The California Energy Commission has issued its final permit for the Willow Rock Energy Storage Center, a 500 MW/4 GWh project, that will deploy compressed-air technology by Hydrostor, marking a major milestone in long-duration energy storage deployment. It is a first-of-its-kind energy storage system capable of discharging at full power for up to eight hours
Hydrostor plans to break ground on the project in mid-2026. Once operational, the Willow Rock facility is expected to have a lifespan of 50 years.
Technology
The project will utilize Hydrostor’s underwater compressed air energy storage (UWCAES) technology. This innovative approach stores energy in the form of compressed air held underwater in accumulators at a pressurized state.
Electricity drives an air compressor that converts electrical energy into compressed air, which is then stored at a specific depth. The compressed air displaces water inside the accumulators and remains stored until electricity is needed. When demand arises, the system reverses the air flow – the water pressure pushes the air back to the surface under pressure, driving a generator to efficiently convert the stored energy back into electricity.
This reversible process enables large-scale, long-duration energy storage using sustainable principles and minimal environmental impact, the company notes.
About the Company: Hydrostor
Hydrostor is a Toronto-based energy storage developer and operator founded in 2010 by Curtis Vanwalleghem, Cameron Lewis, Yannick Beaudoin, and Esther Pinto. The company specializes in long-duration energy storage systems designed to support grid resiliency and integration of renewable energy.
Hydrostor’s key competitors used to include Form Energy, Northvolt, and Ambri, all of whom have been going through difficult times.
Presence
Hydrostor operates a commercial utility-scale facility contracted with the Independent Electricity System Operator (IESO) in Goderich, Ontario. It also has two advanced projects under development – one in Kern County, California, and another in New South Wales, Australia.
Beyond these, Hydrostor maintains an early-stage pipeline exceeding 6 GW across renewables-rich regions such as Australia, the United Kingdom, and the southwestern United States, as well as additional prospects in North America and Europe. The company manages offices in Toronto (Canada), Melbourne (Australia), and Denver (United States).
Funding
Hydrostor has raised a total of USD 615 million across 11 funding rounds since November 16, 2016. The company’s most recent funding round was its Series D on February 13, 2025, raising USD 200 million from investors including CPP Investments, Goldman Sachs, BDC, and Sustainable Development Technology Canada.
Its investor base comprises 18 institutional backers, including major partners like Goldman Sachs Asset Management, the Canada Pension Plan Investment Board (CPPIB), and the Canada Growth Fund (CGF).
