Willow Rock A-CAES by Hydrostor: status and timeline

Willow Rock A-CAES

500 MW / 4 GWh A-CAES facility in Kern County, California. Uses underground air caverns and thermal storage, with no fossil fuels during discharge.

PilotStage 3 of 5

Permit obtained December 2025; construction begins mid-2026; online 2030.

Updated Feb 2026Checked 10 Oct0 updates this week

Milestones

Next · Construction begins

California Energy Commission issues final permit19 Dec 2025Complete.
Kern County Board franchise agreement approvalDec 2025Complete.
Conditional letter of support from Kern County BoardApr 2025Complete.
Construction beginsTarget 2026Current milestone.
Facility comes onlineTarget 2030Not yet reached.

Most important updates

Upcoming

2026Construction begins (next)
2026Construction breakground at Willow Rock
2030Facility comes online
2030Facility begins operations

Current obstacles

Permitting and sitingGeological surveys and local approvals for cavern siting take years. Only certain regions have suitable geology.
Proof of long-term durabilityOntario facility is commercial-scale; proving thermal storage and cavern integrity over decades is critical.

Physics limits

Geology limits sitingSuitable deep caverns with stable rock and sufficient water tables exist only in certain regions. This is unlike lithium-ion batteries, which can be built almost anywhere.
Capital cost per kWh is highA-CAES costs roughly $3,000 per kW for a 10-hour system, or $300/kWh. Lithium-ion batteries have dropped below $100/kWh, though they degrade within 10–15 years, while A-CAES lasts 50+ years.

How it works

3 parts

Compression

Air goes underground

Compressors inject air into a 2,000-foot cavern. Compression heat is captured in thermal storage tanks on the surface.

Storage

Heat and air remain stable

The cavern keeps compressed air stable for decades. Thermal tanks store heat, ready to reheat on demand.

Discharge

Reheated air drives turbines

Compressed air releases, reheats using stored thermal energy, and drives turbines for 8 hours at 500 MW.

Spec sheet

Spec	Willow Rock A-CAES
Power capacity	500 MW^R (reported)
Energy storage	4 GWh^R (reported)
Continuous discharge duration	8 hours^R (reported)
Air cavern depth	~2,000 feet^R (reported)
Site footprint	89 acres^R (reported)
Estimated capital cost	~$1.5B^E (estimated)

^R reported by the company · ^E our estimate

Update log

2 updates

Feb 2026

Major: Hydrostor signs 50 MW / 400 MWh offtake agreement with California Community Power for Willow RockPressCommercial

Dec 2025

Major: California Energy Commission approves final permit for Hydrostor's 500 MW Willow Rock A-CAES facilityPressRegulatory

About Hydrostor

The team behind Willow Rock A-CAES

Hydrostor

Compressed air energy storage, thermal energy storage

Hydrostor develops compressed air energy storage (A-CAES) for long-duration grid storage. It compresses air into underground caverns, captures heat, and uses stored heat to generate power. The company operates a facility in Ontario and develops 6+ GW of projects globally.

Willow Rock stores 4 GWh over eight hours at 500 MW, the largest standalone grid storage project in California.
Construction begins mid-2026; facility comes online 2030; capital cost ~$1.5B.
The system eliminates fossil fuel combustion during discharge by capturing and storing compression heat in thermal tanks.

hydrostor.ca (opens hydrostor.ca)Company profile →

Founded: 201016 yrs
Headquarters: Canada
Status: Private
Valuation: $658MprivateJan 2022
Raised: $250M1 round
Last round: Series C · $250MJan 2022
Works in: EnergyGrid storage
Coverage: 1 program · 2 updateslatest 1 Feb 2026checked 10 Oct
Lead investors: Goldman Sachs Asset Management
Customers: Southern California Edison (opens sce.com)California Community Power (opens ca-power.org)
Investors: Export Development Canada (opens edc.ca)Canada Pension Plan Investment Board (opens cpp-cri.ca)
People: Curtis VanWalleghem (opens hydrostor.ca)Founder & CEO

Spec

Willow Rock A-CAES

Power capacity

500 MW^R (reported)

Energy storage

4 GWh^R (reported)

Continuous discharge duration

8 hours^R (reported)

Air cavern depth

~2,000 feet^R (reported)

Site footprint

89 acres^R (reported)

Estimated capital cost

~$1.5B^E (estimated)