Hydrostor, Hawaiian Electric, and Wärtsilä Storage Projects: Scope and Status
This page compares three dated storage-project announcements that use different technologies and serve different grids. It is retained as secondary context, not as a current procurement comparison. The project owners’ records establish announced ratings and status on their cited dates; they do not establish delivered economics, lifetime performance, or present operating status without later filings and operating data.
Willow Rock compressed-air storage
Hydrostor’s current Willow Rock project page describes a late-stage-development advanced compressed-air energy storage facility in Kern County, California. The owner lists a 500 MW power rating, 4,000 MWh of stored energy, and an intended discharge duration of more than eight hours. These are owner-reported project specifications, not independently verified operating results.
The project record does not establish commercial operation, measured round-trip efficiency, availability, construction cost, water use, schedule performance, or degradation in revenue service. Those questions require current California licensing and interconnection records, executed contracts, construction evidence, and later operating data.
Waena battery project on Maui
As a historical project snapshot, Hawaiian Electric’s December 17, 2025 release stated that construction of the Waena battery energy storage system was scheduled to begin in January 2026. The utility described the project as a grid-connected 40 MW/160 MWh system on company property along Pulehu Road in Central Maui, approved by the Hawaii Public Utilities Commission in December 2023 and anticipated to come online by 2027.
Those dates and ratings are a historical project snapshot. Verify current construction, commissioning, cost recovery, permits, equipment, safety plans, contract changes, and commercial-operation status in later Hawaiian Electric and Hawaii PUC records. The announcement’s connection to planned Kahului generating-unit retirements does not prove that every replacement-resource or reliability milestone has been completed.
Mallard battery project in Texas
Peregrine Energy Solutions’ December 2025 announcement identifies Mallard Energy Storage as a 250 MW/500 MWh project northeast of Dallas using Wärtsilä’s Quantum2 system, with WHC Energy Services named as engineering, procurement, and construction contractor. The page says construction was advancing and refers to a tolling agreement, but it is a company announcement rather than independent operating evidence.
Verify the project entity, county permits, ERCOT registration and interconnection, equipment delivery, construction status, safety approvals, tolling terms, and commercial-operation date through current records. Do not infer actual availability, market revenue, renewable-energy integration, or grid-reliability value from the announced nameplate rating alone.
Why the projects are not directly comparable
| Project | Technology and intended duty | Evidence boundary |
|---|---|---|
| Willow Rock | Advanced compressed-air storage intended for long-duration discharge. | Owner project page; late-stage-development specifications, not operating results. |
| Waena | Lithium-ion battery described for load shifting and Maui system support. | Utility construction announcement and dated anticipated schedule. |
| Mallard | Grid-scale battery described for ERCOT market and reliability services. | Developer announcement; contracts and construction claims require current verification. |
A useful comparison needs the same definitions for usable power and energy, discharge duration, round-trip efficiency, auxiliary load, availability, degradation, safety boundary, interconnection service, construction cost, operating cost, contract term, and comparison date. The cited records do not provide that complete common dataset.
Reader verification checklist
- Confirm the current project owner, legal entity, site, permit, and interconnection status.
- Separate announced, approved, under-construction, commissioned, and commercially operating states.
- Request usable rather than nameplate energy, auxiliary loads, warranty conditions, degradation, and availability guarantees.
- Check fire protection, emergency response, environmental review, decommissioning, and community-notification records for the exact site.
- Use actual operating and contract data before comparing cost or grid value.
Sources and status
- Hydrostor — Willow Rock Energy Storage Center: owner-reported project design and development status.
- Hawaiian Electric — Waena BESS construction announcement, December 17, 2025.
- Peregrine Energy Solutions — Mallard project announcement, December 2025.
Last source check: August 10, 2026. Recheck each owner and regulator record because construction and commissioning status can change after publication.
Research evidence framework
What was demonstrated: This page identifies the reported experiment, project, product announcement, or engineering result. What was not demonstrated: A reported result does not by itself establish a finished commercial system, field performance, or buyer outcome. Scale of evidence: Check the sample, test cell, pilot, project, or deployment boundary. Measured result: Separate measured values from estimates, forecasts, and promotional targets. Comparison baseline: Identify the control, reference design, or prior result before comparing performance. Commercial status: Confirm whether the subject is research, pilot, announced, available, or independently verified. Known durability: Look for operating duration, cycling, environmental exposure, and maintenance evidence. Known cost: Treat cost as unconfirmed unless the source states the system boundary, date, geography, and currency. Remaining engineering barriers: Consider manufacturing, qualification, safety, supply chain, installation, and service constraints. When this matters to a buyer: Use the page as a question list and verify exact product or project documents before making a decision.