The Guam Power Authority is reviewing whether to require energy storage systems for all future solar power customers in light of intermittency, or power fluctuations, within the power grid.
"It would be substantially cheaper for the utility to provide (energy storage systems) than for individual customers to install theirs," GPA General Manager John Benavente told The Guam Daily Post.
When asked to clarify if this meant the utility was contemplating providing batteries to customers, Benavente said there is no decision at this time.
GPA, through a notice on its website, stated ratepayers may experience short outages due to loss of a generator or solar farm. This notice explains intermittency. Because solar power relies on the availability of the sun, it can lead to sudden drops and rises in energy depending on the weather. Issues arising from that intermittent power become exacerbated when there is a large amount of solar energy fed into the power grid.
"Guam’s islandwide power system operates as an isolated grid, making it more vulnerable to issues that may result in a total system blackout," the notice stated. "One issue includes the sudden loss of generation. This may arise from a generator going offline or a large reduction in power output from solar photovoltaic systems, due to passing clouds or inverter trips."
To prevent an islandwide blackout, the grid automatically relinquishes load in order to make up the balance from a sudden drop in power. GPA uses an "under frequency load shedding" scheme to manage these fluctuations. The utility groups feeders in 20- to 25-megawatt blocks, which are rotated by engineers to provide some relief to customers placed into the scheme.
Some critical feeders, such as those serving hospitals, are not part of the scheme.
There are about 22 megawatts of distributed solar power on island coming from individual customers.
"They are causing intermittency issues because almost all of them don’t have energy storage," Benavente said, leading to the mandatory storage proposal.
But GPA also has utility-scale solar projects that cause intermittency, such as the Dandan solar farm in Inarajan. Renewable integration experience, especially for solar photovoltaic systems, was limited when the project was awarded, Benavente said.
"Conventional wisdom at the time was that systems could absorb up to 15% of intermittent renewables without major issues. This proved to be wrong for small systems like Guam," Benavente said.
The utility was procuring a 40-megawatt battery to alleviate outages due to the loss of baseload generators in 2015 – due to the explosion at Cabras 3 and 4.
But GPA has adjusted those plans to address the intermittency issue stemming from Dandan and distributed solar, Benavente said. Batteries will now be used to steady the frequency within the grid.
A 24-megawatt battery is being installed in Hagåtña to help smooth the intermittency from distributed solar, Benavente said. A separate 16-megawatt battery is being installed in Talofofo to alleviate fluctuations coming from Dandan.
"Both (battery energy storage systems) totaling 40 megawatts are targeted to begin operational testing within the next three months and commissioning by Oct. 31," Benavente said.
All future utility-scale solar projects must include energy storage to eliminate sending intermittent energy into the grid, he added.
Cabras 3 and 4 were never used to regulate frequency in the grid, Benavente said, because they can't. Units 3 and 4 were slow-speed diesels, and GPA's intermittency issues today would remain even if these units hadn't been compromised, he added.
Currently, GPA is using its combustion turbines in the daytime to help regulate intermittency during peak times. The remaining Cabras units, 1 and 2, regulated grid frequency for decades but were unable to keep up with photovoltaic technology, which fluctuated too suddenly for the units to handle.
The aging units regulate frequency at night, however, when photovoltaics are not operating.
Cabras 1 and 2 are to be replaced in the future, by a new power plant that also would be better capable of regulating intermittency from solar technology, according to the utility. That plant is expected to come online by 2022.