Willow Rock Energy Storage Center will be located in a nearly 60-acre area in Eastern Kern County. (Credit: © 2022 Hydrostor Inc.)
The Energy Storage Center will store power supplied by solar and wind projects of California. (Credit: Maria Maltseva from Pixabay)
Hydrostor secured $250m investment from Goldman Sachs Asset Management to advance its A-CAES projects in Australia and California (Credit: © 2022 Hydrostor Inc.)

The Willow Rock is a long duration energy storage centre that is being developed in the US state of California. The project will be located in a nearly 60-acre area in Eastern Kern County, outside Rosamond in California.

Willow Rock was proposed by Gem A-CAES, a wholly owned subsidiary of long-duration energy storage solutions provider Hydrostor.

The facility will leverage Hydrostor’s Advanced Compressed Air Energy Storage (A-CAES) technology and will provide up to 500MW of emissions-free electricity to increase grid reliability in California during peak demand hours.

In July 2022, the California Energy Commission (CEC) determined that Hydrostor’s Application for Certification for the Willow Rock Energy Storage Center is Data Adequate. The move officially began the public environmental review process for the project.

The construction of the project is expected to commence in the third quarter of 2023 after the receipt of all permissions. Commercial operations are slated to begin in 2026-27.

Once complete, the facility will become the largest stand-alone energy storage project in California.


The Willow Rock Energy Storage Center will be designed as a 500MW, 4,000 MW-hour (MWh), A-CAES facility, capable of charging and discharging every day.

Particularly, the facility will store excess power generated by Californian solar and wind projects and supply it in times of need.

The energy stored in the centre will be enough to power up to 400,000 homes for more than eight hours.

Hydrostar’s patented A-CAES technology supports long term energy storage with minimal environmental impact, and facilitate the transition to renewable energy.

The technology creates a scalable energy storage system using proven components from gas and mining operations. It can store energy from five hours to multiple days, when needed.

Project details

Key Willow Rock Energy Storage Center infrastructure will include five all electric air compressor trains, five 100MW air-driven power turbine generators, thermal heat storage, an underground compressed air storage cavern, a water reservoir aboveground, heat exchangers and other support facilities.

The facility will also have a 10.9-mile interconnection to the existing Southern California Edison Whirlwind Substation (SCE).

The centre will compress air into an underground cavern by using electricity from the grid. The air will be stored in the cavern using the pressure of a hydrostatic head created by a water reservoir above the ground.

The heat generated from the air compressor will also be stored in a thermal storage system above the ground.

In times of need, the compressed air will be released under the hydrostatic pressure, re-heated using the stored thermal energy and then passed through turbine generators above the ground to produce electricity.

As the facility will be located in northeast of the Los Angeles basin, the electricity produced will be deliverable to both the Los Angeles Department of Water and Power (LADWP) and California Independent System Operator (CAISO) grids.


Willow Rock will have a life of more than 50 years.

During peak construction, the project is expected to employ around 800 skilled workers. After it enters operation, the facility will employ 25-40 full-time staff.

The project is also expected to deliver $500m in regional direct and indirect economic impacts.

The energy storage centre will also support California to achieve its objective of reducing greenhouse gas (GHG) emissions 40% below 1990 levels by 2030. The facility is also aligned with the state’s target of delivering 100% carbon free electricity by 2045.


In January 2022, Hydrostor secured $250m investment from the Private Equity and Sustainable Investing businesses within Goldman Sachs Asset Management.

The company will use the proceeds for its A-CAES projects in Australia and California.

Canada Pension Plan Investment Board (CPP) committed $25m in April 2022 for Hydrostor’s facilities across the world.

Hydrostor also received $10m from the Business Development Bank of Canada (BDC) in August 2021 and $37m in growth financing in 2019.


In August 2022, Kiewit Engineering Group signed an agreement with Hydrostor regarding front-end engineering and design studies for the Willow Rock Energy Storage Center.