PG&E launches clean energy generator at its Angwin microgrid

From the outside, the new linear generator Pacific Gas & Electric Co. will be using at its microgrid in Angwin looks like just another piece of electrical equipment, a large diesel generator or maybe an array of lithium-ion energy storage.

But the generator, developed by Mainspring Energy of Menlo Park, uses renewable biogas, or natural gas, to generate cleaner electricity with less nitrogen oxide emissions.

On Monday, PG&E, in partnership with Mainspring and NextEra Energy, launch the novel generator as part of the Northern California utility’s microgrid in the Napa County community. To be used during emergencies and public safety power shut-offs, the generator is expected to reduce the microgrid’s reliance on diesel fuel generators.

“What we’re really trying to study is whether this hybrid solution can reduce diesel use and improve air quality during emergencies and (power shut-off) events in 2021,” PG&E spokesman Paul Doherty said.

Monday’s deployment comes months after PG&E and Mainspring tested the equipment at Mainspring’s Menlo Park site in the spring. The technology was developed by Mainspring CEO Shannon Miller, and two other engineers, while the three were doing doctoral studies at Stanford University.

That work took the trio back to the fundamentals of thermodynamics, as they searched for a way to generate power that produces less carbon. Mainspring’s generator produces electric energy through the linear movement of pistons, as opposed to conventional engines that use crankshafts.

Also, unlike conventional engines, Mainspring’s linear generator does not ignite and burn fuel at high temperatures, Miller said. Consequently, the generator does not create the kinds of nitrogen oxide emissions that other engines produce, she said.

Miller said the generator used in Angwin, which is a 240-kilowatt system, is ideal for producing clean energy during a public safety power outage or as a way to bolster other energy sources.

“Having local distributed power generation that can ramp up and down, turn on and off whenever you need it, support and fill in for things like solar and wind really can enable a cleaner and more reliable grid,” Miller said in an interview Friday.

PG&E said it now deploys mobile, diesel generators across its service area during emergencies and preventative power shut-offs to rapidly reenergize thousands of customers. These generators are connected to the grid at substations, distribution microgrid sites, community resource centers, and critical sites as needed and available, the company said.

The Angwin microgrid, one of PG&E’s first, can supply power to about 54 customers that include a fire station, a market, a medical site and a student housing building at Pacific Union College. The power requirement, between 200 and 250 kilowatts, is supplied by a 500-kilowatt diesel generator.

The growth of microgrids in California, as well as some parts of the western United States, has been mainly driven by years of wildfires and the ongoing threats of more disastrous infernos, Michael Burr, director of the Microgrid Institute, told The Press Democrat in 2019. The national organization supports the planning and implementation of microgrids.

Mainspring’s linear generator will kick in when the microgrid’s energy load reaches a certain threshold, Doherty said. For the Angwin deployment, the linear generator is connected to both PG&E’s electric distribution system and its natural gas distribution system.

The third partner in the pilot program, NextEra Energy, is acquiring biogas fuel that is renewably produced at another location. The procured biogas is processed and injected into the existing gas distribution infrastructure, offsetting the linear generator’s use of natural gas.

You can reach Staff Writer Martin Espinoza at 707-521-5213 or On Twitter @pressreno.

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