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Editor’s note: This story is reprinted with .

Hawaii shut down its last coal plant on Sept. 1, 2022, eliminating 180 megawatts of fossil-fueled baseload power from the grid on Oahu — a crucial step in the state’s first-in-the-nation commitment to cease burning fossil fuels for electricity by 2045.

But that’s becoming increasingly urgent as clean energy surges across the United States: How do you maintain a reliable grid while switching from familiar fossil plants to a portfolio of small and large renewables that run off the vagaries of the weather?

Now Hawaii has an answer: It’s a gigantic battery,  that have been built before.

The Kapolei Energy Storage system actually began commercial operations before Christmas on the industrial west side of Oahu, according to Plus Power, the Houston-based firm that developed and owns the project. (The company just had the good sense to wait to announce it until journalists and readers had fully returned from winter holidays.)

Now, Kapolei’s 158 Tesla Megapacks are charging and discharging based on signals from utility Hawaiian Electric. The plant’s 185 megawatts of instantaneous discharge capacity match what the old coal plant could inject into the grid, though the batteries react far more quickly, with a 250-millisecond response time. Instead of generating power, they absorb it from the grid, ideally when it’s flush with renewable generation, and deliver that cheap, clean power back in the evening hours when it’s desperately needed.

“It feels incredible to be part of what Hawaii and Hawaiian Electric are doing to get to 100% renewable energy and to play this enabling role to help them get one step closer,” Plus Power Executive Chairman Brandon Keefe told Canary Media.

Canary Media’s Julian Spector visited Oahu in September 2021 to report on Hawaii’s planned shift from coal to batteries.

The construction process , as did the broader effort to replace the coal plant with a .

The Kapolei battery was  to come online before the coal plant retired. Covid disrupted deliveries for the grid battery industry across the board, and Kapolei’s remote location in the middle of the Pacific Ocean didn’t make things easier. By summer 2021, Plus Power was hoping to , but it ended up taking another year. Even then, it has joined the grid before several of the other large solar and battery projects slated to replace the coal plant’s production with clean power.

Batteries Replace Key Coal Plant Functions

Grid batteries operate in a fundamentally different way than coal plants, so Hawaiian Electric and Plus Power crafted a new framework to replace what needed to be replaced. The old coal generator provided three key values to Oahu, Keefe explained: energy (the bulk volume of electricity), capacity (the instantaneous delivery of power on command), and grid services (stabilizing functions for the grid, wonky but vital to keeping the lights on).

The battery directly replaces the latter two: It matches the coal plant’s maximum power output (or ​“nameplate capacity,” in industry parlance), and it is programmed to deliver the necessary grid services that keep the grid operating in the right parameters. The grid runs within a certain frequency, but events can cause the frequency to stray out of bounds, say if another power plant trips offline or a sudden rush of solar production outstrips consumption. The Kapolei project provides a first line of defense, called ​“synthetic inertia,” responding to and correcting grid deviations in real time. If the situation continues to deteriorate past a specified threshold, the battery’s fast frequency response kicks in as a second line of defense.

With 565 megawatt-hours of storage, the battery can’t directly replace the coal plant’s energy production, but it works with the island’s bustling solar sector to fill that role. ​“We’re enabling the grid to add more clean renewable energy to the system to replace the energy from the coal plant,” Keefe said.

Hawaiian Electric’s modeling suggests it can reduce curtailment of renewables by an estimated 69% for the first five years thanks to Kapolei Energy Storage, allowing surplus clean electricity that would otherwise go to waste to get onto the grid.

The utility also requested ​“black-start capability.” If a disaster, like a cyclone or earthquake, knocks out the grid completely, Hawaiian Electric needs a power source to restart it. The Kapolei batteries are programmed to hold some energy in reserve for that purpose. Plus Power located the project near a substation connected to three other power plants so the battery ​“can be AAA to jump-start those other plants,” Keefe said.

The combination of all these abilities in one site — capacity, grid services, black start — leads Keefe to call Kapolei ​“the most advanced battery energy storage facility on the planet.”

Model For A Reliable Clean-Energy Grid

The new battery is just the latest dispatch from Hawaii’s long-held spot at the vanguard of the energy transition. This is the state that hit mass rooftop solar adoption first and crafted the  in Kauai (not coincidentally, Plus Power CCO Bob Rudd had a hand in that project during his tenure at Tesla).

But when renewables growth and fossil-plant retirements pass a certain threshold, as they have in Hawaii, simply adding more wind, solar or batteries isn’t sufficient. The clean technologies, which run on digitally controlled inverters, have to start maintaining the grid, not just feeding it.

Plenty of other batteries provide frequency services to other grids, and a few of them are larger than Kapolei. But this is the only large-scale battery that we’ve seen capable of combining the basic peak capacity, frequency response, synthetic inertia and grid-rebooting tasks. That’s because Kapolei plays a more central role in its grid than battery plants do elsewhere.

After years of construction, California’s grid battery fleet  last year, but that only equates to 7.6% of the mammoth nameplate capacity of the state’s grid. Kapolei alone constitutes about 17% of Oahu’s peak capacity. Hawaiian Electric needed it to take on more responsibility than batteries elsewhere have ever had to.

Take , which stabilizes grid frequency, as one example. Old plants provide this passively, through the spinning mass of their turbines; inertia didn’t need to be defined and compensated for separately in bygone decades because it was part of the package of running a power plant.

Now, across the country, the grid is moving to a model of maximizing cheap renewables when they are available and burning fuel when renewables aren’t. But the thermal plants need to be spinning to provide inertia — sometimes, on the mainland, renewables get curtailed to keep old coal plants running so they can deliver these grid services, Keefe said. This can be a bad deal for electricity customers, not to mention the climate.

Advanced batteries provide a synthetic version of this inertia through savvy programming of their inverters. This offers a more economic alternative while avoiding unnecessary carbon emissions. They also are faster and more precise — Keefe likened the Kapolei battery to a zippy electric sports car compared to the lumbering diesel bus of old thermal plants. That makes batteries a good technical fit for grids that are becoming increasingly volatile due to the fluctuations of renewable production.

Longer-term, U.S. climate goals require a phaseout of fossil fuels from the electric grid. Hydropower and nuclear plants help deliver valuable grid inertia without carbon emissions, but they aren’t on track to grow.

That’s why this project matters to the clean energy shift everywhere: It’s one of the first real-life examples of how to shift critical grid functions from fossil-fueled plants to clean energy plants. And eventually, the kind of grid services Kapolei has pioneered will have to scale nationwide.