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Here's how intermittent vs. firm renewable energy sources factor into Hawaiʻi's power grid

Governor David Y. Ige

Hawaiian Electric is looking to bring more firm renewable power to the grid in the next decade. But as new utility solar plus storage projects come online, the definition and role of firm generation remain up for debate.

On Oʻahu, Kapolei Energy Storage by Plus Power is still under construction, but it will be the largest storage facility in the state when completed, according to Polly Shaw, director of policy and communications for Plus Power.

"This Kapolei Energy Storage facility at scale will provide a great absorption of the noonday solar energy to be ready when the evening peak comes on," she told HPR.

Battery storage is an important piece of the energy puzzle because it adds stability to variable energy sources, like wind and solar. Traditionally, those resources only provided energy when the wind was blowing or the sun was out.

Renewable energy usually falls into two camps: intermittent or firm. Intermittent sources like solar and wind are weather dependent and energy limited. Firm sources can generate power 24/7, whenever needed.

The waste-fueled H-POWER on Oʻahu is considered a firm renewable, according to state law, as would a biomass, biodiesel or geothermal plant. But storage technology like Kapolei Energy Storage is making those definitions more complicated.

crane energy storage construction
Savannah Harriman-Pote
The Kapolei Energy Storage facility is scheduled to be completed by Spring 2023.

"Battery plus solar is a pretty reliable resource. If you just buy a battery, well it's not a renewable at all. But if you're gonna feed solar into it, and solar plus batteries, is that a firm renewable? Well, depends on your definition," said Matthias Fripp, an associate professor of electrical engineering at the University of Hawaiʻi at Mānoa.

He also serves on the technical advisory board for Hawaiian Electric's integrated grid planning process. Fripp pushes back against any hard line in the sand between what is and is not a firm renewable.

"I would say that there's not a clear definition. Anytime someone comes to you using that kind of terminology — firm, renewable — you need to probe a little more deeply and ask how are they defining it?" Fripp said.

Hawaiian Electric maintains that solar and wind paired with battery storage do not meet its definition of firm power. This year, HECO put out the call for bids for 500 to 700 megawatts of new firm generation on Oʻahu.

"Storage is typically limited to about four hours and that's what we seek in our request for proposals," said Rebecca Dayhuff-Matsushima, vice president of resource procurement at Hawaiian Electric. "Long-duration storage, things that would last days or weeks on end, it's very expensive and still in its infancy in technology development. So there is still that need for firm generation that's available 24/7."

Fripp agrees there’s a role for this kind of firm power, but thinks people should be careful not to overestimate the power needed. Even though the main advantage of firm power is that it can be used 24/7, Fripp says it would be incredibly expensive to actually use a new renewable firm generation plant all of the time.

"I don't even want to think how expensive that would make power. It would be bad. But again, if you're only using it 3% to 5% of the year, it's not going to have a big effect on bills," Fripp said. "If you use my definition of what they need, which is something you can turn on when you need to, but aren't going to run all the time, then it looks to me like we need something like 150 megawatts of that."

Unlike Hawaiʻi's current fossil fuel plants, Dayhuff-Matsushima says HECO does not plan on using new firm renewable plants around-the-clock.

"What we're looking for is flexible, firm, renewable generation that we can turn up and down, that we can turn off when it's not needed, but is there when it is needed so that we can ensure that we're able to provide safe, resilient, reliable power for the island," she told HPR.

Solar and wind will still be out in front, says Dayhuff-Matsushima, but firm generation will be able to pick up the slack.

FILE - Clearway Energy Group's utility-scale solar and storage facility near Mililani on Oʻahu.
Office of Gov. David Ige
FILE - Clearway Energy Group's utility-scale solar and storage facility near Mililani on Oʻahu.

But some say Hawaiʻi needs to consider if that role can be filled by battery storage technology instead.

"Battery technology is already very functional, and it's improving and changing rapidly. And battery technology can store power from any source, including sources that are affected by the weather like wind and solar. And they can turn what we normally call intermittent energy into firm energy," said Colin Yost, the chief operating officer of RevoluSun, a locally owned and operated solar plus battery installation company that specializes in rooftop solar.

Kapolei Energy Storage can capture solar energy during the day for use in the early hours of the night. Yost asks, why should it stop there?

"Right now, we don't think of batteries as operating 24/7. They are usually thought of as like a four-hour battery or some other amount of storage that's guaranteed," Yost said. "When you connect thousands of batteries into a network, some portion of that power can always be available 24/7."

"So it's really a question of scale and a question of technology and how you integrate those resources into the larger grid. And that's where more discussion I think is needed. Some imagination is needed in terms of how we're really going to make it work," he told HPR.

Dayhuff-Matsushima says storage just isn’t there yet. If something were to go really wrong, like a natural disaster, traditional firm generation facilities could fair better and provide power sooner.

Bringing on new firm renewable power addresses other concerns, like limits on how much land we have for new solar and wind farms. But Dayhuff-Matsushima says that solar plus storage still has an important role to play in Hawaiʻi's energy strategy.

"Being able to use those facilities, solar plus storage, or any other type of intermittent plus storage facility, allows us to lessen the amount of firm generation that we have to use. And that's why even though we're going out and we're seeking new firm generation that's renewable, we are going to be retiring more fossil fuel generation," Dayhuff-Matsushima said.

Firm versus intermittent isn’t just semantics. These definitions are important because they shape what the grid will fundamentally look like as Hawaiʻi tries to meet its clean energy goals. Dayhuff-Matsushima says it can be a bit of a high-wire act trying to measure the needs of the future against the technologies of today.

"I think a lot of people when they think of that 2045 goal of 100% renewable energy that we're going to just be done — like we're just going to have all these systems in place," she said. "But really, it will be constant because facilities that we're putting into place today with 20-year contracts will be coming up to the end of their term in 2045. Those are either going to have to be renegotiated or they're gonna have to be replaced."

"This will be an ever-evolving journey, just like any power system is. It's just that we're moving from what was traditionally an ever-evolving journey of fossil fuel to new forms of renewable technology," she added. "Right now, we also have to plan for what is available."

Can solar and wind be considered firm power? HPR's Savannah Harriman-Pote takes a closer look.
Extended segment on The Conversation - Sept. 14, 2022

Savannah Harriman-Pote is the energy and climate change reporter. She is also the lead producer of HPR's "This Is Our Hawaiʻi" podcast. Contact her at sharrimanpote@hawaiipublicradio.org.
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