en.Wedoany.com Reported - On August 12, U.S. construction contractor Granite Construction disclosed that it has been awarded a contract by Hydrostor for early site grading works at the Willow Rock Energy Storage Center, valued at approximately $31 million. The project is located north of Rosamond in Kern County, California, with related works having commenced in July 2026 and scheduled for completion in March 2027.
Granite will construct early infrastructure on the 89-acre project site, including building an approximately 9-acre working platform to provide construction conditions for subsequent underground cavern drilling and excavation. The planned caverns are located approximately 2,000 feet underground, with an excavation volume of approximately 1.3 million cubic yards. The contract also covers off-site road improvements, on-site stormwater retention basins, construction material storage yards, and equipment and material work areas for the underground mining contractor and the surface EPCM contractor.
The Willow Rock project, developed by Hydrostor, plans to build a 500 MW/4,000 MWh advanced compressed air energy storage facility with a discharge duration exceeding 8 hours at full power, utilizing four turbine units. The project is designed for an operational life of more than 50 years and, once completed, will provide power equivalent to the needs of over 400,000 households, while also supporting renewable energy grid integration and transmission system regulation.
The project obtained its California Energy Commission permit and Kern County franchise agreement in December 2025. Site works initiated in July 2026 constitute early construction, with Hydrostor planning to advance the main works following financial close. On August 6, the project signed a 60 MW offtake agreement with Clean Energy Alliance, a community power provider serving San Diego County.
Hydrostor completed a $230 million equity financing on August 12, with funds to be used to advance its global portfolio of advanced compressed air energy storage projects and to support the Willow Rock project in achieving financial close within 2026. Investors include Canada Growth Fund, Goldman Sachs Alternatives, Canada Pension Plan Investment Board, Baker Hughes, and Hatch, among others.
Hatch serves as the project's EPCM partner, with Hydrostor contracting directly with suppliers and contractors. The publicly disclosed procurement scope includes heavy civil works, electrical and mechanical installation, ready-mix and structural concrete, coatings, insulation materials, industrial surface finishing, construction equipment rental, logistics and site support, as well as modular temporary facilities. The project has opened supplier registration and organized contractor and supplier engagement events in Kern County.
