Hydrostor Signs 60 MW Offtake for 500 MW Willow Rock Compressed Air Storage in Kern County
- Energy Storage News Utility-Scale BESS
Marin Holt
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- 7 minutes read
- Offtake Signed: 60 MW
- Project Size: 500 MW / 4,000 MWh
- Contracted to Date: 110 MW
- Round-Trip Efficiency: 60% to 65%
- Expected Online: 2030
Hydrostor has signed a 60 MW offtake agreement with Clean Energy Alliance. The buyer takes power from the Willow Rock Energy Storage Center in Kern County, California. Willow Rock is a 500 MW / 4,000 MWh compressed air plant now under construction. The Canadian developer announced the deal on August 6, 2026.
Clean Energy Alliance is a community choice aggregator. It serves more than 255,000 people across seven San Diego County communities. On-site work started in July 2026. The plant should reach commercial operation in 2030.
The contract is the second for the site. Hydrostor sold 50 MW to California Community Power in February 2026. That joint powers agency acts for six aggregators. Members include CleanPowerSF, Peninsula Clean Energy and Silicon Valley Clean Energy Authority. The two deals together cover 110 MW. That is 22% of rated output, four years before first delivery.
What Is Being Built
Willow Rock stores energy as compressed air in mined underground caverns, not in cells. Crews excavate the cavern with hard-rock mining methods and flood it with water. Injected air then pushes that water up to a surface reservoir. The water column holds the cavern at constant pressure through every charge and discharge. The design trades chemistry for civil works. So the plant should not degrade on a cycling duty the way a lithium system does.
The efficiency penalty is the trade. Jon Norman, president of Hydrostor, spoke to pv magazine in May 2026. He put round-trip efficiency at 60% to 65%. Utility-scale lithium plants routinely clear 85%. Each stored megawatt-hour at Willow Rock therefore costs about a third more input energy. The 8-hour discharge duration has to earn that penalty back.
The permits are in hand. The California Energy Commission issued the power plant license in December 2025. Kern County granted franchise approval the same month. That clears the regulatory risk that has stalled other long-duration projects in California.
Why It Matters
Buyers now treat an unproven technology as procurable capacity. The only operating Hydrostor asset is a 2 MW demonstration plant in Ontario, built in 2020. Willow Rock is 250 times that size. It is also the first utility-scale build by the company anywhere. Two California aggregators have committed load to a design that has never run at grid scale.
The reason is duration. Greg Wade, chief executive of Clean Energy Alliance, described the value plainly. Long-duration storage lets the provider absorb surplus solar output at midday. It then discharges into the evening peak. The four-hour lithium fleet in California already saturates that window. So the marginal value of a fifth and sixth hour keeps rising.
An 8-hour asset bids into a part of the curve that lithium does not reach economically. Procurement teams should watch the remaining 390 MW. Hydrostor still has to place 78% of plant output. The contracts so far are small slices from aggregators, not large utility offtakes. Whether investor-owned utilities follow will say more about bankability than either announcement does.
Critical Perspective
Two utility-scale compressed air plants have run anywhere in the world in nearly fifty years. Huntorf in Germany started in 1978 at 290 MW. McIntosh in Alabama followed in 1991 at 110 MW. Both burn natural gas to reheat the air on discharge. Willow Rock does not, and that difference carries the whole investment case. It is also the part nobody has tested at 500 MW.
The 60% to 65% round-trip efficiency deserves harder scrutiny than a press release gives it. That figure comes from the developer, not from a metered plant. Hydrostor has one 2 MW unit in Ontario behind it. A shortfall of a few points across 4,000 MWh of throughput becomes a large recurring cost, and these offtake contracts run for years.
The contracting pattern rewards a close read. Two community choice aggregators have bought 110 MW between them, in slices of 50 MW and 60 MW. No investor-owned utility has signed. Aggregators answer to state reliability mandates that reward long-duration procurement, and they carry smaller balance sheets than an IOU. Does the absence of a large utility offtake, four years from commercial operation, reflect timing or a bankability judgment the market has already made?
