PG&E, Energy Vault Seek OK for Biggest U.S. Long-Duration Storage System

The microgrid would be owned, operated and maintained by Energy Vault, and would use both lithium-ion batteries and hydrogen fuel cells to supply the northern California town. Calistoga has been affected by multiple wildfires, including the Glass Fire in 2020, as well as a series of public-safety power shutoffs in advance of high winds that officials feared might cause utility equipment to spark, leading to more wildfires. Microgrids can be used as emergency backup, whether for wildfires, PSPS events or other outages. In addition to resilience and reliability, they add flexibility and can be tailored to meet specific customer needs (California Energy Markets No. 1715).

The proposed system is designed to supply power to more than 2,000 electric customers for a minimum of 48 hours, serving all the load within a safe-to-energize area. The footprint will include critical facilities such as fire and police stations, and shared services in the downtown and surrounding area of Calistoga.

Calistoga began developing a citywide resilience plan in 2019 (California Energy Markets No. 1550); this proposal will replace the mobile diesel generators currently used to energize PG&E's Calistoga microgrid during broader grid outages.

"PG&E selected Energy Vault's innovative hybrid architecture and design to create a cost-effective, community-scale, fully carbon-free microgrid that can store and dispatch on-demand renewable energy," Ron Richardson, PG&E regional vice president for the North Bay and North Coast, said in a news release. "This breakthrough collaboration between PG&E and Energy Vault provides a template for future, renewable community-scale microgrids that successfully integrate third-party distributed energy resources, which is expected to cost customers less than the benchmark set by state regulators based on the alternative use of mobile diesel generators."

The project requires the approval of the California Public Utilities Commission. In a Dec. 30 letter submitted to the CPUC outlining how the project will be structured, Sidney Bob Dietz II, director of regulatory relations at PG&E, said it will have an 8.5-MW capacity and expected generation of 293 MWh over a 48-hour period. The energy storage system will be run by Energy Vault while providing dispatchable power under a 10.5-year tolling agreement with PG&E. Dietz asked for final signoff by May 15, with construction starting before the end of this year and commercial operation before June 1, 2024.

PG&E and Energy Vault state that the entire system will be developed on less than 1 acre of land and cost less than $50 million. They have also requested approval for the system's capacity to grow to 700 MWh at a later date, saying this would let it operate for longer without refueling, and allow more flexibility for PG&E and the City of Calistoga.

"We are setting a new benchmark for what can be achieved with an innovative design that integrates the most advanced energy storage mediums in order to deliver a fully renewable green hydrogen battery energy storage system," Robert Piconi, chairman and CEO of Energy Vault, said in a news release. "Our engineers designed [the] hybrid energy storage system [using a] technology-neutral integration platform and energy management software. This project [shows our] ability to bring the most innovative short, long and ultra-long duration energy storage technologies to our customers."

Energy Vault will operate the facility, providing energy service using grid-forming generation as well as storage resources. This could be in combination with demand-side resources to provide energy, fault-current contribution and voltage and frequency regulation within PG&E's established parameters to enable islanding of the Calistoga microgrid during planned outages. The company will also deploy software for full system control and optimal dispatching across the different aspects of the project, including batteries, hydrogen tanks and fuel cells.

Green hydrogen is produced when the process of electrolysis—using water to separate hydrogen from oxygen—is powered by zero-carbon electricity like solar or wind. In the case of the Calistoga project, the hydrogen then powers fuel cells, which employ a chemical process to cleanly and efficiently produce electricity; the only products are electricity, water and heat. The entire community-scale microgrid will be 100-percent renewable, the two companies said.

The microgrid's black-start capabilities involve recovering from a blackout. Systems with this feature can start up and gradually reconnect parts of the grid to one another to form an interconnected system again without relying on the external electric transmission network to recover from a total or partial shutdown.

On its side, PG&E will provide a microgrid controller, metering services, networking equipment and a pole and line extension, and will interconnect the Energy Vault facility in order to allow it to meet station-power requirements during normal grid conditions.

This article has been updated with a new estimated cost for the project, at the request of Energy Vault.