KEKAHA, Kauai — The Kauai Island Utility Cooperative, the island’s community-owned electricity company, has signed final agreements with AES Corp., a developer of a wide variety of generating plants, to build Hawaii’s first power plant relying on “pumped storage” technology.

Together, KIUC and AES will redevelop two existing reservoirs — one a couple thousand feet higher than the other. It will be on the west side of Kauai, near the U.S. Navy’s Pacific Missile Range Facility.

During the day, water will be pumped from the lower reservoir, Mana, to the higher one, Puu Loa, using solar power from a generating station to be built on site. Then at night, the water will flow back down by gravity, running through turbines. They will create a third reservoir in the middle, called Puu Opae.

The resulting hydro power will produce 240 megawatts overall — about a quarter of Kauai’s total electricity needs. It will have 12 hours per day of storage time to release power overnight.

While the installation will be a first in the state, Hawaiian Electric, the provider for the rest of Hawaii, will be watching the project closely, even though its own flirtations with pumped storage have yet to produce results.

Hawaiian Electric, according to spokesman Jim Kelly, considers pumped storage a proven technology. He said Hawaiian Electric commissioned a number of studies looking at its potential, most recently on Maui in 2014.

Kelly said there was an extensive study looking at Koko Crater on Oahu in the 1980s.

Though nothing has come of the studies, other energy companies endorse the idea. A huge Danish energy infrastructure venture capital firm and a Montana start-up that hopes to develop a half-dozen pumped storage stations in the Northwest both say the technology holds great national promise in the rush to replace fossil fuel generating technology — especially coal — with cleaner alternatives.

The Montana start-up Absaroka Energy, based  in Bozeman, believes that pumped storage can seamlessly replace coal-fired plants by using wind to generate the power necessary to run the pumped storage installations and connect them to complex electricity transmission networks that already exist. In a sense, that’s what KIUC is doing because it has a high-voltage network already in place on the west side.

Hawaiian Electric faces the challenge of replacing its coal-fired generating station on Oahu, the only coal plant in the state. Ironically, the Oahu plant — which Hawaiian Electric intends to shut down by late 2022 — is operated by AES, the company that will build Kauai’s pumped storage facility.

“I’m not sure anyone in the country is as advanced” as the plant AES will build on Kauai, said Woody Rubin, AES clean energy development officer.

The company has already constructed two solar facilities for KIUC. “There is solar and battery in Hawaii, but not in this unique configuration,” he said.

Brad Rockwell, KIUC’s executive manager for utility operations, said Kauai is out in front of the rest of the state — and much of the country — because it is a small organization that can afford to experiment with new technologies and be nimble in the way it brings them into the island’s energy mix.

“I always go back to our structure,” he said, “being a co-op and having locally elected board members who are more in tune with the community. There’s a clear line you can draw between what the community wants and is best for the community and the understanding the board has.”

Rockwell said remaining regulatory approvals may occupy most of 2021, with actual construction beginning in 2022. KIUC’s application was filed with the state Public Utilities Commission on Dec. 31, and is currently pending commission review.

Correction: An earlier version of this story incorrectly stated that final PUC approval has already been secured.

He said he knows pumped storage technology is being pursued elsewhere in the country, but the technology is taking off gradually and Kauai will have the most advanced operation in the nation.

That won’t remain the case for very long if Absaroka Energy and Copenhagen Infrastructure Partners have their way. Absaroka is trying to build an extensive network of wind-based pumped storage plants in a large area of the Northwest including Montana, Washington and Oregon.

None of the facilities have started construction and most still don’t have the required federal government permits to come into existence, said Rhett Hurless, Absaroka’s vice president. The Absaroka projects would be situated along existing high-voltage transmission networks that are currently fed by coal-fired generating stations.

Copenhagen Infrastructure Partners, said Hurless, has been snapping up proposed pumped storage projects in several states. The Danish firm has invested heavily in Absaroka’s projects along with other groups vying to replace coal-fired power. The company did not respond to requests for comment.

Absaroka’s most advanced project — still several years from actually going into service — is in the Gordon Butte area of Montana and would be able to generate 400 megawatts.

Like all hydro power, said Hurless, pumped storage has an advantage in that it can be turned off and on as if operated by a light switch. The Gordon Butte project, he said, will be able to ramp up from running almost at idle to working at full capacity in about 20 seconds.

KIUC has a couple of hydro plants — very old facilities that date to the sugar plantation eras — that also have the same fast turn-on capability.

Kelly said Kauai is fortunate in that it has remaining expanses of former agricultural lands on which new renewable power systems can be constructed. That’s not so true on Oahu and elsewhere in Hawaiian Electric’s territory, but what is clear is that Kauai is moving ahead to actually build a pumped storage plant while the rest of the country is still trying to catch up.

Actual construction on the Kauai system might start in as little as a year. KIUC’s CEO, David Bissell, said in a media statement that “the project’s integration of pumped storage with large-scale solar power is unique in the energy industry.”

“Our concept is the same” as what KIUC is building on Kauai, said Hurless. “You have a lower swimming pool and an upper swimming pool and there are a lot of wind farms feeding into a very large transmission line that comes from a coal power plant.”

KIUC, Amaraska and Copenhagen Infrastructure Partners are playing a very long game. All think they have hit upon one of the holy grails of the national challenge of weaning the country off of fossil-fuel powered electricity.

How long might it be before the country sees the dramatic change that KIUC is about to bring to Kauai? “About 15 years,” said Hurless.