capacity remains uncontracted. A conditional DOE loan guarantee was put under review in 2025 and its current status is unclear. Hydrostor holds a global A-CAES pipeline of 7 GW and has raised capital from CPP Investments, Goldman Sachs Alternatives, and Canada Growth Fund.
Scalability & Strategic Context
Willow Rock is the first U.S. compressed air energy storage project in over 30 years and, at 500 MW / 4,000 MWh, would be the world's largest non-pumped-hydro long-duration storage facility when completed. The California Public Utilities Commission has recommended 10 GW of eight-hour storage by 2031, providing a clear regulatory pull for subsequent projects. The project's three offtake amendments and multi-year permitting delays are a cautionary signal on execution risk for A-CAES at this scale in California's regulatory environment. Hydrostor's other advanced projects — including the 1.6 GWh Silver City facility in Australia — are proceeding in parallel, providing geographic diversification of the company's commercial pipeline even as Willow Rock navigates continued timing uncertainty.
Project Timeline
Hydrostor’s Willow Rock Energy Storage Center Obtains Final Approval for Power Plant License
Hydrostor Signs Offtake Agreement with California Community Power
Further Reading
Hydrostor Signs Offtake Agreement with California Community Power
Hydrostor has signed a 50 MW offtake agreement with California Community Power for its Willow Rock Energy Storage Center, which will provide long-duration compressed air energy storage to support clean and reliable electricity for six community choice aggregators in California.
Hydrostor and 3CE amend Willow Rock ESA for third time, permitting and interconnection challenges to blame
Hydrostor and Central Coast Community Energy have amended their Willow Rock Energy Storage Agreement for the third time due to ongoing permitting, interconnection, and economic challenges, resulting in project delays and increased costs.
Willow Rock Energy Storage Center
The Willow Rock Energy Storage Center is a 500 MW advanced compressed air energy storage facility under development in Kern County, California, designed to provide long-duration, reliable, and clean energy storage to support the integration of renewable energy and grid reliability for decades.
First Offtake Deal Signed for 500MW / 4,000MWh Advanced Compressed Air Energy Storage Project in California
Hydrostor has signed a 25-year power purchase agreement for a 200 MW/1,600 MWh portion of its planned 500 MW/4,000 MWh advanced compressed air energy storage project in California, which will support renewable energy integration and grid stability.
A Peek Inside the Cavern: Building the Willow Rock Energy Storage Center and Powering California’s Energy Future
The article details the construction and significance of the Willow Rock Energy Storage Center in California, highlighting how Hydrostor's advanced compressed air energy storage technology will provide long-duration, emissions-free energy storage to support the state's renewable energy future and grid reliability.
Momentum Building for Hydrostor’s Willow Rock Energy Storage Center as Company Reaches Key Permitting and Interconnection Milestones
Hydrostor has reached key permitting and interconnection milestones for its Willow Rock Energy Storage Center in California, advancing the development of its 500 MW, 4,000 MWh advanced compressed air energy storage project to support the state's clean energy needs.
Novel energy storage project launches in California
A novel long-duration energy storage project by Hydrostor, which uses compressed air in underground caverns, has received state approval to begin construction in California’s Mojave Desert, but will only proceed once enough customers are secured to finance the $1.5 billion, 4-gigawatt-hour facility.
Hydrostor’s Willow Rock Energy Storage Center Obtains Final Approval for Power Plant License
Hydrostor’s Willow Rock Energy Storage Center has received final approval from the California Energy Commission for its power plant license, allowing the 500 MW advanced compressed air energy storage project in Kern County to move forward and support grid reliability and resiliency starting in 2026.
Hydrostor’s $1.5B Willow Rock Project Faces Permit Hurdles But Could Revolutionize California’s Power Grid
Hydrostor’s $1.5 billion Willow Rock project in California, which aims to revolutionize the state’s power grid with long-duration, fossil-free compressed air energy storage, faces significant permitting delays and policy uncertainties that threaten its timeline and federal incentives.
Willow Rock Energy Storage Center
The Willow Rock Energy Storage Center is a proposed 500 MW net advanced compressed air energy storage facility in Kern County, California, currently under review for certification by the California Energy Commission.
Willow Rock Energy Storage Center
The Willow Rock Energy Storage Center is a 500 MW advanced compressed air energy storage facility under development in Kern County, California, designed to provide long-duration, reliable, and clean energy storage to support the integration of renewable energy and help California achieve its carbon-free electricity goals by 2045.
Technology Strategy Assessment: Compressed Air Energy Storage
This document is a U.S. Department of Energy assessment that analyzes the current state, challenges, and research and development pathways for compressed air energy storage (CAES) technology, with the goal of identifying innovations and investments needed to reduce costs and enable long-duration energy storage for the electric grid by 2030.
California Moves Closer to Carbon-Free Electricity Goals as Central Coast Community Energy Signs 25-Year Power Purchase Agreement with Hydrostor
Hydrostor and Central Coast Community Energy have signed a 25-year agreement for long-duration energy storage from Hydrostor’s Willow Rock facility, advancing California’s goal of carbon-free electricity by providing clean, reliable power using advanced compressed air energy storage technology.
California Moves Closer to Carbon-Free Electricity Goals as Central Coast Community Energy Signs 25-Year Power Purchase Agreement with Hydrostor
Central Coast Community Energy has signed a 25-year, nearly $1 billion power purchase agreement with Hydrostor to provide long-duration, emission-free energy storage using advanced compressed air technology, helping California move closer to its carbon-free electricity goals.
Hydrostor Reaches Key Milestones for its 500 MW Advanced Compressed Air Energy Storage System in Southern California
Hydrostor has achieved significant progress on its 500 MW advanced compressed air energy storage project in Southern California.
Hydrostor Secures US$250 Million Investment from Goldman Sachs Asset Management to Accelerate Growth
Hydrostor has secured a US$250 million investment from Goldman Sachs Asset Management to accelerate the development and global expansion of its advanced compressed air energy storage projects.