Hawaii shut down its last coal plant on September 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 the move posed a question 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, unlike the gigantic batteries 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.

The construction process had its setbacks, as did the broader effort to replace the coal plant with a roster of large-scale clean energy projects. The Kapolei battery was initially intended 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 complete Kapolei by the end of 2022, 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.