CALISTOGA, Calif. — A quaint northerly outpost of Napa Valley wine country, Calistoga has struggled to keep the lights on when wildfires strike the region. Now it’s got a brand-new microgrid to run the whole town for days on end without any onsite fossil fuels, just batteries and liquid hydrogen.
After disastrous conflagrations in 2017 and 2018, utility Pacific Gas & Electric began preemptively shutting off power lines to avoid sparking fires amid dangerously dry, windy conditions.
“We were the first community in all of PG&E’s network that was getting our power shut off to protect us,” said Calistoga City Council member Lisa Gift. “By 2019 we were one of the first communities to have a microgrid in all of PG&E’s network, and that was being powered by diesel generators.”
PG&E arranged a bank of truck-based diesel generators to sit in the town during fire season. When the utility cut grid power, the generators kicked on, belching smoke in a particularly beloved pocket of the 5,000-person community.
“We’re a small town, so they would come up and they’d be polluting the environment, taking up our dog park — loud, gross, noisy,” Gift recalled.
Now the diesel generators are gone and the park has been turned back over to Calistoga’s canine companions.
On a slim parcel of city land next door, publicly traded energy-storage company Energy Vault installed lithium-ion batteries and a 234-foot, reinforced-steel tank for liquid hydrogen (designed to withstand a roaring fire, should it ever come to that) that runs a bank of hydrogen fuel cells. Altogether, this compound should be able to meet Calistoga’s electricity needs without any power from the broader grid. It’s contracted to produce up to 8.5 megawatts for 48 hours, whenever PG&E shuts off grid power due to fire concerns. Refilling the hydrogen tank could let it run for several days more.
“Even though we’re taking elements — fuel cells, batteries, liquid hydrogen storage and distribution — that have been used before in commercial settings, they’re coming together for the first time as resiliency,” said Craig Horne, Energy Vault’s senior vice president for advanced energy solutions, in an interview before the project’s unveiling in early August.
Fans of hydrogen hail it as a solution to just about any entrenched decarbonization challenge, from heavy transport to steelmaking to on-demand power. But how hydrogen is produced makes a huge difference in its climate impact; seemingly clean sources can actually rack up major carbon emissions for negligible benefit. For now, the clean hydrogen economy remains largely speculative, with hardly any truly clean hydrogen being produced or any real projects using it. Many planned clean hydrogen projects have vanished without a trace, following a short-lived boom fueled by Biden-era support.
In Calistoga, Energy Vault has tapped hydrogen to deal with a very specific set of constraints — delivering energy without local emissions, over multiple days, in a tight footprint — but the cleanliness of that hydrogen is a more complicated issue than public descriptions of the microgrid suggest.
The key players all have a lot riding on the project.
Energy Vault, which previously raised several hundred million dollars in a singular bid to store energy with multi-story robotic cranes that stack blocks, wants to build a new long-duration storage business around this hydrogen microgrid showcase. Plug Power, the financially challenged hydrogen company, points to Calistoga as its largest deployment of hydrogen fuel cells (a beefy 8 megawatts, after 28 years of hard work). And PG&E has orders from regulators to add more clean energy microgrids in communities where it regularly cuts off power — Calistoga was its first delivery on that directive, after a few years of soliciting proposals and a couple more years of permitting and construction.
“Community microgrids are the future of the energy system,” said Craig Lewis, who advocates for such projects as executive director of the Clean Coalition nonprofit. The Calistoga microgrid is “a commercial-scale experiment, and I’m grateful for it.”
The results of that experiment will take time to analyze. It could unleash a new, replicable model for premium-priced community-level backup power. Or the quirkiness of the design and the murkiness of hydrogen’s supply chain and emissions could make it a quixotic outlier of questionable climate value.
Compact and cleaner backup power in a fire zone
The Calistoga microgrid poses an answer to the question of how to provide a few days of backup power to a small town in a small space, without worrying too much about cost. The limitations drove the design, which turned out quite unlike anything built thus far.
Energy Vault had to figure out how to pack 293 megawatt-hours of storage into just two-thirds of an acre. The lot used to hold debris from city works, like old bits of sidewalk and pipes, Horne said.
Lithium-ion batteries have proven themselves capable of storing power, be it as a Powerwall in someone’s garage or as a large-scale grid storage facility. But to store nearly 300 megawatt-hours, grid battery enclosures need more acreage than was available to lease from the city. Even if enough batteries could fit, the auxiliary power consumption for keeping them safely cooled would pose a challenge for a project that’s supposed to mostly sit around waiting for an emergency event.
Hydrogen gas can be liquefied by cooling it to ultra-low temperatures, which unlocks greater energy density. When converted back to gas and run through fuel cells, it produces a stream of electricity and no byproduct besides water vapor. That core technology powers hydrogen vehicles, though their cost and inconvenience make for a widely derided car-ownership experience. At Calistoga, the hydrogen flows directly to six Plug Power GenSure 1540 fuel cells, boxy containers with cooling units stacked on top, making them about two stories tall.
The engineers added a small lithium-ion battery (7.7 MW/11.6 MWh) to perform “black start,” the complicated and crucial task of rebooting an electrical system after a complete blackout, Horne noted. The battery also buffers the output of the system while the hydrogen gets up and running. Then the power flows to Calistoga’s grid, which, when PG&E shuts off the transmission lines, will be fully islanded from the surrounding network.
The hydrogen is stored onsite in an 80,000-gallon tank, manufactured in Minnesota by Chart Industries. The tank holds enough to power the fuel cells for about two days, but Energy Vault will try its best to keep the lights on beyond the contracted timeframe, Horne said. So the company made sure the tank can be refueled while it’s in active use.
“The task is to squeeze toothpaste into a toothpaste tube that was being squeezed,” Horne said. “That’s what we proved in our acceptance testing, running for multiple hours while the fuel cells were running and a tank trailer here in the driveway is pushing liquid hydrogen into the tank itself.”
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