When Hawaii’s coal-fired power plant in Kapolei triggered a massive disruption of power on Oahu’s electrical grid earlier this week, it caused a big snafu for Hawaiian Electric Co.

The AES facility, which was producing 180 megawatts of power, suddenly dropped to 80 MW on Monday afternoon. HECO responded by cranking up a unit at its Kahe generating station, which produces about 89 megawatts.

But after about 15 minutes, the Kahe generator tripped off line, too. With the energy supply on the grid suddenly plummeting relative to demand, HECO’s system cut power to thousands of customers spread out on Oahu to avoid a bigger grid problem.

The result: Some 20,000 customers lost power for about six minutes, says Jim Kelly, HECO’s vice president of corporate relations.

Although this week’s incident isn’t unique to AES Hawaii, the outage underscores the plant operator’s importance to Oahu. The facility provides as much as 25 percent of the island’s electricity. So when AES Hawaii trips off the grid, it can create big challenges for the state’s biggest utility.

Unfortunately for HECO, in the not-too-distant future, AES Hawaii is poised to leave the grid –permanently.

AES’s contract to provide power to HECO is set to expire in 2022, a result of state policy to produce 100 percent of its electricity from renewable sources by 2045. While HECO faces the tough question of how to replace a facility that produces 180 megawatts of steady power, AES is trying to figure out whether it can convert its power plant from coal to renewables.

“AES Hawaii is currently exploring options to re-purpose the AES Facility to utilize greater amounts of renewable materials such as biomass,” the company said in a filing to the Hawaii Public Utilities Commission earlier this year. “Re-purposing the existing AES Facility to utilize biomass or other fuel conversion may provide a better value for ratepayers because the infrastructure has already been paid for.”

AES has expressed a desire too switch to a cleaner fuel but has provided no details.

For clean energy advocates, the pending closure is hardly bad news.

“This is a good thing for Hawaii and Oahu,” said Jeff Mikulina, executive director of the Blue Planet Foundation, a local nonprofit pushing for Hawaii’s transition to renewable energy.

Based on data from the Hawaii Department of Business, Economic Development and Tourism, Mikulina estimates AES has burned about 39 billion pounds of coal since it opened in 1992. The plant’s annual consumption of about 781,000 tons annually, breaks down to about 4.3 pounds of coal burned at AES per Oahu resident per day.

And Mikulina adds, “Coal is absolutely the worst source of electricity in terms of greenhouse gas emissions.”

Coal Is Dirty But Cheap

But as bad as coal might be for the environment, there’s one thing about coal that’s attractive: it’s cheap, a fraction of the cost of oil or renewables like biomass, says Mattias Fripp, a professor of electrical engineering at the University of Hawaii Manoa. And that’s hard to ignore in a state where electricity rates are the nation’s highest.

In 2016, Hawaiian Electric paid AES Hawaii an average of 5 cents per kilowatt hour. During the same period, wind was about 20 cents per kilowatt hour. Solar from different facilities at that time was about 21 to 23 cents, although under a recently announced contract some of HECO’s solar prices are expected to fall to about 11 cents per kilowatt hour.

Those costs, Fripp says, justify keeping the coal plant running.

As Hawaii pushes toward its 100 percent renewable goal, HECO has the enormously complicated challenge of figuring out how to cobble together energy sources that include its own facilities, plus private wind and solar farms and tens of thousands of rooftop solar systems.

Adding to the complexity is that the grid needs a generally stable flow of electricity to run smoothly. Ideally, HECO can keep a steady flow, known as frequency, on the grid.

The problem is that the frequency can rise and fall depending on how many people are using electricity and what sources are pumping power out onto the grid. Renewables like wind and power go up and down based on the time of day and the weather, which makes them both variable and unpredictable, Fripp says.

And that poses a challenge for HECO operators trying manage the power on the grid.

Dave DeViney, HECO’s supervisor of load dispatch who tries to keep the frequency steady, likened the challenge to the runaway bus in the movie “Speed,” which has to be kept running at 50 miles per hour or it will explode. If the frequency gets too low, HECO has to cut power to customers — or “shed load” in utilities jargon — to get the frequency back up to speed.

“When we have a sudden failure to one or more of our larger generation units we have a situation of having too much load and not enough generation, so the frequency starts to decrease,” DeViney said. “Then at a certain (frequency) value, the system automatically starts to shed load to save what generation we have. This has to happen in a matter of seconds.”

In contrast to the variable and unpredictable power produced by wind and solar, the power produced by big power plants like AES Hawaii, which burns coal to run a steam turbine, is usually steady. That makes it appealing for people like DeViney.

Fripp said he has created an analytical model to analyze the best mix of resources for Oahu. When Fripp doesn’t factor in Hawaii’s renewables policy, the result clearly favors the status quo, he said.

“And when I run that model,” Fripp said, “it says to keep running that plant burning coal to keep costs down.”

The problem, of course, is that the Hawaii policy makers don’t want coal.

HECO has not said explicitly how it plans to make up for the 180 megawatts it will lose if AES Hawaii shuts down, as planned.

In a power supply plan submitted to the Public Utilities Commission, HECO has proposed two new fossil fuel generation sources on military bases: a 100-megawatt Pearl Harbor-Hickam plant and a 54-megawatt plant at Kaneohe Marine Corps Base. But the commission noted that numerous parties had objected to the proposal, including the Hawaii Department of Business, Economic Development and Tourism.

“DBEDT is concerned that the manual selection of these plants ‘will box out/influence the renewable options chosen in the future,’” the commission wrote.

Plus, there’s another factor, said Kyle Datta, general partner with the Ulupono Intitiative, a Hawaii-based social investment firm focused on renewable energy, local food production and waste management. The military projects, Datta says, are simply more expensive than other options.

UH’s Fripp says that it might make sense intuitively to replace the big AES Hawaii plant, which could take a day to start up, with numerous smaller plants, which can start and stop much more quickly.

But Fripp said he does not believe the smaller plants make economic sense in light of Hawaii’s push to rewewables. A more cost effective plan, Fripp said, is to use renewables and batteries to store energy for later use.

Plus, he said, HECO can manage demand in a way that more customers will use renewables like solar when they are available. For example, industrial activities like heating and pumping water can be done during the day when the sun is out, he said.

In the meantime, what AES plans to do is a mystery.

At one point, HECO had proposed that AES phase in the use of biomass, typically agriculture products that can be burned, to partially replace coal. That plan, which would have let AES use 50 percent coal and 50 percent biomass, would have extended AES’s contract to 2030. But HECO’s revised power supply plan indicated the AES contract simply would not be renewed in 2022.

In 2016, HECO paid AES Hawaii an average of 5 cents per kilowatt hour. During the same period, wind was about 20 cents per kilowatt hour, solar about 21 to 23 cents.

In its statement to the Public Utilities Commission, AES offered no specifics, although it raised the idea of using a fuel that the state has clearly rejected.

“To the extent the Commission provides for the ability to use LNG (liquefied natural gas) in the future,” the company wrote, “AES Hawaii would like the opportunity to re-purpose the AES Facility to use LNG.”

But AES’s specific plans, if it has any, are anyone’s guess. In a statement, AES said simply, “AES Hawaii is committed to meeting the State’s goal of generating electricity through renewable sources that are reliable and low cost. We are working diligently on plans to transition to a more environmentally friendly fuel source for production of electricity at our Kapolei facility past 2022.”

HECO officials say they have not heard a specific plan from AES.

“It surprises me that I’ve never heard a word from AES about what they think they should be doing,” Fripp said.