The largest energy utility in the US has applied for regulatory approval to build a “groundbreaking” 48-hour green hydrogen/battery energy storage system that would power a Northern California city if transmission lines must be switched off for safety reasons due to a high risk of local wildfires.

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The 8.5MW/293MWh zero-carbon project would replace the mobile diesel generators used to energise Pacific Gas & Electric's (PG&E) microgrid in the small city of Calistoga, in the wine-producing Napa Valley region, during grid outages.

Swiss company Energy Vault would own, operate and maintain the project, dubbed the Calistoga Resiliency Center, and provide dispatchable power to more than 2,000 PG&E customers under a ten-and-a-half-year agreement.

“The system’s capacity may be expanded to 700MWh, which would allow it to operate for longer without refueling, enabling further flexibility for PG&E and the City of Calistoga,” the two companies said a joint statement released yesterday (Thursday).

“PG&E submitted the project contract for review and approval to the California Public Utilities Commission (CPUC) on December 30, 2022, with a request for the issuance of a final resolution approving the project by May 15, 2023.”

If the facility is approved, the partners plan to begin construction in the fourth quarter of this year, with commercial operation expected in the second quarter of 2024.

“Upon completion, this project is expected to be the first-of-its-kind and the largest utility-scale green hydrogen project in the United States,” the companies said.

PG&E filed for bankruptcy protection in 2019 after the state of California blamed it for two devastating wildfires in 2017 and 2018, leaving it liable for billions of dollars of damages.

It is not clear how much green hydrogen would be required for the Calistoga project or what the ratio of lithium-ion battery to H2 energy storage might be. Nor is it stated where the hydrogen would come from, although the 135-page “request for approval” document submitted to the CPUC says that it would be produced via electrolysis derived from renewable energy.

“The entire system will be developed on less than one acre [0.4 hectares] of land and is expected to serve as a model for Energy Vault’s future utility-scale hybrid storage system deployments,” said the joint statement.

Until recently, Energy Vault's sole focus was its proprietary gravity-based energy storage system in which a crane lifts composite blocks into the air and then lowers them to generate power — a concept that critics described as “crazy”.

Despite raising $380m in nine funding rounds since 2017, it has yet to commission a single commercial gravity storage system, although the company says that two small projects are under construction in Texas and China.

Lithium-ion battery storage space now represents the majority of its income, according to the most recent company results.

Incidentally, Calistoga — which has a population of a little over 5,000 and is well known for its hot springs and wineries — got its unusual name from a drunken speech in the 1860s by one Sam Brannan, the leader of the first permanent non-native settlers in the area, who developed a spa similar to one in Saratoga, New York state. He meant to say that he would make the area the new Saratoga of California, but it came out as “the Calistoga of Sarafornia”.