Compressed Air Storage

The World’s First Grid-Scale Compressed Air Storage System in Hard Rock

The Willow Rock Project Could Reshape the Economics of Grid-Scale Storage

Jai Duhan

Subsurface Energy Storage Consultant

jai.duhan@respec.com

Project at a Glance

Technology: Advanced Compressed Air Energy Storage (A-CAES) in hard rock
Depth: Caverns excavated nearly 2,000 feet below ground
Innovation: First-of-its-kind application of compressed air storage in hard rock

Key Benefits

Lower cost than grid-scale battery storage
No reliance on rare earth materials
Emission-free energy solution
Minimal surface footprint

Where RESPEC Made the Difference

Conducted detailed geotechnical analysis to assess granite cavern suitability
Modeled long-term cavern stability under repeated pressure cycling
Designed excavation approach and optimized underground cavern layout
Validated system design to ensure long-term safety, integrity, and performance

Willow Rock: Hydrostor’s hard rock A-CAES facility will deliver long-duration, zero-emission storage for California’s carbon-free future.

As California accelerates its shift to carbon-free electricity and the US focuses on energy security, long-duration energy storage has become not just important, but essential. The Willow Rock Energy Storage Center, developed by Hydrostor, is positioned to meet these priorities in a groundbreaking way.

Located in Kern County, Willow Rock will be the first facility in the world to store compressed air in hard rock formations. Using Hydrostor’s proprietary Advanced Compressed Air Energy Storage (A-CAES) technology, the project will convert surplus electricity into compressed air, storing it nearly 2,000 feet underground in purpose-built granite caverns. When electricity demand rises, the system will release the stored air to drive turbines, delivering clean, dispatchable power back to the grid.

Willow Rock represents a bold step forward in energy innovation, offering a cost-effective, zero-emission solution to balance California’s increasingly renewable energy mix.

Mechanical Storage, Minimal Footprint

Unlike conventional battery technologies, A-CAES systems provide a durable and low-impact solution for grid-scale energy needs. By relying on proven mechanical processes and purpose-built underground caverns, A-CAES eliminates the need for rare earth materials, reduces surface land use, and delivers long-term operational reliability.

At Willow Rock, this approach turns underground space into a strategic asset—addressing surface-level energy challenges with a non-combustible, emissions-free solution. The result is a sustainable, low-footprint system designed for resilience and scalability.

A Global First in Hard Rock

The challenge? Compressed air energy storage has never before been implemented in hard rock.

RESPEC is leading the geotechnical analysis, seismic design, excavation planning, and system validation to ensure the facility operates reliably—under literal pressure. Leveraging decades of expertise in cavern storage and hard rock mining, the team is using proven underground engineering methods in an entirely new context.

“We’re applying decades of underground design and energy storage expertise to help prove that compressed air in hard rock isn’t just possible—it’s practical,” says RESPEC Project Manager Jai Duhan. “Our focus is ensuring the geology supports a system that is safe, stable, and scalable for the energy transition.”

Designed to Outlast Batteries and Lower Costs

Compared to battery systems or surface tank storage, A-CAES provides a more cost-effective and durable solution for grid-scale energy storage, making grid-scale storage more affordable to build and capable of serving communities for decades without the environmental costs of conventional technologies.

For energy developers facing the dual challenge of scaling clean power and minimizing ecological footprint, A-CAES offers a compelling alternative, particularly in regions where land is limited, material sourcing is under scrutiny, and infrastructure must be designed for long-term resilience.

If you’re evaluating long-duration storage, RESPEC can help assess cost, risk, and long-term performance.